Multinuclear Solid-State NMR of Inorganic Materials

PERGAMON MATERIALS SERIES VOLUME 6 Multinuclear Solid-State NMR of Inorganic Materials PERGAMON MATERIALS SERIES Ser...

Author: Kenneth J.D. MacKenzie | M.E. Smith

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PERGAMON MATERIALS SERIES VOLUME 6

Multinuclear Solid-State NMR of Inorganic Materials

PERGAMON MATERIALS SERIES Series Editor: Robert W. Cahn VRS Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, UK

Vol. 1 Vol. 2 Vol. 3 Vol. 4 Vol. 5 Vol. 6

C ALPHAD by N. Saunders and A. P. Miodownik Non-Equilibrium Processing of Materials edited by C. Suryanarayana Wettability at High Temperatures by N. Eustathopoulos, M. G. Nicholas and B. Drevet Structural Biological Materials edited by M. Elices The Coming of Materials Science by R. W. Cahn Multinuclear Solid-State NMR of Inorganic Materials by K. J. D. MacKenzie and M. E. Smith

A selection offorthcoming titles in this series: Underneath the Bragg Peaks: Structural Analysis of Complex Materials by T. Egami and S. L. J. B illinge Phase Transformations in Titanium- and Zirconium-Based Alloys by S. Banerjee and P. Mukhopadhyay Thermally Activated Mechanisms in Crystal Plasticity by D. Caillard and J.-L. Martin Nucleation by A. L. Greer and K. F. Kelton Non-Equilibrium Solidification of Metastable Materials from Undercooled Melts by D. M. Herlach and B. Wei The Local Chemical Analysis of Materials by J. W. Martin Synthesis of Metal Extractants by C. K. Gupta Structure of Materials by T. B. Massalski and D. E. Laughlin Intermetallic Chemistry by R. Ferro and A. Saccone

P E R G A M O N MATERIALS SERIES

Multinuclear Solid-State NMR of Inorganic Materials by Kenneth J.D. MacKenzie School of Chemical and Physical Sciences, Victoria University of Wellington, and N e w Zealand Institute for Industrial Research and D e v e l o p m e n t

Mark E. Smith Department of Physics, University of Warwick, U K

2002

PERGAMON An Imprint of E l s e v i e r S c i e n c e A m s t e r d a m - Boston - London - New Y o r k - Oxford - P a r i s S a n D i e g o - S a n F r a n c i s c o - Singapore - Sydney - Tokyo

ELSEVIER SCIENCE Ltd The Boulevard, Langford Lane Kidlington, Oxford OX5 1GB, UK 9 2002 Elsevier Science Ltd. All rights reserved. This work is protected under copyright by Elsevier Science, and the following terms and conditions apply to its use:

Photocopying Single photocopies of single chapters may be made for personal use as allowed by national copyright laws. Permission of the Publisher and payment of a fee is required for all other photocopying, including multiple or systematic copying, copying for advertising or promotional purposes, resale, and all forms of document delivery. Special rates are available for educational institutions that wish to make photocopies for non-profit educational classroom use. Permissions may be sought directly from Elsevier Science Global Rights Department, PO Box 800, Oxford OX5 1DX, UK; phone: (+44) 1865 843830; fax: (+44) 1865 853333; e-mail: [email protected]. You may also contact Global Rights directly through Elsevier's home page (), by selecting 'Obtaining Permissions'. In the USA, users may clear permissions and make payments through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA; phone: (+ 1) (978) 7508400; fax: (+ 1) (978) 7504744, and in the UK through the Copyright Licensing Agency Rapid Clearance Service (CLARCS), 90 Tottenham Court Road, London W1P 0LP, UK; phone: (+44) 207 631 5555; fax: (+44) 207 631 5500. Other countries may have a local reprographic rights agency for payments. Derivative Works Tables of contents may be reproduced for internal circulation, but permission of Elsevier Science is required for external resale or distribution of such material. Permission of the Publisher is required for all other derivative works, including compilations and translations. Electronic Storage or Usage Permission of the Publisher is required to store or use electronically any material contained in this work, including any chapter or part of a chapter. Except as outlined above, no part of this work may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without prior written permission of the Publisher. Address permissions requests to: Elsevier Science Global Rights Department, at the mail, fax and e-mail addresses noted above. Notice No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be made. First edition 2002 Library of Congress Cataloging in Publication Data MacKenzie, Kenneth J. D. Multinuclear solid-state NMR of inorganic materials / by Kenneth J.D. MacKenzie, Mark E. Smith. p. cm. -- (Pergamon materials series ; v. 6) ISBN 0-08-043787-7 1. Nuclear magnetic resonance spectroscopy. 2. Inorganic compounds--Analysis. I. Smith, Mark E. II. Title. III. Series QD96.N8 M28 2002 543'.0877--dc21

2002016354

British Library Cataloguing in Publication Data A catalogue record from the British Library has been applied for. Q The paper used in this publication meets the requirements of ANSI/NISO Z39.48-1992 (Permanence of Paper). Printed in The Netherlands.

Preface Although the concept of solid state nuclear magnetic resonance with magic angle spinning (MAS NMR) has been with us for many years, it is only over the last two decades that the technique has become increasingly used in other disciplines. The advantages of being able to probe the atomic environment of poorly crystalline materials were quickly appreciated by mineralogists and zeolite chemists, who exploited the fact that the two nuclides of major interest to mineral studies (29Si and 27A1) are readily amenable to MAS NMR study. Other subject areas, including ceramics, glasses and cements, have now followed this lead, and today MAS NMR is being applied to most of the major topics in the area of inorganic materials research. In the early stages of its development, MAS NMR tended to be the preserve of specialist spectroscopists and theoreticians, but, as its possibilities have become better known, it has become very much a technique used by engineers and scientists to solve real problems. The literature, which was once found mainly in fundamental and highly technical texts, is now just as likely to be found in applied science journals, as increasing numbers of non-specialist researchers turn to MAS NMR to solve problems in real and often non-ideal systems. Arising from questions directed to us by students and colleagues wishing to use MAS NMR in just such studies, especially of inorganic materials, for some time we have been conscious of the need for a book addressed to non-specialist researchers. Ideally, such a book would provide accessible answers to the most common questions about the theory and practice of MAS NMR asked by novices, and it should also provide a more specialised and up-to-date treatment of the most important areas of inorganic materials research to which MAS NMR has application. Such a book would bring together all the theory necessary to appreciate the scope (and limitations) of the technique, together with practical details and hints which are often not published elsewhere because they seem too self-evident and mundane to established practitioners. In short, the book we have envisioned is precisely that which we would have found invaluable when we first began to use MAS NMR to study inorganic materials, and into which we would continue to delve for background information as the need arose to branch out into the study of new systems. This, then, is the philosophy behind the present book, which we hope above all will prove useful to MAS NMR users whatever their level of expertise, and whatever inorganic materials they wish to study. MAS NMR is constantly being extended to a more diverse range of materials, pressing into service an ever-expanding range of nuclides including some previously considered too intractable to provide usable results. At the

vi

Preface

same time, new developments in both hardware and software are being introduced and refined. In this book we have tried to cover the most important of these new developments without spreading ourselves too thinly over the subject matter. For this reason, the book does not include any of the extensive literature on carbon-based polymers which is already catered for by comprehensive texts. Materials science is very much an interdisciplinary exercise, and its practitioners are likely to come from a variety of scientific backgrounds. It is expected that the readers of this book will be at least slightly acquainted with some aspects of physics, chemistry, mineralogy or engineering, and some may be deeply knowledgeable in one or more aspects of these subjects. Because of the diverse nature and range of inorganic materials which have been subjected to NMR investigation (from cements, catalysts and glasses to superconductors and metals), it is beyond the scope of this book to treat in depth the most advanced chemical and physical aspects of the various materials. We have therefore confined ourselves to discussion and illustration of how solid state NMR techniques have been applied to a wide range of systems, and have provided an extensive bibliography to allow a reader with particular interests a convenient entry into the literature to gain more advanced information on any specific topic. The plan of the book is straightforward; the second chapter lays the theoretical groundwork for an understanding of the NMR experiment, including the factors which must be taken into account both when establishing the appropriate experimental conditions and when interpreting the data. Particular attention has been given to the theoretical basis of new experimental techniques such as multiple-quantum MAS NMR, which holds considerable promise for a wide range of inorganic solids. The third chapter covers the practical aspects of solid state NMR spectroscopy, while in the following chapters, the foregoing principles are applied to specific nuclides, with particular emphasis on applications of the technique to solving problems in inorganic materials science. A range of illustrative examples has been included, together with tabular data summarising the present state of the literature on the various nuclides in inorganic materials. Finally we should note that this book is possible only because of the ingenuity of the NMR researchers who have contributed to the rapid development of the subject. We are especially grateful to the many NMR colleagues who have shared their enthusiasm and expertise with us, and to members of our research groups both past and present, whose efforts have been very important. The copyright holders who gave their permission to use figures are gratefully acknowledged. MES is grateful for the support of funding bodies such as EPSRC, the Royal Society and the Leverhulme Trust, and particularly thanks Susan Holmes for her support and understanding during the preparation of this book. KM is indebted to the Royal Society of New Zealand for a James Cook Research Fellowship allowing two years to be spent in Oxford during which time the book became a reality, and to the Materials Department of Oxford

Preface

vii

University for being gracious hosts and providing the necessary facilities for this undertaking. We also wish to record our gratitude to the Series Editor, Prof. Robert Cahn, and the Editorial staff of Elsevier Science for their assistance in bringing this book to fruition. KENNETH MACKENZIE Email: kenneth.mackenzie@ vuw. ac.nz MARK SMITH Email: m.e.smith, [email protected] Oxford and Warwick, October 2001

This Page Intentionally Left Blank

Contents Preface CHAPTER 1 INTRODUCTION 1.1. Methodology of Materials Characterisation by NMR 1.2. Historical Aspects of NMR Spectroscopy 1.3. Brief Description of the NMR Experiment 1.3.1 General Principles 1.3.2 Overcoming NMR Spectral Broadening in Solids by MAS 1.3.3 Other NMR Experiments used with Solids 1.3.3.1 Decoupling 1.3.3.2 Cross-Polarisation (CP) 1.3.3.3 Spin-Echo Experiments 1.3.3.4 Two-Dimensional Experiments 1.3.4 Nuclei Suitable for NMR Spectroscopy 1.4. Further Reading References

3 6 7 7 10 11 12 12 12 12 13 17 18

CHAPTER 2 PHYSICAL BACKGROUND 2.1. Fundamental Interaction with External Magnetic Fields 2.1.1 A Quantum Mechanical Description of the Zeeman Interaction 2.1.2 Bulk Magnetisation 2.1.3 The Rotating Frame and the Application of RF Pulses 2.1.4 Observation of the NMR Signal 2.2. Internal Interactions 2.2.1 The Dipolar Interaction 2.2.2 Scalar Coupling 2.2.3 Paramagnetic Coupling 2.2.4 Chemical Shielding 2.2.5 Knight Shift 2.2.6 Quadrupole Interaction 2.2.7 Nature of Interactions

23 25 26 29 34 35 37 40 43 44 48 50 57

x

Contents

2.3.

58 59 59 61 63 64

One Dimensional Methods for Improving Resolution 2.3.1 Magic Angle Spinning and First-Order Effects 2.3.1.1 Physical Principles 2.3.1.2 Formation of Spinning Sidebands 2.3.2 Magic Angle Spinning and Higher-Order Effects 2.3.2.1 MAS of Second-Order Quadrupole Effects 2.3.2.2 Residual Coupling Effects due to Quadrupolar Nuclei in MAS Spectra 2.3.2.3 Nonequivalent Homonuclear Spins 2.3.3 Variable Angle Spinning 2.3.4 Double Angle Spinning 2.3.5 Multiple Quantum Transitions 2.3.6 Ultrasonically-Induced Narrowing 2.4. Dipolar Decoupling 2.4.1 Heteronuclear Dipolar Decoupling 2.4.2 Homonuclear Dipolar Decoupling 2.5. Spin-locking 2.6. Cross-Polarisation 2.7. Two-Dimensional Methods 2.7.1 Dynamic Angle Spinning 2.7.2 2D MQMAS 2.8. NMR Relaxation 2.8.1 Introduction to Relaxation 2.8.2 Mechanism for Relaxation Processes References

71 74 74 75 77 78 78 78 79 83 85 90 92 93 98 98 101 105

CHAPTER 3 EXPERIMENTAL APPROACHES 3.1. Basic Experimental Principles of FT NMR 3.2. Instrumentation 3.2.1 Overview of a Pulsed FT NMR Spectrometer 3.2.2 Magnets 3.2.3 Shimming 3.2.4 Transmitters 3.2.5 Probes 3.2.6 Connection of the Probe 3.2.7 Signal Detection 3.2.8 Additional Equipment

111 112 112 113 115 116 120 122 124 127

Contents

3.3.

Practical Acquisition of NMR Spectra 3.3.1 Processing the FID to Produce a Spectrum 3.3.1.1 Window Functions 3.3.1.2 Shifting of the Time Origin and Linear Back Prediction 3.3.1.3 Zero Filling 3.3.1.4 Phase Correction 3.3.1.5 Baseline Correction 3.3.2 Complications in Recording Spectra 3.4. Static Broad Line Experiments 3.4.1 Pulsed Echo Experiments 3.4.2 Stepped Experiments 3.5. One-Dimensional High Resolution Techniques 3.5.1 Magic Angle Spinning (MAS) 3.5.2 Extraction of Parameters from MAS NMR Spectra 3.5.3 Suppression of Spinning Sidebands 3.5.4 Special Considerations for MAS of Quadrupolar Nuclei 3.5.5 Magic Angle Spinning Observation of Satellite Transitions 3.5.6 Double Angle Rotation of Quadrupolar Nuclei 3.5.7 Practical Implementation of CRAMPS 3.6. Two-Dimensional Experiments 3.6.1 Nutation NMR 3.6.2 Off-Resonance Nutation 3.6.3 Order-Resolved Sideband Spectra 3.6.4 Dynamic Angle Spinning (DAS) 3.6.5 Two-Dimensional Sequences Developed from Solution NMR 3.6.6 Multiple Quantum Experiments in Dipolar Coupled Systems 3.6.7 Multiple Quantum NMR Experiments of Non-Integer Spin Quadrupolar Nuclei 3.6.8 2D XY Correlation Methods 3.6.9 Correlation of Tensor Information- Separated Local Field Experiments 3.7. Summary of Approaches for Examining Quadrupole Nuclei 3.8. Multiple Resonance 3.8.1 Cross-Polarisation (CP) 3.8.2 SEDOR, REDOR and TEDOR 3.8.3 TRAPDOR and REAPDOR 3.9. Techniques for Determining Relaxation Times and Motional Parameters 3.9.1 Measurement of T1

xi 127 128 128 129 129 130 130 130 133 133 136 138 138 143 143 144 149 150 152 153 153 154 155 156 157 160 161 168 170 172 172 173 178 182 183 183

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Contents

3.9.2 Other Spin-Lattice Relaxation Times (TI~, T1D) 3.9.3 Transverse Relaxation Times (Y2) 3.9.4 Molecular Motion 3.9.5 Diffusion Measurements 3.10. NMR Under Varying Physical Conditions 3.10.1 Variable Temperature NMR 3.10.2 High Pressure Experiments References

CHAPTER 4 298I NMR 4.1. General Considerations 4.1.1 Broadening Effects in 298i Spectra 4.1.2 Relaxation Effects in 298i Spectra 4.1.3 Effect of Structure on 298i Spectra 4.2. Si-O Compounds 4.2.1 Relationships between 29Si NMR Spectra and Structure/Bonding 4.2.2 Four-Coordinated Si-O-Compounds 4.2.3 Tetrahedral 298i Chemical Shifts in Silicates 4.2.4 29Si Chemical Shifts in Aluminosilicates 4.2.5 Effects of Other Nearest Neighbours on the 298i Shift 4.3. Order-Disorder Effects in Minerals 4.4. Identification of Silicate Minerals 4.5. Thermal Decomposition of Silicate Minerals 4.6. Relationships between 298i Chemical Shift (d) and Structure 4.6.1 Relationships between 6 and the Si-O Bond Length 4.6.2 Relationships between 6 and the Si-O-Si Bond Angle 4.6.3 More Complex Relationships between 6 and the Structure 4.7. Five and Six-Coordinated Si-O Compounds 4.8. Cross-Polarisation (CPMAS) Experiments 4.8.1 Cross-Polarisation between ~H and 298i 4.8.2 Cross-Polarisation between 19F and 298i 4.8.3 Other Cross-Polarisation Experiments with 298i 4.9. Glasses, Gels and Other Amorphous Materials 4.9.1 Silicate Glasses 4.9.2 Deconvolution of 298i NMR Spectra 4.9.3 Connectivities in Glass 4.9.4 Chalcogenide Glasses

184 185 186 187 187 187 189 190

201 201 202 204 205 205 205 205 206 208 208 212 214 217 218 219 223 225 227 227 229 229 230 231 235 236 238

Contents

xiii

4.9.5 Gels 4.9.6 Other Amorphous Materials 4.10. Si-N and Si-N-O Compounds 4.11. Si-A1-O-N Compounds 4.11.1 fl-Sialon, Si6_zAlzOzNs_z 4.11.2 O-Sialon, Si2_xAlxOl+xNz_x 4.11.3 X-Sialon, nominally Si12AllsO39Ns 4.11.4 Polytypoid Sialons, (Si,A1)m(O,N)m+l 4.11.5 oL-Sialons,MxSilz_(m+n)Alm+nOnN16-n 4.12. Other Metal Silicon Nitrides and Oxynitrides 4.13. Si-C, Si-C-O and Si-C-N Compounds 4.13.1 Silicon Oxycarbide Species 4.13.2 Silicon Carbonitride Species 4.14. Other Materials 4.14.1 Biologically Compatible Glasses 4.14.2 Cements 4.14.3 Inorganic Polymers References

240 242 244 247 247 25O 251 253 253 253 255 256 257 257 257 257 259 260

CHAPTER 5 27AL NMR 5.1. General Considerations 5.2. Chemical Shifts in 27A1 Spectra 5.2.1 27A1Chemical Shifts in A1-O Environments 5.2.2 27A1Chemical Shifts in Aluminosilicates 5.2.3 Relationships between 27A1 Chemical Shift (giso) and Structure 5.3. Five-Coordinated A1-O 5.3.1 A1(v) in Well-Defined (Crystalline) Environments 5.3.2 A1(v) in Non-Crystalline Environments 5.3.3 A1(v) in Zeolites 5.4. Aluminium Oxides 5.5. Amorphous Aluminium Compounds 5.5.1 Aluminate Gels 5.5.2 Glasses 5.5.3 Other Amorphous Systems 5.6. Aluminophosphates 5.7. Aluminium Borate and Molybdate 5.7.1 Aluminium Borate 5.7.2 Aluminium Molybdate

271 272 273 274 279 281 281 283 287 291 294 294 299 303 304 307 307 307

xiv

Contents

Aluminium Fluorides 5.9. Thermal Decomposition Reactions 5.10. Cements 5.11. Nitride and Oxynitride Compounds 5.12. Sialon Compounds 5.12.1 Polytypoid Sialons 5.12.2 [3-Sialons 5.12.3 O-Sialons 5.12.4 X-Sialons 5.12.5 oL-Sialons 5.12.6 Sialon Glasses References

308 310 313 316 317 317 318 320 321 322 323 324

CHAPTER 6 170 NMR 6.1. Introduction 6.2. Background 6.2.1 Enrichment Schemes 6.2.2 Experimental NMR Methodology 6.2.3 Relationships between NMR Parameters and Structure 6.3. Binary Oxides 6.3.1 Crystalline Materials 6.3.2 Sol-Gel Produced Samples 6.4. Crystalline Ternary Ionic Systems 6.5. Silicates and Germanates 6.5.1 Crystalline Materials 6.5.1.1 Silica and Germania 6.5.1.2 Ternary Silicates 6.5.1.3 Silicates and Germanates of Zirconium and Titanium 6.5.2 Amorphous Materials 6.5.2.1 Silica and Germania 6.5.2.2 Metal Silicate and Germanate Glasses 6.5.2.3 Gel-Based Silicates 6.6. Aluminium- and Gallium-Containing Systems 6.6.1 Alumina and Aluminates 6.6.2 Crystalline Alumino- and Gallosilicates 6.6.3 Amorphous Aluminosilicates 6.7. Boron-Containing Systems 6.7.1 Borates

333 334 334 337 346 349 349 352 355 359 359 359 361 365 366 366 367 369 372 372 375 379 381 381

5.8.

Contents

6.7.2 Ternary and Quaternary Systems 6.8. Other Systems 6.9. Hydrogen-Containing Samples 6.9.1 Crystalline Hydroxides and Other Hydrogen-Containing Materials 6.9.2 Hydrous Gels and Glasses 6.10. High Temperature Ceramic Superconductors References

CHAPTER 7 NMR OF OTHER COMMONLY STUDIED NUCLEI 7.1. 23NaNMR 7.1.1 General Considerations 7.1.2 23NaNMR Spectra of Sodium Compounds 7.1.3 Relationships between the 23Na Chemical Shift and Structural Parameters 7.1.4 23NaNMR of Crystalline Materials 7.1.5 23NaNMR Studies of Thermal Reactions 7.1.6 23NaNMR of Glasses 7.1.6.1 Silicate and Aluminosilicate Glasses 7.1.6.2 Sodium Borosilicate Glasses 7.1.6.3 Sodium Borate, Germanate and Tellurite Glasses and Melts 7.1.6.4 Phosphate Glasses 7.1.6.5 Miscellaneous Glass Studies 7.1.7 23NaNMR of Zeolites 7.2. llBNMR 7.2.1 General Considerations 7.2.2 11B NMR of Crystalline Compounds 7.2.3 I~B NMR of Glasses 7.2.4 ~B NMR of Zeolites 7.3. 31PNMR 7.3.1 Relationships between 31p NMR Parameters and Structure 7.3.2 31p NMR of Glasses 7.3.2.1 Binary Phosphate Glasses 7.3.2.2 Phosphosilicate Glasses 7.3.2.3 Alkali Borophosphate Glasses 7.3.2.4 Borosilicophosphate Glasses 7.3.2.5 Phosphoaluminosilicate Glasses 7.3.2.6 Alkali Phosphoaluminoborosilicate Glasses

XV

382 384 386 386 387 388 390

399 399 399 403 406 412 413 413 414 415 415 416 418 420 420 421 424 431 432 438 441 441 443 445 445 446 447

xvi

7.3.3 7.3.4 References

Contents

7.3.2.7 Phosphorus Chalcogenide Glasses 31p NMR of A1PO4 Molecular Sieves 31p NMR of Biomaterials

CHAPTER 8 NMR OF LOW-~/NUCLIDES 8.1. General Considerations 8.1.1 Problems Associated with Low-y Nuclei 8.2. NMR of Spin-1/2 Nuclei 8.2.1 89y NMR 8.2.2 l~ and l~ NMR 8.2.3 183WNMR 8.3. Quadrupolar Nuclei 8.3.1 laN NMR 8.3.2 25Mg NMR 8.3.3 33S NMR 8.3.4 35C1and 37C1 NMR 8.3.5 39K NMR 8.3.6 43CaNMR 8.3.7 47Ti and 49Ti NMR 8.3.8 67ZnNMR 8.3.9 91ZrNMR 8.3.10 95Mo and 97Mo NMR 8.3.11 135Baand 137Ba NMR 8.3.12 Other Miscellaneous Low-y Nuclei References

CHAPTER 9 NMR OF OTHER SPIN-l/2 NUCLEI 9.1. Introduction 9.2. Abundant High-~/Nuclei 9.2.1 ~H NMR 9.2.1.1 Background to Proton Studies in Inorganic Materials 9.2.1.2 Studies of Stoichiometric Protons in Crystalline Materials 9.2.1.3 Non-Stoichiometric Proton Environments in Crystalline and Glassy Materials

447 448 450 452

461 461 462 462 469 473 475 475 479 488 491 495 502 505 511 514 516 522 525 526

535 536 536 536 539 542

Contents

9.3.

9.2.1.4 1H NMR of Hydrous Glasses 9.2.1.5 Biomineral-Related Materials 9.2.2 19F NMR 9.2.2.1 Introduction 9.2.2.2 Simple Inorganic Fluorides 9.2.2.3 More Complex Fluorides 9.2.2.4 Applications to Fluoroapatite Studies 9.2.2.5 Fluorine in Aluminosilicate Minerals and Related Materials 9.2.2.6 Surface Interaction of Fluorine with Silica- and Alumina-Based Materials 9.2.2.7 Fluorine in Alumino- and Gallophosphates 9.2.2.8 Fluorine in Oxygen-Containing Glasses 9.2.2.9 Fluoride Glasses 9.2.2.10 Fluorine in Other Materials 9.2.2.11 Fluorine as a Source of Cross-Polarisation 9.2.2.12 Summary of 19F Shift Trends and Other NMR Properties Dilute or Medium-~/Nuclei 9.3.1 13C NMR 9.3.1.1 13C NMR of Elemental Carbon 9.3.1.2 Silicon Carbide 9.3.1.3 Other Binary Carbides 9.3.1.4 Ternary and Quaternary Carbides 9.3.1.5 Carbonates 9.3.2 15NNMR 9.3.2.1 Nitrides 9.3.2.2 Silicon Aluminium Oxynitride Ceramics and Glasses 9.3.2.3 Nitride Ceramics from Polymeric Precursors 9.3.2.4 Nitrates and Nitrites 9.3.3 77Se NMR 9.3.4 111Cd and 113CdNMR 9.3.5 ll5Sn, ll7Sn and ll9Sn NMR

9.3.6

9.3.5.1 Crystalline Oxygen-Containing Materials 9.3.5.2 Oxide Solid Solutions and Glasses 9.3.5.3 Non-oxide Materials 123Te and 125TeNMR 9.3.6.1 Crystalline Tellurides 9.3.6.2 Crystalline Tellurites and Tellurates 9.3.6.3 Glassy Tellurium-Containing Materials

xvii 545 550 550 550 551 554 555 556 557 559 559 560 562 562 562 563 563 563 568 570 572 572 574 575 576 579 582 583 587 591 591 594 595 598 598 599 601

xviii

Contents

9.3.7 129Xe NMR 9.3.8 195pt NMR 9.3.9 199Hg NMR 9.3.10 2~ and 2~ NMR NMR 9.3.11 2~ 9.3.11.1 Correlations between 2~ Chemical Shifts and Structure 9.3.11.2 2~ NMR of Crystalline Lead Compounds 9.3.11.3 2~ NMR of Lead-Containing Glasses 9.3.11.4 2~ in Sol-Gel Prepared Ceramics References

CHAPTER 10 NMR OF OTHER QUADRUPOLAR NUCLEI 10.1. 6Li and 7Li NMR 10.1.1 General Considerations 10.1.2 6'7LiNMR of Crystalline Solids 10.1.3 Relation between 6Li Chemical Shifts and Structure 10.1.4 6'7LiNMR of Fast Lithium Ion Conductors 10.1.5 6'7LiNMR of Glasses 10.2. 9Be NMR 10.3. 5iV NMR 10.3.1 General Considerations 10.3.2 5iV NMR of Vanadium Oxides and the Vanadates 10.3.3 5~V NMR of Zeolites and Catalysts 10.4. 63CHand 65CHNMR 10.4.1 63CHNMR of Superconductors and Superfast Ionic Conductors 10.5.

10.6.

10.7. 10.8.

69Ga and 71Ga NMR 10.5.1 General Considerations 10.5.2 69'71GaNMR of Crystalline Compounds 10.5.3 69'71GaNMR of Other Compounds 87Rb NMR 10.6.1 General Considerations 10.6.2 87RbNMR of Crystalline Compounds 10.6.3 87RbNMR of Rubidium Fullerides 93Nb NMR 133Cs NMR 10.8.1 General Considerations

601 603 604 604 607 607 609 613 615 616

629 629 630 634 636 638 639 642 642 642 646 649 650 653 653 655 657 658 658 658 661 662 665 665

Contents

10.8.2 10.8.3 10.8.4

133CsNMR of Crystalline Caesium Compounds

133CsNMR of Minerals and Zeolites 133CsNMR of Fullerides, Superionic Conductors and Semiconductors 10.9. 139LaNMR References

xix 666

669 673 674 678

CHAPTER 11 SOLID STATE NMR OF METALS AND ALLOYS 11.1. Introduction 11.2. Experimental Approaches 11.3. Metallic Elements 11.4. Intermetallic Alloys 11.5. Phase Transformations, Ordering and Defect Sites 11.6. Phase Composition and Precipitation 11.7. Atomic Motion References

687 689 691 693 696 698 700 701

SUBJECT INDEX MINERAL INDEX

703 721


Chapter 1

Introduction 1.1. Methodology of Materials Characterisation by NMR 1.2. Historical Aspects of NMR Spectroscopy 1.3. Brief Description of the NMR Experiment 1.3.1 General Principles 1.3.2 Overcoming NMR Spectral Broadening in Solids by MAS 1.3.3 Other NMR Experiments used with Solids 1.3.3.1 Decoupling 1.3.3.2 Cross-Polarisation (CP) 1.3.3.3 Spin-Echo Experiments 1.3.3.4 Two-Dimensional Experiments 1.3.4 Nuclei Suitable for NMR Spectroscopy 1.4. Further Reading References

3 6 7 7 10 11 12 12 12 12 13 17 18


Chapter 1

Introduction Solid state NMR spectroscopy is a powerful technique capable of providing information both about the structure of materials and about the dynamics of processes occurring within those materials. The focus of this book is on the application of NMR spectroscopy to structural studies of materials rather than their dynamics. NMR spectroscopy has been applied to the study of a wide range of materials including polymers, organic compounds, organometallics and foodstuffs, but these are outside the scope of this book, which has been restricted to the range of diverse inorganic materials amenable to multinuclear NMR spectroscopy in its various forms.

1.1. METHODOLOGY OF MATERIALS CHARACTERISATION BY NMR

A number of experimental techniques exist for investigating the structures of materials (Figure 1.1). On the largest scale, materials are characterised by measurements of their bulk properties including density, surface area, chemical composition and thermal properties. At the next level, the macroscopic and mesoscopic properties can be probed by optical and electron microscopy, with small angle X-ray and neutron scattering (SAXS, SANS) also used to probe the mesoscopic properties. In the medium range of atomic ordering, structures can be probed by conventional X-ray, neutron or electron diffraction. Diffraction of amorphous materials yields the radial distribution function which is being increasingly used to characterise such materials. Structures are investigated on the atomic scale by vibrational spectroscopies (infrared, ultraviolet-visible, Raman), by X-ray absorption (EXAFS, XANES) or by the various multinuclear NMR techniques. Materials can exist in an almost infinite variety of states of structural disorder and heterogeneity (Figure 1.2). Unlike techniques such as X-ray diffraction which conventionally require the presence of long-range atomic order, NMR spectroscopy is almost equally useful for probing the atomic environments of the most disordered (melts and colloid gels) as for the most ordered single crystal systems. NMR is thus able to monitor the changes occurring in the atomic environment when materials change from one state of structural disorder or heterogeneity to another and has proved to be a powerful technique for studying the transition from glasses to melts, nucleation of polycrystals from glasses and the formation and growth of crystalline phases from colloids or gels. NMR is also an ideal method for studying the intermediate phases formed when minerals react or transform to other phases by heating or

Multinuclear Solid-State NMR of lnorganic Materials

Atomic scale probes

Bulk measurements

Multinuclear NMR:

MAS, CP, MQ

i

X-ray absorption: EXAFS, XANES

>

Density Surface area Chemical composition Thermal analysis

I

Vibrational spectroscopy: IR, UV-visible, Raman

Macroscopic and mesoscopic probes

Medium range probes X-ray diffraction Neutron diffraction Electron diffraction Radial distribution function

Electron microscopy Optical microscopy

Mesoscopic probes Small angle X-ray scattering Neutron scattering

Figure 1.1. Relationship between the various methods for investigating the structures of materials.

Melt

Colloid (Gel) /

I Glass transition

/

Agglomeration

O oN

Glass

Nanocrystal /

,4~

Nucleation ~

~//Crystal growth

Polycrystai

i / ~ Single Crystal

Composite

Structural heterogeneity Figure 1.2. Schematic relationship between disorder and heterogeneity of complex materials. mechanical grinding, which often lack the long-range order necessary for conventional diffraction studies. An important element in the use of NMR techniques to determine the structural characteristics of materials is to establish relationships between the experimental

Introduction

5

NMR spectrum and recurring structural motifs. In its simplest form this is done by a "fingerprinting" procedure in which an extensive database of NMR spectra is established for related materials of known composition and structure, and the characteristics of an unknown material are deduced by comparison, (shown schematically in Figure 1.3, pathway 3). The parameter most readily determined from an NMR spectrum is the position of the resonance peak, and although this may not necessarily represent the true value of the isotropic chemical shift ~iso (see below), empirical relationships have been developed for a limited range of similar compounds between the resonance position and some characteristic feature of the structural unit (e.g. the mean bond angle or bond length). To put these empirical relationships on a sounder theoretical basis and make them applicable to a wider range of compounds, other spectral parameters such as the nuclear quadrupolar coupling constant (• or the chemical shift anisotropy (CSA) may have to be invoked; these are deduced from a more in-depth analysis of the NMR spectrum, in some cases making measurements at more than one magnetic field and using specialised techniques to improve the resolution of the spectral lineshapes. Another approach to deducing the structures of materials from their solid state NMR spectra is to make an ab initio theoretical calculation of the electric field gradient at the nucleus of an atom in a known crystal structure environment (Figure 1.3, pathway 1), from which the nuclear quadrupolar parameters and hence the expected NMR spectrum can be calculated and compared with the experimental spectrum (Figure 1.3. pathway 2). The difficulties inherent in making such ab initio calculations are such that relatively few real compounds have so far been solved completely, but developments in ab initio theory, and mathematical algorithms and computing techniques should make this approach much more widely accessible in future.

I

' Sam le 'i P I Structure I

(~

~- Nuclear l, Spin / Hamiltoniau

.... J

] xperlmeut INMR ~ Spectrum J Figure1.3. Relationship between experimental and theoretical approaches for establishing the structural characteristics of materials by NMR spectroscopy.


1.2. HISTORICAL ASPECTS OF NMR SPECTROSCOPY This book is about an experimental technique which has been a mainstay of solution chemistry for many decades. Since very well resolved and structurally informative 13C NMR spectra can readily be obtained from solutions at modest magnetic field strengths, organic chemists have traditionally been the principal beneficiaries. In earlier times inorganic chemists attempting to apply the technique to solid samples did not fare well, as these spectra were too broad to provide much useful information. Furthermore, the interesting nuclei are often quadrupolar (e.g. 27A1), ideally requiring much higher magnetic fields. The need for higher field strengths was eventually satisfied by the development of superconducting magnets, the field of which could not, however, readily be varied. Thus the traditional technique of irradiating the sample with a continuous source of radiofrequency radiation while scanning over a range of the magnetic field had to be modified; the field was now kept constant and the radiofrequency was applied as a sequence of pulses. An important breakthrough in NMR spectroscopy of solids came when spectroscopists in the UK (Andrew et al. 1959) and America (Lowe 1959) deduced that some of the factors causing broadening in these materials could be minimised by rapidly rotating the sample at a particular angle to the magnetic field axis (the magic angle, 54.73~ However there were still difficulties associated with observing materials of interest such as ~3C in organic polymers as the sensitvity of ~3C was low and the relaxation time was long. In addition the ~H-~3C dipolar coupling was too strong to be narrowed by MAS at the spinning speeds then available. A number of other experimental breakthroughs were necessary. The concept of polarisation transfer from an abundant to an insensitive nucleus was demonstrated by Hartman and Hahn (1962). This was combined successfully with decoupling for ~H-13C (Pines, Gibby and Waugh 1973). The final piece of the jigsaw was to combine cross-polarisation, decoupling and MAS (Schaefer and Stejskal 1976) to reveal the fine structure in ~3C NMR spectra of solid materials. These developments opened the way for commmercial probe development to allow usefully narrow solid state NMR spectra to be obtained from nuclei such as 298i and 27A1, a fact rapidly exploited by mineralogists and materials scientists working on zeolite catalysts. The study of the latter received considerable impetus and funding from the oil crisis of the 1970s, resulting in a great deal of detailed work being done to establish practical relationships between the structural units of the silicates and zeolites and their 298i NMR spectra (summarised in Engelhardt and Michel 1987). Although 298i NMR has historically received the greatest amount of attention for solid state NMR of inorganic materials because of its ubiquity and its technical importance in materials science, other nuclei (especially quadrupolar nuclei such as 27A1, 23Na and ~B) have become increasingly accessible with the availability of higher magnetic fields, faster spinning speeds and new methodologies for improving resolution and

Introduction

7

offsetting the effects of quadrupolar broadening (double rotation, double angle spinning and multiple quantum techniques). All these aspects are treated in depth in the following chapters.

1.3. B R I E F D E S C R I P T I O N O F T H E N M R E X P E R I M E N T

1.3.1 General principles Many nuclei possess a quantised property called nuclear spin which can be adequately described only by quantum mechanics (see Chapter 2) but can usefully be thought of as being caused by the physical spinning of the nucleus. Nuclei with even mass number and even charge (e.g. 12C, 160) have zero spin and are therefore of no use for NMR spectroscopy. The angular momentum of a spinning nucleus is a function of its spin quantum number I which can have either integer or half-integer values. When such a nucleus is placed in a strong magnetic field, the energy levels between the various spin states are split (the Zeeman interaction). The differences between the various energy levels are small compared with spectroscopies involving electronic energy states, and transitions are only possible between adjacent energy levels, with the absorption or emission of a photon in the radiofrequency (rf) range. It is the frequency of this rf radiation which is measured in an NMR experiment. The nuclei in different structural environments in a solid may experience slightly different magnetic fields because they are shielded by the surrounding electrons and consequently absorb photons of slightly different frequency. Since the absolute NMR frequencies are difficult to measure with sufficient accuracy, the resonance frequencies are normally reported as chemical shifts (~) relative to an external standard compound. Unless the nucleus is in a very symmetrical environment, it experiences anisotropic shielding which will be reflected in its ~ value. A more useful parameter for comparison purposes is the isotropic chemical shift ~iso, the average shift which is experienced by the nucleus. The goal of all NMR experiments is to determine the change in the separation of the energy levels for different environments. In its simplest form, an NMR experiment consists of three parts, the preparation of the nuclear spin system by placing it in an external magnetic field, its perturbation by applying a pulse of rf radiation, and the detection of phenomena accompanying its return to the initial state when the perturbation is removed.

Preparation. When the nuclear spin system is placed in the external magnetic field Bo, its magnetic moment precesses about the magnetic field axis similar to the manner in which a gyroscope precesses about the orientation of a gravitational field (Figure 1.4A). In the NMR experiment we are dealing with many spins simultaneously and the observed response is an average of the behaviour of the individual spins. The


Z

Z' M

"~Xx

A. Preparation

B. Perturbation

C. Detection

(evolution)

N

/ \

/ \

I \

/ /

N N

/

Figure 1.4. Schematic representation of the three stages of a simple NMR experiment. In the preparation stage the magnetic moment of the nucleus precesses about the principal axis of the applied magnetic field Bo. In the evolution stage the sample is irradiated with plane-polarised rf radiation at the Larmor frequency containing a magnetic field component Bo which causes the net magnetisation M to incline, becoming oriented perpendicular to Bo in the case of a -rr/2 pulse. In the detection stage the spin system coherently precesses in the transverse plane inducing a voltage in the coil producing a signal which is detected (the FID). frequency of the precession is called the Larmor frequency and is a characteristic of the particular nucleus (since each nuclide has its own characteristic Larmor frequency, it follows that in any particular experiment the technique acquires information only about the nuclide for which the system is specifically tuned). The magnetic moments of all the nuclei in a particular sample can be visualised as forming a cone about the z-axis. yielding a net magnetisation M parallel to the Bo axis. When viewed in a frame rotating about the z-axis at the Larmor frequency (the so-called rotating frame), the spin system appears to be stationary. When thermal equilibrium is attained between the nuclear spin moments and their surrounding lattice, an equilibrium magnetisation is obtained, which is, however, only a very small fraction of the maximum possible value if complete alignment of the nuclear moments with Bo was achieved.

Perturbation. The sample is now irradiated with a pulse of plane-polarised rf radiation at the Larmor frequency. Plane polarised electromagnetic radiation consists of electric and magnetic fields oscillating in fixed planes perpendicular to each other and to the direction of the radiation. The nuclear spin packets which are already in thermal

Introduction

9

equilibrium with the static external magnetic field interact with the magnetic field component of the radiation, causing the equilibrium magnetisation to incline with respect to the applied magnetic field (Figure 1.4B). In the simplest experiment, the angle of inclination is 90 ~ and provided the pulse is of short duration compared with the time taken for the spin packet to spiral into the 'new' field direction, the precession can be assumed to remain fixed at 90 ~ Such a pulse, which is long enough to rotate the spin packet through 90 ~ is described as a 7r/2 pulse (a 7r pulse is one which rotates the spin packet through 180~

Detection. When the rf field is turned off at the end of the simple 1-pulse experiment,

the spin system dephases in the x-y plane with a characteristic relaxation time T2, and returns to thermal equilibrium along the z-axis with a characteristic relaxation time T1. The transverse magnetisation after the pulse induces a voltage in the coil which is recorded as a function of time and is called a Free Induction Decay (FID) (Figure 1.5) containing frequency information. This can be extracted mathematically by a process called Fourier Transformation (FT) giving a plot of amplitude vs. frequency (Figure 1.5) showing which frequencies would have to be added together with what relative amplitudes to reproduce the time-dependent shape of the FID. Thus, Fourier transformation of a "rr/2 single-pulse NMR experiment gives exactly the same information as would be obtained from a continuous wave spectrum, allowing the entire frequency range to be covered in the time taken to acquire a single FID. Usually the sample is irradiated with many pulses and the resulting signals added together to give an improved signal/noise ratio. A clear and simple description of the processes involved in pulsed NMR has been given by Schwartz (1988). Many of the advances in the application of NMR to solids have involved the development of more complex pulse sequences for manipulating the spin systems in order to improve the spectral resolution or extract other information (bond lengths, atom connectivities, etc.)

~

,~

FT

>

t

v

FID

NMR spectrum

Figure 1.5. Schematic representation of Fourier transformation of a FID in the time domain to an NMR spectrum in the frequency domain.

10


1.3.2 Overcoming NMR spectral broadening in solids by MAS The NMR spectra of solids suffer from broadening due to various interactions between the dipole moments of the nuclei, between the quadrupole moments of quadrupolar nuclei and the electric field gradient (EFG) at the nucleus, and by anisotropy of the electronic shielding at different sites in the structure. These interactions are dealt with in detail in Chapter 2. These broadening effects do not arise in liquids because their atomic motion is faster than the interaction frequency, allowing all the nuclei in a particular atomic environment to experience the same average magnetic field, thus producing extremely narrow NMR spectra. A number of the interactions giving rise to line broadening in solids can be cancelled out or at least reduced by spinning the sample very rapidly (typically 10-15 kHz or 10,000-15000 revolutions per second) at an angle of 54.74 ~ to the axis of the applied magnetic field (the so-called magic angle, Figure 1.6A). Broadening in spin I = 1/2 nuclei arises mainly from interactions between the magnetic dipoles of adjacent nuclei which are anisotropic in powder samples. In quadmpolar nuclei where the spin I > 1/2 (a category containing more than two-thirds of all the NMR-active nuclei) the non-zero nuclear electric quadrupole moment results from the fact that the charge distribution is non-spherically symmetric. Interaction of this quadrupole moment with the electric field gradient at the nucleus causes the broadening associated with these nuclei. As discussed in detail in Chapter 2, some of these interactions contain terms in (3cos 2 0-1) the second-order Legendre polynomial (P2(cos0)), where 0 is the angle between the axis of the applied magnetic field and the principal axis of the interaction. At the magic angle, cos0 = 1/~/3 and the term P2(cos0), becomes zero. Magic angle spinning (MAS) thus removes the dipoledipole and chemical shift anisotropy (CSA) interactions, as well as the first-order quadrupolar interactions, narrowing the resonance lines from both spin I - 1/2 and quadrupolar nuclei to reveal spectral details (Figure 1.6B). Lines equispaced on each side of the central resonance (spinning side bands) are caused by modulation of the interaction by the physical act of spinning the sample, and, since they are harmonics of the spinning speed, they can be distinguished from tree peaks because their position changes with the spinning speed, moving away from the central peak at higher speeds. It should be noted that although the well-resolved NMR spectra of solids obtained by MAS may resemble solution spectra, they are normally broader due to distributions of the isotropic chemical shifts in solids. There is also another important difference between liquid and solid MAS NMR spectra. Unlike the random molecular motion in liquids, magic angle spinning is a coherent averaging process, introducing the possibility of synchronising the pulse sequences with the rotor speed to selectively reintroduce anisotropic interactions and allowing the measurement of internuclear distances. MAS removes the broadening of the spectra of quadrupolar nuclei resulting from first-order quadrupolar interactions, but not broadening due to higher-order interac-

11

Introduction

A

~;o

|

~;o

!

i

o

!

!

-lOO

!

-~'oo

~7AIshift (ppm) w.r.t AI(H20)~3+

Figure 1.6. A. A typical magic angle spinning probe illustrating the orientation at the magic angle of the sample within the stator/coil assembly with respect to the axis of the applied magnetic field B0. B. Typical solid state 27A1 NMR spectra of a sintered mixture of A1203 and Y203. Upper spectrum unspun, lower spectrum spun at the magic angle. The peaks marked with asterisks are spinning side bands.

tions which have angular dependences other than the (3 C O S 2 0 - 1) term eliminated by MAS (see Chapter 2). In complex spectra of samples containing several overlapping sites, the removal of higher-order broadening may be important, and can be accomplished by spinning at more than one angle either simultaneously as in double rotation (DOR) or sequentially as in double angle spinning (DAS) (see Chapter 3). A more recent and extremely promising method which does not require complex and fragile probe hardware involves the excitation of higher order quantum coherences, exploiting differences in their associated evolution under different interactions. This method, called multiple quantum or MQMAS NMR, is described in detail in Chapter 2.

1.3.3 Other NMR experiments used with solids Although MAS NMR with a simple pulse sequence is the technique most generally used to study solid materials, there are a number of other experiments which may be used with or without MAS to provide specific information or improve the quality of the spectrum. Since these are described in Chapters 2 and 3 and will be encountered in many of the practical examples given in the subsequent chapters, they are only treated briefly here.

12


1.3.3.1 Decoupling. In some cases the broadening arising from interactions between the dipole moments of two different nuclei (heteronuclear broadening) is too large to be removed completely by MAS. Typically the second nucleus may be 1H, in which case the protons in the system are continuously irradiated at their Larmor frequency, which continuously modulates their spin state, cancelling the effect of their dipole moment. The spectrum of the other nucleus is collected during this decoupling period and is thus freed of heteronuclear broadening.

1.3.3.2 Cross-Polarisation (CP). In systems containing two nuclei, one with more abundant spins than the other, magnetisation can be transferred from the more abundant (often 1H) to the less abundant nucleus by irradiating both nuclei at their correct Larmor frequencies in fulfillment of the Hartmann-Hahn condition (Hartmann and Hahn 1962) (see Chapter 2). The spectrum of the less abundant nucleus can then be collected at a higher magnetisation level determined by the protons, and, since the relaxation time T~ is now that of the protons, a greater number of scans can be collected in a given time, giving a better signal/noise ratio. Further, since the signal from the less abundant nuclei located in structural sites in closest proximity to the protons is preferentially enhanced, additional structural information can be gained by comparing CP and non-CP spectra of the same sample.

1.3.3.3 Spin-echo experiments. Spectra containing broad resonances can suffer from artifacts and distortion introduced during the first few microseconds of instrumental dead time at the beginning of the FID. To overcome this, the spin system can be given an initial pulse and is then subject to another pulse (or pulse sequence) which causes the dephasing spin system to rephase (the refocussing pulse) and is sometimes termed a spin-echo (Hahn 1950). The FID is then collected and Fourier transformed in the usual way. Spin-echo experiments are also often used for determining relaxation times. 1.3.3.4 Two-dimensional experiments. A wide range of experiments are now available in which a series of FIDs (and hence spectra) are collected, each with a different value of some time-dependent variable (e.g. the evolution period in a multiple-pulse experiment) that modulates the FID. This has been termed two-dimensional spectroscopy and was first suggested by Jeneer (1971) and implemented practically a few years later (Aue et al. 1976). The spectra are first Fourier-transformed in the usual way, then again with respect to the second time variable, and may be plotted as a contour with the chemical shift along one axis and the effects related to the evolution period along the other. 2D experiments can be used to provide a range of structural information, including the identification of sites which are directly connected, and are discussed in detail in Chapter 2.

Introduction

13

1.3.4 Nuclei suitable for NMR spectroscopy To produce an N M R spectrum, a nucleus must possess a nuclear spin. Nuclei with odd mass numbers (e.g. 29Si, 27A1) have half-integer spins and are of most interest for solid state NMR. Nuclei with even mass numbers and odd charge (e.g. 2H, 14N) have integer spins, and although subject to difficulties they can still be useful N M R nuclei. Of the 120 nuclei suitable for NMR, 9 have spin I - 1, 31 are spin I - 1/2, 32 are spin I = 3/2, 22 are spin I = 5/2, 18 are spin I = 7/2 and 8 are spin I = 9/2. One factor influencing the usefulness of an N M R nucleus is its natural abundance, which can range from 100 percent down to the vanishingly small. In the latter case, it may only be possible to acquire an N M R spectrum if the sample has been artificially isotopically enriched, as is normally done for 15N and 170 NMR. The natural abundances of the most useful spin I = 1/2 nuclei are listed in Table 1.1 and those of the quadrupolar nuclei in Table 1.2. The standard substances against which the chemical shifts of the various nuclides are quoted are listed for the spin I -- 1/2 nuclei in Table 1.1 and for the quadrupolar nuclei in Table 1.3. Tables 1.1 and 1.2 also provide information about the N M R sensitivities of the various nuclei, listed as their receptivity relative to Si, with the absolute sensitivity defined by Harris (1984) as @COO + 1)) where C is the natural abundance of the nucleus and ~/is its gyromagnetic ratio. The relative receptivities of the quadrupolar nuclei in Table 1.2 have been adjusted by taking into account the fractional contribution of the central transition.

Table 1.1. NMR properties of the spin I = 1/2 nuclei. Nucleus

Natural abundance (%)

Vo at 7.05 T (MHz)

1H 3He 13C 15N

99.985 1.3 • 10 - 4 1.108 0.37 100 4.70

300.00 228.546 75.435 30.408 282.282 59.601

2.71 • 1.56 • 4.77 • 1.04 ;< 2.26 • 1

100 2.19

121.440 9.714

1.80 • 102 2.01 • 10 -3

7.58 100 100 51.82 48.18 12.75 12.26 0.35

57.213 14.751 9.516 12.141 13.569 63.645 66.579 98.580

1.47 3.22 • 10 -1 8.56 • 10 -2 9.43 • 10 -2 1.33 X 10 -1 3.33 3.66 3.36 • 10 -1

19F 29Si 31p 57Fe

77Se 89y

l~ 1~ l~ 111Cd 113Cd 115Sn

Relative receptivity* 103 10 -3 10 -1 10 -2

103

Standard substance TMS TMS MeNO2 CC13F TMS H3PO4 Fe(CO)5

Me2Se Y(NO3)33aq mer-[RhC13(Sme2)3] Ag+aq Ag+aq CdMe2 CdMe2 Me4Sn


14

Table 1.1. (Continued) Nucleus

Natural abundance (%)

Vo at 7.05 T (MHz)

Relative receptivity*

Standard substance

ll7Sn l l9Sn

7.61 8.58 6.99 26.44 100 14.31 14.40 1.64 33.8 16.84 29.50 70.50 22.6

106.896 111.870 94.647 83.430 24.810 52.830 12.483 6.846 64.242 53.733 173.100 172.899 62.760

9.46 1.22 • 10 6.07 1.54 X 10 1.53 2.12 2.87 X 10 .2 5.42 X 104 9.19 2.66 1.54 X 102 3.79 X 102 5.45

Me4Sn Me4Sn Me2Te XeOF4

125Te ~29Xe 169Tm 171yb

183W 1870S 195pt 199Hg 2~ 2~ 2~

WF6 OsO4 [Pt(CN)6] 2MezHg T1NO3aq T1NO3aq Me4Pb

* relative receptivity normalised to 298i

Table 1.2. NMR properties of the quadrupolar and integer-spin nuclei. Nucleus

I

Natural abundance

Vo (MHz) at 7.05 T

(%) 2H 6Li 7Li 9Be l~ liB 14N

170 21Ne 23Na 2SMg 27A1 33S 35C1 37C1

39K 41K 43Ca

45Sc 47Ti 49Ti 51V

1 1 3/2 3/2 3 3/2 1 5/2 3/2 3/2 5/2 5/2 3/2 3/2 3/2 3/2 3/2 7/2 7/2 5/2 7/2 7/2

1.5 x 10 -2 7.42 92.5 100 19.58 80.1 99.63 0.037 0.27 100 10.0 100 0.75 75.77 24.23 93.26 6.73 0.135 100 7.28 5.51 99.75

46.051 44.146 16.67 42.20 32.239 96.32 21.671 40.71 23.71 79.44 18.39 78.27 23.06 29.44 24.51 14.02 7.70 20.23 73.03 16.95 16.95 79.05

Quadrupole Relative moment receptivity* (mb)

Quadrupole Stemheimer broadening antishielding factor** factor (~)

7.35 x 102 3.76 x 10

-0.808 -40.1 52.9

2.28 X 10-1 1.10

0.2 0.2

3.59 x 10 2 5.22 2.93 x 10 -2 1.80 x 10 -2 2.51 x 10 2 7.29 x 10 -1 5.61 x 102 4.62 x 10 -2 9.71 1.79 1.29 1.46 x 10 -2 2.35 x 10 -2 8.19 x 102 4.16 x 10 -I 5.61 x 10-1 1.04 x 103

40.6 20.44 -25.6 101.6 104 199.4 146.6 -67.8 -81.7 -64.6 58.5 71.0 -49.0 -220 302 247 -52

2.83 x 10 -1 NA 6.40 x 10 -2 7.21 2.64 8.60 1 3.30 3.75 2.80 4.20 11.3 1.39 X 10 -1 1.12 21.40 6.08 5.78 X 10 -2

0.19 -6.7 -13.8 -9.5 -5.5 -4.1 -3.6 -52.2 -42.0 -42.0 -21.8 -21.8 -18.8 -23.1 -9.0 -9.0 -7.6

15

Introduction Table 1.2. (Continued) Nucleus

I

Natural abundance

Vo (MHz) at 7.05 T


(%) 53Cr 55Mn

59Co 61Ni

63Cu 65Cu 67Zn 69Ga 71Ga 73Ge 75As 79Br 81Br 83Kr 85Rb 87Rb 878r 91Zr 93Nb 95Mo

97Mo 99Ru lOlRu lO5pd 113in 115in

121Sb 123Sb 127I 131Xe

133Cs 135Ba 137Ba 139La 141pr 143Nd 145Nd 1478m 1498m 151Eu 153Eu 155Gd

157Gd

3/2 5/2 7/2

3/2 3/2 3/2 5/2 3/2 3/2 9/2

3/2 3/2 3/2 9/2

5/2 3/2 9/2 5/2 9/2 5/2 5/2 5/2 5/2 5/2 9/2 9/2

5/2 7/2 5/2 3/2 7/2

3/2 3/2 7/2 5/2 7/2 7/2 7/2 7/2 5/2 5/2 3/2

3/2

9.50 100 100 1.14 60.11 39.89 4.11 60.1 39.9 7.73 100 50.69 49.31 11.5 72.16 27.84 7.00 11.22 100 15.92 9.55 12.7 17.0 22.33 4.3 95.7 57.36 42.64 100 21.2 100 6.59 11.23 99.91 100 12.18 8.30 15.0 13.8 47.8 52.2 14.8 15.65

17.00 74.56 71.04 26.87 79.65 85.32 18.82 72.24 91.79 10.50 51.57 75.46 81.34 11.59 29.08 98.56 13.06 28.02 73069 19.65 20.07 13.78 15.45 13.80 66.02 66.17 72.30 39.15 60.47 24.79 39.64 30.02 33.58 42.73 91.90 16.35 10.07 12.51 10.31 74.63 32.95 9.28 12.18

2.34 x 10 -1 4.85 x 102 7.57 x 102 1.11 x 10 -1 1.75 X 102 9.59 x 10 3.19 x 10 -1 1.14 X 102 1.55 x 102 2.96 X 10 -1 6.90 X 10 1.09 X 102 1.33 x 102 5.95 x 10-1 2.88 x 10 1.34 x 102 5.16 x 10 -1 2.88 1.32 x 103 1.41 9.03 x 10 -1 3.89 x 10-1 7.35 x 10 -1 6.84 x 10-1 4.09 x 10 9.20 x 102 2.54 x 102 5.39 x 10 2.59 x 102 1.62 1.31 X 102 8.92 X 10 -1 2.13 1.64 x 102 9.09 X 102 1.12 1.78 x 10 -1 6.17 x 10 -1 3.18 X 10 -1 2.32 X 102 2.19 x 10 6.00 • 10 -2 1.40 x 10 -1

Quadrupole moment (mb) -150 330 420 162 -220 -204 150 171 107 - 196 314 313 261.5 259 276 133.5 335 -176 - 320 -22.0 255 79.0 457 660 799 810 -360 -490 -616 -114 -3.4 160 245 200 -59.0 -630 -330 -260 75.0 903 2410 1270 1350

Quadrupole Sternheimer broadening antishielding factor**

factor (Y)

21.9 5.81 4.20 16.18 10.06 8.08 4.76 6.70 2.06 3.36 31.67 21.50 13.92 5.32 10.42 3.00 7.91 6.02 1.28 0.98 12.88 1.80 53.75 125.5 8.9 9.13 8.98 10.36 24.95 8.68 4.93 x 10 -4 14.12 29.60 1.58 1.51 X 10 -1 41.02 18.28 9.13 0.92 43.45 700.9 2878 2478

-6.6 -5.8 -4.7 -25.2 -25.2 -21.9 - 17.0 -17.0 -8.7 -80 -80 - 85.5 -52.8 -52.8 -47.8 -26.6 - 23.0 -20 -20 -28 -28 - 162 -110 -110 -110 - 71 -


16

T a b l e 1.2. (Continued) Nucleus

I

Natural abundance

Vo (MHz) at 7.05 T


Quadrupole moment (rob)

72.12 10.33 14.46 64.07 8.66 14.61 34.28 12.18 7.65 36.40 68.50 69.21 23.64 5.4 5.87 5.22 20.07 49.09 5.85

1.88 X 102 2.46 X 10 -1 8.75 X 10 -1 5.54 X 102 3.11 X 10 -1 5.89 X 10 - l 8.25 X 10 7.07 X 10 -1 2.03 X 10- l 1.02 X 102 1.41 X 102 2.43 X 10 2 1.07 3.04 X 10 -2 6.79 X 10 -2 8.13 X 10 -2 5.32 X 10 - l 3.92 X 102 2.93 X 10 -3

1432 2507 2650 3580 3570 2800 4970 3360 3790 3170 2180 2070 856 816 751 547 386 -516 4936

(%) 159Tb 161Dy 163Dy 165Ho 167Er 173yb 175Lu 177Hf 179Hf 18~Ta 185Re ~87Re ~89Os 191Ir 193Ir 197Au 2~ 2~ 235U

3/2 5/2 5/2 7/2 7/2 5/2 7/2 7/2 9/2 7/2 5/2 5/2 3/2 3/2 3/2 3/2 3/2 9/2 7/2

100 18.9 24.9 100 22.95 16.12 97.41 18.606 13.629 99.988 37.40 62.60 16.1 37.3 62.7 100 13.18 100 0.72

Quadrupole Sternheimer broadening antishielding factor** factor (~/) 470.9 2419 1931 338 2487 2134 1218 1566 1728 466.5 275.9 246.1 513.3 2042 1591 949 122.9 4.99 7038

--73.8 -

* relative receptivity normalised to 298i ** quadrupole broadening normalised to 27A1 Data from Smith and van Eck (1999) and references therein, and Pyykk6 (1992, 2001)

T a b l e 1.3. Standard reference substances for the quadrupolar and integer-spin nuclei, with compounds commonly used as secondary references and their shift relative to the primary reference. Nucleus

Reference substance

Secondary reference compound

2H 6'7Li 9Be l~ 14N 170 23Na 25Mg 27A1

TMS LiCl(aq) BeO(s) BF3.Et20 CH3NO2 H20 NaCl(aq) MgSO4(aq) Al(H20)63+

NaBH4 NH4C1 NaCl(s) MgO(s) Y3A15OI2 (octahedral peak)

33S

CS 2

NH4AI(SO4)2.12H20

35'37C1

NaCl(aq)

CaS KCl(aq)

Shift from the primary compound (ppm)

3.2 342.4 7.2 26 0.7 333 -28.5 3.07

17

Introduction

Table 1.3. (Continued) Nucleus

Reference substance

39'41K 43Ca

KCl(aq) CaC12(aq) 47'49Ti TIC14 51V VOC13 63'65Cu CuC1 67Zn Zn(NO3)2(aq) 69,71Ga Ga(H20)63+ 85'87Rb RbCl(aq) 91Zr Cp2ZrBr2 93Nb NbC15in wet acetonitrile 95'97Mo Na2MoO4(aq) 131Xe Xe(g) at zero pressure 133Cs CsCl(aq) 135Ba BaClz(aq) , 139La LaC13(aq) 2~ (CH3)zHg

Secondary reference compound

Shift from the primary compound (ppm)

CaO(s) SrTiO3 0.16M NaVO3(aq) ZnSe ' _ BaZrO3 Nb2Os(s) Mo(CO)6 BaZrO3 -

128 - 843 -574.4 276 _ 208.1 - 1240 - 1854 279 -

A further factor to be taken into account when assessing the relative ease with which a quadrupolar nucleus may be observed is the width of the line which is determined for a given electric field gradient by the quadrupole moment (also listed in Table 1.2). Often only the central transition is observed, for which the linewidth is determined by its second-order quadrupolar broadening and is proportional to [Q2(I(I + 1)-3/4)2]/ [ ( 2 I - 1)2]. These quadrupolar broadening factors listed in Table 1.2 have been normalised to 27A1. The actual linewidth will be given by this factor multiplied by the electric field gradient and although this property is not easily defined for a particular nucleus, it is amplified by the Sternheimer antishielding factor (Table 1.2) which must therefore also be taken into account when comparing nuclei. Hence to get a feeling for the observability of the central transition of a given quadrupolar nucleus, the secondorder broadening factor should be multiplied by the square of the Sternheimer antishielding factor. Values of the latter are given in Table 1.2 only for cases where meaningful calculations have been carried out for ions with a closed shell structure, e.g. A13+, C1-. Where possible, these values are quoted for a lattice (Schmidt et al. 1980), otherwise they pertain to the free ions.

1.4. F U R T H E R R E A D I N G

This book is intended to be a self-contained introduction to the background of solidstate NMR, its experimental techniques and applications to inorganic materials.

18


Nevertheless there are a number of excellent books dealing with different aspects of NMR spectroscopy and solid state NMR with a different emphasis and perspective. Some of these are listed below, and the interested reader is encouraged to consult them. The background to the physics of NMR can be found in the excellent books by Abragam (1983), Slichter (1990) and Munowitz (1988), the latter providing a very readable quantum mechanical description of solid state NMR experiments. Harris (1983) provides a nice introduction to the chemical aspects of NMR techniques, and the physical background of multidimensional NMR techniques is developed in detail by Ernst et al. (1988). High-resolution solid state NMR is dealt with by Haeberlen (1976) and Mehring (1983). A book devoted to pulsed solid state NMR has been written by Gerstein and Dybowski (1985). Although 13C NMR of organic materials is not dealt with in the present book, there is ample literature on this subject, including the books by Stejskal and Memory (1994) and Fyfe (1983), the latter providing numerous applications of 13C NMR. Schmidt-Rohr and Spiess (1996) present a comprehensive background to modem NMR techniques for characterising solid polymeric materials. Applications to silicates and zeolites were dealt with comprehensively by Engelhardt and Michel (1987). On practical matters, the authors find it useful to have copies of the books by Fukushima and Roeder (1981) and Derome (1987) in the laboratory. Last but not least is the Encyclopaedia of NMR, published in 1996 by Wiley.

REFERENCES

Abragam, A. (1983) Principles of Nuclear Magnetism, Oxford University Press, Oxford. Andrew, E.R., Bradbury, A. & Eades, R.G. (1959) Nature 183, 1802. Aue, W.P., Bartholdi, E. & Ernst, R.R. (1976) J. Chem. Phys. 64, 2229. Derome, A.E. (1987) Modern NMR Techniques for Chemistry Research, Pergamon, Oxford. Engelhardt, G. & Michel, D. (1987) High-Resolution Solid State NMR of Silicates and Zeolites, Wiley, Chichester. Ernst, R.R., Bodenhausen, G. & Wokaun, A. (1988) Principles of Nuclear Magnetic Resonance in One and Two Dimensions, Oxford University Press, Oxford. Fukushima, E. & Roeder, S.W.B. ( 1981) Experimental NMR Spectroscopy, Addison-Wesley, Reading, Mass. Fyfe, C.A. Solid State NMR for Chemists, CRC Press, Guelph. Gerstein, B.C. & Dybowski, C.R. (1985) Transient Techniques in NMR of Solids, Academic Press, Orlando, Florida. Haeberlen, U. Advances in Magnetic Resonance, Supplement 1, Academic Press, San Diego. Hahn, E.L. (1950) Phys. Rev. 80, 580. Harris, R.K. (1984) NMR Spectroscopy, (Pitman, London). Hartmann S.R. & Hahn, E.L. (1962) Phys. Rev. 128, 2042. Jenner, J. (1971) Amp6re International Summer School, Basko Polje, Yugoslavia.

Introduction

19

Lowe, I.J. (1959) Phys. Rev. Lett. 2, 285. Mehring, M. (1983) High Resolution NMR of Solids, Springer-Verlag, Berlin. Munowitz, M. (1988) Coherence and NMR, Wiley, New York. Pines, A., Gibby, M.G. & Waugh, J.S. (1973) J. Chem. Phys. 59, 569. Pyykk6, P. (1992) Z. Naturforschung 47a, 189. Pyykk6, P. (2001) Mol. Phys. 99, 1617. Schaefer, J. & Stejskal, E.O.J. (1976) J. Amer. Chem. Soc. 98, 1031. Schmidt, P.C., Sen, K.D., Das, T.P. & Weiss, A. (1980) Phys. Rev. B 22, 4167. Schmidt-Rohr, K. & Speiss, H.W. (1996) Multidimensional Solid-State NMR and Polymers, Academic Press, San Diego. Schwartz, L.J. (1988)J. Chem. Educ. 65, 752. Slichter, C.P. (1990) Principles of Magnetic Resonance, Springer-Verlag, Berlin. Smith, M.E. & van Eck, E.R.H. (1999) Prog. Nucl. Magn. Resort. Spectrosc., 34, 159. Stejskal, E.O. & Memory, J.D. (1994) High Resolution Solid State, NMR Oxford University Press, New York.


Chapter 2

Physical Background 2.1.

Fundamental Interaction with External Magnetic Fields 2.1.1 A Quantum Mechanical Description of the Zeeman Interaction 2.1.2 Bulk Magnetisation 2.1.3 The Rotating Frame and the Application of RF Pulses 2.1.4 Observation of the NMR Signal 2.2. Internal Interactions 2.2.1 The Dipolar Interaction 2.2.2 Scalar Coupling 2.2.3 Paramagnetic Coupling 2.2.4 Chemical Shielding 2.2.5 Knight Shift 2.2.6 Quadrupole Interaction 2.2.7 Nature of Interactions 2.3. One Dimensional Methods for Improving Resolution 2.3.1 Magic Angle Spinning and First-Order Effects 2.3.1.1 Physical Principles 2.3.1.2 Formation of Spinning Sidebands 2.3.2 Magic Angle Spinning and Higher-Order Effects 2.3.2.1 MAS of Second-Order Quadrupole Effects 2.3.2.2 Residual Coupling Effects due to Quadrupolar Nuclei in MAS Spectra 2.3.2.3 Nonequivalent Homonuclear Spins 2.3.3 Variable Angle Spinning 2.3.4 Double Angle Spinning 2.3.5 Multiple Quantum Transitions 2.3.6 Ultrasonically-Induced Narrowing 2.4. Dipolar Decoupling 2.4.1 Heteronuclear Dipolar Decoupling 2.4.2 Homonuclear Dipolar Decoupling 2,5, Spin-locking 2.6. Cross-Polarisation 2.7. Two-Dimensional Methods 2.7.1 Dynamic Angle Spinning 2.7.2 2D MQMAS 2.8. NMR Relaxation

23 25 26 29 34 35 37 40 43 44 48 50 57 58 59 59 61 63 64 71 74 74 75 77 78 78 78 79 83 85 90 92 93 98

2.8.1 2.8.2 References

Introduction to Relaxation Mechanism for Relaxation Processes

98 101 105

Chapter 2

Physical Background 2.1. FUNDAMENTAL INTERACTION WITH EXTERNAL MAGNETIC FIELDS

The nuclei of interest to the NMR experiment possess a nuclear spin angular momentum ] and consequently a nuclear magnetic moment ~. A classical description of this magnetic moment placed in a static magnetic field B shows a torque z being exerted of

z-ExB

(2.1)

The magnetic dipole moment ~ is related to the spin angular momentum via

= ?'hi and I~[- 17[h4I(I + 1)

(2.2)

where ~/is the nuclear gyromagnetic ratio (units of rads-lT-1). The torque in Eq. 2.1 causes the magnetic moment to change direction such that

dt

=7~xB

(2.3)

If the moment was inclined at 0 to B this would cause a precession of the moment in a conical path at constant 0 about B at an angular frequency O~ogiven by O)~ -?'[B I

(2.4)

COois known as the Larmor or resonance frequency. Strictly this is the angular resonance frequency but the literature uses this term interchangeably with the direct frequency which is of more immediate experimental relevance, Vo = eOo/2~r. This means NMR is a radiofrequency spectroscopy so for protons at B = 1 T, Vo - 42 MHz. ~/can be either positive or negative, although the consequences of this are usually ignored. Levitt (1997) has explored this issue and the consequences it has for phase shifting and detecting the signal. Classically, a magnetic dipole moment in a magnetic field has an energy of

E--~.B~ o --Th[.B

o

(2.5)

which not only depends on ~/, h and the magnitude of B o but also on the relative orientation of ~ and B o. In NMR spectroscopy the energy difference between different spin 23

24


levels (i.e. effectively different orientations of I) is of interest. This is the well known Zeeman interaction and is the basis of magnetic resonance spectroscopy. Although the magnetic moment has been treated classically, and indeed many NMR effects can be conveniently described classically, the spin is quantised. This means that its values and components can only take discrete values. The energy levels can be calculated using the spin Hamiltonian which is an operator giving the energy of the system. In principle all energy levels in a system (nucleus, system of coupled nuclei, molecules) are given by the time-dependent Schr6dinger equation: ih ~)l/t _ HI// (2.6) ~)t where H = Hamiltonian and qJ is the wave function. In NMR the way the system is interrogated (e.g. with an rf pulse) is assumed not to alter the energy levels of the system so that it can be assumed that the Hamiltonian is time-independent. Then the wave function that solves Eq. 2.6 will be of the form

= qg. exp(-iEt / h)

(2.7)

where ~ is time-independent. If these two equations are combined then q~ will satisfy

Hq~= Eq~

(2.8)

where q~ is the eigenfunction of the Hamiltonian H with eigenvalue (energy) E. The Hamiltonian gives all possible energy levels of a system, including rotations, vibrations, bonding etc. However the NMR energy splittings are --~ 1.24 X 10 -6 eV (= 300 MHz = 2 x 10 -25 J = 0.01 cm-~ in various spectroscopic units) which is sufficiently different from other interactions that it is possible to split the Hamiltonian, and hence the resulting wavefunctions, into those associated with the nuclear spin and those with the other interactions (e.g. vibrations). Wave functions that represent the interactions of the nuclear spin alone will give an equivalent equation to 2.8 but relating only to the nuclear spin. This removes all the interactions that do not influence NMR experiments. The Hamiltonian H will henceforth in this book be taken to be the nuclear spin Hamiltonian and the eigenfunction q~refers to this Hamiltonian. The eigenfunctions of the Hamiltonian are defined in terms of a complete set of orthogonal spin functions q~n. A general eigenfunction q~ can now always be written as a linear combination of the spin functions q~n where the coefficients Cn vary for different Hamiltonians (interactions). q9 - Z C,~0n B

(2.9)

25

Physical B a c k g r o u n d

Table 2.1. Nuclear spin operators. Operator

Description

Operator II,m > =

I

Nuclear Spin

~/I(I + l)h l I, m >

12 ---Fx -/-Fy -/-Fz

Square of the total nuclear spin

I ( I + 1)h 2 I I, m >

Iz

z-component of nuclear spin

mhlI, m>

I+ --Ix + ily

Raising Operator

~/I(I + 1 ) - m ( m + 1)h l I, m + 1 >

1_ : Ix- ily

Lowering Operator

~/I(I + 1)- m(m-

1)h l I, m -

1>

In NMR the q~n are characterised by the quantum numbers I and m where I is the spin quantum number (0, 1/2, 1, 3/2,...) and m is the z-component of I with values of m = - I, - I + 1. . . . . I - 1, I. The spins of nuclei are given in Tables 1.1 and 1.2. The state of a nucleus would be described by the pair of quantum numbers I and m and is usually written I I, m > in Dirac bra and ket notation. This text is not intended as a primer in quantum mechanical operators but to understand papers that interpret interactions and develop new pulse sequences it is worth noting some of the most important results. It is immediately apparent from Table 2.1 that II, m > is an eigenfunction of both I 2 and Iz so that II, m > is unchanged. In contrast the raising and lowering operators, I+ and I_, change the spin function so that I I , m > is not an eigenfunction of I+ or I_. The raising and lowering operators describe changes in the z-component of the spin state corresponding to a transition of a spin between energy levels.

2.1.1 A quantum mechanical description of the Zeeman interaction The classical Zeeman interaction between a magnetic moment and an applied magnetic field was defined above. In a quantum mechanical description the operators for the quantities need to be used, such as the nuclear spin I. With an externally applied magnetic field B o the Zeeman Hamiltonian is given by (2.10)

H z =-yh[.B_ o

The usual convention is to let the direction of the externally applied static magnetic field B o define the z-direction in the laboratory frame. Then the Zeeman Hamiltonian can be written as

H z - -~,'h Iy

9

- _~,hBoI z

Bo

(2.11)

26

Multinuclear Solid-State N M R of Inorganic Materials Zero field Bo = 0

Bo finite E = YhBO 2

(,,

./,.,-' """ x\

AE= y Bo

E - -TfiBO

~

I~-~>t I~.~) J

Figure 2.1. Energy level diagram of a spin-l/2 nucleus showing the Zeeman interaction. The Zeeman Hamiltonian can be used to calculate the energy difference between the nuclear spin states. For a nucleus the energy eigenvalues can be obtained by taking the Hamiltonian operator given in Eq. 2.11 so that H_ I I, m >= E m I I, m >= -COol_ I I, m >= - m h ~ o I I, m >

For I

=

1/2, m can be

_+ 1/2

E% - - ~ h

(2.12)

which gives the eigenvalues Em of

COo(m - ~ ) and E_,/: = ~ h COo(m - - ~ )

(2.13)

The separation (Figure 2.1) of m - _+ 1/2 levels is AE = E - l / - El/ - h O~o which is 2 2 . the energy of the m - _+1/2 transition and matches the classical expression given in Eq. 2.4.

2.1.2 Bulk magnetisation A sample on which an NMR experiment is carried out will usually contain a large number of nuclei. They will all precess around the direction of B o but the phase of precession of each individual spin will vary (the so-called random phase approximation). Consequently, in a unit volume the total nuclear magnetisation M is obtained by summing the individual contributions. The components of magnetic moment perpendicular (transverse) to B o cancel leaving M pointing parallel to the applied magnetic field. M acquires a non-zero equilibrium value Mo in this longitudinal direction since the applied field B o tends to align the magnetic moments preferentially in the B o direction through a normal susceptibility mechanism. It is pertinent at this point to reflect on the different ways of describing magnetic fields. They can be characterised by the magnetic induction (or flux density) designated by B and measured in Tesla (T) or by the magnetic field intensity H measured in Am-1. The difference between these two vector fields is that H is a measure of the field from free current alone whereas B depends on the material. B is the more fundamental quantity and is what is meant when the term "magnetic field" is used in this book. The magnetisation (M) results from the

27

Physical Background

additional internal field provided by the sample itself and is also measured in Am-~. The three vector fields are related by B

H = ,~ - M ~

~0

(2.14)

~

Where ~o = 4w x 10 -7 Hm -~ is the permeability of a vacuum. The amount of magnetisation generated by a sample when placed in magnetic field is characterised by the susceptibility Xv. The literature on magnetic susceptibility is littered with confusing units and careful attention needs to be paid to the units being used. Magnetisation is effectively the amount of magnetic moment per unit volume so the most directly useful measure is the volume susceptibility thus

Zv -

~M

(2.15)

~H

In a linear isotropic material B = / z H where p~ is the permeability of the material. The relative permeability ~r = P~/~o or = 1 + • is a measure of how much magnetisation a material will generate. The molar and specific susceptibilities are also quoted in the literature. The equilibrium bulk magnetisation comes from the net moment on summation of contributions from individual spins. A sample containing spin-1/2 nuclei has two states (energy levels) with the nuclear spins distributed between these according to the Boltzmann distribution which gives the probability of occupation of the different states. N1/2

AE = e kT

~-. 9~,~

N-1/2

The total number of spins is given by N = N1/ + N_ and the difference in population is n = N1/z-N 1/2. Then N1/2 -- (N-n)/2 aZnd N_ 1/2' (N+n)/2. Substituting into 1/2 - Eq. 2.16 gives AE

N-n

~ - - e

AE

kT

1 - e kv n - N ~

(2.17)

N+n

l+e

k~

which in the high temperature limit (nearly always a good approximation in NMR) AE AE gives ~ 0"33 and consequently 811 -> ~22 ~ ~33. Particular special cases are (Figure 2.7):

811=~'~2

833 811

822

833 811

822--q~33

Figure 2.7. Example of static CSA powder patterns showing different skews. spherically symmetric where 811 = ~22 = ~33, ~ -- K -- 0 axially symmetric (prolate) where 811 = ~22 5/:~33, ~'~ finite, K= 1. axially symmetric (oblate) where 81~ 4= ~22 = ~33, , [-~ finite, K = -- 1.

2.2.5 Knight shift In a conducting solid the nucleus will couple to electrons of the chemical bonds in the same manner as above for diamagnetic insulating solids, but there is the additional effect of the conduction electrons. Unlike paramagnetic and diamagnetic solids, the electrons are delocalised in conductors and a given nucleus will experience the magnetic field from a range of electrons. The effect of this collective influence must be described. The conduction electrons occupy a Fermi distribution within the electronic states of the material. These states are occupied in a pairwise fashion due to the Pauli exclusion principle (spin up and spin down) from the lowest energy up to the Fermi energy. At Bo = 0 the pairing will mean that there is effectively no net magnetic moment from these electrons. However, under a magnetic field there is a shift in energy of the spin-up and down states. The resulting imbalance in the number of spin-up and down states leads to a net magnetisation of the material and hence an associated susceptibility termed the Pauli susceptibility (Xp). Then, if the change in populations are n + 1/, M = / l e ( n ~ - n_,/2)

(2.89)

if the electron distribution is described by fiE) so that

M - lu2Bf nf(E)dE For a free electron gas, in the limit that Ef > > kT then

(2.90)

49

Physical Background M - IaBBon(E f ) ~ Zp - ~ t o l 2 B 2 n ( E f )

(2.91)

The shift in the field thus produced can be determined by calculating the field caused by the electrons producing the susceptibility. This can be expressed in terms of the hyperfine field produced by the conduction electrons, a contact type of interaction,

4re 3

F

This depends on the value of the wavefunction at the nucleus (r = 0) for electrons averaged over the Fermi surface, where 12 is the volume per electron. This shift of frequency in a metal was first observed in copper in 1949 by W.D. Knight and was subsequently termed the Knight shift (Townes, Herring and Knight 1950, Knight 1956), given by K - AB

2

(2.93)

Since K depends on the wavefunction density at the nucleus, the effect is dominated by s-electrons which is certainly true in metals with unpaired s-electrons. If the Pauli susceptibility and electron density can be independently measured then the Knight shift will give an independent measure of the s-component of the conduction electron spin density. These shifts are positive and are much larger than chemical shift effects, some typical values being L i - 0.025%, A g - 0.52% and H g - 2.5%. In other metals the situation is more complicated when the s-electrons are paired but there are other electrons (e.g. p but especially d). As only s-electrons have significant density at the nucleus the effects of these other electrons are much smaller. The hyperfine fields of these electrons induce polarisation in the s-electrons that subsequently produce a shift, termed core polarisation.

R-orb + (y

Total s h i f t - K + c r - K s + Kcp + "~d

(2.94)

A major problem is that it is the net value of the shift which is measured in the experiment. Two points emerge from this. The zero of the scale needs to be known so that the contribution of the chemical shielding has to be taken into account. Also, in more complicated metals the different terms have different signs, with Ks and Kd ~ positive whereas Kcp is negative. If the symmetry of the site is lower than cubic the full tensor form of the electronnucleus interaction needs to be used, so that in addition to an isotropic term there is an anisotropic contribution. If in the PAS of the Knight shift tensor the components of the tensor are Kx, Ky and Kz, then in the laboratory frame with its orientation in the frame defined by Bo described by the Euler angles 0 and +,

50

Multinuclear Solid-State NMR of lnorganic Materials B - B o { K z cos20+Kx sin2Ocos2~+Kr sin2Osin2~}

(2.95)

In many materials the tensor is axially symmetric so that Kx = Ky - - Kz/2 and the anisotropy is usually axially symmetric Kax. The frequency shift is then given by

AV-

YBo (Kis o + Kax(3 2re

COS 2

O-- 1))

(2.96)

2.2.6 Quadrupole interaction It can be shown that for nuclei with I > 1/2, the electrical charge distribution of the nucleus is non-spherical, giving rise to an electric quadrupole moment. The background to the quadrupole interaction is given in the classic article by Cohen and Reif (1957) and a comprehensive recent introduction has been given by Man (2000). The quadrupole moment is designated eQ and can be prolate (eQ > 0) or oblate (eQ < 0). The energy of a charge distribution in an electrostatic potential is V(r)

E - J

p(r). V ( r ) d r

(2.97)

In the volume surrounding the nucleus V(r) can be expanded as a Taylor series as

V(r)- V(O)+

Z

i--~t

i=x,y,z

+~

ij

r=0

" "

aiaj[

+

(2.98)

r=0

which can be substituted into Eq. (2.97) to give 1

i

(2.99)

t,y

where ~V

Vi - - ~ / . and

V~, -

~2V

Oi---0-]

(2.100)

Vii is a second rank symmetric tensor which is diagonal in its PAS so that Vq - ~ij . Vq

(2.101)

The zero-order term in Eq. 2.100 represents the electrostatic energy which is the same for all orientations. This term will have no influence on the spectrum and can be ignored. The first-order term represents the electric dipole moment and from the fact that the nuclear wavefunction is symmetric (i.e. r(r) 5 r (2 r)), the product rr(r) is antisymmetric and this term will be identically zero for all nuclei. The second-order term is the electric quadrupole moment and this is the most important in giving an

Physical Background

51

orientational dependence to the effect of this interaction on the NMR spectrum. The variation will be determined from the deviation of the charge distribution from spherical symmetry and is defined as

eQ - I P(r)(3z2 - r2 )

(2.102)

For the potential Laplace's equation states VZv = 0, from which it follows that for the electric gradient tensor, s

-0

(2.103)

i

This property means that to fully define V only two of its three components need to be known. It is usually chosen to define these as the largest component

Vzz = eq

(2.104)

and the asymmetry parameter

(Henceforth the term xI will be taken to refer exclusively to the asymmetry parameter in the quadrupole interaction). The energy equation for the quadrupole interaction can be transformed into a form that makes it compatible with the other Hamiltonians above by substituting spatial operators with spin operators using the Wigner-Eckart theorem (Slichter 1990) which after some manipulation gives the quadrupole Hamiltonian in the PAS of this interaction

)]-

3# -

+

(i+ +

where XQ is the quadrupole coupling constant defined as

ZQ=

eeqQ h

(2.107)

The electric field gradient (efg) is set up by the charge distribution outside the ion (e.g. A13+) but the initially spherical charge distribution of inner shells of electrons will become polarised to lower their energy in this efg. This polarisation of the inner electrons produces an additional efg at the nucleus so that eq, = eq (1 - ~/~) where ( 1 - ~/~) is the Sternheimer antishielding factor (Sternheimer 1954). This factor is a measure of the magnification of eq due to distortion of the inner electrons close to a nucleus. Full energy band structure calculations of efgs have improved markedly in recent years with developments of code such as WIEN97 (Blaha, Schwartz and

52


Dederichs 1988, Blaha, Schwartz and Luitz 1999). These show the importance of the contribution of the electrons on the ion itself compared to the lattice. These direct calculations remove the need to apply this correction factor but it is helpful to make it clear why some nuclei experience much larger quadrupole interactions than others. To obtain the quadrupole Hamiltonian of a spin in a magnetic field the Hamiltonian needs to be transformed from the PAS to the LAB frame, keeping only those terms that commute with Iz. This is called truncation of a Hamiltonian and is only valid when HQ < < Hz (the high field approximation). To perform the transformation it is much more convenient if second-rank irreducible spherical tensors are used. The Cartesian and spherical tensor elements (T) can be related (see Schmidt-Rohr and Spiess 1994 and Eq. 8, in Man 2000), with two of the more common elements being (2.108)

~o - i= and "~/-6/~2o- 3~2 _ 12 In these operators, the quadrupole Hamiltonian is

where Vi are the elements of the electric field gradient in the PAS so that

V0 = ,1~ eq, V+~ - 0 , ~2 -

V+_2=

eqJ~

(2.1 10)

2

In the high field limit, where the quadrupole interaction acts as a perturbation of the Zeeman states, the terms of this Hamiltonian which commute with Iz lead to the perturbation of first-order

4(1)_ o

eQ

4 1 ( ~ - 1)

~[3i2 i2]V0

(2.111)

and the second-order term is

(

H~2) _

2

/2

{VIV liz (412 -- 8 # - 1 ) +

eQ

V0 4 I ( 2 1 - I )

VzV2I~( 2 I

2 - 2 I 2 - 1)} ( 2 . 1 1 2 )

These perturbations then lead to a shift in the separation of the Zeeman states Em.m- 1 - E m - E m - 1 which to first-order is E(l) _

3eQ

- 41(21 - 1)

(1 - 2m)V0

(2.113)

PhysicalBackground

53

However second-order effects are also present and for the central (1/2, -- 1/2) transition as there is no first-order energy shift (m = 1/2), these become particularly important. This shift is E(2) =

2[

eQ ]2~V-~Vl(24m(m-1)-4I(I+l)+9)+} I V-2V2

v0 4 I ( 2 I - 1 )

(2.114)

(lZm(m-1)-41(I+l)+6)

The Vi then need to be transformed from the PAS to the Laboratory frame. If the Euler angles (or, [3, ~/) describe the orientation of Bo with respect to the PAS system the tensor elements can be transformed by Wigner rotation matrices Dij (n) (Haeberlen 1976) via

2 (2.115)

_ Z

j=-2

Substituting these values and operating on the states with the spin operators gives the perturbations and the orientation dependence

E(1)=

Zoh 8I(2I-1)

[ cos

0 -1+

0

-

+

The Zeeman splitting gives a set of equally-spaced energy levels, shown for a spin-5/2 system in Figure 2.8. To first-order, the quadrupole splitting gives a set of symmetric transitions with the magnitude of the effect depending on XQ and a characteristic shape determined by 0. The first-order term splits the spectrum into 2I components of which the single quantum intensity is detected. This intensity depends on I<mlIxlm+ 1>12 (proportional to I(I+ 1 ) - m ( m + 1 ) = oL) at frequency V(1)m, m -- 1 away. The intensity distribution between the different transitions has to be carefully accounted for when accurate quantification is required from NMR spectra. The splitting between different transitions also causes a change in the pulse response. If XQ is very small so that the energy levels remain effectively evenly spaced, the response to the if-pulse is as would be expected for a spin-l/2 nucleus with the same ~/(Schmidt 1971, Fenzke 1984, Man 1988). This is called non-selective excitation and the magnetisation precesses as sino~rft. When XQ becomes significant, the separation between adjacent levels is quite different and at each set of energy levels the nucleus behaves as an effective spin1/2 nucleus but with a different ~/. This limit is termed selective excitation. This ability to consider transitions in pairs has become known as the fictitious spin-b2 formalism with each different transition showing its own nutation rate, and its pulse response now being sinoLtOrft. This change of response to the rf pulse means that the effective 90 ~ pulse

et al.

et al.

54

Multinuclear Solid-State NMR of lnorganic Materials B0 only Zeeman interaction

-hv0m

First-order quadrupolar

Second-order quadrupolar

~0~(3cos20_1)

9hz~ 6400vo

5

5(2 sin 2 20+ sin 4 O)

/

-512

-3/2

_~

-1

.3(sin 4 8 - 2sin 228)

-1/2

N

-4

2(sin' O- 2sin' 20)

112

~

-4

/ 2(2sin' 20- sin' O)

3/2

_...

-1

/

3(2sin2 20-sin4 O) 5 5/2

-5(2sin 220+ sin4 O)

/

Figure 2.8. Energy level diagram of a spin-5/2 system showing the Zeeman interaction and the first- and second-order quadrupole perturbation of the energy levels.

Table 2.3. Changes in the 90 ~ pulse and the intensity distribution for different transitions for quadrupole nuclei. I 3/2 5/2

7/2

9/2

mz

1/2 3/2 1/2 3/2 5/2 1/2 3/2 5/2 7/2

1/2 3/2 5/2 7/2 9/2

90~ 0.500 0.578 0.333 0.354 0.447 0.250 0.258 0.389 0.378 0.200 0.204 0.218 0.250 0.333

Int (%) 40.0 60.0 25.7 45.6 28.6 19.0 35.8 28.6 16.6 15.2 29.0 25.4 19.4 11.0

t _ For large • comparedto the case where there is no quadrupole splitting.

55

Physical Background

changes, as shown in Table 2.3. The change of response to the rf pulse with XQ is the basis of the 2D quadrupole nutation technique (Samoson and Lippmaa 1983, Guerts et al. 1985). The differences in the pulse response and the observed intensity for the selective and non-selective cases need to be considered. For non-selective excitation,

2 lm+l,m _ am+l,m "nonsel -- I-1

sin mrf t

Z 2 am+l,m -I

(2.117)

and the equivalent expression for selective excitation is m+l,m i sel

_

am+l,m I-1

Z -I

sin a~o~ft

(2.118)

2

am+l,m

so that in the small pulse angle limit (sin0 --- 0) these two expressions become the same and intensity between sites with very different XQ can be compared directly (Schmidt 1971, Lippmaa, Samoson and Magi 1986). The frequency shift can cause the non-central transitions (i.e. m # 1/2) to be shifted sufficiently far from the Larmor frequency that these transitions become difficult to observe with conventional pulse techniques. Equation 2.116 also shows that there is an orientational dependence of the frequency so that very broad lines occur in a powder. This is important for spin-1 nuclei as there is no central transition and all transitions are broadened to first-order. For the satellite transitions the shape extent will depend on XQ and the shape will depend on ~ (Figure 2.9A). It should be noted that for m - m transitions there is no first-order effect. For the central transition of non-integer quadrupolar nuclei, v(]~ _ i/ - 0 , and "2' 2 . the dominant perturbation is second-order. The rotation of the tensor elements in the second-order equation gives _

1//2

(--~

V1V_1 - - ~ - 3

COS2 20~ + 2 r / c o s 2 a - 3)cos 4 fl

e2q2 q-(~-3/72cos 2 2 0 : - 2 r / c o s 2 a -

1//2 --b3)cos 213

3

(2.119)

1//2 (1 - cos 2 20:) and 1

2

2

1

( - ~ r / cos 2 c t - - r / c o s 2 a 4

V2V_2 -

_3_.e2q2

+(--12//2 COS2

2

1.2

_

3

8 )cOS4fl

2a_ 1 2

77 - 3 )cos

1

3

+ - ,t cos 2 2 a + - 7/cos 2 a + 3 4 8

(2.120)


56 A

B

rt=l

.n=l

n =0.

n =0.9

n =0.

n =0.8 n = 0.7

n---0.4 r--J

~_~~--a

n--~

~ , ~

.... H

.... ,___ '--~ .....

I

-2

,__

n-0.3

....

n =0 I

n-0.4

n =0 I

-2VQ

I

l

-VQ

I

I

I

0

I

I

I

I

2VQ

VQ

I

I

1

I

0.5

0

I

-0.5

I

-1

I

-1.5

I

-2

A

Figure 2.9. The quadrupole perturbed powder patterns for an I = 3/2 nucleus. A. The outer satellite transitions to first-order. B. The second-order quadrupolar broadening of the central transition with A = (I(I+ 1) - 3/4)p~/I10. The second-order line can then be calculated for any transition so that for the (1/2, -- 1/2) transition ~,(2) _ 1 I 3ZQ ~-~ - - 6v----~ 2 I ( 2 I - 1)

[A(a,~)cosa fl+B(a,~)cosZ fl+C(a,~)]

I(I+1)-

(2.121) where 27 9 A ( a , rl) = - - - + - 0 c o s 2 a - 8 4

B(a, rl) =

30

/72

8

2

3 ~2

cos 2 2 a

(2.122)

8

277 cos 2 a +

3 -4~

2

cos2 2 a

(2.123)

Physical Background

57

2

C(a, 7/) - - ~3 + __f_0- - ~-r/cosZct--8 1 3 r/2 cos 2 2o~

(2.124)

To obtain the lineshape of the central transition the second-order perturbation of the quadrupolar interaction must be calculated on the (1/2,-1/2)transition. These give distinctive lineshapes for the centreband (Figure 2.9B) which provide information about the local symmetry of the electrical charge about the nuclear site. For the special axially symmetric case where rl = 0,

,

(2)

"~-~

_

1

3XQ

I ( I + 1)-

(1- cos 2 fl)(9 cos 2 f l - 1)

(2.125)

16v o 2 I ( 2 I - 1)

If the total width of the central transition is Av, an estimate of the quadrupolar interaction is

xQ~r/~ + 22r/Q + 25 - 8 I ( 2 I - 1)l

v~ 3 (I(I + 1 ) - ~)

(2.126)

2.2.7 Nature of interactions All these interactions are quite similar in form, containing an anisotropic part so that in powders the lines are usually significantly broadened. The first-order perturbation shows an angular variation of the form A((3cos213 - 1) + "q sin2[3cos2o0 which is similar for all interactions. Sometimes it is necessary to distinguish between homogeneous and inhomogeneous interactions. The distinction can affect the way their averaging must be approached and also the way the interaction responds to refocusing, e.g. in echo experiments. An inhomogeneous interaction means that a broad spectral line can be regarded as being made up of many individual contributions, with each spin, (e.g. depending on its orientation) contributing intensity to a specific part of the line. For a homogeneous line, a spin via its interactions with other spins (especially via spin diffusion caused by the flip-flop term of the dipolar Hamiltonian) contributes intensity effectively to the whole line. This distinction gives rise to the well known phenomenon in laser and optical spectroscopy of hole burning. If a resonance is strongly irradiated by narrow band radiation at a specific frequency within a broad spectral line, a homogeneous line will show a decrease in intensity across the whole line whereas an inhomogeneous line will show a decrease in intensity only in the vicinity of the irradiation. Most interactions are inhomogeneous e.g. shielding effects, the quadrupolar interaction and Bo inhomogeneity, whereas homonuclear dipolar coupling can be homogeneous. A more rigorous distinction between these two interactions

58


is that given by average Hamiltonian theory. If the Hamiltonian operator is taken as a function of time and the terms commute at different times, the interaction is inhomogeneous. There is also possible intermediate behaviour where irradiation at a specific point affects other parts of the line but not the complete line, for which the term heterogeneous has been coined, although this can be regarded as largely inhomogeneous. For these interactions there is also the concept of coherent and incoherent homogeneous interactions. T2 relaxation is incoherent due to the independently fluctuating local fields whereas coherent effects would be generated by multiparticle interactions such as the dipolar couplings between spins. This is discussed in some detail by SchmidtRohr and Spiess (1994). The inhomogeneous interactions such as shift anisotropy and dispersion, two-spin dipolar interactions and the quadrupolar interaction can be refocused by spin echoes (Hahn 1950) whereas coherent multispin interactions can only be refocused by sequences such as the solid echo (Powles and Strange 1965) and magic sandwich echo. Incoherent effects are essentially irreversible.

2.3. ONE DIMENSIONAL METHODS FOR IMPROVING RESOLUTION

The anisotropic part of the interaction can often provide insight into structure, but is usually regarded as the poor relation of the isotropic information. In materials with relatively few sites static NMR spectroscopy is often worth considering. Its disadvantage is that the anisotropic part causes broadening which can often be very significant so that there is strong overlap between different sites, meaning that different sites cannot really be resolved. In liquids the interactions responsible for line broadening are averaged by the continuous, random tumbling and translational motions of the molecules. This isotropic averaging of the second-rank tensor interactions produces high resolution spectra. However, for solids it is often necessary to improve resolution by deliberately averaging the anisotropic parts of the interactions, thereby obtaining line narrowing. The Hamiltonians representing these interactions have all been seen to contain a spatially-dependent part and a spin-dependent part. To average these interactions, either (or both) of these parts of the Hamiltonian must be manipulated which usually means making them time-dependent. By far the most common approach is to make the spatial part time-dependent, for example by applying magic angle spinning (MAS). Sometimes this is extended to other angles and is then called variable angle spinning (VAS). There are other more complex time dependencies imposed such as double angle rotation (DOR) and dynamic angle spinning (DAS). The spatial part can also be varied by diluting the spins (i.e. increasing r) which reduces the dipolar coupling since the magnitude of the dipolar coupling is proportional to the inverse cube of the distance between spins. By making that distance large, the dipolar coupling can be made small. This situation occurs naturally for 13C where the low natural abundance ensures that carbon-carbon dipolar couplings are not

Physical Background

59

observed in J3C spectra. For nuclei such as protons where the dipolar coupling between protons can be very strong, diluting the protons can be extremely helpful. This can be done by deuterating a sample, which is acceptable provided it does not change the chemical nature of the sample (which might not be true, for example, for hydrogen-bonding). For species that are naturally rare e.g. 13C, aSN, selective isotopic enrichment selectively introduces the dipolar coupling which can be used to determine bond lengths. Techniques for averaging the spin part include decoupling, both for heteronuclear and, by multiple pulse techniques, for homonuclear dipolar coupling cases. Averaging techniques can be used in combination with each other, leading to even higher resolution enhancement. Included in such approaches are the combination of multiple pulse homonuclear dipolar coupling averaging with MAS which is then termed CRAMPS (Combined Rotation and Multiple Pulse Sequence), and more recently for non-integer spin quadrupolar nuclei, multiple quantum excitation has been combined with MAS in MQMAS sequences. For multinuclear studies of solid materials all of these techniques should ideally be available so that high resolution solid-state NMR spectra can be obtained from a range of solids.

2.3.1 Magic angle spinning and first-order effects 2.3.1.1 Physical principles. Magic Angle Spinning is probably the most widely used technique to enhance spectral resolution in solid state NMR spectroscopy (Andrew 1971, 1981). The solid sample, usually a powder, is loaded into a container called a rotor which is inclined at a fixed angle to the magnetic field and rapidly spun about its symmetry axis. The Hamiltonian thus becomes time-dependent and to find its new form the tensor elements have to be rotated, now not directly from the PAS to the Laboratory (LAB) frame, as for a static powder, but first to the rotor frame (ROT). With the Euler angles describing the relative orientations of these frames shown in Figure 2.10 this can be readily achieved by using the Wigner rotation matrices to give in the Laboratory frame

H L~b(t) - CI~AjToo + CIA Z "mO r) (2) (0, O, (_Ort+ q~)Z D(2) m l m (CZ,i~, ~') m

(2.127)

m1

This then leads to a Hamiltonian that can be split up into modified static and timedependent parts

H Lab (t)

= H + H * (t)

(2.128)


60

ZL ZROT~~

io xL~

.....

n ""--~

x Yt~ YROT

X~S

Figure 2.10. The relative orientations of the principal axis system (PAS), rotor (ROT) and laboratory (LAB) frames.

The part which now appears static in the Laboratory frame is

H - Cl_AjToo + ~/-~CI:Aj (3 c~ "

0 - 1 ) A A [(3cos 2 / 3 - 1 ) + r/sin 2/3cos2y]

(2.129)

2

This Hamiltonian is very similar in form to the anisotropic parts of the Hamiltonians found above. Thus, in a powder where [3 and ~/are different for different crystallites, the same broadening effect would occur. However, every crystallite orientation has now acquired a modulation factor 1/2(3cos20 - 1) (= P2(cos0), the second-order Legendre polynomial). If 0 is set to 54 ~ 44' 8", the value of Pz(cos0) is zero and the anisotropic part disappears leaving only the isotropic part as would appear in solution; this is termed the magic angle. It should be emphasised that this is strictly true if only first-order broadening effects are present. An alternative way to envisage the effect of MAS on spatially-dependent Hamiltonians is to consider the dipolar Hamiltonian. For a fixed internuclear vector it is the orientation of this vector that determines the magnitude of the interaction. If the sample is rotated about an axis, for any given internuclear vector its "average" orientation will become the rotation axis (Figure 2.11) which can be set such that

(3C0S 2 0 - 1)- ~-1 (3C0S2 0 m --1)(3 cos 2 1 3 - 1 ) - 0

(2.130)

Physical Background

_

J

61

A>f

2o,.rp.,r

Figure 2.11. Schematic reorientation of a dipolar pair under MAS.

2.3.1.2 Formation of spinning sidebands. The above expression for the effect of MAS has neglected the H(t) part of Eq. 2.128. This is a good approximation if the spinning speed is very high compared to the magnitude of the interaction, or the Hamiltonian is sampled after complete cycles only (termed rotor synchronised acquisition). This is not usually the case so that the full signal produced by the modulation process is obtained. Without losing generality the phase factor + in Eq. 2.127 can be set to zero which gives

H(t)- ./27 CIzAiAA[-cos2~cos20cos(.Ort + sin 2 flsin 20cos2(-Ort ] ~32

L

(2.131)

This time modulation produces spinning sidebands which are signals separated from the isotropic line by integer multiples of the spinning frequency yr. They will extend out to cover a range comparable with the frequency extent of the static anisotropy. The spinning speed necessary to break a static powder pattern up into a set of spinning sidebands must be carefully considered. Sideband formation is discussed in a classic article by Maficq and Waugh (1979). For sample spinning to have any averaging effect, a causality argument can be invoked. If an interaction produces a line that spans a frequency range Av, in the time domain this magnetisation remains coherent for --~1/Av. Any imposed modulation must be occurring faster than this to have any effect. This would apparently limit the applicability of MAS since, even with speeds of 50 kHz becoming available, many lines encountered in solid state NMR are much broader. Fortunately a distinction can be drawn between homogeneous and inhomogeneous interactions. For homogeneous interactions (e.g. strong dipolar coupling between groups of nuclei such as protons) this condition does indeed have to be approached before MAS will have any significant effect since the spins lose complete coherence on the timescale 1lAy. However, for an inhomogeneous line, the transverse magnetisation dephases after a pulse, but this is because the individual spins lose coherence with one another. A 180 ~pulse would refocus this magnetisation and a spin echo would

62


form. This means that the lifetime associated with an individual spin in the transverse plane is quite long, i.e. the intrinsic linewidth associated with any particular spin is small. It is this intrinsic width associated with a given spin that the spinning speed must exceed. In terms of average Hamiltonian theory for an inhomogeneous interaction, the Hamiltonian commutes at different times, so precise averaging will occur at rotation rates small compared to Av. Following an rf pulse, because the tensor orientations of each crystallite are different, the resonant frequency for each crystallite is different and the magnetisation rapidly dephases. This can be envisaged pictorially from the chemical shift interaction. In the static powder pattern the frequency axis could be read as an orientation axis. Then in Figure 2.12 the two sets of spins starting off at A and B have different initial precession rates. The azimuthal phase angle picked up by each of these orientations after a time t is t

(2.132)

qgi(t)- OgoCrisot+ coo H i (t')dt" 0

The time domain signal (ignoring relaxation effects) is then t

gE (t) - - Z exp(i q~i(t)) - exp(i coo~Yisot )exp Z i coi ~ H;(t')dt" i

i

(2.133)

0

f~

"0

TR

Figure 2.12. Schematic variation of the frequency of two spin packets under a chemical shift interaction showing that the frequency varies during the MAS period (TR) and that the same range of frequencies are experienced by the two spin packets.

Physical Background

63

The purely oscillatory nature of H*(t) (Eq. 2.132) means that H*(t) will disappear at times Tr-- P/vr, where P is integer. Thus after one complete revolution the particles will have regained their phase coherence and an echo (a rotational spin-echo) will form. This can be seen since as the particles change orientation due to the spinning their instantaneous precession frequency, which depends on orientation, will change. However, over a complete cycle all spin packets will have experienced the same set of orientations so that at integer multiples of the spinning frequency all spins will have undergone the same average precession governed by the isotropic shielding. Hence in the time domain the signal will be formed by a series of these rotational echoes and it is the number of these rotational echoes, and hence how long the magnetisation lasts in the time domain, that governs the resolution of an MAS NMR spectrum. Spinning sidebands are formed through the accumulated phase with the FID represented by

g(t)- e i~r176176

(2.134)

which can be used to calculate the intensity of the spinning sidebands. The intensities of the spinning sidebands formed provide information about the anisotropic parts of the interactions. Detailed calculations have been made of the sidebands for different interactions, including the chemical shift interaction, by Maricq and Waugh (1979), and Herzfeld and Berger (1980). Nayeem and Yesinowski (1988) have carried out similar calculations for sideband formation in paramagnetic solids.

2.3.2 Magic angle spinning and higher-order effects In many cases it is not sufficient to consider only the first-order broadening terms. It has already been seen that the second-order quadrupole effects often dominate the lineshape of the static spectra of the central transition of non-integer quadrupolar nuclei. It can be anticipated from the more complex angular variation of this interaction that MAS is unlikely to achieve complete averaging of this term. Quadrupole effects often result in more complex spectra than from first-order effects alone. Also, in a system that experiences multiple interactions there is coupling between these interactions and the cross terms of the interactions (the second-order quadrupole interaction can be regarded as the cross term of the quadrupole interaction with itself). These terms can produce both additional isotropic effects and residual anisotropic effects under MAS. Stuart (1994) has dealt in detail with the case of magnetically dilute nuclei that can simultaneously experience the quadrupole effect, shielding anisotropy and heteronuclear spin coupling. There are two specific cases that are more commonly observed

64


than other combinations and will be examined here viz. (i) direct second-order quadrupole effects and (ii) the distorting effects of quadrupole mixing of spin states on spin coupling interactions.

2.3.2.1 MAS of second-order quadrupole effects. The same type of rotations using Wigner rotation matrices can be applied to the second-order effects as for the firstorder interaction which leads to a second-order quadrupole energy of interaction such that

2 Co (l,m)Fo (71)+ II3ZQIc2(I,m)Pz(cosO)F2([3,)',~)+ E~2)lm, m- 1 ) - V--o 21(21-1) C4(I,m)P4(COS 0 ) F 4 (/~, ~',/7)

(2.135)

1 P4(COS0) - ~ (35 cos 4 0 - 3 0 c o s 2 0 + 3)

(2.136)

where

772

F0(r/) - 1 + ~

(2.137)

3

1-y

(cos/3)7/sin2 t3 cos 27'

(2.138)

F4(/3,r, rt,)- 1+ i 8 P4(cos/3)+ 57/ 96 ( 7 c o s 4 f l - 4 c o s 2 f l - 3)cos 2)' + 35rl 2

~(cos4/3 1152

(2.139)

- 4 cos 2/3 + 3) cos 4~'

It is of central importance to realise that this second-order quadrupole broadening has an isotropic part as well as anisotropic terms that are proportional to the second-order and fourth-order Legendre polynomials. The variations of these two functions are sketched in Figure 2.13. It is apparent that there is no single angle which can simultaneously satisfy that P2(cos0) = 0 and P4(cos0) = 0. Spinning around a single axis can

Physical Background

65

T a b l e 2.4. The coefficients Cn(I,m) for second-order

quadrupole effects. I 3/2

M

Co(I,m)

C2(I,m)

C4(I,m)

1/2

2/5 6/5 -- 16/15 - 4/5 20/3 -- 30/15 - 54/15 30/15 294/15 - 48/15 - 108/15 - 60/15 168/15 648/15

- 8/7 0 - 64/21 - 40/7 40/21 - 120/21 - 96/7 - 240/21 168/21 - 192/21 - 168/7 - 600/21 - 336/21 432/21

54/35 - 6/5 144/35 228/35 - 60/7 270/35 606/35 330/35 - 966/35 432/35 1092/35 1140/35 168/35 - 2332/35

30.6 ~

54.7 ~

--

3/2 5/2

1/2

3/2 5/2 7/2

9/2

1/2

3/2 5/2 7/2 1/2 3/2 5/2 7/2 9/2

,... \

1

\ 0.5

X

9 ' x

""

X

I I "[

I"

P4(cosO)\\

I I I

.P2(cosO).

I I

\

70.1 ~

I

I

"'..

I / " I / I/ /

"'.

\1\

" "..

0

Ix I I i

".

/I /"'/ ...

\ \

'/ ~

"-

I

-0.5

I -I

'

0

10

20

30

40

50

60

70

" ""

80

90

0

Figure 2.13. The Legendre functions P2(cos0) and P4(cos0).

c o m p l e t e l y r e m o v e e i t h e r o n e or the o t h e r o f t h e s e t e r m s b u t w i l l t h e n o n l y p a r t i a l l y a v e r a g e the other. T h e a p p r o a c h o f v a r i a b l e a n g l e s p i n n i n g b a l a n c e s this p a r t i a l avera g e to a c h i e v e the b e s t r e s o l u t i o n (see b e l o w ) . B y far the m o s t c o m m o n a p p r o a c h is to spin at the c o n v e n t i o n a l m a g i c a n g l e o f 54.7 ~ as this e n s u r e s that

all

the first-order

effects are r e m o v e d a n d to live w i t h the c o n s e q u e n c e s o f o n l y p a r t i a l l y a v e r a g i n g the P4(cos0) t e r m . T h e n the f r e q u e n c y w i l l be g i v e n b y


66

3[xQ 3[xQ

Q(2), Vm,m+ l -- _

(I(I + 1 ) - 9m(m + 1 ) - 3) 1 +

40v o 1(21-1)

2 (61(1 + 1)- 34m(m + 1)- 13)

80v o 1(21-1)

(2.140)

X [F4 (0~, F])cos 4 ~-k- F2 (a, ~)cos 2 ~--I- Fo (a, F])] where 105 16

35 35,2 8 r / c o s 2 a +~48 ,i cos 2 2 a

(2.141)

F2(a, r/) - - ~45 + 57/2 + 57/cos 2 a - 35r/2 cos 2 2 a 8 12

(2.142)

F4 (a, rl)

9

~+ F~(a' r/) - 16

7/2

5 r/cos 2 a -k-~ 35 7/2 COS2 2 a

.... 3

8

48

(2.143)

For the normally observed central transition then, this frequency in the fast spinning limit is (2)MAS

~'-~

1 [

I(I + 1)-

3XQ

6v o 21(21-1)

F~(a' r/) c~

fl +

(2.144)

F2 (a, q) cos 213+ F4 (a, r/)

where F0 (a, r/) -

~__m21 7 16

8

7

r/cos 2 a + ~ (r/cos2o~) 2 48

F2 ( a, tl ) -- ----9 + ~712 7/COS2 a -- ~(7/COS 7 20;) 2 8

12

(2.145)

(2.146)

24

5 1 7 F4 (a, r / ) - 1---~- ~ r / c o s Z a +-4--~(rlcosZa) 2

(2.147)

The centrebands produced under this limit will also have characteristic lineshapes even though the shapes will be very different from the static ones. The scale of the

67

Physical Background

narrowing can be gauged from the change in ~M2 under MAS which varies from 3.6 ('q = 0) to 2.4 (~q - 1) (Behrens and Schnabel 1981). The ratio of the overall linewidth static to MAS is generally given by the factor 7(1/2 + 2277 + 25) 2(6+//) 2

(2.148)

Again, if the singularities can be observed the quadrupole parameters can be obtained from spectral simulation and the lineshape is a strong function of xl (Figure 2.14). These lineshapes were seen in early calculations of the effect of MAS on the central transition (Kundla et al. 1981, Samoson et al. 1982, Muller 1982). In the case where 11 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 .......

,,

4t)

,

.

20

.

.

.

0

,

-2t) kl-l[z

.2 4;0.... '

20'

I) . . . .

,

,

-40

,

-r

\ -20

'-4'0

-60

8~o.~ ~ Figure 2.14. The MAS centreband of a second-order quadrupole perturbed powder pattern as a function of the asymmetry parameter xl for an I = 3/2 nucleus with XQ = 3.2 MHz at Uo = 95 MHz.


68

the spinning speed is not in the "infinite" speed limit, spinning sidebands appear. There will then be a dependence on the third Euler angle ~/. An interesting and complicating consequence of this is that the lineshape, even from the same transition, will vary according to the order of the spinning sideband. Hence a complex set of spinning sidebands will result which can make interpretation difficult, especially if a sample contains multiple sites with similar chemical shifts. Another consequence of the residual second-order term is that the centre of gravity of the lineshape does not coincide with the isotropic chemical shift. There is an isotropic second-order quadrupolar shift which is given generally by v~2) ( i , m ) _ -

Q,iso

3Z~

40Voi~i~i_ l)2

[i(i+l)_9m(m_l)_3]ll+~_l(2.149)

When only a featureless, usually asymmetric line is observed the peak position is often quoted and this can be significantly different from the isotropic chemical shift. This effect can be so large that a peak is shifted into an isotropic chemical shift range usually associated with another structural unit. There are numerous examples in the literature of mis-assignments based on this effect. Often when a material is not sufficiently crystalline for the singularities to be observed the resulting line is not just simply a featureless symmetric peak but is asymmetric. This asymmetry results from structural disorder producing a range of electric field gradients. Hence, if the sites had very similar isotropic chemical shifts (as expected if the sites are chemically very similar) then the lineshapes start from approximately the same position but the differing values of • stretch the lineshapes to different extents towards lower frequency (Eq. 2.149 is negative for m = 1/2). Hence the range of • stretches the overall lineshape to negative frequency giving the distinctive tail observed in the MAS powder spectra of disordered solids for nuclei such as 23Na and 27A1 (Figure 2.15). The inverse dependence of this effect on the applied magnetic field means that the lines become more symmetric as the magnetic field is increased and the actual peak position becomes closer to the true isotropic chemical shift. It is emphasised that most papers concerning such quadrupole nuclei when a featureless lie is observed quote the peak position. This position is, of course, a characteristic of the peak but must be quoted in the context of the applied magnetic field used since it is not a universal characteristic. Hence for the (1/2, - 1/2) transition the effect of the residual second-order isotropic shift is to produce a shift of the centre of gravity of ~cg, which means the overall position is given by 2'Q ~cg-~c~iso-3F(1)~2 1+ ~' 40 vo

(2.150)

69

Physical Background

A

B

D

i

i

1

'

'i

i

0

'i

i

-1

i

i

-2

i

-3

A Figure 2.15. Effect of a distribution of interactions on the second-order quadrupole powder lineshape of the central transition in detail with the mean interaction XQ = 2 MHz and ~q -- 0 at a Larmor frequency of 80 MHz showing A. no distribution, B. a Gaussian distribution of isotropic chemical shifts with FWHM = 0.17A, C. a Gaussian distribution of the quadrupole interaction of 340A and D. both the chemical shift and quadrupole interactions distributed, (A = 2344Hz). where F(I) is a spin-dependent factor given by Table 2.5. Spin dependent factors of the central transition.

I F(I)

3/2 1/3

5/2 2/25

7/2 5/147

9/2 1/54

The residual second-order effect produces a linewidth which is a function of I and m and then 2

Av(eZ~(i,m)~

ZQ [6I(I+l)-34m(m-1)-13] Voi2(2I-1) 2

For the central transition m = 1/2 a width can be associated with

(2.151)


70

2

2

Av~2) - f (l) ZQ(6 + ~) 224v o

(2.152)

This represents the speed that must be exceeded for MAS to cause even partial averaging of the second-order quadrupolar interaction, but fulfilling this condition does not mean that the fast spinning limit has been achieved. Although it is the central transition that is usually observed, the non-central or satellite transitions (i.e. different values of m) can also be observed. The same situation where MAS removes the Pz(cos0) term, but only partially averages the P4(cos0) term, remains. However it can be noted from Eq. 2.140 that the coefficient is a function of I and m (Samoson 1985). This means that most importantly the residual second-order quadrupole effects under MAS will be different for each transition and each spin. The factors for the relative isotropic chemical shift and residual widths normalised to the central transition are given in Table 2.6. The removal of the second-order quadrupolar width to improve resolution is the most important consideration. The best combinations of I and m for reducing these residual effects are the

(

+ 3 2 9 transitions of I = 2

tranitionsof I

n

~

5 2

nuclei with

and

/ +-

m

nuclei with only 5.6% of the width compared to the central

transition. This dependence was first noted by Samoson (1985) and was then developed extensively by the groups of Jager (see Jiiger 1994 for a summary) and Skibsted (1991), with the technique becoming known as satellite transition spectroscopy (SATRAS). For the above transitions not only are second-order broadening effects reduced, so that resolution is better, but the second-order shift away from the isotropic chemical shift position is smaller. Satellite transition spectroscopy has been most widely reported from I - 5/2 spins because of its applicability to nuclei such as 27A1 and 1 7 0 . The different factors for the different transitions have effects similar to determining the spectrum of thecentral transition at different applied magnetic fields. The m-dependence of the shift for a particular value of I also produces a different net isotropic shift, and this allows the isotropic chemical shift to be determined from a single spectrum. Hence simulation of all transitions provides an internal check on the NMR parameters. For amorphous solids the extent of the sideband manifold can be used to estimate the Table 2.6. The second-order isotropic shifts 8 (Q)isoand residual w i d t h s AV(2)Qin MAS spectra for satellite transitions (I, m) normalised to the central transition for the same spin. x/2, y/2 ~(2)Oiso

Avr

3,3 5,3 5,5 7,3 7,5 7,7 9,3 9,5 9,7 9,9 2.000 - 0.125 - 3.500 0.400 - 1.400 - 4.400 0.625 - 0.500 - 2.375 - 5.000 -0.889 0.292 - 1.833 0.622 -0.511 -2.400 0.764 0.056 - 1.125 -2.778

--

Physical Background

71

AV77

o ,~ I 1 ' 1

+

,~l++ltlN~itlllllill]l~l!lll[[tll)!lll I II ,,t t'1

-

,

,

c

(*10Snz) 8

,

,

4

.....

1'

,

9

0

,

20

.-

0

(*10Snz)

Figure 2.16. Effect of a changing distribution in the main component of the electric field gradientVzz comparing the effect on A. the satellite transition and B. the centreband with a mean quadrupole frequency of 1 MHz and then a distribution of quadrupole frequencies as shown. Only one half of the satellite transition is shown. The Larmor frequency is 104.26 MHz and a spinning speed of 11.5 kHz has been taken. From J~iger et al. 1993 with permission of the copyright owner.

average value of • and the shape of the envelope can be used to estimate the distribution. 27A1 SATRAS has been applied to a range of compounds, including ordered crystalline materials, atomically disordered solids and amorphous solids. A general problem with 27A1 NMR spectra from highly disordered solids is that the centreband peaks tend to be broad and asymmetric, often with extensive overlap of resonances that make the complete lineshape difficult to simulate. However, the inner satellite transition sidebands for I = 5/2 are better resolved and, just as importantly, are significantly more symmetric since the second-order quadrupolar broadening is reduced relative to the chemical shift dispersion broadening, allowing more straightforward deconvolution of the spectra. The effects of distributions on the shape of the centreband and the envelope of the spinning sideband from the inner transition have been calculated (J~iger et al. 1993). The sidebands take on a more monotonic decrease as the distribution becomes broader relative to the mean interaction (Figure 2.16). Even for nuclei where the satellite transitions are broader than the central transition (Table 2.6), if their second-order quadrupolar structure can be observed, they act as an independent check of quadrupolar parameters deduced from the central transition. The larger shift of the satellite transitions for I = 3/2 has been extremely useful in identifying different boron species from 11B NMR of borate glasses.

2.3.2.2 Residual coupling effects due to quadrupolar nuclei in M A S spectra. It was seen that for both dipolar and spin-spin coupling between nuclei the first-order anisotropic effects are proportional to Pz(cos0) so that under MAS if a nucleus experiences both these effects, only the isotropic effects of J remain. However, the

72


case is complicated if one of the coupled spins is quadrupolar. Suppose a spin-l/2 nucleus (I) is being observed which is coupled to a quadrupole nucleus (S). If the quadrupole interaction becomes strong enough, the eigenstates m of the quadrupole spin are not pure Zeeman states and mixing of spin states occurs, leading again to second-order perturbation terms of the Hamiltonian (Bohm et al. 1983, Harris and Olivieri 1992). This leads to a second-order frequency shift of

E

AV~Q2)- 3D'zo_ S ( S + I ) - 3 m 8Vos S ( 2 S - 1)

21

sin20sin/3Icos/3cos~O(3-r/cos2a) ]-r/sinOsin2a

]

(2.153)

This term is anisotropic and produces a powder pattern. It has been derived under the assumptions that first-order perturbation of the S-states is sufficient, that the J tensor is axially symmetric and that the unique axis of J is aligned with the internuclear vector. Under MAS this term will be scaled but, as it is not proportional to P2(cos0), it cannot be completely removed. Hence the MAS spectrum will still have some residual width, but the most profound effect is to leave an isotropic term which can be calculated by averaging the powder lineshape. Hence for a J-coupled system with an axially symmetric quadrupole interaction, the spectrum is shifted from the isotropic chemical shift by: Av- -mJ+

(2.154)

10Vos

S(2S - 1)

Hence the basic multiplet structure remains, although at low field the additional perturbation can strongly disrupt the coupling pattern. The implications of this frequency shift are that for non-integer spin-S the innermost and outermost lines will be shifted in opposite directions. Also the centre of gravity of the entire multiplet pattern is invariant under second-order effects, so that the chemical shift can be estimated from the centre of gravity of the spectrum. For a spin-l/2 nucleus coupled to a non-integer spin quadrupolar nucleus the spacings of the different transitions have been given (see Tables 2 and 3 in Harris and Olivieri 1992). The variation of the spectrum as the parameter ZD' varies is shown schematically in Figure 2.17A. Vos This effect was first reported for 13C-14N(Opella et al. 1979, Frey and Opella 1980, Groombridge et al. 1980) and was apparent as an asymmetric 1:2 doublet. Since that time these residual coupling effects have been observed for a wide range of spin pairs including 119Sn-35'37C1, 1H-14N, 13C-2H, 13C-59C0, 13C-75As, 295i-14N, 29Si-27A1, 3~p_55Mn ' 63,65Cu' 95,97M0" An example of its application is given by compounds of the family [Cu(PPh3)2(4-RC6HaNCN)]2 which have a spin fragment of the type P2CuN2 with the copper in tetrahedral coordination. This provides a cluster of three coupled spins of P2Cu in which the two phosphorus atoms have slightly different

PhysicalBackground

73 Magnetic field (T) total

A

11.7

--~&~-~-,~'~" ,~-#"".~-"~-r- ~sc u ......... ! ..... . ...........

~..!

2000

0

-2000

(Hz)

f

total

w

9.4

-%.

___s- / 1~_s ,~,-u~ , - ~ _ 6SCu 1 ""20~)0....... 6 ...... -2o0o

total ~

~

~

~

~ 2000

~

6

,

S

~

C

0

7.05

63Ci 1

u

-2000

2.11

~

,

total 63Cu

6SCu

"'20'r ...... 6...... -20'00 (nz)

Figure 2,17, A. The effect on a multiplet due to a spin-S/2 nucleus of increasing parameter

(XQD'/voS) with B. an example of 31p multiplets from coupling to copper. Note that there are two inequivalent phosphorus nuclei and that there are 2 copper isotopes (63'65Cu) both with spin-S/2 but with different moments and quadrupole effects that cause distortion. From Hanna et al. (1992) with permission of the American Chemical Society.

chemical shifts. The coupling pattern will be 2 sets of multiplets for the chemically distinct phosphorus (1,2) which are each split into a quartet by the J-coupling to the S = 3/2 copper. An additional complication is that copper has 2 isotopes both with a spin of 3/2 but with different nuclear properties. The scalar couplings will differ by a factor of 0.934 and the quadrupolar perturbation is proportional to the quadrupole moment so the asymmetry will differ by a factor 1.07. The coupling patterns are then weighted by the natural abundances of 69.09% for 63Cu and 30.91% for 65Cu. An example of this is shown in Figure 2.17B for [Cu(PPh3)z(4-PhNCN)]2. Data were recorded at 4 magnetic fields from 2.11 to 11.7 T. From observation of the multiplet coupling the asymmetry can be deduced, which depends on the both the parameter

ZQD"and the relative VoS

orientation of the quadrupolar and coupling tensors.

74


Thus, to obtain definitive information from these measurements requires other input such as a determination of the orientation of the quadrupole tensor relative to the internuclear vector, a knowledge of the internuclear vector or a value of XQ from other determinations. Given these constraints these coupling patterns have provided very useful information about the local structure around metal centres. To calculate these effects average Hamiltonian theory has been applied. There have also been full calculations of these effects, (Menger and Veeman 1982) which supersede average Hamiltonian theory when the Hamiltonian cannot be averaged by MAS. These authors applied an adiabatic variation of the eigenstates. The calculation by Menger and Veeman also showed that the isotropic J-coupling will be affected by the quadrupole interaction but this is a higher order effect and most experiments are carried out at sufficiently high magnetic field that J can be regarded as an invariant. These effects are not common in MAS spectra but are very interesting spin effects that can provide additional structural information. 2.3.2.3 Nonequivalent homonuclear spins. If two homonuclear spins that are not equivalent are coupled, then when other interactions are present the MAS lineshape will depend on the MAS rate (Wu and Wasylishen 1993). In the case of ~SN in cisazobenzene dioxide large changes in the lineshape were observed and simulations showed that the lineshape was dependent on the relative orientation of two chemical shift tensors and their orientation with respect to the internuclear vector.

2.3.3 Variable angle spinning Spinning a sample rapidly around any angle will introduce a scaling of the NMR inhomogeneous lineshape for the central transition. If the quadrupolar interaction is dominant so that effectively only second-order quadrupolar broadening is present in the central transition, there are angles that are more efficient at reducing second-order effects than 54.7 ~ This approach has been termed variable angle spinning (VAS) (Ganapathy et al. 1982). When dipolar and chemical shielding effects can be neglected, VAS can be effective, but as these interactions are not normally negligible, VAS is useful only in a limited range of applications. However, it should be noted that when the second-order perturbation is modulated there is no single angle that can remove both the P2(cos0) and P4(cos0) terms simultaneously. The effect on the lineshape when second-order effects dominate is shown in Figure 2.18. Spinning at 0 ~ produces the same lineshape as the static case. Spinning at the conventional magic angle produces the familiar lineshape, narrowed by comparison with the static case. The total width of the centreband of the central transition has its minimum in the range 60 ~ - 70 ~ depending on xI, the line being approximately twice as narrow as under MAS (Lefebvre et al. 1987, Amoureux et al. 1990). At 43.5 ~ the linewidth is independent of rl. For xl = 0,

75

Physical Background

q -->

0.0

0.3

0.6

N~ 4,

1.0

90

A____A__

___3_2___

- - - - - - - - ~ ----~------ -----.,~

~

70

__h__A___A_ A_ _A___A._ ~ - - - ~

__k._ __/.A_ l,

10

I

I

~

i--

_~

_

9

I

I

,

so

~

A____A

a _ ,

i_

80

40

L_

0

0 -1010 0 -10 10 0 -10 10 0 -10

(v- v,) (kHz) Figure 2.18. Variable angle spinning spectra for different angles as a function of different ~q, from Ganapathy, Schramm and Oldfield (1982) with permission of the copyright owner.

spinning at 36 ~ and 75 ~ gives a line consisting of an extremely intense narrow component with a broader component, so that although the total linewidth is not at its narrowest at this point, in a complex material the resolution of the intense component may be an advantage. This advantage becomes increasingly less evident for higher ~q. Calculations have also been performed for the sidebands of the central transition formed by spinning off the magic angle (Ganapathy et al. 1990).

2.3.4 Double angle spinning Although the quadrupolar information is itself useful, often it is desirable to produce better resolution by complete removal of second-order quadrupolar effects. As spinning about a single axis is unable to remove the P2(cos0) and P4(cos0) effects simultaneously, a more complex time dependence needs to be imposed upon the sample. The most direct solution to this problem is to make the spinning axis a continually varying function of time (Llor and Viflet 1988, Samoson et al. 1988, Zwanziger and Chmelka 1994). A scheme that achieves this uses a spinning rotor (termed the inner rotor) which moves bodily as a function of time by being enclosed in a spinning outer rotor so that the axis of the inner rotor describes a complicated but continuous trajectory as a function of time (Figure 2.19A). This is termed double angle rotation (DOR) and the second-order quadrupolar frequency of the central transition experiences a double modulation in the laboratory flame (similar to the single modulation of MAS). The effect of this modulation can be calculated by using Wigner rotation matrices, but now there are three


76

B ~ z ~ Bo

~ z,o~.~

~_~.~ ~~01 ~

/

Static

-"z,o,

_54,7o 02=30.6~

L

I ....

1 , , ~ , 1 , , , , I , ~ , , I

1 O0

....

0

I,,,,I

....

!

- 1 O0

Frequency(ppm)

Figure 2.19. A. The relative orientations and the motion of the rotors in a DOR experiment. B. The increase in resolution possible for 170 in wollastonite (CaSiO3) comparing static, MAS and DOR spectra from Wu, Sun and Pines (1990) with permission of the copyright owner. consecutive transformations, from the PAS of a given crystallite to the inner rotor axes which are defined by the Euler angles [3 and o~, then from the inner rotor to the outer rotor and, finally, from the outer rotor to the laboratory frame. The angle between the outer rotor axis and the applied magnetic field is 0~ and the angle between the two rotor axes is 02. If the angular velocity of the inner rotor is o~ and +1 describes the azimuthal orientation of the outer rotor in the laboratory, then in the laboratory frame the secondorder quadrupolar Hamiltonian becomes

~ (I,m)Fo(~)

I

+A 2 (I,m)P: (cosP 1)P2 (c~ 02 )F2 (fl, a , 77)

(2.155)

H ~ ) ( t ) - 2 V o ( 2 1 ( 2 1 _ 1)) 2 +A4 (l,m)P4 (c~ 01 )P4 (c~ 02 )/74(fl, a, 77) +terms o~ cos(t~ + ~1 )

where 01 and 02 c a n be chosen so that P2(cos0]) = 0 (01 -- 54.74 ~ and P4(COS02) -- 0 (02 = 30.56 ~ or 70.15~ DOR works well if the quadrupolar interaction is dominant and the sample is highly crystalline, in which case extremely impressive gains in resolution are achieved.

Physical Background

77

Simulation of the complete DOR spectrum (centreband plus the spinning sidebands) will yield the NMR interaction parameters (Sun et al. 1992, Cochon and Amoureux 1993, Amoureux and Cochon 1993). However, it is most usual to perform the experiment to give improved resolution and simply quote the measured peak position which appears at the sum of the isotropic chemical and second-order quadrupole shifts. DOR experiments at more than one applied magnetic field will allow these different contributions to be separated and hence provide an estimate of the quadrupole interaction via the combined quadrupole effect parameter PQ

(2.156)

/72

Po - Z O (1+ T )

This approach is similar to the use of the field variation of the centre of gravity of the MAS centreband, but has the advantage that the narrower, more symmetric line makes determination of the correct position of the centre of gravity more precise. For experiments carried out at two magnetic fields where the Larmor frequencies are vol and Vo2 for the measured DOR peak positions (in ppm) at the two magnetic fields of ~dorl,2 then

(~iso,cs =

(2.157)

V21(~dorl __ V022(~dor2 2 2 V01 -- V02

and I 3(I(I + 1 ) - 3 ) 1 772 4012(21 1)2 Z~(1 + - T ) --

2

2

Vo1Vo2((~d~ --(~dor2)2

(2.158)

V01 -- V02

This means that measurements at multiple magnetic fields will constrain the interactions. Detailed examples given in Chapter 6.

2.3.5 Multiple quantum transitions It has been pointed out that the central (1/2,-- 1/2) transition does not experience any first-order quadrupole interaction. The absence of first-order broadening effects is a general property of symmetric ( m , - m) transitions. There are cases where this can be a distinct advantage, the most direct instance being for integer spin nuclei (e.g. 2D and 14N, both I = 1) where there is no (1/2,- 1/2) transition. The main problem is to excite and detect such higher-order transitions, for which there are two separate approaches. The sample may either be irradiated and detected at the multiple quantum frequency (called overtone spectroscopy) or the MQ transition can be excited arid a 2D sequence used to detect the effect on the observable magnetisation.

78


Overtone spectroscopy developed for 14N irradiates the sample at approximately twice the Larmor frequency (Tycko and Opella 1987). If the quadrupole interaction is sufficiently large that second-order quadrupole effects are significant, the ( - 1 1) transition becomes weakly allowed. In powders the spectrum is still structured, allowing the interactions to be deduced, but is narrowed by a factor of 8Vo/XQ. Symmetric transitions remove first-order quadrupole effects and the coefficients of the anisotropic terms of the second-order broadening depend on I and m. Amoureux (1993) pointed out that for the 3Q transition of I = 3/2 spins the coefficient of the P2(cos0) term is zero. Hence spinning at either 30.56 ~ or 70.12 ~ will remove the P4(cos0) term. The other first-order interactions still have the P2(cos0) variation so that spinning away from 54.7 ~ will only partially scale these. Comparing the widths of these interactions at the roots of P4(cos0) for the MQ transition with the static width of the (1/2, - 1/2) transition shows a scaling of 1.84 at 30.56 ~ and 0.98 at 70.12~ hence the latter is usually preferred. The other combination where there may be an advantage is the m = 7/2 transition for I = 9/2. Although calculations have been carried out there are no experimental data using this approach in the literature to date.

2.3.6 Ultrasonically-induced narrowing Narrowing of NMR lines has been observed in colloidal suspensions of ultrafine particles, presumably by Brownian motion. A suggestion related to this principle is to take small solid particles into a liquid medium and induce sufficient reorientational motion of these particles to produce narrowing (Satoh and Kimura 1990). Ultrasound induces translational motion and collisions of the particles produce rotation. This sonically induced narrowing has been observed for 27A1 in aluminium sulphate, with the advantage that no spinning sidebands were observed (Homer et al. 1991). The experiment depends on the frequency and the power of the ultrasound and the liquid medium. Although the first reports of this approach appeared in 1989 there have been few developments in this field.

2.4. DIPOLAR DECOUPLING

2.4.1 Heteronuclear dipolar decoupling Heteronuclear dipolar coupling can be a major broadening mechanism, especially as in a number of materials protons are present which often have dipolar couplings to other nuclei in excess of 50 kHz. This means that MAS is usually not sufficient to remove the effect of the dipolar coupling in the spectra. ~3C NMR spectra are, for instance, often obscured by dipolar coupling to the protons. In order to remove the carbon-proton coupling the protons are irradiated with an rf field while, at the same time, the carbon spectrum is measured. Consider the CH system where the heteronuclear dipolar

Physical Background

79

interaction is proportional to the factor Sz(carbon)Iz(proton). Initially, the proton magnetisation will lie parallel to the z-axis (Iz). The rf field rotates the proton magnetisation to the xy-plane and then to t h e - z-axis so that the proton magnetisation is then represented b y - Iz. Hence the spin part of the Hamiltonian is manipulated. Representing this schematically, the proton spin will sequentially vary as Iz --a -Iz --~ Iz ~ -Iz ~ Iz ~ -Iz. If this rotation is rapid compared to the timescale of the experiment, the time-average of Iz will tend to zero. Hence the heteronuclear dipolar Hamiltonian oscillates between + IzSz so that the average of the spin operator will be zero. This is an example of averaging interactions by manipulating the spin part of the Hamiltonian. In order for this to work, the rf field strength has to be greater than the strength of the dipolar interaction. This technique is often used in combination with MAS, resulting in almost liquid-like resolution for solids. In recent years there has been a significant amount of theoretical and experimental work on schemes to improve on standard CW decoupling, and some of these are discussed in Chapter 3.

2.4.2 Homonuclear dipolar decoupling Solid-state spectra of many materials containing protons (and fluorine) are usually dominated by the homonuclear proton-proton coupling (or fluorine-fluorine, and in cases where the phosphorus density is high, phosphorus-phosphorus). This is especially true when the nuclei are physically close together so that r is small, making the dipolar coupling strong. The proton spectra are usually featureless due to the fact that protons have a high natural abundance and a high gyromagnetic ratio causing even distant protons (--~10.~) to contribute to the broadening of the spectrum. To remove homonuclear dipolar coupling the lifetime of the magnetisation must be prolonged in the transverse plane (Powles and Mansfield 1962). Several multiple pulse schemes (Mansfield 1971) have been developed to accomplish this, including WHH4 (Waugh et al. 1968), MREV-8 (Mansfield et al. 1973, Rhim et al. 1974), BR24 (Burum and Rhim 1979) and BLEW-12 (Caravatti et al. 1982, 1983). These sequences involve long trains of pulses that cause coherent motion of the magnetisation. A distinction must be drawn as to whether the sequence is being used simply for decoupling and the heteronucleus is being observed, or whether the same nucleus is being observed. In cases where observations of the same nucleus are being made, a so called "windowed" sequence is necessary so that the magnetisation can be recorded. These sequences are quite unlike all those encountered to date as the multiple pulse sequences sample the magnetisation stroboscopically. This means that there are certain times within the pulse sequence, which often involve repeated short cycles where the magnetisation is in the correct state. At each point a single data point is recorded. Just as using the rotating frame oftensimplifies the understanding of many NMR experiments, to understand these multiple pulse sequences it

80


is easier to observe the magnetisation in a flame in which it remains stationary so that the flame's motion compensates for the effect of the rf pulses. In the literature this is termed the toggling flame. Note that to properly understand such pulse sequences, second-averaging and Magnus expansion techniques should be applied (Gerstein and Dybowski 1985). Consider the simplest of these sequences, WHH4, where the cycle has four pulses and five time-intervals as shown in Figure 2.20. If the dipolar coupling is dominant then, in the toggling flame, the magnetisation is rotated successively to different directions, the average dipolar interaction being "" tl

)-

(3Ilxl2x

--/-1" ~/2 )'k-

(3Ilyl2y - 11" [2 )+ (311zl2z - 1-1"1-2)

(2.159)

The factor of 1/3 arises from the fact that the spin spends an equal amount of time along each of the Cartesian directions. It is clear from this expression that the sequence successfully removes the homonuclear dipolar coupling. However, it is important to realise that such manipulation by pulses has the effect of scaling other operators and not completely removing them. The most important is Iz, its average value for the above sequence being

(1 eft) -- 89 x + Iy + lz) ]

(2.160)

This means that there is an effective field along the unit vector 1L/3(1,1,1) direction in the Cartesian basis, producing an effective scaling of the frequency of 1/~/3. This basic sequence was developed into longer sequences such as MREV-8 and BR-24 (Table 2.7).

90~

Magnetisation along

Z

90~

Y

90%

x

90~

Y

z

Figure 2.20. The periods of a WHH4 multipulse experiment for narrowing homonuclear dipolar coupling.

Physical Background

81

Table 2.7. Comparison of some of the more commonly used homonuclear decoupling sequences with a normal one pulse experiment. Sequence

Pulse cycles (phase)

Cycle time

Scale factor for Izeff

One pulse

x

"r

1

WHH-4

x- yy- x

6'r

MREV-8

x - yy - x - x - yyx

12'r

BR-24

xy-y-x -xy-yx yx-x-y -yx-xy xy-x-y -yx-xy

36'r

45

2

As the cycles become longer they have better self-compensation and second-averaging of the interactions which improves the resolution of the spectrum, and they are more robust to frequency offset effects. The downside is that the cycle time increases markedly. The causality argument applied to MAS can also be invoked here in that the cycle time which is doing the averaging must be fast compared with the timescale of the modulation of the magnetisation caused by the interaction to be averaged. Hence this condition is much more easily met when the sequence has a shorter cycle time. When the number of pulses increases, the only way to decrease the cycle time is to decrease the n- spacing, which is governed by the rate of recovery of the receiver system after a pulse and the speed of the electronics. The 90 ~ pulse time can also be reduced which is a stringent test of the power handling capabilities of the probe. When it is simply decoupling that is required (as opposed to observation as well) an off-resonance proton field can be applied such that the proton magnetisation is spin-locked at the magic angle. This is termed Lee-Goldburg decoupling (Lee and Goldburg 1965). A variant of this sequence currently favoured is Frequency Switched Lee-Goldburg decoupling (FSLG) (Bielecki et al. 1990). The ability of NMR hardware to abruptly change the frequency while keeping the phase has meant that this approach is now possible. This approach does not require the use of hard, resonant 90 ~ pulses and is therefore relatively more efficient at decoupling I-spins over a greater frequency. Such considerations have become more important even for 1H as very high magnetic fields are applied. The basic principle of the Lee-Goldburg decoupling concept is that it causes coherent precession of the magnetisation about an effective field inclined to Bo. With B1 (say) along the x-axis, this effective field will be inclined at 0 to Bo, defined by

82


tanO =

BI Bo

~o,

(2.161)

If the effective field is directed along the magic angle (i.e. directed along the direction (111)), 0 = 54.7 ~ This will remove the dipolar coupling since the effective spin direction is inclined at the magic angle to Bo. The offset frequency ALG = tOo - tO1.Rather than continuously irradiate at this single frequency, it has been found to be much more efficient to flip the frequency by _+ ALG with precession for a time "rm corresponding to 2-rr. Between each period the phase is also successfully changed by ~r. The FSLG decoupling has been shown to be one of the most effective decoupling schemes. It will also scale other spin operators, with a factor of (~/3)-1 for the chemical shift. FSLG has been used for the observation of abundant spins as an alternative to the multiple pulse sequences based on hard 90 ~ pulses (Levitt et al. 1993). These sequences have the benefit of short cycle times and high scaling factors but are sensitive to rf homogeneity. These pulse sequences completely remove only the homonuclear dipolar coupling, which means that interactions such as shielding are scaled but still present. The anisotropy can, of course, provide useful information but to achieve the best resolution this should also be removed. This is readily achieved by MAS and the combination of multiple pulse and MAS averaging is termed CRAMPS (Combined Rotation And Multiple Pulse Sequence) (Ryan et al. 1980, Jackson and Harris 1988, Maciel et al. 1990). When such multiple pulse sequences and MAS are combined care must be taken to ensure that the independent averaging processes do not interfere with each another. Hence it has been assumed that there should be at least five multiple pulse cycles per rotor period for this to be approximately true, so that modest spinning speeds (i.e. --~2 kHz) were typically used. In strongly dipolar coupled proton systems CRAMPS gives much better resolution than MAS (Figure 2.21). For Lee-Goldburg decoupling, similar time constraints are necessary so that 2"rLC should be short compared to the period of sample rotation. More recently, consideration has been given to combining multiple pulse sequences with fast MAS. The timing of the fast MAS with a sequence means that symmetry of the sequences allows their combination. The sequences proposed have largely been windowless so that one data point is accumulated at the end of the complete sequence and the sequence is then incremented (Hafner and Spiess 1998, Filip et al. 1993). If indirect detection is being used, as in many 2D sequences, this is not a drawback. The technique has been developed still further to use semiwindowless decoupling so that at points in the sequence quasi-static conditions apply, even under fast MAS. These approaches may offer new impetus to high resolution proton spectroscopy even in systems with a high proton density.

83

Physical Background

\

15

10

5

0

-5

\

PPM

/ \

/ \

r \

/ \

/ \

/ ~

/ /

-~~

~

A

10.7 kHz

j

~

9.2 kI-Iz

5.0 kHz

40

20

0

-20

-40

kHz

Figure 2.21. Example of ~H spectrum comparing static, MAS and CRAMPS from citric acid from Dec et al. (1989) with permission of the copyright owner.

2.5. SPIN-LOCKING A useful concept is the creation of spin-locked transverse magnetisation after a pulse, with the phase of the rf then flipped by 90 ~ This means that in the rotating frame after the phase shift of the rf field, the transverse magnetisation and the B1 field are aligned. The transverse magnetisation will not then dephase under interactions such as chemical shift and dipolar coupling, as would normally be the case. The transverse magnetisation will decay by genuine relaxation processes in the rotating frame, characterised by the relaxation time TI~. In this situation the magnetisation is said to be spin-locked, since the transverse magnetisation lasts much longer than under normal T2 dephasing. For quadrupole nuclei the situation is more complex and it is well known that

84


rotationally induced spin-locked magnetisation can appear when an if-field is applied to a quadrupolar nucleus in conjunction with MAS. The spin-locking behaviour of quadrupolar nuclei on resonance can be described by considering the first-order quadrupolar interaction only, so the Hamiltonian in the rotating frame will be given by the first-order quadrupolar interaction (Eq. 2.111) plus the rf contribution VlIx. Diagonalising this Hamiltonian gives the eigenfunctions (Vega 1981, 1992), which, in the case of the quadrupolar interaction being much larger than the if-field for an I = 3/2 spin, are the single (c+) and triple (t_) coherences, defined as

ic+) = [ ~ ) + - I - ~ ) and It+)= 13/2)+1-3/2) -

(2.162)

-

The energy level diagram for these eigenstates is shown in Figure 2.22 which demonstrates that the eigenvalues change smoothly as the magnitude of the quadrupole interaction changes relative to the rf field. When IVQI > > Vl the states are spin-locked. However, when VQ is comparable to v~, c+ and t+ are no longer the eigenstates but linear combinations are (Vega 1992). In a spinning sample the orientation of the quadrupole nucleus changes continuously. Hence, the splitting between the energy levels which depends on the crystallite orientation through the anisotropic term will vary, and level crossings between the different spin levels will occur (Vega 1992a, Grey et al. 1993, Grey and Vega 1995). During one rotor period, spin density is transferred from the outermost spin levels to a spin-locked state and back again, continuing as the rotation continues. Depending on the orientation of the quadrupole tensor with respect to the spinning axis, either two or four level crossings occur per rotor period. There are three spin-lock regimes depending on the so-called adiabaticity parameter = v~2/VQVr (Vega 1992). For e~ > > 1 the level crossings are adiabatic and spin density will be transferred from one level to another. For e~ < < 1 the change is sudden and no transfer of population occurs, and in this case the normal spin-lock behaviour is expected. In the intermediate regime the transfer of spin density is not efficient but still occurs, giving rise to very short effective spin-lock times. The situation is more complex when there are additional levels for higher spins, but the same reasoning can be used. For 27A1 when • -- 2 MHz, spinning at 5 kHz and an rf field-strength of 60 kHz gives oL = 2.4, which means it is in the adiabatic regime. So for effective spin-locking the system should be well into the adiabatic or sudden regime. The rotationallyinduced level crossings adversely affect the performance of various NMR experiments such as cross-polarisation (Sec. 2.6) and nutation NMR (Sec. 3.6.1). It also lies behind the TRAPDOR effect (Sec. 3.8.3). A more detailed and thorough discussion of MAS and spin-locking of quadrupole nuclei has been provided by Vega (1992a).

Physical Background

85

Spin I=3/2 _

~Eigenvalue (v0 c+

4

t+

Spin I=5/2

Figure 2.22. The variation in the energy level eigenvalues of a quadrupole nucleus as a function of the ratio VQ/V~which is changed by sample rotation since VQ is a function of orientation. The c+ and t+ states are as given in Eq. 2.162 and the diagrams follow ideas from Vega (1992a), and Grey and Vega (1995).

2.6. C R O S S - P O L A R I S A T I O N

Even with the line-narrowing techniques described earlier, NMR experiments on solids with dilute spin-1/2 nuclei are still relatively unattractive on two principal counts. One is the lack of sensitivity due to their low net polarisation and the other is the relatively long spin-lattice relaxation time that is often encountered. In solids where both abundant (I) and dilute (S) nuclei coexist, polarisation transfer techniques can usually be used to overcome both these problems. There are many schemes to effect such a transfer but the most common technique is to create and then spin-lock transverse I-magnetisation. This experiment is best understood using ideas from spin thermodynamics. The magnetisation is given by Curie's Law (Eq. 2.21) and the temperature in

86

Multinuclear Solid-State NMR of Inorganic Materials IH Channel 90, ~ i |

(Spin-lock)y Decoupling

,, i

X Channel F1D

CP

H-spins

X-spins

Lab

Rotating Frame

Bo ~ ~

Bm Blx Hartmann-Hahn matched

Lab Be

C

,

0

~

5

....

, ....

, ....

, ....

10 15 20 25 30 35

msec

Figure 2.23. A. Schematic representation of the CP sequence for ~H --~ X. B. the changes of the energy levels from the laboratory frame and in the rotating frame showing the Hartmann-Hahn match (separations are not to scale). C. The build-up of magnetisation for the 13C signals in the 2 carbons of glycine as a function of contact times with an inset of short contact times showing the effect of different Tis for each carbon.

this equation is the thermodynamic lattice temperature. The pulse sequence has three main steps (see Figure 2.23A), which in terms of the spin thermodynamics are" 1. Cooling down the abundant spin system, 2. Contact between the I and S spins to allow polarisation transfer, 3. Observation of the dilute spins.

Physical Background

87

The transverse I-magnetisation is created and then spin-locked. In the spin-locking frame (which is equivalent to examining the magnetisation in the rotating frame as in Eqs. 2.36 and 2.37) the effective field is only B1. In the spin-locking frame, immediately after the 90 ~ pulse, the I-magnetisation is still MI with the same degree of order, but the energy levels are now much closer (i.e. ~lhB1 as opposed to ~/h Bo). With the new field B1 the system can now be assigned with an effective spin temperature Tpl defined by M1=

NlyZh2I(I + 1)Bo

-

N1YZh2I(I + 1)BlI

(2.163)

As B li < < Bo and the degree of order amongst the spins remains the same (i.e. M~ is constant) it follows that Bli

Tpl - -~o Tc

(2.164)

Hence the I-spins are effectively very cold. The S-spins, in contrast, start off with no transverse magnetisation so, in terms of thermodynamics, are very hot. There is a thermodynamic driving force for the transfer of magnetisation. However, the spin systems have to be allowed to communicate efficiently and this is achieved by applying a second B1 field, this time to the S-spins. If the two spins to be brought in contact are spin-1/2 then the condition the two fields must meet are the Hartmann-Hahn condition (Hartmann and Hahn 1962) given by (Figure 2.23B). ~/IBII -

(2.165)

YsBls

When this term is present the dipole flip-flop terms (e.g. I+S_) are energy conserving so that order can be transferred between the I and the S spins. Thermodynamics means that the transfer of order occurs, tending to give the two systems a common spin temperature. Magnetic energy is conserved, given by

E - N?2h2I(I + 1)B2 3kT

(2.166)

Conservation of total energy amongst the spin system from these flip-flop terms can be invoked on a timescale that is less than the spin-lattice relaxation times so that

N, yZh:I(I + 1)B2I

NiY~h2I(I + 1)B2I + Nsy2h2S(S + 1)B2s

3k 1

3k 2

(2.167)

Using the Hartmann-Hahn condition and rearranging leads to a new temperature of


88

(2.168)

The implication of this equation is that the spin temperature for the I-spin system changes very little, so that I-magnetisation essentially remains unchanged as a result of the contact, and the S-magnetisation created is

Ms=

NsT2h2S(S + l) T1 1 NsT~h2S(S + I)B o = 3kTp2 ~"s 1 + A 3kTL

(2.169)

where A

n

NsS(S+I) NII(I+I)

(2.170) ~I I

Hence in a single contact experiment the gain in signal intensity is approximately

7s compared to a single 90 ~ pulse on the S-spins (which for I - 1H and S - 29Si produces a factor of--~ 5). This produces transverse S-magnetisation. Although this describes the driving mechanism for the transfer of the magnetisation between I and S, it gives no clue as to the dynamics of the process. Mehring has carried out an extensive analysis of the CP process (Mehring 1983), leading to

M ce(t) - )'i Mo exp -~/~ 3 - e x p Ts (P+-P-)

(2.171)

Tis )]

where

,+ [l+I/1 ]

(2.172)

with

x--

1/ 1+a+~7-+~-3--

2

T~p

T~p

andy-

1+

T~p~,

+

s

T~So) Tip

(2.173)

where ot is a factor depending on the number of nuclei of each type in contact with one another. It has been shown that for many CP experiments where there are many more I spins than S and when Tis < < TS~p, the following dynamics approximately hold

Physical Background

exp -

89

-exp -

M ce (t) - MCoe

(2.174) 1

Tip

In this equation T~s determines the rate of polarisation transfer and hence the build up of signal, while Tlo is the relaxation time of the spin-locked I-magnetisation in the rotating frame determining the time scale of the decay of the reservoir of magnetisation. The rate of transfer of the magnetisation is found to depend on the second moments (M2) of the I-S and I-I spin systems as

1

M ,s

Tzs = Czs ~/MzII

(2.175)

C~s is a constant. In cases where the CP occurs between relatively isolated spins (e.g. pairs) the spin thermodynamic description cannot be applied. Rather than a smooth, continuous build-up of magnetisation, the CP curve shows structure, termed dipolar oscillations (Muller et al. 1974). The magnetisation oscillates between the proton and the X-nucleus. In the case where the system is exactly Hartmann-Hahn matched, the magnetisation of the S-spin is given by (Levitt et al. 1986)

M s ( t ) - Mso ~I [I_cos(_P2(cosO)t)],

(2.176)

where Mso is the equilibrium Zeeman magnetisation of the S-spin system. An example is shown in Figure 6.7 for 1H-170 from Mg(OH)x(OCH3)z-x allowing the distance between these nuclei to be determined. As the contact time increases, destructive interference occurs and the homonuclear proton dipolar coupling begins to dominate the CP curve. There is also an interesting contrast between Mg(OH)x(OCH3)z-x and Mg(OH)2 where the more dilute proton bath in the former leads to weaker proton-proton coupling and hence more pronounced oscillations. The above analysis is usually applied to spin-1/2 systems but there is no reason why magnetisation transfer cannot be applied to quadrupole systems. If XQ is small the theory is identical to that given above, but if • becomes significant the HartmannHahn condition is modified to

aiTiBli = aS~tsB1s

(2.177)

90

Multinuclear Solid-State N M R o f Inorganic Materials

where oL- 1 for spin-l/2 systems and a - ~ / [ S ( S + 1 ) - m ( m - 1 ) ] for the ( m , m - 1 ) transition of a spin-S quadrupole nucleus. The enhancement of the signal depends on the heat capacities of the two spin systems, which depend in turn on the gyromagnetic ratios of the two spins and the number of spins in contact. Thus, the CP magnetisation (Mcr,) is related to the normal Zeeman magnetisation (Woessner 1987, Walter et al. 1988) Mcp = M o

7t N s [S(S + 1)- m ( m - 1)]1 NI

I(I + 1)

J

(2.178)

All of the above remains only strictly true for a static sample, but CP is usually combined with MAS. If hvr < < HII, the effect of sample spinning on the cross-polarisation process may be ignored. Once MAS begins efficiently to average the I-I dipolar coupling, CP may be weakened as magnetisation can no longer diffuse freely through the I-spin reservoir. This often leads to modulation in the Hartmann-Hahn-match condition across the spectrum causing distortions in the spectrum. This effect has become more noticeable as faster-spinning probes have been used to overcome problems caused by spinning sidebands arising from CSA as ever higher applied magnetic fields become available. Quantification is often required, and, in CP, signal generation depends on the heteronuclear dipolar coupling which determines the CP rate via Tis. At slow MAS rates a broad matching condition is usually obtained around the central Hartmann-Hahn condition and any mismatch is relatively unimportant since the strong homonuclear dipolar coupling compensates. As the spinning speed increases, the matching profile breaks up into a series of narrowing matching bands separated by yr. The CP rate at the central matching condition is slower than at some of the sidebands (Metz et al. 1994, 1996). The ability to maintain efficient matching under fast MAS has attracted much attention. Sequences have been developed based on either phase and/or amplitude modulation. Ramping the amplitude across one of the matching sidebands with the ramp centred on one of the matching sidebands has been found to improve greatly the CP efficiency and broaden the matching condition. This approach is straightforward to implement on modern spectrometers. An example of the improvements brought by this approach is that of N-t-Boc-alanine (Metz et al. 1996), where there are several different carbons with different dipolar couplings, together with the influence of different match conditions (Figure 2.24).

2.7. TWO-DIMENSIONAL METHODS

Most of the methods described to this point have simply excited single quantum magnetisation and manipulated the Hamiltonian by making it time-dependent. They are all

Physical Background

91

90~ CP

L

Decoupling

,,...f

SACP Centreband

B

SACP Sideband

RAMP-CP Sideband

i~l

e ~

0

1---

-

9

0

-

-

9

-

9

9

9

9

- ,

. . . .

,

~

10

1

,

0

.

.

.

.

.

.

.

,

9

-

9

9

9

9

9

9

9

,

~

1

10

0

10

Contact Time (ms) Figure 2.24. A. One of the pulse sequences for modulating the Hartmann-Hahn match condition by ramping the X-nucleus transmitter amplitude. B. Comparison of 1H --+ 13C CP for conventional CP (SACP) matched on the centreband and a sideband with RAMP-CP on a sideband of N-t-Boc-alanine showing the very much improved quantitative data for the six equally populated carbon sites. Taken from Metz, Ziliox and Smith (1996) with permission of the copyright owners. essentially one-dimensional techniques, the only time domain associated with the experiment being that of the normal FID. A whole range of further experiments becomes available when a second time domain is introduced. There are many experiments that use this concept of two-dimensional (2D) NMR; some of those more commonly used are described in Section 3.2. The concept of the experiment is that the Hamiltonian is manipulated in some way so that the system is prepared in a particular state. This state is then allowed to evolve for a time (tl) and the system is interrogated by applying a pulse that creates observable magnetisation. The FID signal accumulated in this time is designated t2 (Figure 2.25, Ernst et al. 1988). On the basis of one experiment nothing can really be said about the effect of the evolution in tl. However, a whole series of FIDs are accumulated in which the time tl is varied. Fourier transforming this set of FIDs will produce spectra which are modulated in some way by the evolution of the system during the preceding tl-period, and a second FT will produce a second frequency dimension containing information about this modulation, and hence about the interaction causing it. This 2D concept is important in some of the approaches to be described next.

92

Multinuclear Solid-State NMR of Inorganic Materials tvl

t2 v

Evolution Interrogationi

Figure 2.25. Time intervals in a basic two-dimensional pulse sequence.

2.7.1 Dynamic angle spinning Dynamic Angle Spinning (DAS) is a 2D experiment in which the sample is spun about different axes sequentially during different periods (Eastman et al. 1992, Zwanziger and Chmelka 1994). During the first evolution time tl the sample is spun at an angle of 01 degrees. The magnetisation is then stored along the z-axis and the angle of the spinning axis is changed to 02. After the rotor has stabilised (--~ tens of milliseconds) the magnetisation is then brought into the xy-plane again and a signal is acquired. The secondorder quadrupole frequency of an individual crystallite depends on the angle of the spinning axis, so during tl the quadrupole interaction will produce a frequency in the crystallite of v~, and 1)2 during t2. If v2 is of opposite sign to vl the signal from the crystallite will be at its starting position again at some time later during t2. The angles can be chosen in such a way that the signals from each individual crystallite will be at the starting position at exactly the same time so that an echo forms with the secondorder quadrupolar broadening removed at the peak of the echo. The two angles should fulfil the following equations simultaneously P2(cos 01 ) = -kP 2(cos 02 ) and P4 (cos 01 ) --

-ke 4(cos 0 2 )

(2.179)

where k is the scaling factor. There is a continuous set of solutions for 01 and 02, the so-called DAS complementary angles, and each set has a different scaling factor. For these solutions, the second-order quadrupole powder pattern at 01 is exactly the scaled mirror image of the pattern at 02, and an echo will form at t2 = ktl. For the combination 01 = 30.56 ~ 02 = 70.12 ~ the P4(cos0) terms are zero and the scaling factor k = 1.87. For the combination 01 = 37.38 ~ 02 = 79.19 ~ the scaling factor k = 1, the spectra are exact mirror images and an echo will form at t l = t2. Finally, the combination 01 = 0 ~ 02 = 63.43 ~ (k = 5) is special because it allows efficient CP for signal enhancement prior to the DAS sequence. There are several ways in which the DAS spectra can be acquired; one can acquire the entire echo, which means that the resulting 2D spectrum will be sheared. Some additional processing is then required to obtain an isotropic spectrum in F1. The advantage of acquiring the entire echo is that the second-order information is retained. Hence the 2D data set has improved the resolution whilst providing secondorder quadrupolar information as well. Alternatively, the acquisition could also start at

Physical Background

93

the position of the echo so that an isotropic spectrum in F1 is obtained directly. This has the disadvantage that the S/N ratio is less than if the complete echo is acquired. A third possibility is to carry out the experiment as a pseudo-1D experiment where only the top of the echo is acquired as a function of t l. In this case the isotropic spectrum is acquired directly but there is no shortening of the duration of the experiment. It is, however, the best way to combine DAS with 2D heteronuclear correlation. The DAS sidebands can be analysed to deduce interactions (Sun et al. 1992).

2.7.2 2D MQMAS From the DAS experiment it is known that in different time intervals the sample is spun at different angles so there is a point (where the echo forms) where anisotropic effects are refocused. Also it is known that there are no first-order quadrupole effects in symmetric transitions. Using these ideas, in 1995 a new experiment emerged from the group of Frydman that has had an enormous impact on solid state NMR spectroscopy of noninteger quadrupolar nuclei (Frydman and Harwood 1995, Medek et al. 1995). The 2D Multiple Quantum Magic Angle Spinning (2D MQMAS) experiment greatly enhances resolution of the spectra of half-integer spin quadrupolar nuclei. Under MAS for a symmetric transition, as the P2(cos0) term is removed the frequency evolves under the quadrupole interaction as

I

(2.180)

2I(2I-1)

+ v o [C4(i,m)P4(54.7O)F4(fl,)/,r D

where F4(~,'y,'q) is defined in Eq. 2.139. The experiment correlates ( m , - m) (the multiple quantum ( p - 2m) transition) to the central (~/2,-1/2) transition. The resolution enhancement stems from the fact that the quadrupole frequencies for both transitions are correlated. It can be seen from the coefficients in Table 2.8 that both the isotropic and anisotropic parts show different time evolution for the different coherences. The experiment is shown schematically in Figure 2.4, together with the coherence level diagram. The 3Q experiment has so far been the most common since, of the higher coherence transitions, 3Q is the easiest to generate. Initially the 3Q transition is generated and the Hamiltonian evolves for a period h. Another pulse converts the 3Q signal to an observable ( - 1) single quantum coherence (in Figure 2.4 a three pulse sequence where the coherence changes is 0 ~ +_ 3 --~ 0 ~ - 1) so that at t2 = ktl the anisotropic F2 term in Eq. 2.180 is refocused when

k - C2(I'3/2) C2(I,1/2)

(2.181)

94

Multinuclear Solid-State NMR of Inorganic Materials Table 2.8. Coefficients for the second-order quadrupolar interaction and the ratio for the echo periods. I

3/2

m

p

Co

C2

C4

k

1/2

1 3 1 3 5 1 3 5 7 1 3 5 7 9

3 - 9 8 6 - 50 15 27 - 15 - 147 24 54 30 - 84 - 324

- 12 0 - 32 - 60 20 - 60 - 144 - 120 168 -- 96 - 252 - 300 - 168 216

- 27 21 - 72 - 114 150 - 135 - 303 - 165 483 -- 216 - 546 - 570 - 84 116

NA - 7/9 NA 19/12 - 25/12 NA 101/45 11/9 - 161/45 NA 91/36 95/36 7/18 - 31/6

3/2 5/2

7/2

9/2

1/2 3/2 5/2 l/2 3/2 5/2 7/2

1/2 3/2 5/2 7/2 9/2

NA - not applicable.

At this point an e c h o forms but the coefficients of the isotropic terms are different and so are not r e f o c u s e d at this point. This has i m p o r t a n t c o n s e q u e n c e s for the e x p e r i m e n t . T h e isotropic part VoQ of the q u a d r u p o l e f r e q u e n c y is

vQ - - _

(2.182)

,~'~(3 if-/7 2 )

4012 (21 - 1)2V0 and the anisotropic part vaQ([3,~/) is g i v e n by

7

4

i - ~ ( 3 - ~ c o s 2 ~ ' ) 2 sin fl 9Z~ 44812 (21 - 1)2Vo

/7 2

+ 2 ( r / c o s 2 y - 2 - --~-) sin 2 fl

(2.183)

2//2

4 +-45 5

As the 2D e x p e r i m e n t is carried out at different e v o l u t i o n times the echo m a x i m u m will always be free from anisotropy but will s h o w m o d u l a t i o n due to isotropic effects. A double F T will then contain both isotropic and anisotropic information. After a 2D F T the signals s h o w up as ridges lying along the Q u a d r u p o l e A n i s o t r o p y (QA) axis w h i c h has a gradient d e p e n d i n g on k (Figure 2.26). T h e isotropic s p e c t r u m can be obtained by projection of the entire 2D s p e c t r u m on a line t h r o u g h the origin p e r p e n d i c u l a r to

95

Physical Background Site

1

2

3

vl ~ p

~ F1

Co(P) Co(l) -'"

ganiso--"

C4(P )

C2(1---~ F2

Figure 2.26. A schematic 2D MQ MAS data set of a sample containing 3 sites- 1 (large XQ, no distribution), 2 (moderate XQbut with some distribution) and 3 (very small XQ and no distribution). 3 lies on line Vl = PP2, 2 and 3 displaced from this line along a line with gradient giso,Qand 3 showing a lineshape along a line with gradient ganiso.

the QA-axis (vide infra). Cross-sections along the ridge will retain the second-order quadrupolar lineshape. The sign of k is varied for different coherence orders and has important consequences for the way the experiment will be carried out. If the value is negative, anisotropic broadening is refocused at a positive value of t2 and the correlation must be between - p and - 1 (the observable coherence). However, for positive k, to get refocusing at positive t2 the correlation has to be between the + p and - 1 coherences. A scheme that gives refocusing at positive t2 values is termed an echo pathway whereas refocusing at negative values is an antiecho pathway. Under MAS the terms involving P2(cos0) (for both the quadrupole and chemical shift) and P4(cos0) are refocused along the ridge but there are isotropic terms. The isotropic parts of both the chemical shift Aviso and the isotropic second-order quadrupole effects v Q, are scaled. These scale factors (SF) are for the chemical shift

SFcs(i,m)_

p+ k

l+lkl

(2.184)

96


and for the quadrupolar term

C~

-k

SFQ(I,m) - CO(I,- 1/2)

(2.185)

l+lkl The MQ experiment involves excitation of higher order coherences and selection of particular pathways by phase cycling. These effects lead to considerable reduction in intensity compared to normal single quantum experiments. Hence, much attention must be paid to efficient generation of the MQ coherences and the efficient reconversion of the MQ signal to observable single quantum magnetisation. If the quadrupole interaction is significant, single pulse excitation is a comparatively efficient way of generating MQ coherences. For samples where there is no quadrupole interaction there can be no MQ signal generated, and these sites are missing from the MQ spectra. There have been a number of investigations to optimise these experiments. Generation of MQ transitions depends upon both the ratio r -- VQ/Vl and the flip angle of the pulse. The optimum pulse tip angles are almost independent of the ratio r but depend on the coherence order to be maximised. However, the efficiency of the generation of the MQ transition is a peaked function (Figure. 2.27), having an optimum value of r. The key is that with typical • values there is usually a gain from using higher rf fields. This is increasingly true as the order of the coherence to be generated increases. This has meant that most work has concentrated on the lower coherence orders. Most of the calculations have assumed a static sample but MAS imposes a time dependence on the Hamiltonian. As spinning speeds increase the generation of the MQ coherence decreases, but even at the highest speed available (--~45 kHz) the intensity is reduced by a factor of only --~3 and the optimum pulse angle is not much changed. Numerical calculations have been carried out on the effect of the angle on the conversion efficiency. The work of Amoureux and Fernandez (1998) is summarised in Table 2.9. One of the key observations is that the efficiency of the flip angles shows no real dependence on r. However, the conversion is quite an inefficient process so that the overall sensitivity of the experiment is poor relative to one-pulse experiments and much effort has gone into improving the excitation and reconversion efficiency. As I increases, the optimum flip angle for maximum excitation decreases. Since both excitation and conversion are dependent on r, MQ spectra are normally not directly quantitative, so great care has to be taken to interpret the spectra. The situation will generally improve as the applied rf field increases. Since the 2D data set contains the anisotropic information, lineshapes can be analysed and the quadrupolar parameters deduced. The lineshape will show distortion as there will be crystallites whose orientation is such that their quadrupolar frequency is zero and will not contribute. Apart from the excitation and the conversion, the actual coherence pathway

97

Physical Background 60./'~3a i\ '

-

40

20"

3Q

/=\

$

/ /i i '............ . ' ~ '"~'" '~\" ''\

/;. ,o7~ \:\

0"~

'

I

'

I

,

! ~

,I

'

I

30-

20

""

5Q

i~,- ,t2

.. ,,..

~aS!s

/

20

il\i

"~'~

10-

fl

._n

-4

-2

0

2

4

6

VQffS(S+I)-3/~4/V1 =2" F i g u r e 2.27. The overall efficiency of a 2-pulse MQ pulse sequence for different excitation (3Q, 5Q, 7Q, 9Q) for different spins with the quadrupole frequency scaled by the spin factor to allow direct comparison of the different spins with the optimum pulse angle as given in Table 2.9, after Amoureux and Fernandez (1998).

Table 2.9. Numerically calculated optimum flip angles and the relative efficiency of the sequences after Amoureux and Fernandez (1998) for VQ/Vl = 1.25. I

3/2 5/2 7/2 9/2

Pulse

1 2 1 2 1 2 1 2

Ipl = 3

Ipl = 5

Ipl = 7

Ipl = 9

0~

Eft

0~

Eft

0~

Eft

0~

Eft

240 55 180 60 120 45 90 35

1.50 0.30 1.50 0.30 1.50 0.30 1.50 0.26

170 72 120 65 100 50

0.32 0.05 0.49 0.05 0.64 0.07

220 80 150 72

0.21 0.03 0.29 0.03

250 86

0.14 0.02

The relative efficiency (Eft) is one for the central transition observedafter a perfectly selective90~pulse.

98

Multinuclear Solid-State N M R of Inorganic Materials

will affect the lineshape, so, for example, the coherence pathway 0 --~ p --~ - 1 will produce an echo but there will be absorptive and dispersive components. Ideally the sequence 0 --~ _+ p --> - 1, (especially if both pathways are equal) will markedly improve the lineshape. For I = 3/2, echo and anti-echo pathways can be equalised but this is not possible for higher spins. Triple quantum experiments are the most popular because they are most readily generated, but with higher spins there is the possibility of generating higher order coherences. The sensitivity of such experiments decreases rapidly for the higher order coherences. There has been much work to improve the efficiency of such experiments. Instead of single pulses both composite pulses (Marinelli et al. 1998) and shaped pulses (Ding and McDowell 1997) have been investigated. The group of Kentgens has used amplitude modulation of the rf field which amounts to a sweep of the frequency (DFS) and causes an adiabatic transfer between coherences. Through DFS good enhancement of the signal and excitation of an improved range of XQ were obtained (Kentgens and Verhagen 1999, Iuga et al. 2000, Schiifer et al. 2001, Iuga and Kentgens 2001). A more readily implemented scheme is the fast amplitude-modulated conversion pulses of Madhu et al. (1999, 2000) (FAM) which again produce coherence transfer through adiabatic passages. Wu et al. (1996) showed that as for spin-locking, rotation of the samples produces interconversion between 1Q and 3Q coherences, this approach being termed RIACT. The conversion of the single quantum to triple quantum coherence by the first long pulse and its reconversion by the second pulse depends on two main factors. The situation is most closely described for I = 3/2. High power rf and relatively slow MAS are required to ensure the conditions are as close as possible to adiabatic. Under adiabatic conditions, for spins to contribute, the system must undergo an odd number of passages or zero-crossings during the period of the spin-locking pulse. An adiabatic zero-crossing will the convert central-transition coherence (1Q) to 3Q, but after either two or four passages, the system will return to its initial state. The RIACT(II) sequence in which the excitation and conversion of the coherences is achieved by a spin-lock of duration Tr/4 instead of a hard pulse, has the advantage of more uniform excitation and conversion, which is essential in obtaining quantitative information. Schemes for sensitivity enhancement based on the selective inversion of the satellite transitions have been proposed (Haase et al. 1994). This can be combined into the MQ sequence as has been successfully demonstrated for 23Na and 87Rb (Gan 2000, Yao et al. 2000).

2.8. NMR RELAXATION

2.8.1 Introduction to relaxation

The spectroscopy described above is effectively the static part of the interaction, but there are dynamical processes that determine how rapidly the spins lose coherence and

99

Physical Background

how rapidly thermodynamic equilibrium is achieved between the spin states. This aspect of N M R is termed relaxation. Relaxation can provide information on the processes that cause transitions. First, a description of the process is necessary and a classical approach is to adopt the phenomenological B loch equations for an ensemble of spins in an external magnetic field. This model describes the behaviour of the magnetisation through the torque as in Eq. 2.3. Parallel to the main magnetic field the time dependence of the magnetisation is given by (2.186)

dMz = M~ - Mz dt

T1

where T1 is the longitudinal or spin-lattice relaxation time. If the sample is placed in the Bo field and initially has no magnetisation, the magnetisation along the direction of the magnetic field builds up as

exp/;//

(2.187)

The transverse components of the magnetisation decay as dM x = -M x dt

(2.188)

dMy _ -My

T2

dt

T2

where T2 is the transverse or spin-spin relaxation time. Then, in the laboratory frame the overall equation of motion of the magnetisation in an applied magnetic field B will be given by dM

---=-- = ?'M_x B dt

-

M x i + Myj

- -

M~ - M o k

T2

T1

(2.189)

-

which can be rewritten in the rotating frame as

dM

_ ) ' M x B_eff

MX'i + My,j

(2.190)

Mz' - M~ k

dt

T1 CO

In the rotating frame at v - Vo if there is no rf field present then Beff= B + -o = 0 and the B loch equations can be written as -o ), dM~, =

dt

M X,

r2'

dMy______.~=, _ My________[, dMz._____~,= M o - M~,

dt

dt

The solutions for the transverse (x',y') magnetisation are

T,

(2.191)

100

Multinuclear Solid-State NMR of Inorganic Materials M,(t)- M:,,(O)e-t/7"2 and My,(t)- My,(O)e-t/T2

(2.192)

Hence any magnetisation in the x'y'-plane (and xy) decays exponentially to zero. The transverse relaxation time is a result of the dephasing of the magnetisation in the xy-plane of the magnetic moments precessing about Bo. In principle, if each magnetic moment of the same nuclear species precessed with the same Larmor frequency about Bo no dephasing would occur. However, the moments do not experience the same magnetic field due to interactions and inhomogeneities in the static magnetic field Bo. The result of these effects is that each magnetic moment precesses with a different frequency which will result in the dephasing of the magnetisation in the xy-plane. These processes can be coherent or incoherent so that the T2* seen through the decay of the magnetisation in the FID after a pulse can be regarded as being made up of different contributions and written as 1

1

1

---r = - - - t - ~

(2.193)

where T2 is the decay of the magnetisation due to the interactions, and T'2 is due to the the effect of inhomogeneities in the field. The real physical interest lies in T2, since it can provide information about the interactions and the nature of the motion that modulates the interactions and causes relaxation. Spin echo techniques allow T2 to be measured rather than T2*. In this technique a sequence of two rf pulses is used as shown in Figure 2.28. The first pulse is a 90 ~ pulse and following this the individual nuclear moments spread out around the transverse plane due to their different precession rates. After a time "r a second pulse of 180 ~ is applied. All the spins are thus flipped about B1 so that those moments which had been precessing faster than average prior to --~2

~

3 ----~ 4

J

i ' v

AA~

'

5

L,

I I" 5 ,~..___m

Figure 2.28. The basic spin-echo pulse sequence and effect on spin packets showing the formation of a spin-echo.

Physical Background

101

application of the second pulse, and had thus got ahead of the other moments now find themselves, after the second pulse, behind the others. They are, however, still precessing faster than average and so, after a further time "r, they catch up with the slower moments. The same argument applies in reverse to the slower moments, the nett effect being to restore phase coherence to the precession at a time 2"r after the first pulse, thus forming a spin echo. The echo shape itself decays away with the characteristic time Y2* as the moments continue to precess at different rates. During the time 2T irreversible T2 processes will have been going on so that the re-phased magnetisation is less than Mo. Consequently the echo amplitude is reduced and varies with 2"r according to the B loch equation (Mx,(0) = Mz,(O) = O)

dMy, _

My,

My, (2z')- My,(O)e -2"c/T2

dt

(2.194)

By measuring the amplitude of the signal as a function of the echo time 2-r, T2 may be determined. This method assumes that each nucleus remains in the same field for the duration 2"r of the experiment. If, however, the nuclei move into a different field e.g. through diffusion, the echo amplitude will be further reduced because the precession frequency of the individual nuclei will have changed during the echo as they move from a region with one particular value of the magnetic field to another region. The signal amplitude at t = 2-r is now

2"c Mxy(2"c) = M ~ ( 0 ) e x p ( - T 2

2y2G2D'c 3 ) 3

(2.195)

where G is the magnetic field gradient in the z-direction and D the diffusion coefficient. To measure T2 in the presence of diffusion in an inhomogeneous field the Carr-Purcell spin-echo technique (a train of 180 ~pulses) can be applied. Carrying out the experiment with the echo spacing constant means that field gradient effects are much less significant than in a experiment with varying "r delays, but the disadvantage is that each 180 ~ pulse has to be very accurate. Diffusion measurements can also be made by deliberately applying a pulsed field gradient during both -r periods of a spin echo, and measuring the signal as a function of the gradient strength.

2.8.2 Mechanism for relaxation processes Transitions between Zeeman energy levels, and hence nuclear magnetic relaxation, are caused by fluctuations (time variations) in the local interactions at the nucleus that can cause transitions. The transition rates between these energy levels that cause nuclear spin relaxation depend on two factors: Rate of relaxation - (Strength of interaction) • (Number of fluctuations at Vo)

(2.196)


102

The fluctuations are often caused by atomic motion e.g. Brownian motion in liquids, ionic hopping, molecular rotations, librations and atomic vibrations. These motions are often complex and it is the range of frequencies that are present in the motions that determine relaxation. The spectral density function describes the relative intensities of different frequencies in the motions and can be used to calculate relaxation rates. Let y(t) be some function of time, such as the orientation of the internuclear vector and some other function f (e.g. the dipolar interaction) that depends on y. It is possible to define a probability function p(y,t) which is the probability that at a time t, the internuclear vector has some orientation y. Then (2.197)

f (t) - ~ p(y,t)f (y)dy

This assumes that y(t) is a stochastic function (i.e. y(t) varies completely randomly) but often y(t) shows some correlation. It is useful to define the probability function P(y~,t~;y2,t2) which means that if y = y~ at t = t~ this value is the probability p of y = Y2 at t -- t2. Then the probability that y = Y2 at t -- t2 and y = y~ at t = t~ and y = y~ at t -- t~ is p(y~,t~)P(y~,t~;y2,t2). The correlation function G(t~,t2) is

G(tl ,t2 ) - f (tl ) f * (t2) - ~ P(Yl ,tl )P(Yl ,tl ;Y2,t2 ) f (Yl ) f * (Y2)dYldY2

(2.198)

Since G is the correlation of the function with itself it is termed the auto-correlation function of f(y). If the function y(t) is stationary then it is invariant to changes of the time origin and G depends only on the difference, a" -- t2 - t~, so that G(r)

~ P(Y~ )P(Y~,Y2, v)f(y~ ) f * (Y2)dyldy2

(2.199)

The spectral density function is defined to be the temporal Fourier transform of G('r) so that ~x~

oo

--~

0

J(m) represents the "strength" (or energy) of fluctuations in f(y) at the frequency m and, as an example, the dipole interaction can be considered using the alphabet expression of Eq. 2.52. In Eq. 2.52 the terms A, B, C, D, E and F can be considered as having the form (Geometric term i.e. depends on the polar angles (0,~) fly)) • (Spin-operator term). The spectral density functions are

J(P)(m)- f f where

Y2p(Oa, ~a ) 3

J~,

Y2p(Ob'g~ 1~,3

,r b , m) dr_ dr b

(2.201)

Physical B a c k g r o u n d

103

oo

P(r~a ' ~ b , co) - FT[P(r~a ' r_~, r)] - I P(ra'~ rb , oo)e -iarcdr

(2.202)

--oo

with Y2p(Oi, (~i) the spherical harmonic functions. As an example for homonuclear dipolar coupling

1

-~1

l

3

3,2

-

-~

= -~'4h2I(I

(//0) 2

[jO) (CO~ )+ j(2) (2r ~)],

(2.203)

[J(~ (2(.01) + 10J(1) ((.0o) + J(2) (2(.0o)],

(2.204)

h 2 I ( I + 1) G

+ 1)

(/./O ~2 \4~J

and

l = 3 /4}~2i(I -t-1)(1"/o ~2[J(~ T2 8 \4,cJ

- 10j(1)((.Oo)--kJ(2)(2(.0o)].

(2.205)

The functional form of J(m) in turn depends on the form of P(G, G, ~) which will depend on the details of the atomic motion. The function P can often either be derived analytically or can be calculated numerically. The key is to understand that these expressions show that the different relaxation times probe different frequencies. The spectral density function is a measure of the distribution of fluctuations. For relaxation to be efficient there needs to be significant spectral density at the frequencies in Eqs. 2.203-2.205. Often a simplified functional form can be assumed, of which the most common is the BPP approximation (Bloembergen, Purcell and Pound 1948). This model assumes that the autocorrelation function is exponential

exp( /

206,

where % is a correlation time and is related to the jump rate. Then J(P) ((.o) - 2 G (p) (0)

vc

1 + (o)~"C)2

(2.207)

The relaxation rates are dominated by the behaviour of the spectral density function at frequencies related to the spectral densities at ~o (T1), o~1 (T~p) and 0 (T2). The correlation time % follows an Arrhenius-type activation so that r c - r o exp I Ea ]

(2.208)

104


where Ea is an activation energy for the motion and % is the inverse of the jump attempt frequency. If an experiment is carried out at constant Bo (and hence too) while the temperature (T) is changed then the form of the spectral density will change. This is shown schematically in Figure 2.29 so that as the temperature is increased the spectral density is spread out over a wider frequency range. There is a temperature where there will be a maximum amount of spectral density at COo. The correlation time can be represented by

Ea

ln(r C) - ln(~') + ~ kT

(2.209)

Therefore, a plot of In(relaxation time) against T - 1 gives a characteristic curve (Figure 2.30) with the position of T1 being minimum at ~Oo'rc - 1 since this is the maximum of j(1) (COo). Similarly, the minimum of Tlo occurs at tol"rc = 1/2. The slopes at high and low temperatures for T1 can be used to estimate Ea of the motion since

j(~)' Tx '

Tv

COO

O~

Figure 2.29. Schematic representation of the spectral density functions J(~o) at three temperatures where Tx < Tu < Tz.

\

/B01

Rigid Lattice Limit -T2

1/T Figure 2.30. Plots of the relaxation times In T1, T~p and T2 as a function of inverse temperature T-1

Physical Background

105

/

ln(T1) ~ in 1 + 0) 2 2 Tc

(2.210)

then

T --->,,o :cor 0"coy >> 1" ln(T1) ~: -ln(CO2T) ,~ ln(CO2) + E. kT

(2.212)

This symmetry is lost if there is a range of activation energies, as in a glassy material (Brinkman 1992). Hence relaxation measurements can provide significant insight into atomic scale motion. T2 has the same slope at high temperature as T1, but at low temperature the J(0) term leads to a constant value termed the rigid lattice limit. The use of relaxation time measurements can have important effects on constraining local motion in materials (Strange 1987, Brinkmann 1992).

REFERENCES Amoureux, J.P., Fernandez, C. & Lefebvre, F. (1990) Mag. Resort. Chem., 28, 5. Amoureux, J.P. (1993) Solid State Nucl. Mag. Reson., 2 83. Amoureux, J.P. & Cochon, E. (1993) Solid State Nucl. Mag. Reson., 2, 223. Amoureux, J.P. & Fernandez, C. (1998) Solid State Nucl. Mag. Reson., 10, 211. Andrew, E.R. (1971) Progress Nucl. Mag. Reson. Spectrosc., 8, 1. Andrew, E.R. (1981) Int. Rev. Phys. Chem., 1, 195. Behrens, H.S. & Schnabel, B. (1981) Physica B, 114, 185. Bielecki, A., Kolbert, A.C., de Groot, H.J.M., Griffin, R.G. & Levitt, M.H. (1990) Advances in Mag. Reson., 14, 111. Blaha, P., Schwartz, K. & Dederichs, P.H. (1988) Phys. Rev. B, 37, 2792. Blaha, P., Schwartz, K. & Luitz, J. (1999) Technical University of Vienna, Austria, ISBN 3-95010031-0-4. Bloembergen, N., Purcell, E.M. & Pound, R.V. (1948) Phys. Rev., 73, 679. Brinkmann, D. (1992) Progress Nucl. Mag. Reson. Spectrosc., 24, 527. Bohm, J., Fenkze, D. & Pfeifer, H. (1983) J. Mag. Reson., 55, 197. Burum, D.P. & Rhim, W.-K. (1979) J. Chem. Phys., 71,944. Caravatti, P., Bodenhausen, G. & Ernst, R.R. (1982) Chem. Phys. Lett., 89, 363. Caravatti, P., Braunschweiler., L. & Ernst, R.R. (1983) Chem. Phys. Lett., 100, 305. Clayden, N.J., Dobson, C.M. & Fern, A. (1989) J. Chem. Soc., Dalton Trans., 843.

106


Cochon, E. & Amoureux, J.P. (1993) Solid State Nucl. Mag. Reson., 2, 205. Cohen, M.H. & Reif, F. (1957) Solid State Phys., 5, 321. Dec, S.F., Bronnimann, C.E., Wind, R.A. & Maciel, G.E. (1989) J. Mag. Reson., 82, 454. Dickinson, W.C. (1950) Phys. Rev., 77, 736. Ding, S. & McDowell, C.A. (1997) Chem. Phys. Lett., 270, 81. Eastman, M.A., Grandinetti, P.J., Lee, Y.K. & Pines, A. (1992) J. Mag. Reson., 98, 333. Ernst, R.R., Bodenhausen, G. & Wokaun, A. (1988) Principles of Nuclear Magnetic Resonance in One and Two Dimensions, Oxford University Press, Oxford. Facelli, J.C. (1996) Encyclopedia ofNMR, Eds. Grant D.M. & Harris, R.K., John Wiley and Sons, Chichester, p. 2576. Farrar, T.C. (1990) Concepts in Mag. Reson., 2, 1. Farrar, T.C. (1990a) Concepts in Mag. Reson., 2, 55. Fenkze, D., Freude, D., Frohlich, T. & Haase, J. (1984) Chem. Phys. Lett., 111, 171. Fernandez, C. & Amoureux, J.P. (1998) Solid State Nucl. Mag. Reson., 10, 211. Filip, C., Hafner, S., Schnell, I., Demco, D.E. & Spiess, H.W. (1999) J. Chem. Phys., 110,423. Frey, M.H. & Opella, S.J.J. Chem. Soc., Chem. Commun., 474. Frydman, L. & Harwood, J.S. (1995) J. Amer. Chem. Soc., 117, 5367. Gan, Z. (2000) J. Amer. Chem. Soc., 122, 3242. Ganapathy, S., Schramm, S. & Oldfield, E. (1982) J. Chem. Phys., 77, 4360. Ganapathy, S., Shore, J. & Oldfield, E. (1990) Chem. Phys. Lett., 169, 301 Geurts, F.M.M., Kentgens, A.P.M. & Veeman, W.S. (1985) Chem. Phys. Lett., 120, 206. Grant, D.M. (1996) Encyclopedia ofNMR, Eds Grant D.M. & Harris, R.K., John Wiley and Sons, Chichester, p. 1298. Grey, C.P., Dobson, C.M., Cheetham, A.K. & Jakeman, R.J.B. (1989) J. Amer. Chem. Soc., 111,505. Grey, C.P., Smith, M.E., Cheetham, A.K., Dobson, C.M. & Dupree, R. (1990) J. Amer. Chem. Soc., 112, 4670. Grey, C.P., Veeman, W.S. & Vega, A.J. (1993)J. Chem. Phys., 98, 7711. Grey, C.P. & Vega, A.J. (1995) J. Amer. Chem. Soc., 117, 8232. Groombridge, C.L., Harris, R.K., Packer, K.J., Say, B.J. & Tanner, S.F. (1980) J. Chem. Soc., Chem. Commun., 174. Haase, J., Conradi, M.S., Grey, C.P. & Vega, A.J. (1994) J. Mag. Resort. A, 109, 90. Haeberlen, U. (1976) High Resolution NMR in Solids: Selective Averaging, Advances in Magnetic Resonance, Academic Press, New York. Hafner, S. & Spiess, H.W. (1998) Concepts in Mag. Reson., 10, 99. Hahn, E.L. (1950) Phys. Rev., 80, 580. Hanna, J.V., Smith, M.E., Stuart, S.N. & Healy, P.C. (1992) J. Phys. Chem., 96, 7560. Harris, R.K. (1983) Nuclear Magnetic Resonance Spectroscopy, Pitman Books Limited, London. Harris, R.K. & Olivieri, A.C. (1992) Progress Nucl. Mag. Reson. Spectrosc., 24, 435. Hartmann, S.R. & Hahn, E.L. (1962) Phys. Rev., 128, 1042. Herzfeld, J. & Berger, A.E. (1980)J. Chem. Phys., 73, 6021. Homer, J., McKeown, P., McWhinnie, W.R., Patel, S.U. & Tilstone, G.J. (1991) J. Chem. Soc., Faraday Trans., 87, 2253.

Physical Background

107

Iuga, D., Sch~ifer, H., Verhagen, R. & Kentgens, A.P.M. (2000) J. Mag. Reson., 147, 192. Iuga, D. & Kentgens, A.P.M. (2001) Chem. Phys. Lett., 343, 556. Jackson, P. & Harris, R.K. (1988) Mag. Reson. in Chem., 26, 1003. J~iger, C., Kunath, G., Losso, P. & Scheler, G. (1993) Solid State Nucl. Mag. Reson., 2, 73. J~iger, C. (1994) inNMR Basic Principles and Progress, vol. 31, Eds. Blumich, B. & Kosfeld, R., Springer-Verlag, Berlin, p. 135. Jameson, C.J. (1989) in Multinuclear NMR, Ed. Mason, J., Plenum Press, New York, p. 89. Keeler, J. (1990) in Multinuclear Magnetic Resonance in Liquids and Solids - Chemical Applications, vol. 322, Eds. Granger, P. & Harris, R.K., NATO ASI, p. 103. Kentgens, A.P.M. & Verhagen, R. (1999) Chem. Phys. Lett., 300, 435. Knight, W.D. (1956) Solid State Phys., 2, 93. Kundla, E., Samoson, A. & Lippmaa, E. (1981) Chem. Phys. Lett., 83, 229. Lee, M. & Goldburg, W.I. (1965) Phys. Rev. A, 140, 1261. Lefebvre, F., Amoureux, J.P., Fernandez, C. & Derouane, E.G. (1987) J. Chem. Phys., 86, 6070. Levitt, M.H., Suter, D. & Ernst, R.R. (1986) J. Chem. Phys., 84, 4243. Levitt, M.H., Kolbert, A.C., Bielecki, A. & Ruben, D.J. (1993) Solid State Nucl. Mag. Reson., 2, 151. Levitt, M.H. (1997) J. Mag. Reson., 126, 164. Llor, A. & Virlet, J. (1988) Chem. Phys. Lett., 152, 248. Maciel, G.E., Bronnimann, C.E. & Hawkins, B.L. (1990) Advances in Mag.Reson., 14, 125 Madhu, P.K., Goldburt, A., Frydman, L. & Vega, S. (1999) Chem. Phys. Lett., 307, 41. Madhu, P.K., Goldburt, A., Frydman, L. & Vega, S. (2000) J. Chem. Phys., 112, 2377. Man, P.P. (2000) in Encyclopedia of Analytical Chemistry, Ed. Meyers, R.A., John Wiley and Sons, Chichester, p. 12224. Man, P.P., Klinowski, Trokiner, A., Zanni, H. & Papon, P. (1988) Chem. Phys. Lett., 151, 143. Mansfield, P. (1971) Progress Nucl. Mag. Reson. Spectrosc., 8, 41. Mansfield, P., Orchard, M.J., Stalker, D.C. & Richards, K.H.B. (1973) Phys. Rev. B, 7, 90. Maricq, M.M. & Waugh, J.S. (1979) J. Chem. Phys., 70, 3300. Marinelli, L., Medek, A. & Frydman, L. (1998) J. Mag. Reson. A, 132, 88. Mason, J. (1993) Solid State Nucl. Mag. Reson., 2, 285. Medek, A., Harwood, J.S. & Frydman, L. (1995) J. Amer. Chem. Soc., 117, 12779. Mehring, M. (1983) Principles of High Resolution NMR in Solids, Springer-Verlag, Berlin. Menger, E.M. & Veeman, W.S. (1982) J. Mag. Reson., 46, 257. Metz, G., Wu, X. & Smith, S.O. (1994)J. Mag. Reson. A, 110, 219. Metz, G., Ziliox, M. & Smith, S.O. (1996) Solid State Nucl. Mag. Resort., 7, 155. Mtiller, D. (1982) Annales der Physike, 39, 451. Mfiller, L., Kumar, A., Baumann, T. & Ernst, R.R. (1974) Phys. Rev. Lett., 32, 1402 Munowitz, M. (1988) Coherence and NMR, John Wiley and Sons, New York. Nayeem, A. & Yesinowski, J.P. (1988) J. Chem. Phys., 89, 4600. Neue, G., Dybowski, C., Smith, M.L., Hepp, M.A. & Perry, D.L. (1996) Solid State Nucl. Mag. Reson., 6, 241, Opella, S.J., Frey, M.H. & Cross, T.A. (1979) J. Amer. Chem. Soc., 101, 5856. Pake, G.E. (1948) J. Chem. Phys., 16, 327. Powles, J.G. & Mansfield, P. (1962) Phys. Lett., 2, 58

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Powles, J.G. & Strange, J.H. (1963) Proc. Phys. Soc., 82, 6. Proctor, W.G. & Yu, F.C. (1950) Phys. Rev., 77, 717. Rhim, W.-K., Elleman, D.D. & Vaughan, R.W. (1973) J. Chem. Phys., 58, 1772. Ryan, L.M., Taylor, R.E., Paff, A.J. & Gerstein, B.C. (1980) J. Chem. Phys., 72, 508. Samoson, A., Kundla, E. & Lippmaa, E. (1982) J. Mag. Reson., 49, 350. Samoson, A. & Lippmaa, E. (1983) Phys. Rev. B, 28, 6567. Samoson, A. (1985) Chem. Phys. Lett., 119, 29. Samoson, A., Lippmaa E. & Pines, A. (1988) Molecular Phys., 65, 1013. Sanders, J.C.P. & Schrobilgen, G.J. (1990) in Multinuclear Magnetic Resonance in Liquids and Solids - Chemical Applications, vol. 322, Eds. Granger, P. & Harris, R.K., NATO ASI, p. 157. Sato, N. & Kimura, K. (1990) J. Amer. Chem. Soc., 112, 4688. Sch~ifer, H., Iuga, D., Verhagen, R. & Kentgens, A.P.M. (2001) J. Chem. Phys., 114, 3073. Schmidt, V.H. (1971) Proceedings of the Ampere International Summer School//, p. 75. Schmidt-Rohr, K. & Spiess, H.W. (1994) Multidimensional Solid State NMR and Polymers, Academic Press, London. Skibsted, J., Nielsen, N.C., BildsCe, H. & Jakobsen, H.J. (1991) J. Mag. Reson., 95, 88. Slichter, C.P. (1990) Principles of Magnetic Resonance, Springer-Verlag, Berlin. Sternheimer, R.M. (1954) Phys. Rev., 95, 738. Strange, J.H. (1987) Cryst. Lattice Defects and Amorphous Mater., 14, 183. Stuart, S.N. (1994) Solid State Nucl. Mag. Reson., 3, 199. Sun, B.Q., Baltisberger, J.H., Wu, Y., Samoson, A. & Pines, A. (1992) Solid State Nucl. Mag. Reson. 1,267. Townes, C.H., Herring, C. & Knight, W.D. (1950) Phys. Rev., 77, 852 Tycko, R. & Opella, S.J. (1987) J. Chem. Phys., 86, 1761. Vega, A.J. (1992) J. Mag. Reson., 96, 50. Vega, A.J. (1992a) Solid State Nucl. Mag. Reson., 1, 17. Vega, S. (1981) Phys. Rev. A, 23, 3152. Walter, T.H., Turner, G.L. & Oldfield, E. (1988) J. Mag. Reson., 76, 106. Waugh, J.S., Huber, L.M. & Haeberlen, U. (1968) Phys. Rev. Lett., 20, 180. Woessner, D.E. (1987) Z. Phys. Chem. Neue Folge, 152, 309. Wu, G. & Wasylishen R.E. (1993) J. Mag. Reson. A, 102, 183. Wu, G., Rovnyak, D., Sun, B. & Griffin, R.G. (1995) Chem. Phys. Lett., 249, 210. Wu, G., Rovnyak, D. & Griffin, R.G. (1996) J. Amer. Chem. Soc., 118, 9326. Wu, Y., Sun, B.Q. & Pines (1990) J. Mag. Reson., 89, 297. Yao, Z., Kwak, H.-T., Sakellariou, D., Emsley, L. & Grandinetti, P. (2000) Chem. Phys. Lett., 327, 85. Yesinowski, J.P., Eckert, H. & Rossman, G.R. (1988) J. Amer. Chem. Soc., 110, 1367. Zwanziger, J.W. & Chmelka, B.F. (1994) in NMR Basic Principles and Progress, vol. 31, Eds. Blumich, B. & Kosfeld, R., Springer-Verlag, Berlin, p. 202.

Chapter 3

Experimental Approaches Basic Experimental Principles of FT NMR Instrumentation 3.2.1 Overview of a Pulsed FT NMR Spectrometer 3.2.2 Magnets 3.2.3 Shimming 3.2.4 Transmitters 3.2.5 Probes 3.2.6 Connection of the Probe 3.2.7 Signal Detection 3.2.8 Additional Equipment 3.3. Practical Acquisition of NMR Spectra 3.3.1 Processing the FID to Produce a Spectrum 3.3.1.1 Window Functions 3.3.1.2 Shifting of the Time Origin and Linear BackPrediction 3.3.1.3 Zero Filling 3.3.1.4 Phase Correction 3.3.1.5 Baseline Correction 3.3.2 Complications in Recording Spectra 3.4. Static Broad Line Experiments 3.4.1 Pulsed Echo Experiments 3.4.2 Stepped Experiments 3.5. One-Dimensional High Resolution Techniques 3.5.1 Magic Angle Spinning (MAS) 3.5.2 Extraction of Parameters from MAS NMR Spectra 3.5.3 Suppression of Spinning Sidebands 3.5.4 Special Considerations for MAS of Quadrupolar Nuclei 3.5.5 Magic Angle Spinning Observation of Satellite Transitions 3.5.6 Double Angle Rotation of Quadrupolar Nuclei 3.5.7 Practical Implementation of CRAMPS 3.6. Two-Dimensional Experiments 3.6.1 Nutation NMR 3.6.2 Off-Resonance Nutation 3.6.3 Order-Resolved Sideband Spectra 3.6.4 Dynamic Angle Spinning (DAS)

3.1. 3.2.

111 112 112 113 115 116 120 122 124 127 127 128 128 129 129 130 130 130 133 133 136 138 138 143 143 144 149 150 152 153 153 154 155 156

3.6.5 3.6.6 3.6.7

Two-Dimensional Sequences Developed from Solution NMR Multiple Quantum Experiments in Dipolar Coupled Systems Multiple Quantum NMR Experiments of Non-Integer Spin Quadrupolar Nuclei 3.6.8 2D XY Correlation Methods 3.6.9 Correlation of Tensor Information- Separated Local Field Experiments 3.7. Summary of Approaches for Examining Quadrupole Nuclei 3.8. Multiple Resonance 3.8.1 Cross-Polarisation (CP) 3.8.2 SEDOR, REDOR and TEDOR 3.8.3 TRAPDOR and REAPDOR 3.9. Techniques for Determining Relaxation Times and Motional Parameters 3.9.1 Measurement of T~ 3.9.2 Other Spin-Lattice Relaxation Times (Tip, T1D) 3.9.3 Transverse Relaxation Times (T2) 3.9.4 Molecular Motion 3.9.5 Diffusion Measurements 3.10. NMR Under Varying Physical Conditions 3.10.1 Variable Temperature NMR 3.10.2 High Pressure Experiments References

157 160 161 168 170 172 172 173 178 182 183 183 184 185 186 187 187 187 189 190

Chapter 3

Experimental Approaches 3.1. BASIC EXPERIMENTAL PRINCIPLES OF FT NMR

In principle the NMR experiment is straightforward in that it has relatively few requirements. The equilibrium magnetisation must be produced by the application of a high static magnetic field which creates the Zeeman states lifting the degeneracy of the nuclear spin energy levels. The central task of an NMR spectroscopy experiment is to determine the separation of these energy levels. This measurement is carried out by the application of a usually much smaller magnetic field oscillating at a frequency that can induce transitions between the energy levels. This condition dictates that with magnetic fields in the range 4.7 to 18.8 T typically available today, the corresponding resonance frequencies are in the range 10-800 MHz, and it will not be long before 1H NMR is carried out at 1 GHz. The original method employed to detect a signal was to scan either the frequency of the exciting oscillator or the applied magnetic field until resonant absorption occurred. However, compared to simultaneous excitation of a wide range of frequencies by an rf pulse, scanning either the frequency or Bo is a very time-inefficient way of recording the spectrum. In the linear approximation there is a direct Fourier relationship between the time domain signal after a pulse (the FID) and the spectrum so that the spectrum is produced by Fourier transformation (FT) of the FID. Hence, with the advent of computers that can be dedicated to spectrometers, and efficient Fourier transform algorithms pulsed FT has become the normal mode of operation in NMR experiments (Ernst and Anderson 1966). Operating at constant field and frequency also produces big advantages in terms of the stability of the whole experiment. With the development of persistent superconducting magnets very stable applied magnetic fields are now used. In an FT NMR experiment a pulse of duration Tp is applied close to resonance. The discussion of the rotating frame in Chapter 2 shows that the pulse causes a coherent motion of the magnetisation, tipping the magnetisation through an angle 0p. The signal observed in the NMR coil by electromagnetic induction is termed the FID. To improve the signal/noise ratio (S/N), a number of FIDs (n) are usually coherently added, increasing the signal, with the S/N improving as ~n. It is a tacit assumption that everything behaves in a linear fashion, for example, that the excitation (or effective rf field) is uniform across the entire spectrum. In many cases this situation is closely approximated but distortions can occur for the broad lines that may be encountered. The frequency spectrum A(v) of a pulse of duration Tp applied at Vo is given by a sinc function sin ~Tp (V o - V) A(v) -

rC(Vo - v ) T p

111

(3. l)


112

A(v) rf

/'~

T Tp Fouriertransform,. field[~ ] ]Envelopeof ' ~

sincoet Vo 1

Frequene3 1

Figure 3.1. Fourier relationship between an rf pulse of duration Tpand the amplitude distribution A(v) of the frequency components present.

The sinc function is simply the FT of a rectangular pulse. Hence, from the bandwidth theorem, as the duration of the pulse decreases the frequency range covered increases. In Figure 3.1 the central lobe of this sinc function increases its frequency coverage as the pulselength decreases. To obtain an undistorted spectrum it should ideally be confined to the central portion of this lobe so that the irradiation is then uniform across the spectrum. Unfortunately there are examples in the solid state NMR literature of wide lines where the resonances are clearly much broader than the central lobe so that effectively only the irradiation envelope is reported and not the true spectral lineshape. The sinc 2 function describes the best possible case, with often a much stronger frequency dependence of power output delivered at the probehead. (It should be noted here that alternative excitation schemes to pulses are possible such as adiabatic passage (Kentgens 1991) and stochastic excitation (Yang et al. 1998) but to date these are only very infrequently applied).

3.2. INSTRUMENTATION 3.2.1 Overview of a pulsed F T N M R spectrometer The basic components of a pulse FT NMR spectrometer are shown schematically in Figure 3.2. There is a high field magnet which these days is nearly always a superconducting solenoid magnet. The probe circuit containing the sample is placed at the centre of this magnetic field. The probe is connected to the transmitter. All parts of the spectrometer are coupled together via transmission lines whereby the rf power/signal are transferred. Typically the coaxial cable which acts as the transmission line has a characteristic impedance determined by its capacitance and inductance per unit length. In NMR experiments, the convention is to standardise on an impedance of 50 l). The transmitter consists of a synthesiser to produce the central frequency, which is gated to form the pulses. These pulses must be amplified so they can be made sufficiently short to cause broad enough frequency excitation for the frequency width to be observed.

Experimental Approaches

ii ii

@

I

113

Magnet

Synthesiser r

j Ampl~'~r

Figure 3.2. Basic components of FT NMR spectrometer.

The probe is also connected to the receiver, requiring careful design to ensure that the receiver that is sensitive to IxV does not see any of the large excitation voltages produced by the transmitter. The induced voltage detected in the receiver must be digitised for storage in a computer where it can be manipulated and converted into a spectrum. The relatively simple concept of both the experiment and the instrument belies the extensive research and development effort that has gone into NMR spectrometers.

3.2.2 Magnets NMR demands ever higher magnetic fields, the highest currently commercially available for solid state NMR being 18.8 T. Standard instruments are now considered to be 4.7-9.4 T. The drive for higher fields is based on the increased chemical shift dispersion (in Hz) and the increase in sensitivity via both the Boltzmann factor and higher frequency of operation. Superconducting solenoids dominate the magnets sold and are usually based on Nb3Sn or NbTi multifilament wire maintained at liquid helium temperatures. However, fields and current densities now used are close to the critical limits of these materials, demanding improved materials technology. The demand for ever higher magnetic fields has seen the extension of the superconductor performance by operating at lower temperature. This is achieved by pumping on part of the helium cryostat which reduces the boiling point of helium. The principle of operation is very simple; a high current is circulated through a long (several km) coil of wire at a typical current of at least 50 A. This means that the magnet stores significant amounts of energy (up to 10 MJ) in its field (= 1/2LI2, where L is the solenoid inductance and I is the current flowing). A superconducting magnet consists of a cryostat, main coil, superconducting shim set and a means for attaching the current supply to the main coil (Figure 3.3). The

114


Longitudinal cut through a vertical cryostat ~ ] A - liquid helium (-269~ B - superconducting coil C - l i q u i d nitrogen (-196~

D - vacuum space E - room temperature bore

Figure 3.3. Schematic view of a high field superconducting magnet.

cryostat consists of two vessels for the liquid cryogens, the inner one for helium and the outer one for nitrogen. These are insulated by several vacuum jackets with a radiation shield. The aim is to reduce heat leakage to the inner chamber to conserve helium. NMR superconducting magnets are usually operated in persistent mode, which means that after a current is introduced, the two ends of the main coil are effectively connected so that the current follows a continuous path within the superconductor, allowing the power supply then to be disconnected. To attach and detach the coils (main and shim) to the power supply, the circuits within the cryostat have a superconducting switch. The coil circuits are also designed to cope with a sudden irreversible loss of superconductivity, termed a quench. There are resistors present (called dump resistors) to disperse the heating effect and prevent damage to the main coil when a quench occurs (Laukien and Tschopp 1993).


115

Magnet technology is continually advancing, even for "standard" 7.05 T solid-state NMR magnets which have been available for --~20 years. In early versions the field was typically generated by a current of 50A, whereas modem versions run at nearer 90A since the inductance of the main coil in newer magnets is much lower. This brings the advantage that modem magnets can be charged much more rapidly. The complexity and size of the associated cryostat is greater than in the more conventional magnets. Although higher magnetic fields are available than those used in NMR, the NMR experiment imposes additional constraints on the field. An NMR spectroscopy experiment also demands homogeneity and stability of the magnetic field. Long term stability is aided by persistent-mode operation and the drift should be < 2 X 10 - 7 per day. Homogeneity requirements for solid state NMR experiments are typically 2 X 10 - 9 over a volume of--~ 1 cm s. The main coil alone is unable to produce this level of homogeneity so in the main cryostat there is a set of smaller superconducting coils called cry0shims. The number of these cryoshims depends on the design and also the purpose of the magnet (e.g. solid state NMR, high resolution NMR, imaging), but typically varies between three and eight. For many solid state NMR experiments the homogeneity produced by the cryoshims is sufficient. However, most commercial spectrometers also have a room-temperature shim set which further improves the homogeneity of the magnetic field. A final consideration for the magnet is the accessible room-temperature bore size of the magnet. A standard magnet has a bore of 52 mm diameter, but most solid state NMR spectroscopists prefer an 89 mm bore as this gives much more room for the probe, allowing the use of larger, more robust high-power electrical components, and accommodating some of the more specialised probe designs (e.g. double angle rotation, dynamic angle spinning etc.). Superwide bore magnets also exist, with an accessible diameter of 150 mm. For imaging experiments, even wider-bore magnets are made, and often rather than the bore being in a vertical orientation, as is favoured in spectroscopy, the bore is horizontal.

3.2.3 Shimming For most solid-state NMR spectroscopists, shimming is a relatively minor consideration. For many of the studies discussed later in this book the resolution provided by a well cryoshimmed magnet is more than sufficient. Nevertheless, there will be occasions when shimming is important. The resolution required for high-resolution solution experiments is typically --~0.1 Hz. At a resonance frequency of 400 MHz this corresponds to a magnetic field homogeneity of one part in 2.5 X 10 ~~ this resolution ideally achieved over a cylindrical volume typically 20 mm high and 10 mm in diameter. Most commercial spectrometers are bought with a room temperature shim set. Shimming to this level of precision can be difficult, as the process amounts to trying to find a global minimum in the linewidth determined by the inhomogeneity in the

116


magnetic field which is a multidimensional function. For an ideal shim set all shims would be orthogonal with each gradient having no interaction with other gradients. No shim set is perfect but often local minima are only relatively shallow, although this is not always so. Each shim gradient (the direction and shape of gradient that results when a current is applied to that specific coil) has a shim centre. In a cryomagnet there will be quite a strong field dependence (i.e. a gradient) along the z-axis. If a shim current is used to apply an additional field to cancel out this gradient there should be a change in the value of the field at all points except one; this unchanged point is termed the shim centre for that gradient. The determination of this point for the z direction is made in the factory and is recorded in the magnet handbook as a distance from the magnet top and bottom flanges. Different magnet manufacturers supply different cryoshims, some making only x, y and z shims available but others offering xy, xZ--y2, XZ and yz as well. Cryoshimming is carried out during installation so that most operators never come into contact with this procedure, which needs not be repeated unless the magnet is moved to a significantly changed environment (e.g. another magnet is located close by), or a cryoshim quenches. The authors' experience is that even after a full quench the old cryoshim currents are usually satisfactory. Cryoshimming should ideally be carried out on the largest sample volume likely to be used. Most NMR spectrometers have 12 to 18 shim controls (Churmny and Hoult 1990). Each user will adopt their own procedure but the aim is to produce the minimum linewidth consistent with a good lineshape. In practice, some shims are much more significant than others and for particular probes different shims will be important. For solid-state operation, shimming usually needs to be carried out relatively infrequently. One possible procedure for probes tuned to ~H is to crudely shim on H20. If there is no proton channel most multinuclear probes will tune to 2D, so D20 can be used. For CPMAS probes that tune to 13C, adamantane is a useful compound which should be shimmed under spinning and 1H decoupling conditions. A typical resolution for 13C in admantane of 3-4 Hz at 7.05 T and --~10 Hz at 11.7 T should be achieveable. In any shimming procedure the gradients should be varied systematically (Churmny and Hoult 1990) and it should be borne in mind that even with very good shim sets the gradients will be interdependent. The speed of the Fourier transformation in modem spectrometers allows the spectrum itself to be used for shimming, rather than the FID. There will always be a compromise between the very narrowest lines that can be produced and a good lineshape (free of shoulders and humps).

3.2.4 Transmitters The frequency required to observe a particular nucleus is the first consideration. This frequency was formerly synthesised by analogue methods. Frequencies below 200 MHz

ExperimentalApproaches

117

were synthesised directly, whereas higher frequencies were synthesised via frequency mixing. Synthesising the frequency in a stable and accurate way is very important. The technology of synthesisers has advanced, with digital synthesisers now available. In some experiments the frequency of the synthesiser needs to be switched rapidly. There are two alternative schemes for switching the frequency. One possibility is that it is phase-continuous, whereby, when the frequency changes, the phase continues instantaneously with the same value or phase coherently (i.e. the phase of the second frequency is as if it had started at the same point in time in phase with the other frequency). This frequency is normally continuously synthesised and then gated to form the pulse. Solid-state NMR experiments often call for short pulses and rapid phase switching, and in earlier spectrometers the most commonly-used quadrature phases (0 ~ 90 ~ 180~ 270 ~ or x, y , - x , - y) were simultaneously present in a hardwired modulator. Each of the four channels had separate amplitude and phase controls, and the rf was generated simply by taking the appropriate output. This set-up requires maintenance to correct for changing conditions and the frequency dependence of the operation means that different settings will be necessary for the different channels at different frequencies. Such a set-up produces the maximum switching speed for the phases, but in recent years digital synthesis has become more common. Here, the computer memory holds a sine wave in digital form. The required frequency is then produced by the speed at which the computer memory is read out, the rf being generated by a digital-to-analog converter. The phase shifts are generated by simply varying the memory address from which the signal read-out commences. This means that pulse phases can be generated continuously with typical settability currently of--~0.1 ~ Hence, non-90 ~ phase shifts are readily generated, an important consideration since an increasing number of experiments routinely demand such phase differences (e.g. MQ-MAS of quadrupole nuclei). The pulse from the modulator, which is typically at the --~V level, is then amplified to a level that depends on the type of experiment. The power of a transmitter can be determined by examining the pulsed voltage on an oscilloscope (but NEVER plug a high power transmitter directly into an oscilloscope). Attenuation is measured in decibels (dB) and in terms of voltage:

10

(3.2)

Hence 3 dB represents a factor of 1.4 in voltage while 20 dB is a factor of 10. If Vpp is the measured peak-to-peak voltage after taking into account the attenuation, the power is

P(watts) = 200-,~

(3.3)

118


Amplification is achieved via solid-state transistors or vacuum tube amplifiers. Transistors have always been used for power of less than 100 W, but for amplification up to the 1 kW level the philosophy has changed in recent years. Originally, tube amplifiers were the only real choice for high power rf applications. However, solid-state transistor technology has advanced so that up to 200 MHz 1 kW transistor amplifiers are now a serious option. At higher frequencies, tube or cavity amplifiers are still the only real possibility. For more demanding high-power applications, tube amplifiers are preferred since, if the duty cycle increases, producing a significant heating effect, transistors are susceptible to changes in their characteristics, whereas tube amplifiers already run hot. Tube amplifiers are usually part of an active tuned circuit so that as the frequency of operation is changed the amplifier circuit has to be retuned. By contrast, a transistor amplifier is broadbanded, requiting only the new frequency to be set. This makes the transistor design increasingly popular and the spectrometer easier to use. The behaviour of rf amplifiers falls into one of several classes (normally termed A, AB, B or C) depending on the relationship between the input and the output voltage. Figure 3.4 gives a diagrammatic representation of this relationship. Class A is termed linear, the output being directly proportional to the input. This relationship can make resetting of the CP condition at different power settings relatively straightforward and is necessary for the application of high-power shaped pulses in solid-state NMR. The disadvantage of linear amplifiers is that without careful circuit design the output can be less stable (e.g. due to heating effects). Also, as the circuit operates in the active region it generates significant noise even in the quiescent state, and so needs to be actively blanked by a gating circuit. For some applications (e.g. CRAMPS), the speed of the Anode current

Anode current

Class A

Class C

. . . .

Output

0 ,---i .....

o Grid voltage

i

i i

Grid voltage

i

'Input Input Figure 3.4. Schematic representation of the input-output relations of Class A and C amplifiers with

O the operating point of the grid voltage.


119

Gate

TR i-,,

. v~

I -~-...................... i................................. j ............ [ ,,/~ "--,, i]~Droop i," ~ A-""-^-.....~--~- ....... ~.........................

i/'

.

.

.

.

.

.

.

Ripple

Figure 3.5. Schematic representation of pulse imperfections with TR the rise time and TF the fall time. blanking circuit becomes an important consideration. Class C operation is highly nonlinear but has several advantages. In its quiescent state, the amplifier is biased hard off so that there is very little noise generated by the transmitter between pulses. It then turns on at a fixed level with relatively little change in the output with input level, making the transmitter very stable. Often class AB operation is preferred which, at the extremes, shows the advantages of class C behaviour while nevertheless having a linear region. The pulses from the transmitter are often far from ideal. To check their non-ideality they should be observed on an oscilloscope via a broadband 50 ~ impedance, termed a dummy load. Typical characteristics are the rise and fall times, the on-off ratio and the amounts of tipple and droop. These are shown schematically in Figure 3.5. The turn-on time is usually defined as the time to go from 10 to 90% of the maximum power. The on-off ratio is especially important for determining the noise generated by the pulse during the FID. The effect of droop becomes more noticeable when longer pulses are applied, as for spin-locking. All these effects which alter the shapes of pulses mean that the irradiation profile is often much more complicated than the sinc function of Eq. 3.1. Also, for a pulse incident on a narrowed-banded mismatched load (e.g. a probe), the effects and the irradiation are more complicated still. Even if the bandwidth of the pulse is high, any narrow bandwidth components in the spectrometer sequence will cut this down and will also give rise to transient oscillations.

120


3.2.5 Probes

The probe is the heart of the NMR experiment. It is essentially a tuned resonant circuit with the sample contained within the main inductance (the NMR coil). The inductance of the coil is proportional to p~l,Zon21A where n is the number of turns per unit length on the coil (i.e. N (total turns)/1 (coil length), IX~o is the permeability of the sample and A is the cross-sectional area of the coil. The details of the circuits are often complex, and a typical circuit is shown (Figure 3.6). The circuits often use inductive coupling so that different parts of the circuit need not be physically linked (Kuhns et al. 1988). Both series and parallel circuit designs are used (Fukushima and Roeder 1981, Stejskal and Memory 1994) but they all consist of capacitances and inductances which have frequency-dependent reactive impedances. Usually a parallel-tuned circuit is preferred, and in simplified form the inductance (L) and capacitance (C) can be related to the required resonant frequency (Vo) via Vo - 1/(2-rrL ~ ) . The resonance frequency is the most important parameter, but the input impedance, which should be 50 12, also has to be satisfied. Furthermore, the quality factor Q, (a measure of the sharpness of the resonance of the probe circuit, one definition being the resonance frequency/half width of the resonance response of the circuit, also = tooL/R, R is the coil resistance), must also be considered. The probe must satisfy competing requirements which can be characterised in terms of Q, as summarised in Table 3.1. It can be immediately seen from Table 3.1 that a probe cannot be designed with all the properties optimised because of their differing Q-dependencies. This means that most probes are a compromise, focussing on the most important aspects for a specific application. Several probe designs exist which have advantages for different applications. 74 nH

10 nH 22 pf . . . 1 _

1-10 pf 13C - tdead (3.15) Fourier transforming this gives the FT of the FID (i.e. the expected spectrum) convoluted with sinc(o~- tOo)t~ead, the latter manifested as a rolling of the baseline. An example is shown in Figure 3.9A. The effect of this deadtime-induced baseline roll is not a significant problem if the spectral lines are narrow compared to the frequency of the

132


A

Baseline = A

6000

2000

4000

0

sink(v-I/o) k(v- Vo)

-2000 -4000 -6000

Shift ppm B line

....

I

I

-800

I

I

I

-1000 -1200 -1400 -1600

sgy chemical shift in ppm Figure 3.9. A. The spectrum over a 1.66 MHz spectral range at 130.32 MHz for 27A1 with the initial deadtime producing a sinc roll of the baseline (taken from Alemany et al. 1991 with permission of the copyright owner). B. At 17.64 MHz the more complex baseline produced by ringing effects at the low frequency with the 89y just visible (taken from Smith and van Eck (1999) with permission of the copyright owner). baseline roll. The practical situation is often more complex than this analytic function, and an example is shown in Figure 3.9B. If the resonance was much broader, the lineshape would be significantly distorted by the baseline roll and it would be difficult to know what the true lineshape was. Working at higher frequency decreases tdead. However, practice often shows that the advantage of removing distortion by working at higher frequency is not as marked as might be anticipated, usually because the start of the FID is corrupted by ringing. Ringing is an all-embracing term to describe effects that induce FID-like voltages in the


133

coil. Typical sources of this effect are the electromagnetic effects of the pulse interacting with the surroundings (e.g. the probe body) so that either mechanical and/or re-radiation effects occur. Acousto-mechanical effects can also occur in the coil and can continue after the pulse is turned off, producing a short-lived pseudo-FID. The overall efficiency (Ec) of this process is given by

klB~

EC =

(3.16)

d v s 1+

4

Vs

where kl and k2 are constants, e is the coil resistivity, d is the bulk density and Vs is the acoustic wave velocity (Fukushima and Roeder 1979). Eq. 3.16 shows that for a constant Bo the situation is worse at lower frequency, and at a constant frequency it is worse at higher Bo. These short-lived voltages can look like broad resonances or baseline distortion. Careful probe design can reduce ringing effects and the considerable work put into reducing these effects has been extensively reviewed by Geronthanassis (1987). In addition to the effects from the probe there is the electronic deadtime, including pulse ringdown (100 ns), preamplifier recovery (800 ns), filter overdrive recovery (1 txs) and ADC conversion droop (200 ns) (Hoult 1979). Magnetoacoustic ringing (up to 200 ~s) can be very significant if careful probe design (e.g. coil wire) is not considered. Samples that exhibit peizoelectric behaviour can lead to very long response times of up to 10 ms.

3.4. STATIC BROAD LINE EXPERIMENTS

3.4.1 Pulsed echo experiments Static powder patterns offer one way of characterising a material, and if spectral features can be observed and the line simulated, accurate determination of the NMR interaction parameters is possible. A simple consideration not to be overlooked is that the theoretical powder lineshape assumes the powder sample to be randomly oriented. This means that the powder should be finely ground. Coarse powders show structure in the lineshape which can complicate the lineshape analysis. Despite the effects outlined above, one-pulse acquisition experiments allow relatively narrow spectral features such as singularities to be recorded over a broad spectral range (Figure 11.3A). Detailed analysis of the singularity positions of the different transitions has allowed the different interactions (e.g. quadrupolar, shift) to be estimated (Baugher et al. 1969). Spectral intensity between the singularities is mostly lost so that true lineshape is not observed.

134


A common way of overcoming deadtime problems is to form a signal with an effective time-zero point outside the deadtime, i.e. an echo. There are many methods for forming such echoes. Most involve two-pulse sequences with the classic spin-echo (Sec. 2.8). In general, the loss of transverse magnetisation is incoherent, but under some circumstances this phase loss can be reversed and echoes observed in solids. If this dephasing is due to effects such as inhomogeneity of the applied magnetic field, chemical shift effects or heteronuclear dipolar effects, all these interactions are proportional to the operator iz. A 180 ~ pulse reverses the direction of iz and hence the magnetisation refocuses. In the other important cases of homonuclear dipolar and quadrupolar interactions echo phenomena cannot really be described classically and the loss of phase coherence is properly described by quantum mechanics (Solomon 1958) by which a complete density matrix description of the system is developed. The lineshape will only be accurately reproduced with the spin-echo if iz is accurately refocused. This will certainly be true at short refocusing times. Since the dipolar coupling and chemical shift do not commute, under some circumstances if there are dipolar couplings, the echo will not necessarily be correctly refocused. The spin-echo for an I spin will also not refocus well if the S-spin undergoes spin fluctuations on the timescale of the experiment. Such interactions will occur for strong homonuclear dipolar coupling or if S nuclei show short T~. Hence if there is strong homonuclear dipolar coupling or short S T1, the I spin is not fully refocused by a spin echo via the heteronuclear dipolar coupling. The efficiency of signal recovery in the echo is greatly reduced as a- is increased and the optimum echo signal amplitude will be observed when -r is equal to the spectrometer dead-time following the 90 ~ rf pulse. The echo decay shape is a good replica of the original FID and its observation can be used to obtain more reliable and quantitative information about solids. Echo methods can be used to observe just the central transition of quadrupolar nuclei, allowing the determination of sites or nuclei with larger quadrupole interactions, or the whole satellite transition manifold can be determined. In forming echoes it is convenient to consider two cases depending on the spacing a- relative to the length of the FID (Tf). A so-called Solomon echo forms if a- < < Tf, whereas a Hahn type of echo forms if a- -> Tf. Early calculations of this behaviour assumed a "hard" pulse regime in which the rf nutation frequency is much greater than the effective frequency of the interaction, so evolution during the pulse is solely governed by the if-pulse and the effect of the interaction can be ignored. With the advent of computers, numerical calculations of echo formation have become possible. These effects have been most widely calculated for the quadrupole interaction (initially the second-order quadrupole interaction) and can be readily extended to the regime where the first-order quadrupole interaction is significant, so evolution during the pulse occurs under both the rf and first-order quadrupole interactions. Calculations have been made for a variety of two-pulse sequences and for varying if-field strengths (Haase and Oldfield 1993, 1993a, Man 1993, 1995). These calculations have been


135

extended to include other interactions as well (Bodart et al. 2000, Man 2000). The general conclusion of this work is that echoes will generally be formed but that to obtain quantitatively reliable information great caution has to be exercised. For quadrupole nuclei it is useful to note that VQ can be deduced from the intensity variation as a function of the length of the second pulse (Haase and Oldfield 1993a, Man 1995). In practice, hard if-pulses are used for uniform excitation of broad lines. Our own work has tended to use an echo sequence with the phase cycling first proposed by Kunwar, Turner and Oldfield (1986) which combines quadrature phase cycling with further cycling designed to cancel direct magnetisation (the remaining FID) and ringing effects: Phase pulse 1: Phase pulse 2: Receiver phase:

xxxxyyyy - x- x- x- x- y- y - y- y xy- x- yxy - x- yxy- x- yxy- x- y - yy - yy - xx - xxy - yy - yx- xx- x

Other phase cycling to prevent distortion due to mis-set pulse lengths (Rance and Byrd 1983) has pointed out that response of a spin system covering a wide frequency range is not simply a reflection of the power distribution in the pulse, but that there are spectral distortion effects with some spins remaining in the same state before and after the pulse. The rotation produced by the second pulse in the two-pulse echo experiment is not critical. In practice, the best choice is to make the second pulse twice the length of the first, the actual length being a trade-off between sensitivity and uniformity of the irradiation. The uniformity can be checked by decreasing the pulselength and seeing if the lineshape changes. The excitation has only sufficient bandwidth to allow detection of the first two singularities of the satellite transitions, but compared to the one-pulse experiment, it provides a better record of the satellite transitions between the singularities, although still below the theoretical expectation. An important practical consideration in recording echoes is that the reason for applying the echo is to move the effective t -- 0 position for the FID outside the region where the signal is corrupted. However, so that phasing problems do not re-emerge, the data sampling rate should be sufficient to allow this point to be accurately defined. If T2 is sufficiently short that an echo time can be used that allows the whole echo (both before and after the maximum) to be accurately recorded without an unacceptably large loss of intensity, there is no need accurately to define the new t = 0 position. Fourier transformation of the whole echo (which effectively amounts to integration between +_w), followed by magnitude calculation removes phasing errors and produces a pure absorption lineshape with a signal-to-noise ~/2 larger than that obtained by transforming from the echo maximum. Rather than using a single echo it has recently been suggested that a train of refocusing w-pulses (selective on the central transition), similar in form to the Carr-PurcellMeiboom-Gill (CPMG) sequence, could have significant advantages. Early work,

136


termed quadrupolar CPMG or spikelet spectroscopy, was developed in Aarhus (Larsen et al. 1997, 1998, 2000). The w-pulses refocus the quadrupolar-induced dephasing. At the start of the sequence an optimised quadrupole echo is used. Hence, the refocused magnetisation decays relatively slowly compared to the timescale of the decay of an individual echo. A string of echoes is recorded and the complete echo train is Fouriertransformed to give a series of sharp bands that follow closely the static lineshape. The inhomogeneous interaction is refocused at the echo maxima. FT of the full echo contains both the homogeneous and inhomogeneous interactions. The homogeneous interaction affects the lineshape of the individual bands and the envelope provides information about the inhomogeneous part of the interaction. An extensive phase cycle is applied for the 90~ - 180~ -- [ 180~ ]n -REC+rec sequence (Larsen et al. 1997).

+2

+3 +rec

xy-x-yxy-x-yxy-x-yxy-x-y yxyxy- x- y- xyxyxy- x- y- x yxyx - y- x - y- x - y- x- y- xyxyx - x- yxy - x- yxy- x- yxy- x- yxy

This sequence provides significant signal enhancement, since the intensity from the powder lineshape is concentrated into a series of sharp bands. This gain is especially true if T~ is long compared to the timescale of the echo train. For example, if T~ required a recycle time of 16 s but 60 echoes could be collected without significant magnetisation decay, an experiment using single echoes would take --~30 minutes compared to a few ms using an echo train. The interpulse delay ('ra) determines the frequency spacing of the sharp lines (1/a'a) that make up the spectrum. The delays must be adjusted so that baseline artefacts are minimised, to give significant improvement in S/N but also to provide enough lines to define accurately the powder lineshape allowing its simulation. Splitting the spectra into a series of narrow bands also has the advantage that low-intensity second sites can be observed, and even if baseline artefacts are not completely removed, functions can be used to correct any residual problems (Section 3.3.2) because the baseline can be observed.

3.4.2 Stepped experiments Several alternative approaches have been developed for recording broad spectral lines, based on the philosophy that although a line is broad it can be recorded stepwise using a series of narrow-banded experiments to overcome the distortions introduced into a broad spectrum. One of these approaches is to carry out a spin-echo experiment using relatively weak rf pulses, recording only the intensity of the on-resonance magnetisation and repeating the experiment at many frequencies to map out the lineshape. The system has to be retuned at each new frequency so that the lineshape is mapped out


137

point by point. This is an extremely laborious process, but the lineshapes are accurately recorded, as for high temperature superconductors and 91Zr in ZrO2 (Bastow and Smith 1992). The titanium signal from YzTi207, with a lineshape 250 kHz wide recorded by this method is shown in Figure 3.10, which also shows a pulse echo experiment for comparison. In the direct experiment, the singularities of the powder lineshape can be readily seen but bandwidth effects (excitation and response) result in the centre of the spectrum having much enhanced intensity compared to the edges, distorting the lineshape. Although these static experiments which step the frequency produce accurate lineshapes, they are relatively time inefficient. The time efficiency can be improved by slowly rotating the sample (--~ 1 rpm) to bring different crystallites into resonance. This approach, termed ROTISSERIE, has been shown to give large savings in the time necessary to produce 14N spectra from ceramics (Yesinowski and Hill 1999). The modulation of the echo amplitude produced by the rotation itself contains

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Ti shift in kHz Figure 3.10. Titanium NMR spectrum of Y2Ti207 where the resonance covers a frequency range of "~250 kHz comparing frequency stepping (bottom) with a spin-echo (top) and a simulation in the centre.

138


information about the interaction and has been used in an experiment termed STEAMER (Yesinwoski and Hill 1999). Another approach is to use low power, frequency selective pulses (Sindorf and Bartuska 1989) with phase-coherent sampling. A long (1 ms) pulse at --~10 mW acquires a single data point, with each point shifted by a frequency of 1 kHz. The spectrum is obtained directly without the need for Fourier transformation but this approach has not been widely applied. An alternative is to sweep the main magnetic field. There are several examples of solid spectra obtained in this way dating from the earliest days of NMR, but only a limited number of reports have used superconducting magnets. Much of the work to date has used relatively low-resolution magnets. Examples of studies made using superconducting magnets include work on model alumina-containing compounds (Xu et al. 1994), studies of the aluminium sites in catalyst-related materials and on high temperature ceramic superconductors. There is no doubt that this approach will work in all cases. It is possible for a single NMR spectrometer to be capable of both conventional high resolution spectroscopy and also field-sweep operation at relatively little extra cost. The field sweep in the 7.05 T instrument described by Poplett and Smith (1998) was limited to + 0.5 T, sufficient to cover many broad lines. Control of the field is completely automated and integrated with the pulse programme. As with the stepped-frequency experiment, relatively soft pulses are applied, and although strictly the on-resonance part of the magnetisation should be used, direct use of the spin-echo intensity accurately reproduces the lineshape. 27A1 spectra of ot-A1203 obtained by one pulse, spin-echo and field-sweep methods are shown in Figure 3.11. The intensity between the singularities in the field sweep spectrum is much higher and much more closely matches the theoretical expectation. There is also essentially no bandwidth limit on the field sweep experiment so that the outer singularities are easily recorded. The total frequency width of this lineshape is --~1.4 MHz but the approach can be applied to much broader lines, as demonstrated for 27A1 in A13Zr where the frequency width is well beyond 3 MHz (Figure 11.4A). A field-sweep approach could be extremely useful as an alternative method for examining low-y nuclei, where narrowing techniques do not yet offer widespread opportunities for improved resolution.

3.5. ONE-DIMENSIONAL HIGH RESOLUTION TECHNIQUES

3.5.1 Magic angle spinning (MAS) The most widely used technique for solid state NMR observation is MAS. The early design of MAS rotors by Andrew (1981) had a single bearing/drive surface over which compressed gas was forced (Figure 3.12A). This provided a very low friction gas bearing allowing rapid rotation. These rotors were typically --~ 1 cm diameter, and, using compressed air, could be spun to 3-5 kHz. On spinning at 5 kHz the peripheral


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kHz A1(1)1

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kI-Iz Figure 3.11. 27A1static NMR spectrum of oL-A1203 comparing A. One pulse spectrum, B. a spinecho with C. a field-stepped experiment; note the much wider frequency range of the field-stepped spectrum showing the outer singularities Al(1)3 (from Poplett and Smith (1998) with permission of the copyright owner).

acceleration is - 5 • 105g, so only very strong materials are employed. For inorganic solids these rotors were typically made of plastics such as Delrin and Torlon. Commercially available systems for rapid spinning have tended towards variations of the double bearing (DB) design (Doty and Ellis 1981, Figure 3.12B). A large range of diameters of rotor is available commercially (2.5 to 14 mm) with 1.8 mm diameter rotors currently providing MAS of - 50 kHz. The driving force is provided by compressed gas, usually air or nitrogen, forced through a series of nozzles on to vanes, usually on the rotor cap. Very careful design of the optimum nozzle diameter and angle of incidence on to the vanes is needed. The very tight clearance also requires accurate

140


Figure 3.12. MAS rotor design showing A. A single beating Andrew-Beams (Andrew 1981a) and B. a double bearing (Doty and Ellis 1981) with permission of the copyright owner.

machining of tough ceramics. For example, the optimum gap to the beating in a 12 mm rotor is 0.027 mm (Doty 1994). The nature of the gas supply is important, the minimum requirement being that it is dry and free from oil. Nitrogen boil-off is undoubtedly the best drive gas but is much more expensive than using compressed air for this purpose. The very high spinning rates create considerable pressures across the wall from the resulting centripetal force, and material strength is often the limiting factor for safe spinning speed. Careful note needs to be taken of the sample density as this will determine the fastest speed that can safely be used. Polymeric materials have now largely given way to advanced engineering ceramics such as Si3N4 and stabilised ZrO2. For many sequences it is often better to have a uniform rf field over the sample. This can be achieved by decreasing the volume occupied by the sample within the rotor, and spacers can be used. In CRAMPS experiments spacers with spherical cavities often give


141

the best results. Other more specialised designs have been suggested, some for spinning air-sensitive samples (Gay 1984, Lee et al. 1984, Carpenter 1986, Merwin et al. 1989). Even though air bearings are used, allowing cooling by the gas stream to occur, there has been considerable discussion of the magnitude of the frictional heating effects in such systems. In addition to frictional heating, rapid expansion of the gas through the nozzles in the stator brings about the Joule-Thompson effect. There is no doubt that for spectroscopy at accurately known temperature, careful calibration is required. Elements with large shift ranges (such as 2~ or paramagnetic compounds with a large Curie temperature dependence of shift such as 31p in (VO2)2P207(Pan and Gerstein 1991) and ~3C in samarium acetate (Haw 1988) are ideal for this purpose. Lead in PbNO3 has been widely used to determine the temperature in MAS experiments (Bielecki and Burum 1995, Mildner et al. 1995). The 119Sn signal from Sm2Sn207 is also very sensitive to temperature changes (van Moorsel et al. 1995, Grimmer et al. 1997, Langer et al. 1999). It is apparent that the temperature is a strong function of Yr. For 7 mm rotor the temperature rises by --~ 4 K as the spinning rate is increased from 2-5 kHz (Bielecki and Burum 1995) and for a 2.5 mm rotor spinning at 35 kHz the temperature rise is 30-40 K (Langer et al. 1999). Mildner et al. (1995) have modelled the temperature difference as 2

Tsample - Tinle t

--

3~2FlsR2 Vr

(3.17)

where "qs is the viscosity of the gas, R is the radius of the spinner and X is the thermal conductivity of the gas. In addition to the average rise there will also be temperature gradients which must be well characterised. The temperature rise can change the observed chemical shift tensor and the gradient can limit resolution (Brus 2000). To achieve efficient narrowing by MAS, the magic angle has to be accurately set and fine adjustments can be achieved by the quadrupole satellite technique (Frye and Maciel 1982). This technique is based on materials which contain quadrupole nuclei in a nominally cubic local environment, some of which are distorted, causing first-order quadrupole broadening of the non-central transitions at some sites. As the narrowing factor is ~(3COS2[~ -- 1) for a deviation of d[3, the residual broadening factor is 3cos[3sin~3d[3. Hence, the first-order quadrupolar broadening in a compound to be used to set the magic angle should not be too severe, to enable the spectrum to be narrowed into a visible set of sidebands over a limited but not too restrictive a range of angles about the magic value. Far away from the angle, only the sidebands from second-order broadening of the central transition are observable. Then, as the magic angle is approached, the sidebands from the satellite transitions are observed, and become progressively narrower. The bromine resonance of KBr is typically used for this purpose, but with increasing demand for more accurate setting of the magic angle, other compounds are now often used. Other commonly used angle references are 23Na in

142


NaNO2, and a favourite of the authors is 27A1 in Y3A15012, in which the A106 site with a small XQ value can be used for initial setting of the angle (as good as KBr and the carbonyl carbon in glycine). Fine adjustments are made using the A104 site with a much larger XQ. Figure 3.13A shows the changes in the 27A1 FID as the magic angle is approached. Initially, away from the magic angle, only a few sharp rotational echoes are seen. As the magic angle is approached these become sharper, and more importantly they can be seen at longer times. Typically with 24 scans, using a 4 mm rotor, these should be visible out to --~ 5 ms if the angle is well set. In the corresponding spectra (Figure 3.13B) as the correct angle is approached, the satellite transition sidebands become much sharper. For lower frequency probes that will not tune to the region of

q~

r

o~ A~ @ g~

.
> I)Q. The nutation spectra display distinct features in the region where vl --~VQ. Simulation of nutation behaviour has shown that the useful range of rf field-strengths lies with VQ/Vl in the range 0.05-1. The distinct features of nutation spectra allow the quadrupole parameters to be obtained. This means that only quadrupole frequencies up to a certain limit are accessible with currently available rf-fields of 500 kHz in specially dedicated probeheads, and in fields of around 100 kHz in most commercially available systems. This constraint especially limits the investigation of spin I = 3/2 nuclei such as 23Na. In cases where more than one resonance could be observed, one would like to increase the resolution in the F2-dimension by applying MAS. However, as was discussed previously, rotationally-induced spin-locked magnetisation can appear when an

154


rf-field is applied in conjunction with spinning. This process severely distorts the nutation spectrum as it gives rise to strong dispersive components in the 2D spectrum and a large signal at zero frequency. The spinning speed should therefore be kept low (2-4 kHz) so that the nutation signal has decayed within a quarter rotor period at most. It is possible however to simulate the nutation behaviour of quadrupole nuclei under the influence of MAS, from which the quadrupole parameters can be extracted (Nielsen et al. 1992). A 27A1 quadrupole nutation experiment on A1PO4-8 under fast MAS revealed higher resolution in the cross-section at 3v~f compared to that at Vrf. This was attributed to a smaller distribution of orientations contributing to the 3Vrf cross-section (Rocha et al. 1992).

3.6.2 Off-resonance nutation Off-resonance nutation has been developed (Kentgens 1993) to increase the upper limit of quadrupole couplings that are accessible by nutation NMR. In this experiment the nutation behaviour of the spins is monitored in the effective field rather than in the rffield, the effective field being the vector sum of the if-field and the resonance offset. This greatly increases the upper limit of accessible • values since the resonance offsets can be freely chosen and are limited only by the Q of the probe. For increased sensitivity and for the reduction of the zero frequency signal it is beneficial for the magnetisation to nutate perpendicularly to the effective field. Therefore, one can either apply a frequency-stepped adiabatic half passage (Veenedaal et al. 1998) or a soft 90 ~ pulse to bring the magnetisation into the xy-plane; the former option gives the best results. The if-phase is then shifted 90 ~ and the frequency is switched to the required resonance offset. The useful range of resonance offsets is between one to eight times v~, depending on the ratio VQ/V~. After a time t~ the signal is detected on resonance. The phase-coherent frequency switching (switching times of around 200 ns) of the if-field required in this experiment is within the capabilities of most modem spectrometers. An extensive review has dealt with many of the details of off-resonance nutation spectroscopy (Kentgens 1998). Whereas the on-resonance nutation spectra are amplitude-modulated and can be properly phased in the F1 dimension, the off-resonance nutation spectra are phasemodulated and can no longer be phased properly. Therefore, a magnitude calculation is performed after Fourier transformation. Furthermore, because the magnetisation evolves during t~ around the effective field, the sine and cosine components of the modulation are different and the amplitudes of positive and negative F1 signals are unequal. Figure 3.20 shows an example of the strength of the off-resonance nutation technique applied to various samples of ~-alumina impregnated with phosphorus and/or molybdenum. The nutation spectrum of the tetrahedral resonance is very similar to the nutation spectrum of "7-A1203. The octahedral resonance however shows a nutation

155


A

B

-400

C

0

400

Nutationfrequency(kHz) j

~~~~~

|

-400

|

i

'l

!

0

j

u

i

!

400

Nutationfrequency(kHz) Figure3.20. Off-resonance nutation experiments on Mo(12)p(2)/~/-A1203 with an rf field strength of 41 kHz. The normal 1D MAS spectrum A. is shown and the nutation spectra of the tetrahedral B. and octahedral region C. with the tetrahedral region made up of only ~/-A1203 but the octahedral region requiring a superposition of ~/-A1203 and Alz(MoO4)3 from Krauss et. al. (1996) with permission of the American Chemical Society.

spectrum that can only be simulated using two subspectra, one the nutation spectrum of ~/-A1203 and the second with quadrupole parameters similar to those of A12(MoO4)3. This led to the conclusion that impregnation of ~/-A1203 with molybdenum results in an interaction with the 7-A1203 surface to form A12(MoO4)3 (Kraus et al. 1996).

3.6.3 Order-resolved sideband spectra Techniques have been developed for modulating the phase of sidebands according to their order through a series of pulses. These 2D experiments separate the centreband and sidebands in the second dimension even if they strongly overlap in the normal 1D spectrum. In these sequences, sidebands can often be summed by using either hopping

156


or synchronised acquisition. Combining the individual slices then produces in the isotropic dimension effectively an infinite spinning speed spectrum. Such approaches are especially useful for systems where the fastest spinning speeds cannot really narrow the spectra. Examples include chemical shift distribution of heavy elements such as l l9Sn and 2~ in glasses. There are also nuclei which show considerable second-order quadrupolar broadening of the central transition (e.g. 69'71Ga). In the case of spin-l/2 nuclei, approaches to separate the sidebands include magic angle turning or MAT (Gan 1992, Hu et al. 1994), and phase-adjusted spinning sidebands or PASS (Antzutkin et al. 1995). In these sequences the rotor period is split up by a series of pulses. The timing of the pulses is varied and the different sideband components pick up differing phases depending on their order. MAT forms a tl-modulation of information which contains the infinite spinning speed spectrum. PASS has 6 periods during the rotation time, creating a pitch (i.e. phase) of the magnetisation that scales with sideband order. PASS has the advantage over MAT in that it minimises the number of t~ experiments necessary to fully describe and hence separate the sidebands. Modification of PASS with a shifted-echo (Grandinetti et al. 1993) produces an isotropicanisotropic correlation experiment yielding an isotropic spectrum free from spinning sidebands (Fayon et al. 1999). For quadrupolar nuclei a sequence has been developed called QPASS (Massiot et al. 1997) with nine 180 ~ pulses. Usually the magnetisation of the central transition is manipulated by this sequence. One of the most important practical considerations of this approach is the requirement for highly stable spinning. In practice, the solution for sorting the sidebands is unaffected by an increased delay prior to forming the echo by integer multiples of the spinning speed, commonly done to free the echo formation from deadtime effects. The pulses can constitute a significant fraction of the rotor period, demanding the use of quite high power. This becomes even more of a problem with the use of higher spinning speeds with this sequence. Recent work has shown that the order sorting can be carried out over more than one rotor period, and experiments using two and four periods have been published (Vogt et al. 2000). This alleviates the more stringent power requirements and allows the irradiation profile of the pulses to be improved by the use of composite pulses (Vogt et al. 2000).

3.6.4 Dynamic angle spinning (DAS) In DAS the spinner axis is changed sequentially using a stepper motor and a pulley system (Figure 3.21) to produce the more complex time variation of the spinner axis and remove second-order quadrupole effects. The angle switching must be as fast as possible and reproducible. The complete stator is moved by a stepper motor between 37.38 ~ and 79.19 ~ in 36 ms, triggered by pulses from the pulse programmer. The procedure is to set the conventional magic angle, and then, knowing the number of steps


157

Figure 3.21. Design of a DAS probehead from Eastman et al. (1992) with permission of the copyright owner. required by the stepper motor the other angles can be reached, with motor resolution of--~ 0.16 ~ It should be noted that the 90 ~ pulses will depend on the angle from the quadrupolar influences and the orientation of the coil in the magnetic field. The rf coil can be fixed to the rotor, thus changing its orientation as the angle is switched. This gives a good filling factor, but the rf field will vary with the angle and the rf leads must be flexible and resistant to metal fatigue. The coil may also surround the whole assembly and remain static. In this case the filling factor will be poorer but pulse lengths will remain relatively constant, and most importantly, cross-polarisation at an angle of 0 ~ with the static field is now possible, by contrast with the other design. Appropriate phase cycling and processing allows a pure absorption spectrum to be obtained. A limitation of the DAS technique is that it cannot be used for compounds with a short T a because of the time needed to reorient the spinning axis. Furthermore, the experiment requires a dedicated probe of sufficient reliability to be able to mechanically flip accurately between the pair of angles for thousands of transitions. Detailed calculations of the spinning sidebands in DAS spectra have been carried out using average Hamiltonian and irreducible tensor approaches (Sun et al. 1992). In DAS spectra the sideband intensities and their moments depend on the relative rotor phase between the two evolution periods. The sideband intensities additionally depend on the ratio of the time spent at each angle. The 2D 170 DAS spectrum of zeolite Sil-Y (Figure 3.22) shows three lines in the ratio 2:1:1 (Bull et al. 1998). Simulation of the anisotropic slices from the 170 2D DAS spectrum for each peak allows extraction of XQ and xl for each resonance.

3.6.5 Two-dimensional sequences developed from solution NMR Two dimensional sequences are commonly used in solution state NMR spectroscopy to elucidate connectivity of atoms within molecular structures. Sequences are available for heteronuclear shift correlation, homonuclear correlation (COSY and INADEQUATE) and other sequences for longer-range connectivities. In general the basis of these

158


Figure 3.22. Full 2D 170 DAS spectrum of siliceous-Y zeolite (Bull et al. 1998) with the isotropic projection showing the increased resolution and the individual slices providing the anisotropic interaction, with permission of the American Chemical Society. 2D techniques is that transverse magnetisation is created, and evolution is permitted under some of the coupling(s) present. The phase of the magnetisation at the end of the evolution period will depend on the coupling and the time. Thus, if spectra are accumulated for different evolution times, an FT gives the connectivity information in the F1 dimension. In principle, solution techniques can be carried over directly to solids. The criterion for their success is that the magnetisation persists for sufficiently long times that the weak interactions will cause modulation of the magnetisation. Standard 2D solution sequences can be combined with line-narrowing methods such as MAS and decoupling to lengthen T2. The transverse magnetisation can also be created by CP rather than directly from a 90 ~ pulse. COSY has proved very useful in examining the silicon connectivity in aluminosilicate frameworks such as zeolites and minerals, and has been used with 3~p to study organometallic complexes. A COSY experiment on a 29Si-enriched sample of Na2Si409 glass showed small off-diagonal cross-peaks revealing the connectivity of the Q3_Q4 units present (Knight et al. 1990). Heteronuclear correlation is now being used more widely in 13C-~H work on solids since the development of more robust sequences. The great advantage of this sequence is that 1H information is much more spread out by the ~3C shift dispersion than in its own shift dimension. The presence of anisotropic interactions in solids leads to a greater variety of 2D sequences. One of the simplest is the isotropic/anisotropic correlation, where high resolution is achieved in one dimension while preserving anisotropic information in the second dimension. The dipshift and J-resolved experiments correlate isotropic chemical shift with dipolar and J-coupling information respectively. In these sequences the

159


evolution period remains fixed and an integer number of rotor periods is used with decoupling divided between multiple-pulse and high power decoupling. The latter removes all coupling while the former eliminates only the coupling between the protons, allowing the direct X-1H coupling to remain. The time of multiple-pulse coupling is then incremented at the expense of high power decoupling. The F1 dimension can be simulated to extract coupling information to distinguish CH, CH2 and CH3 groups. The multiple-pulse decoupling is as described in Section 2.4.2, but since the sequence does not require observation to be made, more efficient sequences such as semi-windowless MREV-8 and completely windowless BLEW-12 can be used. 2D sequences can also be used to study various exchange processes. The magnetisation produced is labelled by chemical shift and stored so that exchange can occur before it is sampled. Where exchange occurs and a nucleus moves to a chemically distinct site, the nuclear magnetisation is now labelled with a different shift value (i.e. frequency) and appears as a cross-peak in the 2D plot. The same is true for spin-diffusion and has been used with 1H to study domain sizing in polymers. These sequences are largely based on the ideas of Goldman and Shen (1966) in which domains are labelled according to T2 prior to the start of spin diffusion. Spiess, Schmidt-Rohr and co-workers have developed a series of pulse sequences for 2D and more recently 13C, to study slow motions by the modulation of anisotropic contributions of the NMR interactions since these depend on orientation. As the molecular orientation changes, the resonant frequency also changes, resulting in off-diagonal intensity. For simple motions, the off-diagonal intensity is in the form of ellipses whose axial ratio (b/a) equals Itan oLI where oLis the angle of the jump motion (Schmidt-Rohr and Spiess 1994). These are just some examples of the burgeoning field of 2D applications to solids. Other references are given in Table 3.4. In order to provide an even greater spread of information, three-dimensional sequences have now been developed which are essentially combinations of 2D sequences. Their rapid development has arisen through

Table 3.4. Some 2D pulse sequences that have been applied to solids. Sequence COSY INADEQUATE Chemical exchange Spin diffusion HETCOR DIPSHIFT

Purpose

Reference

Fyfe et al. (1989), Kohn et al. (1991), Hanna et al. (1992) Benn et al. (1988) Homo/heteronuclear connectivity when coupling is weak Szeverenyi et al. (1982) Physical exchange of groups Homonuclear connectivity

Domain sizing e.g. polymers, connectivity Heteronuclear correlation (e.g. 13C-1H) Separated local fields to determine dipolar/J-coupling interaction at sites.

Caravatti et al. (1983), (1982) Burum & Bielecki (1991) e.g. Webb & Zilm (1989), Jakobsen et al. (1982)

160


Table 3.4. (Continued) Sequence NOESY SUPERCOSY VACSY J-resolved J-scaled COSY INEPT TOCSY

Reference

Purpose Local spin exchange (physical and diffusion) Longer-range connectivity Correlation of anisotropy with isotropic shift with fast sample spinning. Correlate isotropic shift with 1H-X coupling Connectivity Connectivity through J coupling Through-bond connectivity

Szeverenyi et al. (1982), Twyman & Dobson (1990) Kolodziejski & Klinowski (1992) Bax et al. (1983), Yang et al. (1990), Lee et al. (1994) Kolodziejski & Klinowski (1992) Kolodziejski et al. (1991) Fyfe et al. 1995, Kao & Grey (1998) Hartmann et al. (2000)

their application to biological solids in which an understanding of connectivity information is important.

3.6.6 Multiple quantum experiments in dipolar coupled systems Although single quantum coherence is the only magnetisation that can be directly observed, for a collection of N strongly dipolar coupled spins, it is possible to induce multiple quantum coherences. A number of pulse sequences have been developed for this purpose (Gleason 1993), for which one of the most common generators is 1

< H'a >= - - 2-

"~'~'i<jDij(l+il+j + l - i l - j ) '

(3.21)

where Dij is the dipolar coupling between spins i and j. This is achieved by applying an eight-pulse sequence of four x-pulses followed by f o u r - x pulses with spacings of A and A', where 3,' = 2A + Tp. Multiple quantum experiments are 2D in nature, the preparation time creating the multiple quantum coherence by an integer number of eight pulse cycles. Evolution can then be permitted, but this is usually set to zero if the interest is only in the number of spins in a cluster. The mixing period is the same number of cycles but time-reversed (effectively with y and - y pulses) so that longitudinal magnetisation is recreated, with the change of intensity reflecting the creation of a multiple-quantum coherence. This is sampled by applying a 90 ~ pulse after a short delay to allow transients to decay. The preparation time can be incremented, but the experiment is commonly performed by keeping the time constant and changing the phase of the preparation sequence using the TPPI principle (Ernst et al. 1988). The different coherences are then distinguished by the fact that for a phase increment A+ an n-quantum coherence will pick up a phase change nA+. Hence, by performing the experiment as


161

a function of +, the signal intensity will be S(+) (if'r is a constant at an optimum value). Fourier transforming with respect to + then gives an intensity I(n), i.e. a function of the coherence order, which from the Nyquist criterion means that Inl < ~r/~+. As in multiple-pulse line-narrowing experiments, these experiments require high stability over long pulse trains with the pulses accurately set for both intensity and phase. It is also necessary to be able to set small phase increments A+ < 2.8 ~ Experiments have been developed to create double quantum (DQ) dipolar coherences under MAS. One of the most useful applications is the elucidation of cluster sizes of protons in materials such as the semiconductor a-Si:H. The nature of the experiment requires strong homonuclear coupling and high sensitivity. The intermediate range order in materials can be very difficult to characterise accurately. In silicate and phosphate glasses, 1D MAS NMR gives an accurate picture of the number of different SiO4 and PO4 tetrahedra which are distinguished by their connectivity (i.e. Qn-type discussed below in Chapters 4 and 7). However, these data do not reveal how the units are joined together to produce the intermediate-range order, neither do they show the pair connectivity of the structure. Phosphates are ideal for study since 31p is 100 percent naturally abundant and has a substantial dipolar coupling which provides the mutual interaction to convey the information. In the 2D DQ data set there will be cross-peaks if there is connectivity. The DQ effectively reintroduces dipolar coupling even under MAS. In the 2D data set, the peaks in the DQ (F1) dimension appear at the sum frequency of the two peaks which are coupled. This type of experiment can readily be extended to glassy silicates and phosphates with greater intrinsic linewidths.

3.6.7 Multiple quantum NMR experiments of non-integer spin quadrupolar nuclei Numerous schemes exist to obtain 2D MQMAS spectra. The simplest form of the experiment (Figure 3.23A) is the excitation of the MQ transition by a single, high power if-pulse, after which the MQ-cohercnce is allowed to evolve for a time h. After the evolution time, a second pulse is applied which converts the MQ coherence into a p = - 1 coherence, observed during t2. The signal is then acquired immediately after the second pulse and the echo will form at a time t2 -- IQAI.h, where QA (Quadrupolc Anisotropy) - C 4 ( p ) / C 4 ( 1 ) , with the Cs given in Table 2.8. The information about the MQ coherence is conveyed by the way it modulates the observed echo, either via the amplitude or phase. The modulation depends on the coherence pathway employed. In amplitude modulation, pathways are present such that the signal formed will always be in a particular mode, usually chosen to be absorption. Then, as t~ is varied the lineshape remains constant, changing only in intensity (amplitude modulation). Alternatively, the selected pathways result in the weighting between the different components of the magnetisation varying with t~ and changing the lineshapc between absorption and

162


3

po-! -2

Acquisition

-3

n

3 1 .2

Acquisition

-3

C '~

3

p0

~ ~j

-t -2

Acquisition

-3

D

3 p0

Acquisition

-1 -2 -3

Figure 3.23. Examples of MQ-MAS pulse sequences that are commonly used A. two pulse, B. z-filter, C. split-t~ and D. RIACT(II). dispersion (phase modulation). Both pulses are non-selective and will excite all coherences to a varying degree. After a 2D Fourier transformation the resonances will appear up as ridges lying along the QA-axis. The isotropic spectrum can be obtained by projection of the entire 2D spectrum on a line through the origin (Vl = v2 = 0) perpendicular to the QA-axis. Figure 3.23B shows one of the MQ pulse sequences commonly used, which includes a z-filter pulse (selective, low if-power) and has the advantage of a symmetrical coherence transfer pathway (Amoureux et al. 1996). Other sequences have used 180 ~ pulses to obtain pure absorption lineshapes (Vosegaard et al. 2000). Figure 3.23C shows a split-t~ experiment (Brown and Wimperis 1997, 1997a) in which the isotropic spectrum is directly obtained without shearing of the data (see below). The disadvantage of the split-t~ experiment is that the echo formation must be dominated by the quadrupole interaction. In practice split-t~ experiments give good lineshapes. Figure 3.23D shows the RIACT(II) sequence (Wu et al. 1996) in which excitation and conversion of the coherences is achieved by a spin-lock of duration Tr/4 instead of a hard pulse, having the advantage of more uniform excitation and conversion which is essential to obtain quantitative information. Significant analytical and


163

experimental work comparing the efficiency of various schemes has been carried out (e.g. Amoureux et al. 1996, Amoureux and Fernandez 1998, Pruski et al. 2000). In an MQ experiment the overall efficiency depends on the initial excitation of the MQ coherence and its conversion back to observable magnetisation. Using density matrix methods, Goldbourt et al. (2000) calculated what they termed the echo efficiency parameter, and also examined the effect of the different schemes on the anisotropic lineshape. This lineshape can itself impose very important constraints on the quadrupole interaction parameters. Direct excitation, RIACT, double frequency sweeps and fast amplitude modulation were compared. This work concluded that fast amplitude modulation gave superior conversion efficiency. Although it is not the most sensitive approach and causes more lineshape distortion than some other methods, RIACT has a very weak dependence on XQ and so produces more reliable site distribution information (Lim and Grey 1998). Generation of 3Q and 5Q for I = 5/2 nuclei by RIACT has been compared (Mildner et al. 1999, 1999a) and showed a broader response to XQ than normal pulsed experiments. On balance, the majority of MQ NMR reported in the literature is obtained using hard pulses. This has implications for the choice of probe. For typical XQ values, the operating conditions are shown to the right of the maximum in Figure 2.27. Hence better sensitivity can be achieved by using higher if-fields. This means that smaller coils can be used which have the advantage of providing faster MAS rates, while their high Vr reduces the effect of sidebands in the MQ direction (Amoureux et al. 1998). However, the reduction of sample eventually outweighs these advantages, making a compromise necessary. Depending on the sensitivity of the nucleus being examined, probes of 2.5 to 4 mm diameter are ideal for MQ MAS. There is no doubt that higher order coherences become more difficult to generate, decreasing the sensitivity of the experiment and making it a much more important consideration. In principle, MQ could be combined with spinning at other angles (Duer and Stourton 1997). The two methods usually employed for the phase sensitive detection in the F1 dimension both depend on amplitude modulation. A straightforward TPPI can be incorporated in the pulse sequence where the phase of the first pulse is incremented by 360/4p degrees for each tl value. This means that each tl value contains alternately real (in phase) and imaginary (out of phase) signals (Marian and Wutrich 1983). The effective spectral width in the F1 dimension must be halved in this case. The other method is the hypercomplex States TPPI (States et al. 1982) where for each t~ value a real and imaginary data point is obtained. In this case the effective spectral width in the F1 dimension is still the inverse of the tl dwell time. The difference between the two methods does not seem large. However, if rotor-synchronisation in the F1 dimension is required, the difference in effective dwell time reduces the maximum spectral width. For example, with a rotor speed of say 20 kHz, rotor synchronisation (Massiot 1996) demands a dwell time of tl = Tr requiring a spectral width of 2/Tr, (10 kHz) for TPPI

164


but 1]Tr (20 kHz) for the States TPPI method. As the former might be too small, the latter is clearly preferred. The first report of MQ from non-integer spin quadrupole nuclei appeared in 1995, generating much subsequent research activity and development of a number of schemes for processing and presenting the data. The relationship between the measured peak positions and the NMR interaction parameters depends crucially on the processing and referencing conventions adopted. There are two main approaches. In one, the MQ evolution is regarded as having taken place only in the evolution time (tl). This is the convention adopted for example by Medek et al. (1995) and Hanaya and Harris (1997). In the other approach, the period up to the echo is regarded as also being part of the evolution time which is then (1 + QA)tl, as adopted by Massiot et al. (1996) and Wang et al. (1997). A detailed critique of these two approaches and the consequences of each has recently been given by Man (1998). The isotropic shift and the quadrupole induced shift (QIS = Co(p)/Co(1), with Cs defined in Table 2.4) can readily be obtained from the data. Firstly, the 2D spectrum must be referenced correctly. To obtain the correct ppm scale in the F1 dimension one must set the spectrometer frequency in the F1 dimension to pro. The shift reference is most readily set at the carrier frequency, i.e. the shift in ppm at the carrier frequency in the multiple quantum dimension F1 is the same as the shift in ppm at the carrier frequency in the single quantum dimension F2. This is valid for data obtained with or without delayed acquisition and processed with or without shearing. The isotropic chemical shift is the same for both the single quantum and the multiple quantum dimensions. The QIS is different in both dimensions, however, and is given by vQ x 106 (in ppm) (3.22) p. v o The position of the centre of gravity ~ g with p = 1 for the single quantum dimension and p = Am for the multiple quantum dimension, is given by (~qPs -- C~

(3.23)

(~Pcg= (~cs,iso + (~qPs

Hence the isotropic shift can readily be retrieved using 6cs,iso - C~

- P'C~ Co(p)-p'Co(1)

(in ppm)

(3.24)

and the isotropic quadrupolar shift is

v0 =

' v 0 (in Hz) Co(1)-Co(p) / p

(3.25)

165


A graphical method to obtain the isotropic shift and QIS from unsheared 2D spectra can give a quick assessment of these parameters. A line is drawn through the estimated centre of gravity with a slope equal to the QIS axis crossing the isotropic chemical shift line, g(F1) - g(F2). The intersection of these two lines gives the isotropic shift, and the difference in the shift between the centre of gravity and the isotropic chemical shift gives QIS for both transitions. Note that the slope of the QIS axis is in Hz per Hz and must be adjusted to the ppm scale, i.e. it is less steep by a factor of 1/p. The key to determining the quadrupole parameters is the accurate location of the position of the centre of gravity. Both the excitation efficiency as well as the presence of significant intensity in the spinning sidebands can adversely affect this accuracy. It is therefore advisable to obtain the quadrupole parameters by simulation of the 1D MAS spectrum, using the constraints on • and ~q dictated by QIS, and the isotropic chemical shift as starting parameters. The inhomogeneously-broadened line in the unsheared data will be directed along v l = kv2. The isotropic shifts due to chemical shift and second-order quadrupolar effects are directed along different directions, each different from the inhomogeneous broadening direction. Pike et al. (2000) have considered the resolution of the MQ spectra and have shown that the chemical shift-like terms (i.e. isotropic chemical shift and offset frequency) are scaled by a factor SFcs where P+k Sfcs(i,p ) - - ~

(3.26)

l+lkl

The isotropic second-order quadrupolar shift SFQs(I,p) can be derived directly from SFcs using a factor of -(10/17) (Equation 14 in Pike et al. 2000). The chemical shift scale factors are given in Table 3.5. Table 3.5 implies that for I = 5/2, the 5Q transition should give better resolution than the 3Q transition. Some samples show better resolution at higher coherence order Table 3.5. Chemical shift scale factors for different spins and coherence orders and the relative factor compared to 3Q for the spin. I 3/2 5/2 5/2 7/2 7/2 7/2 9/2 9/2 9/2 9/2

P

SFcs(I,p)

ISFcsl relative to 3Q

- 3 3 - 5 3 5 - 7 3 5 7 - 9

17/8 - 17/31 85/37 - 17/73 - 17/10 238/103 - 17/127 - 85/131 - 119/25 85/37

1 1 155/37 1 73/10 1022/103 1 635/131 889/25 635/37

166

Multinuclear Solid-State N M R of Inorganic Materials F2

E1

4 0

300

200

Hz

F1

C

kHz

L

Q

F1

-

i

6

!

4

'

'

-i

!"

3

2

kHz

kHz Figure 3.24. Improved resolution achieved comparing A. 3Q, B. 5Q and C. 7Q experiments of 45Sc in Sc2(SO4)3 from Pike et al. (2000) with permission of the copyright owner.

(Figure 3.24, Pike et al. 2000), with exceptional gain found for some samples (Rocha et al. 1996). The gain in resolution depends on both the dataset and also the relative contribution of homogeneous and inhomogeneous interactions. A key point made by Pike et al. (2000) is that the inherent resolution is often not the problem, but poor S/N which can truncate the data in the t~ dimension, causing broadening (Ernst et al. 1998, Callaghan 1991). Decoupling has also been applied in conjunction with MQ MAS (Hanaya and Harris 1997, Lacassagne et al. 1998). In the MQ direction the dipole interaction becomes magnified and provides distance information (Duer 1997). Shearing of the data is carried out to obtain isotropic spectra in the F1 dimension and to facilitate easy extraction of the 1D slices for different peaks. Shearing is a projection of points lying on a line of slope equal to the anisotropy axis, on to a line parallel to the F2 axis. The lines intersect at the F2 zero frequency (i.e. the carrier frequency)

167


line. In effect, this means that a point that had a frequency of (1)1,1)2) will now lie at frequency (1)1 - kv2, 1)2), with k equal to QA (see C4(p), C4(1) from Table 2.4). If the absolute frequencies are taken relative to the carrier frequency in the F1 and F2 dimension: 7

V1 -- Co(P)V Q - --~C4 (p)vQ (O,d/)) + p(Viso + Voff )

(3.27)

and v : - Co(1)VoQ -

7

c4(1)vQ4 (o,0)+ Veto+ VoH

(3.28)

then the sheared frequency Vl' = Vl - kv2 in F1 becomes V; -- [C 0 ( p ) - kC 0 ( l ) ] v Q -k- [p - k](Vis o nt- Voff )

(in Hz)

(3.29)

The scale of the Fl-axis remains unaltered and the frequency of the isotropic peaks can readily be obtained. Converting the Fl-axis from frequency units to ppm, such that 1 ppm equals [p - k]vo Hz, will give the peak position C O(p) - kC o (1) vQ (in ppm)

- 'Lo +

(3.30)

[ p - k]Vo

when referenced correctly (i.e. ~ at the carrier frequency in F1 is ~ at the carrier frequency in F2). Shearing can be achieved in two ways. After the 2D Fourier transformation, the position of the data points is recalculated to obtain the sheared spectrum. The disadvantage of this method is the need for interpolation. Alternatively, prior to Fourier transformation with respect to t l, first-order phasing is applied to the data in the t l dimension by multiplying all points by e x p ( - 27rivztlk), where v2 is the frequency from the carrier frequency in F2. Shearing essentially achieves the same as the split-t1 experiment or delayed acquisition of the echo. Although sheared spectra may look more attractive, they do not add extra information and are not necessary for the extraction of the isotropic quadrupole and chemical shifts. Moreover, shearing introduces an extra processing step, which may introduce artefacts. The centre of gravity of the lineshapes in the two dimensions F1 and F2, designated ~1 and ~2, can be related to the isotropic chemical shift (~cs,iso) and second-order quadrupole isotropic shifts (~Q,iso(2)). 9 e(2)

~1,2 -- a(~cs,iso + D~

(3.31)

168


Table 3.6. The relation of the positions in the F1 and F2 dimensions for unsheared and sheared data sets for different MQ transitions. P

3/2

3

5/2

3 5

gl,2

g~ g2 gl ~2 ~ g2

Unsheared data Sheared and split-tl data a

b

a

b

3 1 3

6/5 -2/5 -4/5

17/8 1 17/31

1

-16/15

1

5 1

20/3 -16/15

85/37 1

1/2 -2/5 32]93 -16/15 160/111 -16/15

The coefficients a and b will differ for unsheared and sheared data (Table 3.6)

3.6.8 2D X Y correlation methods Heteronuclear XY correlation experiments are very useful for determining connectivities between different nuclei when many resonances are present. To measure a 2D heteronuclear correlation (2D HETCOR) spectrum, magnetisation must be transferred between the heteronuclei. This can be achieved by CP (Section 3.8.1) or TEDOR (Section 3.8.2). A variable time t~ is inserted prior to the magnetisation transfer from, say, spin I to spin S. By measuring the FID of spin S as a function of the time tl the S-spin FID is modulated by the signal intensity of spin I at tl. A 2D FT will give the IS 2D correlation spectrum. Phase sensitive detection in F1 can be achieved, for instance, by TPPI. The main difference between 2D HETCOR (which involves only spin-l/2 nuclei) and cases involving quadrupolar nuclei, is the attention that must be given to the transfer of the magnetisation. Overlapping lines in the 1D spectrum of the quadrupolar nucleus can be resolved in the 2D correlation spectrum. However, this does not always occur because of the second-order quadrupole broadening which can hinder the assignment of the correlation peaks. One would therefore like to incorporate a line-narrowing method such as DOR, DAS or MQMAS into the 2D HETCOR experiment. DAS has been used to obtain a high resolution 2D HETCOR spectrum of sodium trimetaphosphate (Jarvie et al. 1995). This experiment involved spinning at two different angles (79.19 ~ and 37.38 ~ during tl such that the isotropic echo is formed at the end of the t~ evolution period. The magnetisation was stored along the z-axis and then transferred from 23Na to 31p at an angle of 0 ~ the 31p signal being detected at the magic angle. Three reorientations of the spinner axis were used during the experiment, which is not suitable for compounds with short T~ values. However, the number of reorientations during the experiment can be reduced to only two by selecting a different set of DAS complementary angles, in which case 01 = 63.43 ~ and 02--0 ~ eliminating the need for an extra reorientation for the CP sequence.

169


Another possibility is to incorporate the MQMAS experiment into the 2D HETCOR sequence (Wang et al. 1997). In this experiment one performs a split-t1 MQMAS experiment during the tl evolution period, such that at the start of the magnetisation transfer the isotropic echo is formed. Figure 3.25 shows the results of a MQMAS/ HETCOR experiment on sodium trimetaphosphate, in which the resolution in the 23Na dimension was greatly increased compared to the straightforward HETCOR experiment. This sequence has been improved by changing the position in the sequence at which CP occurs (Steuernagel 1998). It should be noted that in both types of experiment the results are not quantitative. A

-12.5 ~ . . . . . . . . . . . . .

o

I

-lO -2o ppm from solid NaCI

-17.5

- 2 0 ~ . ~ ~ -22.5

~'~

31p

0 -10 -20 ppm from solid NaCI Figure 3.25. 23Na-31p2D HETCOR spectra of Na3P309A. Obtained with the normal HETCOR pulse sequence and B. with MQ-MAS incorporated in the pulse sequence to obtain higher resolution from Wang, De Paul and Bull (1997) with permission of the copyright owner.

170


3.6.9 Correlation o f t e n s o r i n f o r m a t i o n - s e p a r a t e d local field experiments

Valuable structural information is contained in the orientation and magnitude of tonsorial interactions that can be measured in NMR spectra. Determination of the principal components of such interaction tensors by simulation of the normal 1D NMR spectra is reasonably straightforward. However, it is much more difficult to relate the principal components of these interaction tensors to the molecular frame. By examining two interaction tcnsors (e.g. the dipolar tensor and the chemical shift tensor), separated local field experiments (Hestcr et al. 1976, Linder, H6hener and Ernst 1980) have the potential to relate the tensor orientation to the molecular frame. It is necessary for the orientation of one of the tensor interactions to be known in the molecular frame. Since the dipolar tensor is axially symmetric and usually collinear with the internuclear vector, measuring the relative orientation of the CSA tensor provides the possibility of relating the CSA tensor to the molecular frame. Various experiments have been developed for dipole and CSA tensor correlation in samples under either static conditions (Wu et al. 1994) or MAS (Munowitz et al. 1981, Munowitz and Griffin 1982). The success of this approach is illustrated by the numerous publications using these separated local field experiments to obtain the CSA tensor orientation for the structural characterisation of various materials (Nakai et al. 1988, Schmidt-Rohr et al. 1993). Most separated local field experiments concentrate on the CSA. In spectra of halfinteger nuclei with I > 1/2, the CSA is often much less important than the secondorder quadrupole interaction. Static or MAS spectra display a typical quadrupolc powder pattern from which the quadrupolc coupling constant and the asymmetry parameter can be determined. These quadrupolc parameters can then be used to determine the principal components of the electric field gradient tensor, which arc related to the local environment of the quadrupole nucleus. Knowledge of the local geometry is extremely valuable, and correlation of the quadrupole and dipole information of OH groups provides useful insights into structure. Lindcr et al. (1980) have indicated the possibility of correlating the quadrupole and dipole tensor by quadrupolc separated local field experiments on powder samples. This has been applied to the 1H-170 system in Mg(OH)2 and Mg(OH)• (van Eck and Smith 1998). Lce-Goldburg dccoupling during the dipolar evolution provides better resolution in the dipolar dimension. The normal chemical shift dimension produces the second-order quadrupolar lincshape and the dipolar dimension allows the dipolar interaction to be calculated. The 2D data contain more information since the intensity distribution depends critically on the relative orientation of the two tcnsors. Since the dipolar interaction is axially symmetric, only the azimuthal and polar angles are necessary to describe the relative orientation, and the intensity distribution changes markedly with this orientation (Figure 3.26). MQ can be used in correlation experiments, allowing the orientation of the quadrupole and dipolar tcnsors to be deduced (Duer and Painter 1999). A modified version of an MQ-MAS experiment has been used to cross-correlate quadrupolc interactions at different sites (Dowellet al. 2001).

171


20 .

.

.

......

.

I I

12

o

0

r

800

.

.

.

.

.

.

.

,

400 0 -400 -800 C h e m i c a l shift ( p p m )

20 16 12

1 0

800

460

6

-400

-800

C h e m i c a l shift ( p p m )

Figure 3.26. A quadrupole-dipole separated local field experiment for the OH group in Mg(OH)x(OCH3)z-x with A. the experimental data compared with B. the simulation of the intensity assuming the tensors are collinear from van Eck and Smith (1998) with permission of the copyright owner.

When anisotropic interactions are present, the angle can be changed during an experiment such as DAS to scale or recouple these interactions without removing them, providing a two-dimensional data set (Frydman et al. 1992). Such an approach for 29Si in a silicate glass has provided an isotropic/anisotropic correlation giving a large improvement in the quantification of the different Qn species (Zhang et al. 1996). An experiment has also been developed to enable the correlation of the quadrupole and CSA tensors by using switched angle spinning (Shore et al. 1996). A 2D correlation experiment using MAS and off-MAS has been applied to a study of phosphates, 23Na in crystalline NazSO3 and 11B in a potassium borate glass (Hartmann et al. 1999). In a glass with

172


a high potassium content, the borate network was sufficiently broken up that there was effectively little correlation between the orientation of the BO3 units present. This type of experiment has been used to correlate the quadrupole interaction (Joo et al. 2000).

3.7. SUMMARY OF APPROACHES FOR EXAMINING QUADRUPOLE NUCLEI

There now exists a large range of techniques for observing quadrupole nuclei. A combination of techniques must be used to deduce the NMR interactions (specifically the quadrupole interaction) and the distribution of sites. The situation is evolving rapidly (cf. Smith 1993), the current possibilities being: (i) (ii) (iii) (iv) (v)

One pulse static, One pulse MAS, DOR, MQ or DAS, Nutation.

The improved resolution afforded by DOR, MQ and DAS by removing second-order effects can be used to reveal the number of sites. The field dependence of the isotropic position of the high resolution data allows 8iso,cs and PQ to be deduced. To constrain the data, DOR and MQ can be combined (Anup~ld et al. 1998). Very extensive data sets have been collected using MQ and DOR on single compounds, and the combination of this with one dimensional data provides a powerful approach, as demonstrated for 23Na (Engelhardt et al. 1999), 27A1 (Dupree and Smith 2001) and 170 (Section 6.2.2, Figure 6.7). The quality of MAS spectra has continued to improve with the availability of higher magnetic fields. When nuclei such as 23Na, 27A1 and 170 can be observed at 1 GHz, direct high resolution MAS of such quadrupole nuclei will be possible.

3.8. MULTIPLE RESONANCE

Many multiple resonance methods such as cross-polarisation (CP), Rotational Echo Double Resonance (REDOR), and Transferred Echo Double Resonance (TEDOR) are available. Their application can be differentiated between systems involving exclusively spin-l/2 nuclei and those containing quadrupolar nuclei. The presence of quadrupolar nuclei may prohibit straightforward application of these experiments and can affect the interpretation of the results. There are, however, benefits arising from the presence of quadrupolar nuclei, as illustrated by the Transfer of Populations by Double Resonance (TRAPDOR) experiment which will only work for quadrupolar nuclei.


173

3.8.1 Cross-polarisation (CP) Cross-polarisation has played a very important role in the development of solid state NMR, giving improvements in sensitivity compared to one pulse operation allowing ~3C spectroscopy in polymers and other organic solids to become feasible. Normally the abundant spin is 1H or 19F, for which a general procedure can be followed to set up the CP experiment. It is helpful if a well-defined standard set-up compound can be identified for each particular pair of nuclei. Steps to setting up a CP-MAS experiment: 1. 2. 3. 4. 5. 6. 7.

Spin the well-defined set up sample relatively slowly (1-3 kHz). Tune both the X and 1H/19F channels. Observe the 1H/19F, set on resonance by shifting the transmitter frequency and set the power to produce the desired 90 ~ pulse. Observe the X-nucleus and if a signal can be seen in decoupled one pulse operation, set X close to resonance also. Switch to CP mode and increment the transmitter power of the X nucleus until the maximum signal is observed (i.e. the optimum match is obtained). Adjust the offset frequency of the 1H/19F channel until the optimum CP and decoupling is observed. The system may then need to be rematched. Change to the compound to be studied and ensure that the system is accurately retuned as for the set-up compound.

Not all samples will provide a signal readily in one pulse operation. However for ~H-~3C there are a number of compounds which are helpful for optimising CP operation. The authors always use a combination of adamantane and glycine. Adamantane can be observed with only decoupling and it is sensitive to the shimming of the system. Glycine is sensitive to the angle setting through the CSA of the carbonyl and to the decoupling efficiency (offset and power) through the oL-carbon. Both these compounds can also be used as secondary references for the chemical shift scale. CP between spin-~/2 is most straightforward and well-known set-up compounds are available. There are many considerations that determine optimum compounds. The relaxation characteristics are very important; Eqs 2.171-174 show that the best compounds for CP have short T~s values and long ~H T~0 values. For the best results, the ~H T~ should preferably be short enough for the S/N to be improved rapidly by keeping the recycle time short. The lack of a CP signal from a particular compound may be due to its unfavourable relaxation characteristics. CP to ~H has been extensively applied to more unusual combinations such as heavy metals and low-~/spin-~/2 nuclei (Sebald 1992). Each of these groups has particular difficulties associated with CP. The heavy metals have large chemical shift ranges, so the offset for each compound may be very different. Such nuclei also tend to have large

174

Multinuclear Solid-State N M R o f lnorganic Materials

CSA, splitting the signal into many sidebands and significantly lowering the intensity of the centreband. Low-~/nuclei present inherent sensitivity difficulties, and to satisfy the Hartman-Hahn condition for low-~/nuclei, considerable power must be applied on the X-side of the probe to secure a reasonable ~H 90 ~ pulse. Care must be taken in using high power that too much strain is not placed on the preamplifier and probe. Much of this work was done during the mid-eighties, before high power automated linear amplifiers became widely available, making the determination of the set-up conditions an extremely intensive process. For low-~/spin-1/2 nuclei the optimum contact times tend to be long (10--100 ms).

Table 3.7. Some set-up compounds for CP between Spin-1/2 X-1H. X

Set-up compounds

Shift (ppm)

CT (ms)

Rec (s)

13C

Adamantane Glycine

5 1

10 5

~5N

5

l0

298i

Enriched glycine Kaolinite QsM8

38.4 176.03 (a-carbon) -345 -91.7 12.1 (t), - 180(o)

2 5

10 10

31p 778e

(NH4)zHzPO4 (NH4)SeO4

-1.7 -- 1040.2

1 3

4 4

10

10

89y l~

Y(NO3)3.6H20 - 53.2 Silver lactate

345.9

50

l0

ll3Cd

Cd(NO3)z.4H20

- 100

15

8

Jl9Sn

Sn(cyclohex)4

-97.35

5

10

Xe in 222 hydroquinone clathrate 171yb Yb(PPh2)2(thf)4 440.6

30

5

6

183W

(NH4)2WS4

3639.6

100

10

195pt

K2Pt(OH)6

8024.5

1

199Hg

Hg(Oac)2

- 2487 -2493

5

129Xe

10

Comments

Reference

Good for shimming Good for checking the angle and decoupling efficiency Natural abundance is visible in one scan Cheap Large signal, good Engelhardt & secondary reference Michel (1978) but expensive Many compounds will do A narrow line Collins et al. (1987) Visible after one scan Merwin & Sebald (1990) Four signals most Merwin & positive shift given Sebald (1992) Charles et al. (1983) A large signal Harris & Sebald (1987) Lee et al. (1988) Needs many 1000 Rabe & Sebald transients (1996) 400 transients were acquired Merwin & Sebald (1992) Harris & Sebald (1987) 16 scans necessary, Harris & Sebald has a large CSA but is (1987) poisonous

175


Table 3.7. (Continued) Shift (ppm)

CT (ms)

Rec (s)

199Hg (NEt4)Na[Hg(CN)4]

-434

15

3

(NBu4)z[Hg(SCN)4]

-615

20

2.5

Pb(C7H7)4

-148.8

5

12

X

2~

Set-up compounds

Comments

Reference

Signalobservable Eicheleet al. in one scan (1995) Eichele et al. Signal observable (1995) in one scan P o i s o n o u s Harris& Sebald (1987)

CT is a typical optimum contact time and Rec is a typical recycle time.

There are many variants of the basic CP sequence. These days the most common variants include switching the power between the matching and decoupling. The probe is much more likely to break down when both rf fields are on so the 1H power can be lower during the match period and increased for the decoupling. Other procedures for adjusting the power requirements include CP-TAPF (Takegoshi and McDowell 1986), in which the decoupler phase is varied between _+ y with + y for tl a n d - y for t2, scaling the power requirement by ( t l - t2/t~ + t2). Another such sequence is TPPM, in which the decoupling is split into two and the phase of the rf is switched between _+ +, giving considerably improved decoupling. When the phase modulation frequency matches the rate at which the magnetisation nutates in the rf field, additional decoupling results (Bennett et al. 1995). Other variants have been suggested, including frequency modulation and combinations of amplitude and phase modulation (Gan and Ernst 1997). Ramped CP is now used to broaden the match condition and reduce the effects of fast MAS (Section 2.6). If the T~p of the ~H is long enough and T2 short enough, the residual transverse proton magnetisation after the contact can be flipped back to the z-direction (Pines et al. 1973). This shortens the recycle time, and although the sequence may not provide much gain it cannot do any harm and so can be used routinely. CP can also be used for spectral editing. The relative spatial position of nuclei can be deduced simply through the effectiveness of the CP. Dipolar dephasing is a specific sequence for discriminating species (Opella and Frey 1979) in which the 1H channel is interrupted for 30-100 p~s during the decoupling period. Often a 180 ~ pulse is introduced in the centre of the interruption period for the X nucleus to refocus the chemical shift effects (Murphy 1983). The sequence depends on the reappearance of the dipolar coupling when the decoupler is turned off, so any X nuclei strongly coupled to ~H will dephase rapidly and be lost from the spectrum. Signal remains from those nuclei which are not directly coupled, which for 13C are the quaternary carbons, giving this sequence the name non-quaternary suppression (NQS). In ~3C spectroscopy, other

176


mobile species, especially the methyl groups, will also survive due to their rapid rotation which averages the dipolar coupling. Other spectral editing schemes for discriminating CHx fragments have been proposed (Wu and Zilm 1993). There is also a number of sequences that allow the dynamics to be probed. The CP curve itself can be mapped out by using variable contact time, and fitted to determine TIs and Tlw Other sequences for determining the various relaxation times (X T1 and TI~, 1H T~ and T~o) are shown in Figure 3.27. The IH T~ can be detected via the carbon which sometimes allows the T~ of the different protons to be distinguished. Even if T1x is long, it can be determined via the proton magnetisation, provided T1x > > T1H (Torchia 1978)

900

J

CP 90*

900

CP

I~.

r 900

cP

L v

v

~

90*

180 ~

Dee

L

Dee

L

90 ~

CP

~ v r . ~-

Figure 3.27. Pulse sequences (upper channel 1H lower channel X-nucleus in each pair) for determining relaxation times in CP-related experiments A. T~ of the X-nucleus, B. Tlo of the X-nucleus, C. Tip of IH and D. T1 of 1H.

177


Although CP experiments are predominantly between 1H----~X,other combinations of nuclei exist. In particular, 19F can be used as the source of magnetisation (Sebald et al. 1992). 19F can be more difficult to set-up since its chemical shift range is that much greater than in 1H, so the decoupler offset must be recalibrated. It is also recommended that the 19F is observed directly to check where the decoupler frequency is set relative to the chemical shift for that compound. Appropriate set-up compounds are necessary for optimisation of the CP condition. NazSiF6 is a good set-up compound for 298i. CP to quadrupolar nuclei often produces only weak signals because of the difficulty of spin-locking the magnetisation discussed in Section 2.5. This is especially important under MAS, which rapidly destroys the spin-locked magnetisation. However, since some of these nuclei (e.g. 23Na, 27A1) produce strong signals, CP can be used to distinguish sites. Optimum compounds for setting up CP and the effect of the conditions have been extensively discussed (Table 3.8). Because of the complex changes occurring in the eigenstates during rotation for a spin-locked quadrupolar nucleus there can be significant distortion of the observed lineshape in the CP of such nuclei (Barrie 1993, Hayashi 1994). 19F can be cross polarised to quadrupolar nuclei, and has been used in 19F----~ZVA1studies of fluorine-bearing aluminosilicate glasses (Kohn et al. 199 l a) and a ~9F---~23Nastudy of NasW309F5 (Duet al. 2000). There are now experiments to generate MQ from CP coherences (Pruski et al. 1997, Ashbrook et al. 1998, Lim and Grey 1999, 2000, Ashbrook and Wimperis 2000, 2000a). A two-dimensional experiment has combined CP and MQ MAS (Ashbrook and Wimperis 1999). The CP experiment can be extended to double CP e.g. ~H---~X--~Y. The initial CP is used to enhance the sensitivity of the experiment and the second stage investigates the connectivity of the X and Y nuclei. This approach has become particularly well developed for 13C and ~SN to probe the connectivity of biomolecular solids (Schaefer et al. 1984). Spectrometers now often come equipped with at least two channels (designated X and Y) and often three lower frequency channels. This allows polarisation transfer experiments between X and Y nuclei. Examples of X---~Ymagnetisation transfer are rapidly increasing and some examples are given in Table 3.9.

Table 3.8. Suggestedset-up compounds for CP 1H-quadrupolenuclei. Nucleus 11B 170 23Na 27A1

43Ca 95M0

Set-up compound

References

Woessner (1987) kernite Smith (unpublished) NaB(OH)4.2H20 Mg(OH)2 Walter et al. (1988) Harris & Nesbitt (1988) NaBH4,NazB407.19H20 boehmite(A10(OH)) Blackwell & Patton 1984, Morris et al. (1989), (1990), Kellberg et al. 1991, Rocha et al. (1991), Mortuza et al. (1993) Ca(OH)2 Bryant et al. (1987) Edward & Ellis (1990). (NH4)6M07024

178


Table 3.9. Some examples of X---~Ymagnetisation transfer used in NMR investigations of inorganic solids. X---~Yand application

Reference

27A1,23Na---->29Sienhancing 295i signal in aluminosilicates 27Al31Pin molecular sieve VPI-5 31p-->~Hspectral editing 23Na11B,27A111B,7LiZVA1 3~P-->~3Cd, 3~P-->29Siconnectivity in semiconductors 11B27AIconnectivity in aluminoborate glasses 31P----)TVSein [3-P4Se3 DQ CP ~1B4--~Z3Na,~B+--~2VA1to examine connectivity in aluminoborate glass

De Paul et al. (1997) Fyfe et al. (1992) Crosbie et al. (1988) Eastman (1999) Franke et al. (1992) Van Wullen et al. (1996) Pietrass et al. (1997) Chan et al. (1998)

3.8.2 SEDOR, REDOR and TEDOR

The Spin Echo Double Resonance (SEDOR), REDOR and TEDOR experiments are all multiple resonance experiments that exploit heteronuclear dipolar coupling. Qualitative information regarding the proximity of spins I and S, and even quantitative information regarding the I-S distance can be derived. Examples of these approaches applied to materials will be given in later Chapters. Their application to specific areas such as aluminosilicates has been reviewed (Ba et al. 2000). The SEDOR experiment (Ehmswiller et al. 1960, Wang et al. 1984, van Eck and Veeman 1992) uses a static sample, in which an echo pulse sequence is applied to one nucleus, e.g. spin I (Figure 3.28A). The echo refocuses the heteronuclear dipolar coupling and the CSA. During the echo period a single 180 ~ pulse is applied to the other nucleus (spin S). This inverts the sign of the dipolar coupling which perturbs the dipolar refocusing process, and consequently the echo intensity is diminished. The amount of signal intensity lost (the SEDOR fraction) depends on the magnitude of the I-S dipolar coupling, the echo time, and the position of the perturbing S-spin 180 ~ pulse. By measuring a SEDOR curve as a function of echo time or pulse position one can determine the IS distance through the dipolar coupling constant D-1

y; ys h

2n"

1"/3

~to (in Hz)

(3.32)

4n

The magnitude of the dipolar coupling that can be readily determined by this method is limited by T2. MAS will readily average away the heteronuclear dipolar coupling but the REDOR sequence (Gullion 1998, Gullion and Schaefer 1989, 1989a, van Eck and Veeman 1993) is designed to reintroduce this coupling in an MAS experiment, making it the MAS equivalent of SEDOR. Spinning the sample greatly enhances the sensitivity and


,H

179

F] !

0

....

i

2

Aq

.

.

.

.

|

r

2r

3

s

~

Nc

i

o

-

~ -

.... F ] _ F L F L .

i

.

,R~FI...~ M~__FI Nc,

-

o

~

0

'

.

~

r.

i

.

2

!

l

.

.

.

.

.

j

.

.

R_ FL_ I;1 _ F! .. H~Q

.

.

.

.

.

J

LI-

~/ ............. Nc

.

.

,i

L

i

J

2

3

,

4

Figure 3.28. Double resonance experiments for providing connectivity/proximity information

A. SEDOR, B. REDOR, C. TEDOR and D. TRAPDOR.

resolution, and allows the echo to be measured for longer times. There are various pulse schemes to implement REDOR. In one, a rotor-synchronised echo sequence is applied to spin I which is detected after a time 2t equal to an even number of rotor periods (Figure 3.28B). Two dephasing 180 ~ pulses per rotor period are applied to spin S, ensuring that the sign of the dipolar coupling at the start of each rotor period is the same so that the dipolar dephasing adds up for each rotor period. In contrast to the static SEDOR experiment, where the echo was necessary to refocus the CSA and dipolar dephasing, the REDOR experiment employs MAS to induce rotational echoes which refocus the CSA and the dipolar interaction. The refocusing 180 ~ pulse is still required as it ensures the detection of an in-phase signal. The dipolar coupling can be obtained by measuring the intensity loss as a function of the number of rotor periods (echo time),


180

or as a function of the position of the dephasing 180~ pulses on spin S. Dipolar couplings as small as 25 Hz have been measured by the REDOR experiment (Merrit et al. 1998). REDOR, like SEDOR, is a difference experiment where the attenuated signal is subtracted from the full echo to obtain the REDOR fraction. The magnitude of the dipolar coupling (D) is characterised by the difference of the signal detected with (S) and without (So) in the presence of the 180 ~ pulse in the sequence (Figure 3.28B) with AS = S o - S. AS depends on D and the time for which evolution is permitted, an experimentally adjustable parameter. For an isolated spin-l/2 pair,

AS=l_ 1 SO

2re 1r/2

2re ; ~ c~ 0

(3.33)

0

where ot and 13 are the polar angles that define the orientation of the PAS of the dipolar tensor in the laboratory frame (Pan et al. 1990). Fitting the REDOR curve then gives D. There has been considerable work in solving this equation to deduce the distance numerically (Mueller 1995) and to use a transform for the two-spin case (Mueller et al. 1996). Quantitative distance information is relatively easy to extract for isolated two-spin systems, which often requires specific labelling of samples. The interpretation of the results becomes much more complex when more spins are involved, e.g. IS2 IS3 or IS4 spin systems. It is possible to obtain distance information for these more complex systems but to do so it is necessary to assume a structural model (van Eck and Veeman 1992). Analysis can also be applied only to the start of the REDOR curve (Bertmer and Eckert 1999) to simplify the problem. A modified REDOR sequence that overcomes the effects of imperfections at the start has been used (Chan and Eckert 2000). To circumvent this problem, theta-REDOR, an elegant variation of the REDOR experiment has been developed (Gullion and Pennington 1998) in which a pulse with a small flip angle is applied instead of a dephasing 180 ~ pulse. In a multispin system, for example IS4, this means that the probability of all 4 S spins flipping and hence affecting the dipolar dephasing of spin I is very small. By contrast, the probability of only one spin undergoing a transition is highest (for small flip angles). Thus, it is not necessary to consider a complicated 5-spin system but to analyse four isolated 2-spin systems. For a given order of spin system (e.g. ISx for a particular x) at short evolution times, the geometry dependence of the S-spins is negligible. For quadrupolar nuclei it is assumed that the analysis in the limit/4~ ~"

(3.36)

If the condition B1 > > Bloc is not obeyed, quantisation is no longer correctly described along this B1 direction. However, it is possible to perform an experiment in which the B 1-field is reduced to zero following the spin-locking sequence. This is adiabatic demagnetisation in the rotating frame (ADRF). The sample can be remagnetised by restoring B1 provided this is done before relaxation occurs. Relaxation during the 'demagnetised' state occurs now in the dipolar field and can be calculated theoretically using a spin-temperature approach. This relaxation time, T1D, may be thought of as the ultimate value of T10 when eo~ -+ 0. An alternative method for T1D measurement employs a Jeener-Broekart pulse sequence which consists of 90x~ 45y ~ 'r 45y ~ pulses (Jeener and Broekart 1967), but the signal strength is at best only about 50 percent of that achieved by ADRF.

3.9.3 Transverse relaxation times (1"2) The transverse relaxation time was discussed in Section 2.8.1. As atoms in a solid acquire greater mobility with, for example, increased temperature, an averaging of the local field seen by any one nucleus occurs, resulting in the phenomenon of motional line-narrowing. The onset of narrowing occurs when there is significant modulation (due for example to molecular reorientation or atomic diffusion) of the rigid lattice local field. This occurs typically at 104 s-1. Below this rate of motion, T2 is constant at its rigid lattice value, but as motion increases and T2 lengthens, the relaxation can be described in terms of the spectral density function J(o~) (Section 2.8) with to = 0. In the well-narrowed region T2 can be written 1

2

- Bmrc

(3.37)

Again, T2 can be used to measure motion. Measurement of T2 in solids is usually made directly from the FID, assuming magnet inhomogeneity effects can be neglected (Eq. 2.192). The solid echo can be used to advantage to produce effectively zero time resolution. Since T2 in solids is rarely exponential, the FID shape should be recorded. The working definition often adopted for T2 is the time for the FID to drop to 1/e of its initial value. Following the onset of motional narrowing, the decay usually becomes exponential and Y2 is more easily defined. A

186


spin-echo sequence (90~ - 180~ ,r echo) is used to remove magnet inhomogeneity decay. If translational diffusion is also present, the echo can suffer additional attenuation at long "r. This can be reduced using the Carr-Purcell (1954) echo sequence (90 ~ -r 180 ~ [a" echo "r 180~ by making n" sufficiently short, since the echo height at t = 2n'r is given by -2 n/r@~ M,,, (2nr) - Moe

-

(3.38)

The Meiboom and Gill (1958) modification (CPMG) to the above sequence prevents accumulation of errors due to mis-setting of the sequence. It simply requires a 90 ~ if-phase shift between the 90 ~ and the 180 ~ pulses. In solids, where motion is usually relatively slow, care must be taken to know whether this sequence is measuring T2 or Tip in an 'average' B1 field, since the CPMG experiment effectively spin-locks the magnetisation. Alternate 180 ~ pulses can be phase-reversed to avoid this effect, but such a strategy also spoils the corrective aspect of the sequence. Phase reversal after every second 180 ~ is much more satisfactory (Suh et al. 1994). 3.9.4 M o l e c u l a r motion

The two distinct types of motion common in materials are reorientation and translation (diffusion). Lattice vibrations are usually too high in frequency and produce too small a field fluctuation to be effective in relaxation unless the nucleus in question has a large electric quadrupole interaction. Reorientational motion is easily distinguished from translational diffusion by its effect on the linewidth (or T2). Since reorientation about one or more molecular axes will only partially average nuclear dipole-dipole interactions, the resulting line-narrowing is also partial, the rigid-lattice value changing by a factor of typically three or four. Translation usually produces extreme narrowing, and, provided the motion is sufficiently rapid, the change in linewidth (or T2) can be several orders of magnitude. The application of relaxation time measurements to study segmental motion (in polymers) as well as diffusional chain motion is very well documented but is still a subject of study, particularly using the frequency dependence of relaxation times to test the detailed predictions of models (McBriety and Packer 1993). The anisotropy of reorientation can also be studied conveniently, and recent interest in motion of molecules on surfaces (e.g. water on porous silica) has been investigated with great success (Gladden 1993). Since the dipolar interaction is usually both intermolecular and intramolecular, the relaxation of spin-l/2 nuclei (e.g. 1H) in the same molecule as a quadrupolar nucleus (e.g. 2H) can permit a complete study of reorientation and translation at a microscopic level (Schmidt-Rohr and Spiess 1994).


187

3.9.5 Diffusion measurements Whereas relaxation measurements are sensitive to microscopic aspects of molecular motion in that % is usually the quantity determined, translational diffusion can be measured on a macroscopic scale by NMR using field gradient methods (Callaghan 1991). When nuclei are placed in a uniform magnetic field they lose phase coherence which can be recovered by a spin-echo (Section 2.8). If, however, diffusion carries nuclei to a different position (Bo value) during the refocussing period, recovery will be incomplete (Eq. 2.194). Pulsed gradients switched on during the time intervals between the 90 ~ and 180 ~ pulses and the 180 ~ pulse-echo period, allow larger gradients to be used and thus enable the study of slower diffusion rates (Stejskal and Tanner 1965). The range of diffusion constant, D, available is typically 10 -6 to 10-12 mZs - 1, but has been extended to even slower diffusion and applied to the study of polymeric systems by Kimmich and co-workers by using the strong static peripheral gradients at the edge of a superconducting solenoid magnet. D values down to 10-14 mZs - 1 have recently been measured by this method, which has a predicted practical limit of 10-16 mZs - 1 ( K i m m i c h

1997).

3.10. NMR UNDER VARYING P H Y S I C A L C O N D I T I O N S

It is usual to vary the conditions in any physical experiment. Since the earliest days of NMR spectroscopy, the sample temperature has been varied in order to gain information about the activation energy of motional processes and for the in situ study of materials processing. The effect of varying the sample pressure is now also being more widely studied by NMR.

3.10.1 Variable temperature NMR Most commercial spectrometers provide variable temperature facilities that operate from --~120 to 400 K using a cooled/heated gas flow passing over the sample and rf coil assembly, and contained in a thermally insulated environment. A more efficient and stable low temperature probe assembly uses heat conduction to a cooled heat bath or direct injection of liquid cryogen. Stable temperatures down to - 80 K can be achieved using liquid nitrogen. Lower temperatures require liquid helium which has a relatively low latent heat and is expensive, with the additional problem that it has a relatively low breakdown voltage. A variety of designs are available for different experimental applications. NMR equipment for use in the range 1 to 120 K has been reviewed by Conradi (1993). A gas-flow technique is usually favoured but must be combined with elaborate dewar vessel provision, radiation shielding and careful attention to heat leaks

188


via the electrical connection to rf coils etc. Samples are usually remote, probes long, and sample exchange is relatively difficult. Very useful practical limits on cryostat design to overcome effects such as Taconis oscillations and acoustic ringing are also discussed by Conradi (1993). While low temperatures increase the signal strength (due to Boltzmann factor) and reduce thermal noise, the opposite is true for elevated temperatures. The construction of probes for high temperatures, (i.e. 500 to 2000 K) requires special attention to the maintenance of a reasonable Q value for the tuned NMR sample coil, and to the thermal problems of protecting the magnet and enabling it to provide a stable and homogeneous magnetic field protected from the high temperatures within it. High temperature probes are usually contained in a heated furnace rather than using a gas-flow system, which becomes very inefficient above about 500~ but has been used successfully with an MAS probe to about 700~ Examples of various methods have been discussed in a comprehensive review giving examples of applications to systems such as coals, oxides, ionic conductors, phase transitions, molten salts, metals and semi-metals (Stebbins 1991). The probe structure is constrained by the requirement to fit within a magnet, to transfer negligible heat to the magnet, and to provide sufficient internal space around the coil to avoid undue rf damping of the resonant circuit. Designs usually employ resistive heating with the heater surrounding the chamber holding the NMR coil. Care has to be taken that the heater coil does not introduce too much noise nor produce an additional magnetic field. The heater coil must be wound non-inductively. Other designs have used a compact water-cooled furnace that fits in the NMR coil (Adler et al. 1990). Variable temperature ~70 NMR studies have included motion in ionic conductors (Adler et al. 1990), in situ observation of sol-gel processing (Poplett et al. 2000) and dehydroxylation (Figure 3.29). Some probe designs have replaced resistive and gas flow heating by optical heating. A probe using bulbs shone on to the sample capable of reaching 1300K has been described (Poplett et al. 2000). Even higher temperature designs using laser heating now appear feasible. Two alternative designs for use in different temperature ranges are being examined. A slightly modified MAS probehead with laser heating can be used up to 600~ (Coutures et al. 1990). For extreme temperatures where ceramics such as alumina could be melted (> 2000~ the system used is based on containerless levitation, with the sample in a split-gap resonator rather than a conventional coil (Florian et al. 1995). Probe design for heating samples is becoming ever more sophisticated, with the advent of specialised equipment such as a probe for the in situ observation of catalysis reactions in flowing reactants, gases, etc., held at temperature for long periods. MAS probes capable of carrying out such flow experiments at temperature now exist (see Carlson et al. 2000, and references therein). In all cases, accurate temperature calibration is best carried out directly on an NMR sample which shows a sharp phase transition and can readily be monitored (Table 3.10).

189


500

400

300

250

110

Room

i,,,,I,,,,l,,,,l,,,ll,,,~l',,I

3000

2000

1000

0

-1000

I

- 2 0 0 0 -3000

ppm Figure 3.29. In situ dehydroxylation of Mg(OH)2 from room temperature to 500~ followed by 170 NMR. Table 3.10. Solid-solid phase transitions useful for calibrating NMR probes. System

Transitiontemperature (K)

Reference

13C in squaric acid 13Cin d-camphor 13Cin CBr4 23Na in LiNaSO4 31p in P453 87Rb in RbNO3

373.2 238 320 791 314 437

Klymachov & Dalal (1996) Haw et al. (1986) Van Moorsel et al. (1995) Massiot et al. (1990a) Van Moorsel et al. (1995) Van Moorsel et al. (1995)

3.10.2 High pressure experiments The pioneering work of Benedek and Purcell (1954) on the observation and measurement of NMR parameters as a function of pressure has opened up the possibility of obtaining structural, dynamic and kinetic information about materials by NMR. Two reviews have reported extensively on the experimental techniques available for NMR

190


at pressure (Jonas 1972, Horvath and Miller 1991). Much less attention has been paid to the solid state, although NMR has successfully been used to measure activation volumes for self-diffusion (Ross and Strange 1978), studies of structure (Bertani et al. 1992) and rotational phase changes (Mackowak and Brown 1983). The diamond anvil method has been used for very high pressures, up to 100 kbar (Lee et al. 1987, Bertani et al. 1992). This specialised technique can only be used with very small samples, a considerable disadvantage for NMR which is inherently insensitive.

REFERENCES

Adler, S.B., Michaels, J.N. & Reimer, J.A. (1990) Rev. Sci. Instruments, 61, 3368. Alemany L.B., Massiot, D., Sherriff, B.L., Smith, M.E. & Taulelle, F. (1991) Chem. Phys. Lett. 177, 301. Alemany, L.B., Steuernagel, S., Amoureux, J.P., Callender, R.L. & Barron, A.R. (1999) Solid State Nucl. Mag. Reson. 14, 1. Alemany, L.B., Callender, R.L., Barron, A.R., Steuernagel, S., Iuga, D. & Kentgens, A.P.M. (2000) J. Phys. Chem. B, 104, 11612. Amoureux, J.P., Fernandez, C. & Steuernagel, S. (1996) J. Mag. Reson. A, 123, 116. Amoureux, J.P. & Fernandez, C. (1998) Solid State Nucl. Mag. Reson. 10, 211. Amoureux, J.P., Pruski, M., Lang, D.P. & Fernandez, C. (1998) J. Mag. Reson. 131, 170. Andrew, E.R. (1971) Progr. Nucl. Mag. Reson. Spectrosc. 8, 1. Andrew, E.R. (1981) Int. Rev. Phy. Chem. 1, 195. Andrew, E.R. (198 l a) Phil. Trans. Roy. Soc. Lond. A, 299, 505. Antzutkin, O.N., Shekar, S.C. & Levitt, M.H. (1995) J. Mag. Reson. A, 115, 7. Antzukin, O.N. (1999) Prog. Nucl. Mag. Reson. Spectrosc. 35, 203. Anup61d, T., Reinhold, A., Sarv, P. & Samoson, A. (1998) Solid State Nucl. Mag. Reson. 13, 87. Ashbrook, S.E., Brown, S.P. & Wimperis, S. (1998) Chem. Phys. Lett. 288, 509. Ashbrook, S.E. & Wimperis, S. (2000) J. Mag. Reson. 147, 238. Ashbrook, S.E. & Wimperis, S. (2000a) Mol. Phy. 98, 1. Ashbrook, S.E. & Wimperis, S. (2001) Chem. Phy. Lett. 340, 500. Ba, Y., Kao, H.M., Grey, C.P., Chopin, L. & Gullion, T. (1998) J. Mag. Reson., 133, 104. Ba, Y., Ratcliffe, C.I. & Ripmeester, J.A. (2000) Adv. Mater. 12, 603. Bak, M., Ramussen, J.T. & Nielsen, N.C. (2000) J. Mag. Reson. 147, 296. Barrie, P.J. (1993) Chem. Phys. Lett. 208, 486. Bastow, T.J. & Smith, M.E. (1994) Solid State Nucl. Mag. Reson. 3, 17. Baugher, J.F., Taylor, P.C., Oja, T. & Bray, P.J. (1969) J. Chem. Phys. 50, 4914. Bax, A., Szeverenyi, N.M., Sullivan, M.J. & Maciel, G.E. (1982) J. Mag. Reson. 47, 462. Benedek, G.B. & Purcell, E.M. (1954) J. Chem. Phys. 22, 2003. Benn, R., Grondey, H., Brevard, C. & Pagelot, A. (1988) J. Chem. Soc. Chem. Commun. 102. Bennett, A.E., Rienstra, C.M., Auger, M., Lakshmi, K.V. & Griffin, R.G. (1995) J. Chem. Phys. 103, 6951.


191

Bertmer, M. & Eckert, H. (1999) Solid State Nucl. Mag. Reson. 15, 139. Bertmer, M., Z~chner, L., Chan, J.C.C. & Eckert, H. (2000) J. Phys. Chem. B, 104, 6541. Bielecki, A. & Burum, D.P. (1995) J. Mag. Reson. A, 116, 215. Bloch, F. & Siegert, A. (1940) Phys. Rev. 57, 522. Bodart, P.R., Amoureux, J.-P., Dumazy, Y. & Lefort, R. (2000) Mol. Phys. 98, 1545. Brown, S.P. & Wimperis, S. (1997) J. Mag. Reson. 124, 279. Brown, S.P. & Wimperis, S. (1997a) J. Mag. Reson. 128, 42. Brus, J. (2000) Solid State Nucl. Mag. Reson. 16, 151. Bryant, R.G., Ganapathy, S. & Kennedy, S.D. (1987) J. Mag. Resort. 72, 376. Bull, L.M., Cheetham, A.K., Anup~ld, T., Reinhold, A., Samoson, A., Sauer, J., Bussemer, B., Lee, Y., Gann, S., Shore, J., Pines, A. & Dupree, R. (1998) J. Amer. Chem. Soc. 120, 3510. Burum, D.P. (1990) Concepts in Mag. Reson. 2, 213. Callaghan, P.T. (1991) Principles of Nuclear Magnetic Resonance Microscopy, Clarendon Press, Oxford. Caravatti, P., Deli, J.A., Bodenhausen, G. & Ernst, R.R. (1982) J. Amer. Soc. 104, 5506. Carduner, K.R. (1987) J. Mag. Reson. 72, 173. Carduner, K.R. (1989) J. Mag. Reson. 81, 312. Carlson, L.K., Isbester, P.K. & Munson, E.J. (2000) Solid State Nucl. Mag. Reson. 16, 93. Carpenter, T.A., Klinowski, J., Tennakoon, D.T.B., Smith, C.J. & Edwards, D.C. (1986) J. Mag. Reson. 68, 361. Carr, H.Y. & Purcell, E.M. (1954) Phys. Rev. 94, 630. Chan, J.C.C., Bertmer, M. & Eckert, H. (1998) Chem. Phys. Lett. 292, 154. Chan, J.C.C. & Eckert, H. (2000) J. Mag. Reson. 147, 170. Chan, J.C.C. (2001) Chem. Phys. Lett. 335, 289. Charles, N.G., Griffith, E.A.H., Rodesiler, P.F., & Aroma, E.L. (1983) Inorg. Chem., 22, 2717. Chopin, L., Vega, S. & Gullion, T. (1998) J. Amer. Chem. Soc. 120, 4406. Churmny, G.N. & Hoult, D.I. (1990) Concepts in Mag. Reson. 2, 131. Collins, M.J., Ratcliffe, C.J., & Ripmeester, J.A. (1986) J. Mag. Reson. 68, 172. Collins, M.J., Ripmeester, J.A. & Sawyer, J.F. (1987)J. Amer. Chem. Soc. 109, 4113. Conradi, M.S. (1993) Concepts in Mag. Reson. 5, 243. Cooley, J.W. & Tukey, J.W. (1965) Math. Comput. 19, 297. Coutures, J.P., Massiot, D., Bessada, C., Echegut, P. Rifflet, J.C. & Taulelle, F. (1990) Compt. Rend. Acad. Sci. Series 2 (Paris), 310, 1041. Crosby, R.C., Reese, R.L. & Haw, J.F. (1988) J. Amer. Chem. Soc. 110, 8550. De Paul, S.M., Ernst, M., Shore, J.S., Stebbins, J.F. & Pines, A. (1997) J. Phys. Chem. B, 101, 3240. Dixon, W.T. (1982)J. Chem. Phys. 77, 1800. Doty, F.D. (1996) in Encyclopedia of Nuclear Magnetic Resonance, Eds Grant, D.M., & Harris, R.K., John Wiley & Sons Limited, Chichester, p. 4475. Doty F.D. & Ellis, P. (1981) Rev. Sci. Instrum. 52, 1868. Doty, F.D., Connik, T.J., Ni, X.Z. & Clingan, M.N. (1988) J. Mag. Reson. 77, 536. Dowell, N.G., Ashbrook, S.E., McManus J. & Wimperis, S. (2001) J. Amer. Chem. Soc. 123, 8135.

192


Du, L.S., Samoson, A., Tuherm, T. & Grey, C.P. (2000) Chem. Mater. 12, 3611. Duer, M.J. (1997) Chem. Phys. Lett. 277, 167. Duer, M.J. & Stourton, C. (1997) J. Mag. Reson. 124, 189. Duer, M.J. & Painter, A.J. (1999) Chem. Phys. Lett., 313, 763. Dupree, R. & Smith, M.E. (2001) Encyclopedia of Chemical Physics and Physical Chemistry, Institute of Physics, Bristol, 1273. Eastman, M.A., Grandinetti, P.J., Lee, Y.K. & Pines, A. (1992) J. Mag. Reson., 98, 333. Eastman, M.A. (1999) J. Mag. Reson., 139, 98. Edwards, J.C. & Ellis, P.D. (1990) Mag. Reson. Chem., 28, $59. Ehmswiller, M., Hahn, E.L. & Kaplan, D.E. (1960) Phys. Rev., 118, 414. Engelhardt, G., Kentgens, A.P.M., Koller, H. & Samoson, A. (1999) Solid State Nucl. Mag. Reson., 15, 171. Ernst, R.R. & Anderson, W.A. (1966) Rev. Sci. Instrum., 37, 93. Ernst, R.R., Bodenhausen, G. & Wokaun, A. (1988) Principles of Nuclear Magnetic Resonance on One and Two Dimensions, Oxford University Press, Oxford. Fayon, F., Bessada, C., Douy, A. & Massiot, D. (1999) J. Mag. Reson., 137, 116. Franke, D., Hudalla, C., Maxwell, R. & Eckert, H. (1992) J. Phys. Chem., 96, 7506. Frye, J.S. & Maciel, G.E. (1982) J. Mag. Reson., 48, 125. Frydman, L., Chingas, G.C., Lee, Y.K., Eastman, M.A., Barrall, G.A. & Pines, A. (1992) J. Chem. Phys., 97, 4800. Florian, P., Massiot, D., Poe, B., Farnan, I. & Coutures, J.P. (195) Solid State Nucl. Mag. Reson.,5,233. Fukushima, E. & Roeder, S.B.W. (1979) J. Mag. Reson., 33, 199. Fukushima, E. & Roeder, S.B.W. (1981) Experimental Pulse NMR, Addison-Wesley. Reading, MA. Fyfe, C.A., Gies, H. & Feng, Y. (1989) J. Amer. Chem. Soc., 111, 7702. Fyfe, C.A., Grondey, H., Mueller, K.T., Wong-Moon, K.C. & Markus, T. (1992) J. Amer. Chem. Soc., 114, 5876. Fyfe, C.A., Wong-Moon, K.C., Huang, Y. & Grondey, H. (1995) J. Amer. Chem. Soc., 117, 10397. Fyfe, C.A., Bretherton, J.L. & Lam, L.Y. (2001) J. Amer. Chem. Soc., 123, 5285. Gan, Z. (1992) J. Amer. Chem. Soc., 114, 8307. Gan, Z., & Ernst, R.R. (1997) Solid State Nucl. Mag. Reson., 8, 153. Ganapathy, S. & Vega, S. (1998) J. Amer. Chem. Soc., 120, 1078. Gay, I.D. (1984) J. Mag. Reson., 70, 355. Geronthanassis, I.P. (1987) Progr. Nucl. Mag. Reson. Spectrosc., 19, 267. Gladden, L.F. (1993) Trans. Inst. Chem. Eng. A, 71,657. Gleason, K.K. (1993) Concepts in Mag. Reson., 5, 199. Goldman, M. & Shen, L. (1966) Phys. Rev., 144, 321. Gouldbourt, A., Madhu, P.K., Kababya, S. & Vega, S. (2000) Solid State Nucl. Mag. Reson., 18, 1. Grandinetti, P.J., Baltisberger, J.H., Llor, A., Lee, Y.K., Werner, U., Eastman, M.A. & Pines, A. (1993) J. Mag. Reson. A, 103, 72. Grey, C.P. & Veeman, W.S. (1992) Chem. Phys. Lett., 192, 379. Grey, C.P., Veeman, W.S. & Vega, A.J. (1993) J. Chem. Phys., 98, 7711.


193

Grey, C.P. & Vega, A.J. (1995) J. Amer. Chem. Soc., 117, 8232. Grimmer, A.-R., Kretschemer, A., & Cajipe, V.B. (1997) Mag. Reson. Chem., 35, 86. Gullion, T. & Schaefer, J. (1989) J. Mag. Reson., 81, 196. Gullion, T. & Schaefer, J. (1989a) Adv. Mag. Reson., 13, 57. Gullion, T. (1995) Chem. Phys. Lett., 246, 325. Gullion, T. (1996) J. Mag. Reson. A, 117, 326. Gullion, T. (1998) Concepts in Mag. Reson., 10, 277. Gullion, T. & Pennington, C.H. (1998) Chem. Phys. Lett., 290, 88. Haase, A. & Oldfield, E. (1993) J. Mag. Reson. A, 101, 30. Haase, A. & Oldfield, E. (1993a) J. Mag. Reson. A, 104, 1. Hanaya, M. & Harris, R.K. (1997) J. Phys. Chem. A, 101,279. Hanaya, M & Harris, R.K. (1997a) Solid State Nucl. Mag. Reson., 8, 147. Hanna, J.V., Smith, M.E., Stuart, S.N. & Healy, P.C. (1992) J. Phys. Chem., 96, 7560. Harris, R.K. & Nesbitt, G.J. (1988) J. Mag. Reson., 78, 245. Harris, R.K. & Sebald, A. (1987) Mag. Reson. Chem., 25, 1058. Hartmann, P., Jfiger, C. & Zwanziger, J.W. (1999) Solid State Nucl. Mag. Reson., 13,245. Hartmann, P., Zwanziger, J.W. & J~iger, C. (2000) Solid State Nucl. Mag. Reson., 16, 189. Haw, J.F., Crook, R.A. & Crosby, R.C. (1986) J. Mag. Reson., 66, 551. Haw, J.F. (1988)Anal. Chem., 60, 559. Hayashi, S. (1994) Solid State Nucl. Mag. Reson., 3, 93. Herreros, B., Metz, A.W. & Harbison, G.S. (2000) Solid State Nucl. Mag. Reson., 16, 141. Herzfeld, J. & Berger, A.E. (1980)J. Chem. Phys., 73, 6021. Hester, R.K., Ackerman, J.L., Neff, B.L. & Waugh, J.S. (1976) Phys. Rev. Lett., 36, 1081. Hing, A.W., Vega, S. & Schaefer, J. (1992) J. Mag. Reson., 96, 205. Hing, A.W., Vega, S. & Schaefer, J. (1993) J. Mag. Reson. A, 103, 151. Hong, J. & Harbison, G.S. (1993) J. Mag. Reson. A, 105, 128. Horvath, I.T., & Miller, J.M. (1991) Chem. Rev., 91, 1339. Hoult, D.I. & Richards, R.E. (1975) Proc. Roy. Soc. A, 344, 311. Hoult, D.I. & Richards, R.E. (1976) J. Mag. Reson., 24, 71. Hoult, D.I. (1979) Rev. Sci. Instrum., 50, 193. Hoult, D.I., Chen, C.-N., Eden, H. & Eden, M. (1983) J. Mag. Reson., 51, 110. Hu, J.Z., Orendt, A.M., Alderman, D.W., Pugmire, R.J., Ye, C. & Grant, D.M. (1994) Solid State Nucl. Mag. Reson., 3, 181. Jackson, P. & Harris, R.K. (1988) Mag. Reson. Chem., 26, 1003. Jakobsen, H.J., SCrenson, O.W., Brey, W.S. & Kanyha, P. (1982) J. Mag. Reson., 48, 328. Janssen, R. & Veeman, W.S.J. Chem. Soc., Faraday Trans., 84, 3747. Jfiger, C. (1994) in NMR Basic Principles and Progress, Eds Blumich, B. & Kosfeld, R., Springer-Verlag, Berlin, vol. 31, p. 135. Jeener, J. & Broekart, P. (1967) Phys. Rev., 157, 232. Jonas, J. (1972) Rev. Sci. Instrum., 43, 643. Joo, C., Werner-Zwanziger, U. & Zwanziger, J.W. (2000) Solid State Nucl. Mag. Reson., 16, 77. Kao, H.M. & Grey, C.P. (1996) Chem. Phys. Lett., 259, 459. Kao, H.M. & Grey, C.P. (1998) J. Mag. Reson., 133, 313. Kellberg, L., Linsten, M. & Jakobsen, H.J. (1991) Chem. Phys. Lett., 182, 120.

194


Kentgens, A.P.M., Lemmens, J.J.M., Guerrts, F.M.M. & Veeman, W.S. (1987) J. Mag. Reson., 71, 62. Kentgens, A.P.M. (1991) J. Mag. Reson., 95, 619. Kentgens, A.P.M. (1993) J. Mag. Reson. A, 104, 302. Kentgens, A.P.M. (1998) Progr. Nucl. Mag. Resort. Spectros., 32, 141. Kimmich, R. (1997) NMR Tomography, Diffusometry and Relaxometry, Springer-Verlag, Berlin. Klymachyov, A.N. & Dalal, N.S. (1996) Solid State Nucl. Mag. Reson., 7, 127. Knight, C.T.G., Kirkpatrick, R.J. & Oldfield, E. (1990) J. Non-Cryst. Solids, 116, 140. Kohn, S.C., Dupree, R., Mortuza, M.G. & Henderson, C.M.B. (1991) Phys. Chem. Minerals, 18, 144. Kohn, S.C., Dupree, R., Mortuza, M.G. & Henderson, C.M.B. (1991 a) Amer. Mineralog., 76, 309. Kohn, S.C., Smith, M.E., Dirken, P.J., van Eck, E.R.H., Kentgens, A.P.M. & Dupree, R. (1998) Geochim. Cosmochim. Acta, 62, 79. Kolodziejski, W., Barrie, P.J., He, H. & Klinowski, J. (1991) J. Chem. Soc. Chem. Commun., 961. Kolodziejski, W., He, H. & Klinowski, J. (1992) Chem. Phys. Lett., 191, 117. Kolodziejski, W. & Klinowski, J. (1992) Solid State NMR, 1, 41. Kraus, H., Prins, R. & Kentgens, A.P.M. (1996) J. Phys. Chem., 100, 16338. Kuhns, P.L., Lizak, M.J., Lee, S.H. & Conradi, M.S. (1988) J. Mag. Reson., 78, 69. Kunwar, A.C., Turner, G.L. & Oldfield, E. (1986) J. Mag. Reson., 69, 124. Lacassagne, V., Florian, P., Montouillout, V., Gervais, C., Babonneau, F. & Massiot, D. (1998) Mag. Reson. Chem., 36, 956. Langer, B., Schnell, I., Spiess, H.W. & Grimmer, A.R. (1999) J. Mag. Reson., 138, 182. Larsen, F.H., Jakobsen, H.J., Ellis, P.D. & Nielsen, N.C. (1997) J. Phys. Chem. A, 101, 8597. Larsen, F.H., Jakobsen, H.J., Ellis, P.D. & Nielsen, N.C. (1998) J. Mag. Reson., 131, 144. Larsen, F.H., Jakobsen, H.J., Ellis, P.D. & Nielsen, N.C. (1998a) Mol. Phys., 95, 1185. Larsen, F.H., Skibsted, J., Jakobsen, H.J. & Nielsen, N.C. (2000) J. Amer. Chem. Soc., 122, 7080. Laukien, D.D. & Tschopp, W.H. (1993) Concepts in Mag. Reson., 6, 255. Lee, F., Gabe, E., Tse, J.S. & Ripmeester, J.A. (1988) J. Amer. Chem. Soc., 110, 6014. Lee, J.N., Alderman, D.W., Jin, J.Y., Zilm, K.W., Mayne, C.L., Pugmire, R.J. & Grant, D.M. (1984) Rev. Sci. Instrum., 55, 516. Lee, S., Lusczynski, K., Norberg, R.E. & Conradi, M.S. (1987) Rev. Sci. Instrum., 58, 415. Lee, Y.K., Emsley, L., Larsen, R.G., Schmidt-Rohr, K., Hong, M., Frydman, L., Chingas, G.C. & Pines, A. (1994) J. Chem. Phys., 101, 1852. Lim, K.H. & Grey, C.P. (1998) Solid State Nucl. Mag. Reson., 13, 101. Lim, K.H. & Grey, C.P. (1999) Chem. Phys. Lett., 312, 45. Lim, K.H. & Grey, C.P. (2000) J. Chem. Phys., 112, 7490. Linder, M., H6hener, A. & Ernst, R.R. (1980) J. Chem. Phys., 73, 4959. Livesey, D.L. (1979) Amer. J. Phys., 47, 558. Lowe, I.J. & Tarr, C.E. (1968) J. Phys. E: Sci. Instrum., 1,320. Maciel, G.E., Bronnimann, C.E. & Hawkins, B.L. (1990) Adv. Mag. Reson., 14, 125.


195

MacKenzie, K.J.D., Smith, M.E., Schmficker, M., Schneider, H., Angerer, P., Gan, Z., Anup~ld, T., Reinhold, A. & Samoson, A. (2001) Phys. Chem. Chem. Phys., 3, 2137. Mackowak, M. & Brown, R.J.C. (1983) J. Mag. Reson., 52, 71. Man, P.P. (1993) Appl. Mag. Res., 4, 65. Man, P.P. (1995) Phys. Rev. B, 52, 9418. Man, P.P. (1998) Phys. Rev. B, 58, 2764. Man, P.P. (2000) in Encyclopedia of Analytical Chemistry, Ed. Meyers, R.A., John Wiley & Sons, Chichester, p. 12224. Mangelsdorf, P.C. (1959) J. Appl. Phys., 30, 442. Marian, D. & Wutrich, K. (1983) Biochem. Biophys. Commun., 113, 967. Maricq, M.M. & Waugh, J.S. (1979) J. Chem. Phys., 70, 3300. Massiot, D., Bessada, C., Coutures, J.P. & Taulelle, F. (1990) J. Mag. Resort., 90, 231. Massiot, D., Bessada, C., Echegut, P., Coutures, J.P. & Taulelle, F. (1990a) Solid State Ionics, 37, 223. Massiot, D., Thiele, H. & Germanus, A. (1994) Bruker Report, 140, 43. Massiot, D., Touzo, B., Trumeau, D., Coutures, J.P., Virlet, J., Florian, P. & Grandinetti, P. (1996) Solid State Nucl. Mag. Reson., 6, 73. Massiot, D. (1996) J. Mag. Reson. A, 122, 240. Massiot, D., Trumeau, D., Touzo, B., Farnan, I., Rifflet, J.C., Douy, A. & Coutures, J.P. (1995) J. Phys. Chem., 99, 16455. Massiot, D., Montouillout, V., Fayon, F., Florian, P. & Bessada, C. (1997) Chem. Phys. Lett., 272, 295. McBriety, V.J. & Packer, K.J. (1993) NMR in Solid Polymers, Cambridge University Press, Cambridge. Meiboom, S. & Gill, D. (1958) Rev. Sci. Instrum., 29, 688. Merrit, M.E., Goetz, J.M., Whitney, D., Chang, C.C.P., Heux, L., Halary, J.L. & Schaefer, J. (1998) Macromol., 31, 1214. Merwin, L.H., Sebald, A., Espidel, J.E. & Harris, R.K. (1989) J. Mag. Reson., 84, 367. Merwin, L.H. & Sebald, A. (1990) J. Mag. Reson., 88, 167. Merwin, L.H. & Sebald, A. (1992) J. Mag. Reson., 97, 628. Merwin, L.H. & Sebald, A. (1992a) Solid State Nucl. Mag. Reson., 1, 45. Mueller, K.T. (1995) J. Mag. Reson. A, 113, 81. Mueller, K.T., Jarvie, T.P., Aurentz, D.J. & Roberts, B.W. (1996) Chem. Phys. Lett., 254, 281. Munowitz, M., Griffin, R.G., Bodenhausen, G. & Huang, T.H. (1981) J. Amer. Chem. Soc., 103, 2529. Munowitz, M. & Griffin, R.G. (1982) J. Chem. Phys., 76, 2848. Murphy, P.D. (1983) J. Mag. Reson., 52, 343. Mildner, T., Ernst, H. & Freude, D. (1995) Solid State Nucl. Mag. Reson., g, 269. Mildner, T., Smith, M.E. & Dupree, R. (1999) Chem. Phys. Lett., 301,389. Mildner, T., Smith, M.E. & Dupree, R. (1999a) Chem. Phys. Lett., 306, 297. Morris, H.D., Bank, S. & Ellis, P.D. (1990) J. Phys. Chem., 88, 6135. Mortuza, M.G., Dupree, R. & Kohn, S.C. (1993) Appl. Mag. Reson., 4, 89. Nakai, T., Terao, T. & Shirakawa, H. (1988) Chem. Phys. Lett., 145, 90. Nielsen, N.C., Bilds0e, H. & Jakobsen, H.J. (1992) J. Mag. Reson., 97, 149. Opella, S.J. & Frey, D.M.H. (1979) J. Amer. Chem. Soc., 101, 5855.

196


Pan, H. & Gerstein, B.C. (1991) J. Mag. Reson., 92, 618. Pan, Y., Gullion, T. & Schaefer, J. (1990)J. Mag. Reson., 127, 147. Pandey, L., Towta, S. & Hughes, D.G. (1986) J. Chem. Phys., 85, 6923. Pietrass, T., Seydoux, R., Roth, R.E., Eckert, H. & Pines, A. (1997) Solid State Nucl. Mag. Reson., 8, 265. Pike, K.J., Malde, R.P., Ashbrook, S.E., McManus, J. & Wimperis, S. (2000) Solid State Nucl. Mag. Reson., 16, 203. Pines, A., Gibby, M.E. & Waugh, J.S. (1973) J. Chem. Phys., 58, 569. Poplett, I.J.F. & Smith, M.E. (1998) Solid State Nucl. Mag. Reson., 11, 211. Poplett, I.J.F., Smith, M.E. & Strange, J.H. (2000) Measure. Sci. Technol., 11, 1703. Pruski, M., Lang, D.P., Fernandez C. & Amoureux, J.P. (1997) Solid State Nucl. Mag. Reson., 7, 327. Pruski, M., Bailly, A., Lang, D.P., Amoureux, J.P. & Fernandez, C. (1999) Chem. Phys. Lett., 307, 35. Pruski, M., Wiench, J.W. & Amoureux, J.P. (2000) J. Mag. Reson., 147, 286. Rabe, G.W. & Sebald, A. (1996) Solid State Nucl. Mag. Reson., 6, 197. Rance, M. & Byrd, R.A. (1983) J. Mag. Reson., 52, 221. Rocha, J., Liu, X. & Klinowski, J. (1991) Chem. Phys. Lett., 182, 531. Rocha, J., Klinwoski, J., Barrie, P.J., Jelinek, R. & Pines, A. (1992) Solid State Nucl. Mag. Reson., 1, 217. Rocha, J., Lin, Z., Fernandez, C. & Amoureux, J.P. (1996) J. Chem. Soc. Chem. Commun., 2513. Ross, S.M. & Strange, J.H. (1978) J. Chem. Phys., 68, 3078. Samoson, A., Lippmaa E. & Pines, A. (1988) Mol. Phys., 65, 1013. Samoson A. & Tegenfeldt, J. (1995) Solid State Nucl. Mag. Reson., 4, 53. Schaefer, J., Stejskal, E.O., Garbow, J.R. & McKay, R.A. (1984) J. Mag. Reson., 59, 150. Schmidt-Rohr, K., Wilhelm, M., Johansson, A. & Spiess, H.W. (1993) Mag. Reson. Chem., 31,352. Schmidt-Rohr, K. & Spiess, H.W. (1994) Multidimensional Nuclear Magnetic Resonance and Polymers, Academic Press, London. Sebald, A. (1994) in NMR Basic Principles and Progress, Eds Blumich, B. and Kosfeld, R., Springer-Verlag, Berlin, Vol. 31, p. 91. Sebald, A., Merwin, L.H., Schaller, T. & Kn611er, W. (1992) J. Mag. Reson., 96, 159. Shore, J.S., Wang, S.H., Taylor, R.E., Bell, A.T. & Pines, A. (1996) J. Chem. Phys., 105, 9412. Sindorf, D.W. & Bartuska, V.J. (1989) J. Mag. Reson., 85, 581. Skibsted, J., Nielsen, N.C., Bilds~e H. & Jakobsen, H.J. (1991) J. Mag. Reson., 95, 88. Smith, M.E. (1993) Appl. Mag. Reson., 4, 1. Smith, M.E. & van Eck, E.R.H. (1999) Progr. Nucl. Mag. Reson. Spectros., 34, 159. States, D.J., Haberkorn, R.A. & Ruben, D.J. (1982) J. Mag. Reson., 48, 286. Stebbins, J.F. (1991) Chem. Rev., 91, 1353. Stejskal, E.O. & Tanner, J.E. (1965) J. Chem. Phys., 42, 288. Stejskal, E.O. & Memory, J.D. (1994) High Resolution Solid State NMR, Oxford University Press, New York. Solomon, I. (1958) Phys. Rev., 110, 61.


197

Suh, B.J., Borsa, F. & Torgeson, D.R. (1994) J. Mag. Reson. A, 110, 58. Sun, B.Q., Baltisberger, J.H., Wu, Y., Samoson, A. & Pines, A. (1992) Solid State NMR, 1, 267. Szeverenyi, N.M., Sullivan, M.J. & Maciel, G.E. (1982) J. Mag. Reson., 47, 462. Takegoshi, K. & McDowell, C.A. (1986)J. Mag. Reson., 67, 356. Torchia, D.A. (1978) J. Mag. Reson., 30, 613. Traficante, D.D. (1990) Concepts in Mag. Reson., 2, 181. Twyman, J.M. & Dobson, C.M. (1990) Mag. Reson. Chem., 28, 163. van Eck, E.R.H. & Veeman, W.S. (1992) Solid State NMR, 1, 1. van Eck, E.R.H. & Veeman, W.S. (1993) Solid State NMR, 2, 307. van Eck, E.R.H., Kentgens, A.P.M., Kraus, H. & Prins, R. (1995) J. Phys. Chem., 99, 16080. van Eck, E.R.H. & Smith, M.E. (1998) J. Chem. Phys., 108, 5904. van Moorsel, G.-J., van Eck, E.R.H. & Grey, C.P. (1995) J. Mag. Reson. A, 113, 159. van Veenedaal, E., Meier, B.H. & Kentgens, A.P.M. (1998) Mol. Phys., 93, 195. van Wfillen, L., Zuchner, L., Muller-Warmuth, W. & Eckert, H. (1996) Solid State Nucl. Mag. Reson., 6, 203. Vogt, F.G., Gibson, J.M., Aurentz, D.J., Mueller, K.T. & Benesi, A.J. (2000) J. Mag. Reson., 143, 153. Vosegaard, T., Florian, P., Grandinetti, P.J. & Massiot, D. (2000) J. Mag. Reson., 143, 217. Wang, S.H., De Paul, S.M. & Bull, L.M. (1997)J. Mag. Reson., 125, 364. Wang, S.H., Xu, Z., Baltisberger, J.H., Bull, L.M., Stebbins, J.F. & Pines, A. (1997) Solid State Nucl. Mag. Reson., 8, 1. Walter, T.H., Turner, G.L. & Oldfield, E. (1988)J. Mag. Reson., 76, 106. Webb, G.G. & Zilm, K.W. (1989) J. Amer. Chem. Soc., 93, 2583. Woessner, D.E. (1987) Z. Phys. Chem. Neue Folge, 152, 51. Wu, C.H., Ramamoorthy, A. & Opella, S.J. (1994) J. Mag. Resort. A, 109, 270. Wu, G., Rovnyak, D. & Griffin, R.G. (1996) J. Amer. Chem. Soc., 118, 9326. Wu, X. & Zilm, K.W. (1993) J. Mag. Reson. A, 102, 205. Wu, X., Juban, E.A. & Butler, L.G. (1994) Chem. Phys. Lett., 221, 65. Wu, Y., Sun, B.Q., Pines, A., Samoson, A. & Lippmaa, E. (1990) J. Mag. Reson. 89, 297. Wu, Y., Lewis, D., Frye, J.S., Palmer, A.R. & Wind, R.A. (1992) J. Mag. Reson., 100, 425. Yang, D.K., Atkins, J.E., Lester, C.C. & Zax, D.B. (1988) Mol. Phys., 95, 747. Yang, Y., Hagemeyer, A., Zemke, K. & Schmidt, H.W. (1990) J. Chem. Phys., 93, 7740. Yesinowski, J.P. & Hill, E.A. (1999) Solid State NMR Spectrosc. Inorg. Mater. Amer. Chem. Soc. Symp. Ser., 717, 358. Zhang, P., Dunlap, C., Florian, P., Grandinetti, P.J., Farnan, I. & Stebbins, J.F. (1996) J. Non-Cryst. Solids, 204, 294. Zwanziger, J.W. & Chmelka, B.F. (1994) in NMR Basic Principles and Progress, Eds Blumich, B. & Kosfeld, R., Springer-Verlag, Berlin, Vol. 31, p. 202.


Chapter 4

zgsi NMR 4.1.

4.2.

4.3. 4.4. 4.5. 4.6.

4.7. 4.8.

4.9.

4.10. 4.11.

General Considerations 4.1.1 Broadening Effects in 298i Spectra 4.1.2 Relaxation Effects in 298i Spectra 4.1.3 Effect of Structure on 298i Spectra Si-O Compounds 4.2.1 Relationships between 298i NMR Spectra and Structure/Bonding 4.2.2 Four-Coordinated Si-O-Compounds 4.2.3 Tetrahedral 298i Chemical Shifts in Silicates 4.2.4 298i Chemical Shifts in Aluminosilicates 4.2.5 Effects of Other Nearest Neighbours on the 29Si Shift Order-Disorder Effects in Minerals Identification of Silicate Minerals Thermal Decomposition of Silicate Minerals Relationships between 298i Chemical Shift (d) and Structure 4.6.1 Relationships between 6 and the Si-O Bond Length 4.6.2 Relationships between 6 and the Si-O-Si Bond Angle 4.6.3 More Complex Relationships between 6 and the Structure Five and Six-Coordinated Si-O Compounds Cross-Polarisation (CPMAS) Experiments 4.8.1 Cross-Polarisation between 1H and 298i 4.8.2 Cross-Polarisation between 19F and 298i 4.8.3 Other Cross-Polarisation Experiments with 29Si Glasses, Gels and Other Amorphous Materials 4.9.1 Silicate Glasses 4.9.2 Deconvolution of 298i NMR Spectra 4.9.3 Connectivities in Glass 4.9.4 Chalcogenide Glasses 4.9.5 Gels 4.9.6 Other Amorphous Materials S i-N and S i-N-O Compounds Si-A1-O-N Compounds 4.11.1 ]3-Sialon, Si6_zAlzOzNg_z 4.11.2 O-Sialon, Si2_xAlxOl+xNz_x

201 201 202 204 205 205 205 205 206 208 208 212 214 217 218 219 223 225 227 227 229 229 230 231 235 236 238 240 242 244 247 247 250

4.11.3 X-Sialon, nominally Si12Al18039N8 4.11.4 Polytypoid Sialons, (Si,A1)m(O,N)m+l 4.11.5 oL-Sialons, MxSil2_(m+n)Alm+nOnN16-n 4.12. Other Metal Silicon Nitrides and Oxynitrides 4.13. Si-C, Si-C-O and Si-C-N Compounds 4.13.1 Silicon Oxycarbide Species 4.13.2 Silicon Carbonitride Species 4.14. Other Materials 4.14.1 Biologically Compatible Glasses 4.14.2 Cements 4.14.3 Inorganic Polymers References

251 253 253 253 255 256 257 257 257 257 259 260

Chapter 4

29Si NMR 4.1. GENERAL CONSIDERATIONS

Silicon is the most abundant element in the earth's crust excluding oxygen (at 26% it is about 3.5 times as plentiful as the next most abundant element, aluminium). It is therefore fortunate for experimental mineralogy, geochemistry, ceramics and inorganic materials generally that 29Si is a nuclide from which useful NMR spectra can readily be obtained. The growth in the development and application of solid state NMR spectroscopy in materials science owes much to the success of the technique with this ubiquitous element.

4.1.1 Broadening effects in 29Si spectra 29Si has a spin 1=1/2, which means it is not subject to quadrupolar peak broadening and distortion. Despite its relatively low natural abundance (4.7%), the spectral resolution of 29Si is high due to its relatively narrow resonance lines. It is, however, subject to the two principal sources of broadening in a spin I= 1/2 nucleus (Chapter 2). These are: (1) Dipole-dipole interactions, in which the magnetic dipoles of the neighbouring nuclei interact with the nucleus under investigation. Since this effect is orientationdependent, and since a powder sample contains all possible orientations, this can be a source of significant broadening. The dipole-dipole interaction is described by an expression containing a term in 3cos20 - 1 which is averaged to zero by magicangle spinning (MAS). The MAS speed must be greater than or equal to the static (unspun) linewidth in Hertz but, because in 29Si compounds this linewidth is narrow, useful spectra can be obtained without having to resort to high magnetic fields and spinning speeds. Typically a spinning speed of about 4 kHz in a field of 4.7 T is adequate for the acquisition of useful 29Si MAS NMR spectra. (2) Chemical shift anisotropy. The nucleus is shielded by the surrounding electrons which give rise to the chemical shift. Since this shielding and its resulting chemical shift is orientation-dependent it causes broadening in a powder sample. The chemical shift anisotropy can be described in terms of the shielding along the three symmetry axes at the nucleus in question. Magic angle spinning averages the chemical shift to a single isotropic value, significantly narrowing the resulting lines even when the spinning speed is less than the static linewidth. 201

202


4.1.2 Relaxation effects in 29Si spectra The relaxation time of e9si can vary enormously; in natural materials containing paramagnetic impurities such as Fe, recycle delays of only a few seconds can be used, whereas some SiC polytypes may take several hours to relax (Hartman et al. 1994), making the acquisition of reasonable SiC spectra a very lengthy process. The 295i relaxation time of pure compounds can be shortened by the deliberate addition of small amounts of paramagnetic ions such as the rare earths. The most effective additive for relaxing both 295i and 89y in yttrium silicon oxynitride compounds was found to be Eu 3+ added at concentrations < 360 txmol g-1 (Meinhold and MacKenzie 1995). Higher concentrations of paramagnetic ions, such as the Fe often present in natural material, caused an increasing amount of the signal intensity to be transferred to the spinning side bands, which grow at the expense of the central peaks, making the spectra unusable in the worst cases. Figure 4.1 illustrates the effect of increasing Fe content on the spectra of two natural muscovite mica samples, both of which contained sufficient Fe to permit a complementary study to be made by 57Fe Mossbauer spectroscopy (MacKenzie et al. 1987). The limiting Fe concentration at which 295i NMR spectroscopy becomes impossible varies somewhat with the structure of the material since this determines the proximity and disposition of the Fe and the S i atoms. As a rule-of-thumb, Fe concentrations greater than about 5% (expressed as FeeO3) are likely to make 295i spectroscopy marginal, since in addition to decreasing the relaxation time, the presence of paramagnetic species also

A

B

4.6% FezO3.

t

100

t

-86

I

-100

,

1

2.4% Fe203

,

t

JI

I

-300 100

,

I

-86

|

-lOO

|

|

,

t

-3o0

zgSi shift (ppm) w.r.t. T M S

Figure 4.1. 295i MAS NMR spectra of two natural muscovite micas illustrating the effect of the Fe203 content on the broadness of the central transition and the size of the spinning side bands (indicated by asterisks). A. Muscovite from Stewart Island, New Zealand, B. Muscovite from North Carolina, U.S.A. Adapted from MacKenzie et al. (1987).

29Si NMR

203

125 Of

100

" 14

cD

7s

9

0

~

~

12 ,~

50

25 J e~ 0

0

0.2

0.4

0.6

-~10 0.8

MnO content (mol%) Figure 4.2. Opposing effects on the 298i relaxation time and peak width of adding paramagnetic MnO to sodium disilicate glass, from the data of Mortuza (1989).

broadens the 29Si spectra. This is illustrated by the addition of Mn 2+ to Na2Si206 glass (Figure 4.2), in which the addition of up to 0.8 mol % MnO decreases the relaxation time from 112 to 1.8 s, but increases the resonance width from 10.5 to 15 ppm (Mortuza 1989). Thus, the beneficial effect of deliberately added paramagnetic species on the relaxation rate must be traded off against spectral broadening. The enhancement of the relaxation process in natural hydrated silicate minerals by paramagnetic species such as Fe may be due to their facilitating the interaction between the 29Si nucleus and the electron spin as a result of the relatively high mobility of the hydrated paramagnetic species. A similar effect has been reported in zeolites (Cookson and Smith 1985, Klinowski et al. 1986), in which the relaxation rate is significantly shortened by the presence of paramagnetic oxygen molecules. The increase in intensity of the spinning sidebands with increasing Fe content (Figure 4.1) can be due to chemical shift anisotropy, but in the case of some natural minerals, it appears to be due instead to magnetic susceptibility broadening arising from the presence of magnetic impurities such as Fe304 (Oldfield et al. 1983). An important practical implication of the wide differences in 298i relaxation rates arises from the fact that in the same sample, various Si units may have quite widely differing relaxation rates. If the recycle delay time chosen for the experiment is not sufficient to allow full relaxation of all the Si species, those with the longer relaxation times will be under-represented in the resulting spectra. These possibilities must be kept in mind, especially when using 29Si NMR spectroscopy to make estimates of the relative abundance of the various Si species present in a sample.

204


4.1.3 E f f e c t o f structure on 29Si spectra 29Si NMR spectroscopy provides direct information about the structure of materials

from measurements of the isotropic chemical shifts. This parameter is influenced most significantly by the coordination number of the Si, which in compounds coordinated by oxygen varies by about 50 ppm in going from Si(IV) to Si(V) to Si(VI) (Table 4.1). The 29Si isotropic chemical shift can also undergo changes of similar magnitude when the element in the first coordination sphere is changed, for example from Si-O to Si-N to Si-C (Table 4.1). Changes in the next-nearest neighbour produce smaller but readily measurable changes in the 29Si chemical shift, typically of the order of l0 ppm when the connectivity changes from Q2 to Q3 to Q4 units in Na2Si206 glass (Table 4.1). A change of similar magnitude is brought about when the next-nearest neighbour element is changed; the magnitude of this effect depends on the chemical nature of the substituted element. Even smaller but still measurable changes (typically ---2 ppm) in the 29Si chemical shifts result from the presence of crystallographic distortions in Si environments which are otherwise similar, for example, the Q4(3A1) site in the mineral natrolite ( - 87.7 ppm) which beomes - 86.3 and - 89.1 in the distorted sites of scolecite (Table 4.1). Thus, the 295i chemical shifts can provide information about a considerable range of perturbations in the Si environment.

Table 4.1. Influence of structure on the isotropic chemical shift of 29Si Effect

Typical magnitude

Example

Coordination number SiOx

----50 ppm for Ax -- 1

SiO4 - 110ppm

---) SiO5 ---) SiO6 - 150 ppm -200 ppm

Nearest neighbour

Depends on element

SiO4 - 110ppm

----) SiN4 ~ SiC4 - 49 ppm - 18 ppm

Next-nearest neighbour (nnn) (i) Connectivity (Q")

--~10 ppm for An = 1

Q2 - 78 ppm

Na2Si206 glass* ~ Q3 ___.) Q4 - 88 ppm - 100 ppm

(ii) Element

Depends on element

thompsonite ~ natrolite** nnn nnn nan 4AI --->3A1 + Si 2AI + 2Si -83.5 ppm -87.7 ppm -95.4 ppm [Q4(4A1) ~ QZ(3A1) + Q4(2A1)]

Crystallographic distortion

Depends on distortion, typically ---2 ppm

Qa(3A1) site in: natrolite ~ scolecite -87.7 ppm -86.3 and -89.1 ppm

* Dupreeet al. (1984) **Lippmaaet al. (1981)

29Si NMR

205

4.2. Si-O COMPOUNDS

4.2.1 Relationships between 29Si NMR spectra and structure~bonding Si in inorganic materials is most commonly bonded to oxygen. Although it prefers fourfold coordination, five-fold and six-fold coordination is not unknown in some glasses and materials produced at high pressures (Stebbins and Poe 1999). The 29Si chemical shifts of Si-O units are sensitive to the coordination number, those of Si(IV) units occurring in the range - 6 0 to - 1 2 0 ppm (with respect to TMS). The shifts of Si(V) and Si(VI) occur at about - 1 5 0 and - 1 8 0 to - 1 9 0 ppm respectively. This is illustrated in Figure 4.3 by an unusual spectrum of a high-pressure triclinic modification of crystalline CaSi2Os, in which all three coordination states are present (Stebbins and Poe 1999).

4.2.2 Four-coordinated Si-O compounds Small variations in the local environment of silicon in tetrahedral Si-O sites of a sample can give rise to broad, somewhat featureless spectra, as in glassy or amorphous SiO2 (Oestrike et al. 1987) (Figure 4.4A). On the other hand, in some circumstances where the various tetrahedral Si sites are crystallographically well-defined, they can be resolved by 29Si NMR spectroscopy, as in the spectrum of the siliceous zeolite ZSM-5 (Figure 4.4B), in which 21 of the 24 crystallographically distinct sites can be resolved (Fyfe et al. 1987). However, most solid-state 29Si spectra fall between these two extremes.

4.2.3 Tetrahedra129Si chemical shifts in silicates The chemical shifts of 29Si spectra are commonly quoted with respect to tetramethylsilane (TMS). Systematic variations of tetrahedral Si chemical shifts with structure are well documented, and can be used for "fingerprinting" (identification of a silicate species ~

,

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.

.

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-50 -100 -150 -200 29Si shift (ppm) w.r.t. TMS Figure 4.3. 295iMAS NMR spectrum of the high-pressure crystalline triclinic phase of CaSi2Os, showing resonances corresponding to 4, 5 and 6-coordinated Si-O. From Stebbins and Poe (1999), by permission of the American Geophysical Union.

206

Multinuclear Solid-State NMR of Inorganic Materials A

IP

-60

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29Si shift (ppm) w.r.t. T M S

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29Si shift (ppm) w.r.t. T M S

Figure 4.4. Extremes in the form of 298i MAS NMR spectra of tetrahedral Si-O compounds. A. SiO2 glass, showing a single broad composite resonance envelope, B. Highly siliceous zeolite ZSM-5, showing resolution of 21 of the tetrahedral Si sites, from Fyfe et al. (1987), by permission of MacMillan Magazines Ltd. Note the difference in the chemical shift scale between the two spectra. or, more usually, the class of structural unit present). Silicate structures may be regarded as being built up of tetrahedral units with varying degrees of polymerisation. These can be described in terms of a "Q" notation, where Q denotes a silicon bonded to four oxygen atoms. A superscript n, where n = 0 to 4 is used to indicate the number of other Q units attached to the unit in question. Thus, QO denotes a silicon bonded through oxygen to no other network-forming elements, whereas Q4 denotes a silicon bonded through oxygen to four other silicons. The 298i chemical shift becomes increasingly negative with each additional Si-O-Si linkage, due to increased electronic shielding of the central Si. QO units in monosilicates have typical shifts of about - 6 5 ppm, changing in steps of about 10 ppm for each additional bonded Si tetrahedron, up to about - 110 ppm for the Q4 units of fully polymerised silica polymorphs (as in quartz or cristobalite). This is shown schematically in Figure 4.5, which also illustrates the degree of overlap occurring between the chemical shifts of each of these groups. This overlap can introduce a degree of ambiguity into the assignments made on this basis alone. The influence of the next-nearest neighbour atoms must also be taken into account, as discussed below for the aluminosilicates, a large group of compounds in which some of the next-nearest neighbours are A1.

4.2.4 29Si chemical shifts in aluminosilicates The aluminosilicates constitute an important class of inorganic compounds. The occurrence of aluminium atoms in the second coordination sphere of the silicon, to which they are bonded through oxygen atoms, produce systematic changes in the 295i chemical shift in a similar way to the changes which are associated with differences in the

207

29Si NMR -

Si ( 4 Z n )

= Si (3Zn)

- - - si (2Zn) Si ( 1 Z n ) -------

Si ( 0 Z n )

Si ( 4 T i )

-

si (3Ti) -- Si (2Ti) ---

Si (1Ti) Si (0Ti)

si (4Al) ----

Si (3A1)

---- Si (2A1) ----

Si (1AI)

.......

Si ( 0 A l )

QO Q1 Q2

Q~ Q4 SiWO

SiCV~ ....

= Si ~

I

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-80

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-120 -140

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-180

,,

t

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29Si shift (ppm) w.r.t. TMS Figure 4.5. Schematic representation of the range of 298i chemical shifts in Si-O compounds of differing coordination and with different nearest-neighbour atoms.

SiO4 polymerisation. In general, the substitution by A1 of each of the four silicons surrounding the central Si of a Q4 unit results in a change in the 298i chemical shift of about 5 ppm towards less negative values. Thus, the 29Si chemical shift range of a Q4 unit with no bonded A1 atoms (denoted Q4(0A1)) is about - 102 to - 116 ppm; this becomes about - 97 to - 107 ppm for Q4(1A1), - 9 2 to - 100 ppm for Q4(2A1), - 85 to - 94 ppm for Q4(3A1) to about - 82 to - 92 ppm for Q4(4A1). As can be seen from the range of these values, shown schematically in Figure 4.5, the shifts overlap somewhat but can be used as a guide to the degree of Al-for-Si substitution, which in turn can provide information about the disordering of an aluminosilicate framework.

208


4.2.5 Effects of other nearest neighbours o n t h e 29Si shift The effect of other nearest neighbour atoms on the 29Si shifts of silicates has been systematically studied in a few systems. Balmer et al. (1997) have reported the effect in titanosilicates, finding that the shift increases with increasing oxygen formal charge in a similar manner to the aluminosilicates. The results, based on observations for a number of crystalline titanosilicates of known structure, are shown in Figure 4.5. The 29Si chemical shifts of titanosilicates were also reported by Labouriau et al. (1998). Caution should however be exercised in making comparisons with other Ti-containing systems, as the effect of the Ti depends on whether it is playing a network-forming or network-modifying role. This is illustrated by the ab initio calculations of Ricchiardi and Sauer (1999) for the substitution of Si by Ti in silicalite indicating such substitutions result in a very small effect of only about 1 ppm on the 29Si chemical shift. A chemical shift relationship has been established for the effect of zinc nearest neighbours in a series of tectozincosilicates (Camblor and Davies 1994). These results are also plotted in Figure 4.5. Studies on crystalline gallosilicate molecular sieves with the beta structure (Occelli et al. 1999) have led to the suggestion that substitution of A1 by Ga in these structures leads to a change of 3 ppm towards higher frequencies, but a systematic determination of the range of shifts in gallosilicate compounds has yet to be made. A series of lead silicate glasses and their crystalline equivalents has revealed a linear relationship between the number of Pb atoms in the second coordination sphere of the structural Si tetrahedra (p) and the isotropic 29Si chemical shift (Bessada et al. 1994), defined by: 6iso --

--

106.005 + 5.537p

(4.1)

In a 2 9 S i NMR study of microporous niobium silicate catalysts, Rocha et al. (1998) tentatively identified resonances at - 95.6 ppm as Si(2Si,2Nb) or Si(3Si, lNb), at - 105.5, - 1 0 7 and - 1 0 8 ppm as Si(3Si, lNb) or Si(4Si,0Nb) and at - 1 1 1 as Si(4Si,0Nb) groups. These assignments were made by analogy with aluminosilicates, and not based on measurements of crystalline niobium silicates of known structure.

4.3. ORDER-DISORDER EFFECTS IN MINERALS

If the various aluminosilicate units present in a mineral can be resolved sufficiently for their relative populations to be simulated, the Al-for-Si disorder can in principle be determined. In practice, the resonances from the various structural units often overlap, making it necessary to deconvolute the broad spectral envelope by curve-fitting procedures. Gaussian peaks are most commonly fitted, since they allow good visual fits to

29Si N M R

209

be achieved, are readily calculated and can be justified in terms of a random distribution of parameters. The curve-fitting process must, however, be carried out with care, bearing in mind the possibility that the various structural parameters may not necessarily combine to give a Gaussian distribution of NMR lineshapes (see section 4.9.2). All five of the Q4(nA1) aluminosilicate units can be identified in the 298i NMR spectrum of the zeolite mineral ultramarine (Na7.sSi6A1602484.5) (Klinowski et al. 1987) (Figure 4.6). In this and similar studies, the information on the Si site occupation is extracted from the spectrum by computer-fitting the component peaks followed by integration of their area. The ultramarine spectrum was simulated by fitting five Gaussian peaks which provided an estimate of the relative numbers of each type of unit. This A1 distribution was then compared with the distributions calculated for a number of structures of varying Al-for-Si disorder. The results show this synthetic ultramarine to be completely disordered, thus apparently disobeying Loewenstein's aluminium avoidance principle that in aluminosilicates, links between A104 tetrahedra are rare or absent, and A1-O-A1 bonds should not be found (Lowenstein 1953). These NMR results have been taken to suggest that Lowenstein' s Rule may only apply to crystalline compounds formed under equilibrium conditions (Klinowski et al. 1987). Determination of the distribution of 298i over the various aluminosilicate units by curve fitting provides an accurate and convenient method of obtaining the Si/A1 ratio of the aluminosilicate framework by applying Loewenstein's Rule. This procedure has been used in conjunction with 27A1 NMR in a structural study of NaY zeolite stabilised

-92.5 -97.6 observed

/ V ~/~

s.,4 yi i! ! ate, ..........J,/

componen--ts ...........

:

;il '~

. . . . . .

"

-80 -100 -1 0 29Si shift (ppm) w.r.t. TMS Figure 4.6. Observed and simulated 298i spectra of the synthetic aluminosilicate mineral pink ultramarine, including fitted peaks, showing resolution of all possible Si(nA1) units. From Klinowski et al. (1987), by permission of MacMillan Magazines Ltd.

210


by lanthanum (van Bokhoven et al. 2000), allowing the framework and non-framework components to be quantitatively distinguished. This procedure has also been used to study the effect of annealing time on the ordering of synthetic cordierite (Putnis et al. 1985). Eight distinguishable tetrahedral sites were observed in the most disordered form, but only two in the most ordered form, allowing changes in the site environments to be determined as a function of annealing time. The ordering data thus deduced from the 298i NMR were also used to provide information on the energetics of the Si-A1 interchange in these cordierites (Putnis and Angel 1985). The 298i NMR spectrum of ordered [3-eucryptite (LiA1SiO4) contains one resonance, but samples crystallised from a glass have been found to contain additional 298i peaks indicating a significant level of short-range Si,A1 disorder which decreases exponentially with annealing time at 1173K (Phillips et al. 2000). 298i has also been used to study ordering effects in a number of other minerals, including albite and oligoclase (Yang et al. 1986), Mg-Si garnets (Phillips et al. 1992), solid solutions of pyrope and grossular garnets (Bosenick et al. 1999), [3-eucryptite (Phillips et al. 1999), gallium fluor-amphiboles (Sherriff et al. 1999), fibrolitic sillimanite (Stebbins et al. 1993), calcic and sodic-calcic amphiboles (Welch et al. 1998), a synthetic leucite analogue (Kohn et al. 1991, Kohn et al. 1995) and in a series of cation-exchanged analcite and leucite minerals (Kohn et al. 1997). The 2-dimensional 298i COSY spectrum of a K-Mg leucite was obtained in an attempt to determine the connectivities between the various Si sites, but this system was too complex for an unambiguous assignment of sites in the structure (Dupree 1991). 298i measurements have been used to establish the Si distribution over the tetrahedral sites in the mica minerals muscovite, phlogopite, vermiculite and margarite. These results indicate that the Si-A1 configuration of the tetrahedral sheets of these minerals is governed by (a) Loewenstein's Rule, which excludes neighbouring A1 tetrahedra, and (b) by the need for local charge compensation, which requires adequate dispersion of A1 over the structure (Herrero et al. 1985). Where the 298i spectra are poorly resolved, as in the case of a series of natural illite-smectite clays (Lausen et al. 1999), a number of equally good computer fits to the spectrum are possible, but strategies have been devised for identifying the most likely fit on the basis of correlations established from well-resolved phyllosilicate spectra between the 298i shift and the A1 substitution (Lausen et al. 1999). An elegant example of the use of advanced curve-fitting procedures to extract structural data from 298i spectra is provided by a study of the various crystal modifications of the silica polymorph tridymite (Kitchin et al. 1996). The room-temperature monoclinic form has 12 Si sites, and the spectrum can be simulated by fitting 12 peaks of equal area (Figure 4.7A). At higher temperatures, the mineral progressively transforms through three orthorhombic forms to a high-temperature hexagonal form. The broad 298i spectrum of the lowest-temperature orthorhombic form (Figure 4.7B) contains the

29Si NMR

211

observed

simulated

A

_ .

..

,

|

-108

|

-

J

.

-112

9

i

_J

-108

. . . . .

!

-112

,

I

,

, ,

-106

|

_L__..a_.__s~

-112

-118

-116

~

_

.

.

.

.

.

.

.

.

.

.

.

.

; ' . . - ' . . . ' - - , . , " . . . . . . . . e.-,,,--..:..-. . . . . . . .

t

-116

-110

-114

-118

A _ .

-108

__

~

,

-112

. . . . . . . .

-116

29Sishift(ppm)w.r.t. TMS

i52-"-"

-112

-" i . . . . . . "

-113

" , "",..

-114

-115

29Si shift (ppm) w.r.t. TMS

Figure 4.7. Observed and curve-fitted 298i spectra of three crystalline forms of the silica polymorph tridymite. A. Room-temperature ordered monoclinic form, showing resolution of nine of the twelve Si sites, fitted to 12 pseudo-Voight lines of equal area with a Gaussian:Lorentzian ratio of 0.3. B. Orthorhombic form at 142~ fitted to six lines broadened by an incommensurate plane-wave modulation. C. Orthorhombic form at 202~ fitted to a single line simulated with a non-linear incommensurate modulation. Adapted from Kitchin et al. (1996), by permission of the Mineralogical Society of America. overlapping resonances from at least six sites, which, however, can only be satisfactorily fitted by a model which takes into account the effect of incommensurate structural modulation. A similar fitting procedure is necessary to accommodate the shape of the next orthorhombic phase to be formed, but the two highest-temperature phases can be fitted with a single narrow resonance, consistent with their expected single Si site (Kitchin et al. 1996). The availability of NMR probes capable of operating at elevated temperatures up to 600-700~ has opened up the possibility of studying temperature-induced processes in minerals. The thermal changes in analcime have been studied in situ by multinuclear NMR including 298i (Kim and Kirkpatrick 1998). Pre-melting processes in lithium and sodium metasilicates have also been studied by 29Si NMR at temperature (George et al. 1998).


212

4.4. IDENTIFICATION OF SILICATE MINERALS By their nature, silicate minerals contain Si in a huge range of environments and variety of structural units. C o m p o u n d s containing structural units corresponding to those found in various minerals are frequently encountered in inorganic materials, either in their own right or as products or intermediates of inorganic syntheses. Because of the high resolution of 29Si N M R spectroscopy and the sensitivity of its isotropic chemical shift to local variations in the atomic environment,

29Si M A S

N M R can be correlated

with the structural units occurring in the various classes of silicate mineral structures. Thus, the orthosilicates (containing the silicate tetrahedra in isolated m o n o m e r i c units or anions) have shifts ranging from about - 6 0 to - 8 6 ppm, the sorosilicates (tetrahedra in linearly connected trimeric structures) have shifts ranging from about - 7 2 to - 9 5 ppm, the inosilicates (in which the tetrahedra are connected in infinite chains) have shifts from about - 8 2

to - 9 2

ppm, the phyllosilicates (containing the

tetrahedra connected to form sheets of two-dimensional single layers) have shifts from about - 7 6

to - 9 7 p p m and the tectosilicates (containing the tetrahedra linked in

infinite three-dimensional frameworks) have shifts of about - 8 3

29Si M A S

to - 114 ppm. The

N M R spectra of a large n u m b e r of silicate minerals have now been reported,

for which a selection of chemical shifts are given in Table 4.2. Since natural minerals have variable compositions and impurity contents, the actual values can vary somewhat; the chemical shift values of Table 4.2 have therefore been rounded, and should be regarded as a guide only.

Table 4.2. Typical 29Si chemical shifts reported for a selection of silicate minerals. Shifts in ppm, quoted with respect to TMS, rounded to nearest whole number. Mineral

29Si shift

Orthosilicates andalusite - 80 datolite - 83 kyanite - 82,- 83 mullite - 86,- 90,- 94 phenacite - 84 sillimanite - 86 topaz - 86

Ref.

Mineral

a b c e a c a

chondrite grandidierite monticellite olivine phenacite sphene zircon

29Si shift -

60 80 66 62 84 80 82

Ref.

a r d a a f a

Sorosilicates akermanite lawsonite rankinite

- 74 - 81 - 75,- 76

d b a,d

gehlinite piemontite thorveiteite

- 73 - 82,- 86,- 90 - 95

d c a

Inosilicates clinoenstatite jadeite orthoenstatite

- 82,- 84 -92 - 82

a,d d e

diopside omphacite pectolite

- 85 - 85 - 86

c c c,d

29Si N M R T a b l e 4.2.

213

(Continued)

Mineral

298i shift

Ref.

Mineral

298i shift

Ref.

spodumene wollastonite

- 91 - 8 6 , - 92

a,c,d c

tremolite

- 8 7 , - 91

a,c,d

Phyllosilicates a

apophyllite - 92 beidellite - 8 8 , - 93 danburite - 89 hectorite - 94 lepidolite - 90 montmorillonite - 94 palygorskite - 9 2 , - 9 4 , - 98 phlogopite - 86 saponite - 9 6 , - 9 1 , - 85 serpentine - 94 vermiculite - 88

g b g a,g g j,s a,i g a

endellite - 93 chlorite - 91,- 87,- 83,- 79 fluorophlogopite - 93,- 89,- 96 kaolinite - 92 margarite - 76 muscovite - 86,- 90 paragonite - 8 9 , - 8 4 , - 81 pyrophyllite - 94 sepiolite - 92,- 94,- 98 - 97 talc

a g g

a,c,h a,i a,Li g a,c j,s a,c

i

Tectosilicates

albite - 9 3 , - 9 7 , - 105 anorthite - 8 3 , - 8 5 , - 89 beryl - 102 cancrinite - 86 coesite - 1 0 6 , - 110 cristobalite - 109 [3-eucryptite - 90.6 heulandite - 95 to - 108 leucite - 9 7 , - 9 1 , - 85 milarite - 102.3 nepheline - 8 5 , - 89 petalite - 1 1 0 , - 111 scolecite - 86,- 89,-96 stilbite - 9 8 , - 1 0 1 , - 108 tourmaline - 88 quartz - 107 Miscellaneous

k k f c,d n n v

f,1 t u

c,q d

f,d 1 c

analcime armenite carnegieite chabazite cordierite emerald gmelilite kalsilite microcline natrolite oligoclase scapolite sodalite thomsonite tridymite

-

92,-

96,-

-

95,-

102,-

101

1

82

u

82

m

99 -79,-10 -

-

103

f

97

d

-

85,96,-

-

89,98,-

88,-

93,-

0

93,-

-

86,-

-

109to

94

m,q

100

k

105

k

95

97,-

d,f o

d

106

p

85 89,-

c 92

d,L1

113

n

- 82.0 - 94.2 - 78.5 - 9 5 . 6 , - 100.0

x x x x

c,n

silicates

benitoite lorenzenite penkvilksite vinogradovite

-

93.4 90.0 95.6 900.6

K e y to references a M a g i et al. (1984) c Sherriff et al. (1991) e M e r w i n et al. ( 1991) g W e i s s et al. (1987) i Sanz and Serratosa (1984) 1 L i p p m a a et al. ( 1981) n Smith and Blackwell (1983) p Sherriff et al. (1987) r Smith and Steuernagel (1992) t K o h n et al. (1997) v Phillips et al. (2000)

fresnoite narsarsukite titanite zorite

b d f h j m o q s u x

Smith et al. (1983) Janes and Oldfield (1985) Sherriff et al. (1991 a) K i n s e y et al. (1985) K o m a r n e n i et al. (1986) Stebbins et al. (1986) Putnis et al. (1985) Hovis et al. (1992) F y f e and K e n n e d y (1986) A r m b r u s t e r (1999) Labouriau et al. (1998)

214


Table 4.3. A selection of 298i chemical shifts for tetrahedral Si in metal silicates, in

ppm with respect to TMS. Compound

298i shift

Li4SiO4 -64, -65, -62, -64.7, - 6 6 BazSiO4 -70 Mg2SiO4 -62 o~-CazSiO4 -70 [3-Ca2SiO4 - 71 ~/-Ca2SiO4 - 74 H-Pb2SiO4 - 94.3 M-Pb2SiO4 -96.6 et-Li2Si205 -91 [3-Li2Si205 -92.5 ot-NazSi205 -93.6 [3-Na2Si205 -87.5, -85.6 ~/-Na2Si205 - 86 ~-Na2Si205 -90 Y2SiO5 -79.8 Li6Si207 -67 Ca3Si207 -75, - 7 6 La2Si207 -83 In2Si207 -88 oL-Y2Si207 - 80.96, - 82.43, - 83.4 l, - 84.95 [~-Y2Si207 -93 [3-Y2Si207 -93.65 "y-YzSi207 -92.8 ~/-Y2Si207 -92.68 8-Y2Si207 - 81.27, - 83.00 ~/-Y2Si207 -83.0 8c28i207 -95 Pb3SieO7 -77.5, -94.3,096 LizSiO3 -75 Na2SiO3 -76.8 MgSiO3 -82 PbSiO3 - 84.2, - 86.5, -94.4

Reference Xu and Stebbins (1995) Magi et al. (1984) Magi et al. (1984) Magi et al. (1984) Magi et al. (1984) Magi et al. (1984) Bessada et al. (1994) Bessada et al. (1994) Mortuza (1989) Mortuza (1989) Mortuza (1989) Mortuza (1989) Mortuza (1989) Mortuza (1989) Dupree et al. (1988) Mortuza (1989) Janes and Oldfield (1985) Magi et al. (1984) Magi et al. (1984) Parmentier et al. (2000) Magi et al. (1984) Parmentier et al. (2000) Dupree et al. (1988) Parmentier et al. (2000) Parmentier et al. (2000) Parmentier et al. (2000) Magi et al. (1984) Bessada et al. (1994) Magi et al. (1984) George et al. (1998) Magi et al. (1984) Bessada et al. (1994)

For a fuller discussion of the effect of the various silicate structural groups on the 298i chemical, the reader is referred to Engelhardt and Michel (1987). A list of the reported chemical shift for other inorganic silicates is given in Table 4.3.

4.5. THERMAL DECOMPOSITION OF SILICATE MINERALS N M R spectroscopy, especially 298i NMR, has proved to be a valuable technique for studying thermal decomposition of minerals, especially in cases where the intermediate phases are poorly crystalline or X-ray amorphous.

29Si N M R

215

A mineral reaction of both academic and practical interest is the thermal decomposition of the related 1:1 clay minerals kaolinite and halloysite. These lose structural (hydroxyl) water at about 550~ forming an essentially X-ray amorphous phase which has been the subject of considerable discussion (MacKenzie et al. 1985a, Watanabe et al. 1987, Lambert et al. 1989, Rocha and Klinowski 1990, Massiot et al. 1995). Much of the discussion centres on the changes in the coordination state of the A1 during dehydroxylation and in the subsequent transformation to mullite, about which 27A1 NMR has provided considerable insight (see Chapter 5). The thermally-induced changes in the 29Si spectra are simpler and less contentious. On dehydroxylation, the sharp kaolinite Si peak at about - 9 2 ppm broadens considerably and shifts to about - 102 ppm (Figure 4.8), consistent with a range of Si environments of which the mean Si-O-Si angle can be estimated from the NMR spectrum (MacKenzie et al. 1985a). Rocha and Klinowski (1990) fitted the broad 298i profile of dehydroxylated kaolinite (called metakaolinite) to five Gaussian peaks, while Lambert et al. (1989) claimed a satisfactory fit with four Gaussians, and monitored the change in their relative intensities as a function of dehydroxylation temperature. Heating to higher temperatures causes metakaolinite to form either a cubic spinel or a low-temperature form of mullite (or both), depending on various factors including the crystallinity of the parent sample, the thermal regime and the impurities present in the system. Decomposition of metakaolinite is accompanied by the separation of amorphous SiO2, resulting in a 298i resonance at about - 1 1 0 ppm (Figure 4.8). The subsequent formation of well-crystallised mullite is also apparent from the evolution of its characteristic 298i NMR spectrum (Figure 4.8). 298i and 27A1 NMR have been used in studies of the effect of LiNO3 mineraliser on the thermal decomposition of kaolinite (Rocha et al. 1991) and the unh

5

650

.

970~

ll00~

.92A-108

-110

540~ ~

~~-91.5

800~j

~2 105~// 9

-60 -100 -140

-60 -100


0

I

-109 i

I

__

I

-60 -100 -140

I_

t

1

k

t

~.

-6O -100 -140

29Sishift (ppm) w.r.t. TMS

Figure 4.8. Changesin the 29Sispectra of kaolinite during its thermal decomposition, showing the progressive formation of the broad metakaolinite resonance envelope(- 99 to - 102 ppm) at 650-800~ the sudden appearance of free SiO2 (- 110 ppm) at 970~ and the formation of mullite (- 88 to - 92 ppm) above 1100~ Adapted from Mackenzie et al. (1985a) by permission of copyright owner.

216

Multinuclear Solid-State N M R of lnorganic Materials

effect of flash calcination, in which the 29Si lineshape was fitted to four Gaussian peaks which changed in relative intensity according to the residence time of the sample in the calciner (Slade and Davies 1991). 29Si and 27A1NMR have also been used to study the thermal decomposition of kaolinite under water vapour atmosphere, which was found to facilitate dehydroxylation and the subsequent formation of crystalline products, and improve the mechanical properties of the fired material (Temuujin et al. 1998e). The thermal decomposition of halloysite, a mineral closely related to kaolinite, in which the plates are rolled up into tubes, has also been studied by 29Si and 27A1 NMR spectroscopy (Smith et al. 1993). Pyrophyllite is another layer-lattice aluminosilicate mineral which decomposes on heating to mullite and cristobalite. Its thermal reactions have been studied by 29Si and 27A1NMR (MacKenzie et al. 1985, Sanchez-Soto et al. 1993). The position of the single sharp 29Si resonance at - 9 5 . 6 ppm in unheated pyrophyllite changes on dehydroxylation to - 1 0 1 ppm, corresponding to a mean Si-O-Si angle of 137.6 ~ in the dehydroxylated phase and leading to a refinement of its structure (MacKenzie et al. 1985). Heating at 1100-1150~ results in the abrupt appearance of the 29Si peaks corresponding to mullite ( - 8 7 ppm) and cristobalite ( - 1 0 9 ppm) (MacKenzie et al. 1985). The effect of grinding on the thermal decomposition of pyrophyllite has also been studied by 29Si and 27A1NMR (Sanchez-Soto et al. 1993). The reactions of several other minerals which thermally decompose to form mullite have been studied by 29Si and 27A1 NMR. These include the mica mineral muscovite, which also contained sufficient iron to permit a complementary 57Fe M6ssbauer study (MacKenzie et al. 1987), the hydroxyfluoride mineral topaz (Day et al. 1995) and the semi-amorphous aluminosilicate minerals allophane (MacKenzie et al. 1991) and imogolite (MacKenzie et al. 1989). The same combination of NMR nuclei has been used to study the thermal decomposition of other aluminosilicates including an illiterich clay (Roch et al. 1998), montmorillonite (Brown et al. 1987), and a related mineral, Fuller' s Earth (Drachman et al. 1997). NMR has also been used to study the effect of water vapour on the thermal decomposition of montmorillonite clay compacts (Temuujin et al. 2000a). 29Si in conjunction with 25Mg NMR has been used to follow the thermal decomposition of several magnesium silicates, revealing unexpectedly complex behaviour of the asbestos mineral chrysotile, in which the 29Si spectral peak at - 9 2 ppm broadens and shifts to about - 73 ppm with the formation of an X-ray amorphous dehydroxylate (MacKenzie and Meinhold, 1994a). This phase transforms to forsterite, MgzSiO4, at about 600-700~ at which temperature a second, Si-rich dehydroxylate forms, characterised by a 29Si resonance at - 9 7 ppm (Figure 4.9). This transforms to enstatite (MgSiO3) and free silica at 770-800~ with further enstatite being formed at 1150~ by solid state reaction between the silica and forsterite (MacKenzie and Meinhold 1994a). Other magnesium silicate minerals whose thermal decompositions have been

29Si NMR

217 5Mg3$1:OjlOH| 4

600~

t -g~~

730~

- 6 !.T

2MI

Mg~Si2OI(OH] i

4MgiSllOs{Ofl) 4 tSl

-61.6

650~

750~

]~

| ~..

9 ,

-60

i

..

i _ = ..... I _ : ,

-100

~

e+.s p ~ l

( ~--61.6plmm|

I

-100


,

,, t "~,o

( ~ ..gTpeml

t"

2SiO l

MgSiO 3

I~ ; , ,,,,.

[ ,ocslerlte' ]

: ...... 1:--~--I--+:

-60

[.+d+...

700 "C

2MgiSiO 4

0 0 . . _+, . . ., / C

_

Q

7M@2$10 4 1(~.

700oc ~-~,5 -,28

MgSi:O r

73 p p m ) ~670

~

7"70 800 "~:

l'MglSlO 4

,l

( ~ .,83plma)

[ (~-.1101~m|

I_ . 5 Mg. :S|O 4

5M9S|O:1

~,""-"+' 1 1 6 - - s+.s ~,m!

l ~ - . 83 mmm)

Figure 4.9. Changes in the 298i MAS NMR spectra during the thermal decomposition of chrysotile (white asbestos). Note the evidence for the two dehydroxylated phases, that at - 72 ppm forming forsterite directly, that at - 97 ppm forming enstatite by the thermal decomposition sequence shown schematically at the fight. From MacKenzie and Meinhold, (1994a), by permission of copyright owner.

studied by 29Si and 25Mg NMR include talc (MacKenzie and Meinhold 1994b) and synthetic hectorite (Mandair et al. 1990, MacKenzie and Meinhold 1994c). 4.6. RELATIONSHIPS BETWEEN Z9Si CHEMICAL SHIFT (~) AND STRUCTURE

The 29Si chemical shift values are directly related to the shielding of the 29Si nucleus by the electronic structure of its immediate environment. The chemical shift will thus be influenced by the disposition and chemical nature of the adjacent atoms. Considerable efforts have been made to relate the chemical shifts which have been reported for a large number of silicates to their structural parameters. These relationships, which are often empirical, have been sought with the Si-O bond length (Smith et al. 1983, Smith and Blackwell 1983, Smith et al. 1984, Grimmer 1985, Higgins and Woesner 1982, Grimmer and Radeglia 1984, Weiden and Rager 1985), the Si-O-Si (or T-O-T) bond angle (Smith and Blackwell 1983, Smith et al. 1984, Engelhardt and Radeglia 1984, Newsam 1987, Thomas et al. 1983, Mortuza et al. 1998), or some trigonometric function of the bond angle (Smith and Blackwell 1983, Smith et al. 1984, Newsam 1987, Mortuza et al. 1998, MacKenzie et al. 1985) and to the electronegativity of the surrounding groups (Janes and Oldfield 1985). Most of these simple relationships have been demonstrated

218


to hold for only limited groups of silicates, but by making other refinements, the chemical shifts of a much wider group of silicates can be predicted (Sherriff et al. 1991). In general it is found that the chemical shifts become less negative with increasing mean S i-O bond length, decreasing mean T-O-T bond angle (where T denotes the tetrahedral atom) or with decreasing electronegativity of the surrounding groups.

4.6.1 Relationships between ~ and the Si-O bond length Since the shielding of the Si nucleus in silicates is influenced by the degree of s-hybridisation of the oxygen bond orbitals, which in turn is directly related to the Si-O bond length, several authors have sought a simple relationship between the chemical shift ~ and the Si-O bond length in silicates. Linear relationships of this type have been reported, usually for small groups of silicates. The most extensive test of this relationship was made on a group of 20 silicates, which included representatives of all the silicate classes (Smith et al. 1983). There is considerable scatter, but the best-fit line, shown in Figure 4.10 is defined by (4.2)

= 875 (dsi-o) - 1509

where ~ is in ppm and dsi-o is the mean bond length in/k. Other small groups of samples give similar relationships with the mean value of dsi-o, but still with considerable scatter. Typical relationships, also shown in Figure 4.10, are 6 - 43.2(dsi_o) - 179 (for four silica polymorphs and a silicalite precursor) (Smith and Blackwell 1983),

(4.3)

Weiden & Rager 1985 -40 r

Grimmer & Radeglia 1984 et al. 1983

Smith et ~ m19i84t h a / "

-80

epm

~

"" -120 r~

-160 ~ m m e r |

,

Smith & Blackwel11983 gins & Woesner 1982 1985

!

1.58

l

t

,

|

i

I

i

1.62

|

!

1.66

Mean Si-O distance (,~) Figure 4.10. Relationships between the 29Si chemical shift ~ and the mean Si-O bond length

reported by various authors for groups of related silicate structures.

29Si N M R

219

6 = 1447(dsi_o) - 2432 (for Na,K feldspars, Smith et al. 1984),

(4.4)

6 = 1218(dsi_o) - 2058 (for five silicates and quartz, Grimmer 1985),

(4.5)

6 = 1372(dsi_o) - 2312 (for albite and natrolite and two silica polymorphs) (Higgins and Woesner 1982),

(4.6)

6 = 1187(dsi_o) - 2014 (for various silicates, Grimmer and Radeglia 1984),

(4.7)

6 = 1126(dsi_o) - 1909 (for Si-O bonds in single-crystal Mg2SiO4) (Weiden and Rager 1985).

(4.8)

If all these results are combined in a single plot, the resulting line is 6 = 999(dsi_o) - 1709

(4.9)

However, the scatter in this simple generalised relationship, especially for the chain silicates, reduces its usefulness, and indicates that other factors such as next-nearestneighbour interactions are playing an important role in determining the chemical shift.

4.6.2 Relationships between 8 and the Si-O-Si bond angle The electronegativity of the Si-O bond is related to the Si-O-Si bond angle (or more generally, the T-O-T angle, where T is a tetrahedral atom). This suggests the possibility of a simple relationship between g and the mean tetrahedra! bond angle oL (in degrees), or some trigonometric function of the bond angle. The linear relationships which have been derived between g and the mean T-O-T angle for small groups of samples are shown in Figure 4.11. These relationships are 6 = -0.127c~ - 91.5 (for four silica polymorphs and a silicalite precursor) (Smith and Blackwell 1983),

(4.10)

6 = - 1.18c~ + 69.2 (for Na,K feldspars, Smith et al. 1984),

(4.~1)

6 = - 0.619c~ - 18.7 (for 21 silica polymorphs and zeolites) (Engelhardt and Radeglia 1984),

(4.12)

- - 0.553c~ - 7.58 (for nine zeolites, Newsam 1987), 6 = -0.579c~ - 25.3 (for 10 zeolites, Thomas et al. 1983),

(4.13) (4.14)

220


a et al.

-80

1998

-..~sa _...~ 1987 ~'

-90 Engelhardt & " ~

a~

,I~ -lO0 Radeglia 1984 r~ -110

~

Engelhardt 1999 Kohn et al. 1997 Smith eta/. 1984

Slvadinarayana

et M "- - yana ~

:1,, 130

. Thomas .19. 9 e. t a l . 140

8

~

150

-- "h pSmit e, Blackwel11983 ,_

160

Mean Si-O-Si angle ot (o) Figure 4.11. Relationships between the 29Si chemical shift ~ and the mean tetrahedral S i - O - S i bond angle oLreported by various authors for groups of related silicate structures.

- - 0.563ce - 9.62 (for three sodium disilicate polymorphs) (Mortuza et al. 1998)

(4.15)

= - 0.79c~ + 18.18 (for 13 leucites and related compounds) (Kohn et al. 1997)

(4.16)

Although these various groups of samples are fitted by their respective lines with reasonable scatter, they are not well fitted by a single line. If such a line is plotted, its equation is given by 6 - - 1.44ce + 107

(4.17)

but the scatter in these results suggests that it would be best not to treat this as a general relationship. This does not however preclude the use of specific angular relationships which are known to hold for restricted groups of compounds. Thus, Engelhardt (1999) has demonstrated that 33 sodalites with different cage contents conform closely to the angular relationship 6 = - 0.62a - 1.09

(4.18)

and Sivadinarayana et al. (1998) have found that for 17 of the 24 crystallographically distinct sites in the monoclinic zeolite ZSM-5 a satisfactory correlation of the chemical shifts with the mean T-O-T bond angle ot is given by 6 = - 0.607c~ - 20.9

(4.19)

29Si N M R

221

This simple relationship with the bond angles was found to be more satisfactory than relationships with various electronic properties calculated by semi-empirical quantum chemical calculations based on cluster models (Sivadinarayana et al. 1998). Since the electronegativities of the s-hybridised oxygen bond orbitals are related to the Si-O-Si bond angles oL by a cosine function of the type COSOL/(COSOL-- 1), relationships of this form, or with sec c~ ( = 1/coset) have been sought. In several cases, the secant relationships shown in Figure 4.12 have been claimed to provide a better linear fit to the data sets than the simple mean angle relationship. Note that the parameter plotted is the mean of the secants of the bond angles and not the secant of the mean bond angle. The straight lines representing each of these data sets are: 6 = - 55.7 sec a - 176 (for four silica polymorphs and a silicalite precursor) (Smith and Blackwell 1983), (4.20) = - 62.7 sec a - 180 (for N a - K feldspars, Smith et al. 1984),

(4.21)

6 = - 23.3 sec a - 116 (for nine zeolites, Newsam 1987),

(4.22)

6 = - 32.9 sec a - 134 (for three sodium disilicate polymorphs) (Mortuza et al. 1998)

(4.23)

Additionally, the secant relationship (4.20) established for silica polymorphs (Smith and Blackwell 1983) has been found to hold to within two ppm for a series of 12 layerlattice aluminosilicates (MacKenzie et al. 1985) provided all T-O-T angles (including

-80 1987 -90 et al. 1998

-100 MacK

mith et al. 1984

-110

~Smith --

-1.8

.

-

'

11]6 ....

,

-1.4

,,,1

,

I

-1.2

-

~

& Blackwel11983 .

!

-1.0

M e a n sec a Figure 4.12. Relationships between the 29Si chemical shift ~ and the mean secant of the tetrahedral Si-O-Si bond angle oLreported by various authors for groups of related silicate structures.

222


the relevant Si-O-A1 angles) are used in the calculation. When all these data sets are plotted together, there is less scatter in the best-fit line (4.24)

= - 67.2 sec ce - 182

than for the simple angle relationship, but the result is still less than satisfactory. It should not be concluded, however, that this and the other empirical relationships have no use. Indeed, individual secant relationships derived for restricted groups of compounds fit the experimental data with little scatter, suggesting that useful structural information can be gained, so long as the relationship appropriate to a particular type of compound is used. Thus, the secant relationship (4.20) with the angular definition modified for aluminosilicates (MacKenzie et al. 1985) has been used to shed light on the possible existence of a Si-A1 spinel suggested to form when the clay mineral kaolinite is heated to 980~ On the basis of the known structure of the closely-related ~/-alumina spinel, the T-O-T angle of the postulated tetrahedral Si site in such a structure predicts a chemical shift of - 79 ppm for this site. Such a resonance would normally be masked by the broad resonance of the amorphous SiO2 also present, but when this material was selectively removed by leaching with KOH solution, the predicted 295i peak was detected (MacKenzie et al. 1996) (Figure 4.13). Measurements of the relative intensity of this

~

-110

befor

-110 after 1

l

20

|

t

-60

t

I

9

-140


Figure 4.13. 29Si NMR evidence for the presence of Si in the w-alumina spinel formed from kaolinite at 980~ After leaching out most of the uncombined SiO2 from the sample with KOH solution, the resonance at - 77.5 ppm indicates the presence of Si in the tetrahedral sites of the Al-rich spinel. Adapted from MacKenzie et al. (1996) by permission of copyright owner.

29Si NMR

223

peak, taken in conjunction with the known amount of SiO2 removed by leaching, suggested that the amount of SiO2 in this particular phase is quite small (3.9 wt % maximum) (MacKenzie et al. 1996).

4.6.3 More complex relationships between ~ and the structure The failure of relationships between simple geometric factors and ~ for a wide range of silicate types has led to a recognition that the electronegativity of the groups of atoms surrounding the Si must be taken more rigorously into account, for example, by utilising a linear correlation between ~ and the nett charge on the silicon atom. The latter is reflected by the electronegativities of the four ligands attached to the Si atom, which can be empirically calculated by assigning characteristic group electronegativity values to all the groups or fragments attached to the silicon (Janes and Oldfield 1985). Allowance is then made for the variation in the Si-O electronegativity due to different Si-O-Si angles by use of a linear scaling procedure and the group electronegativites are summed to give an estimate of the chemical shift ~ from the empirical relationship

(4.25)

6 = - 24.336 X E N + 279.27

where ]~EN is the sum of the group electronegativities for systems such as silicate minerals, containing ~r and ~r-bonding (so-called type P silicon). This approach was able to predict to within about 2 ppm the chemical shifts of 99 sites in 51 compounds (Janes and Oldfield 1985) (Figure 4.14A), but is difficult to use in the reverse direction, i.e. to derive structural information from the chemical shifts. A

B

-150

,~ -60 ~

r~ -100

7

-80

~ -100 ~ Q

-50 -50

. . .

-100

-150

C a l c u l a t e d 29Si shift ( p p m )

Q

-120

....

i ....

-100

Calculated

~'..,,

I,....

-80

i ....

I.

-60

298i shift

(ppm)

Figure 4.14. Observed and calculated 298i shifts for a number of different silicate mineral structural types. A. Shifts calculated by the group electronegativity approach, from Janes and Oldfield (1985), by permission of the American Chemical Society. B. Shifts calculated by the method of Sherriff et al. (1991), used by permission of copyright owner.

224


Shortcomings in the simpler relationships have led to the development of another empirical approach which takes into account the positions of the next-nearest neighbours and makes a correction for the geometry of the S i-O-X coordination triangle (Sherriff et al. 1991). The equation relates ~ to 1)', a parameter which includes a standard geometric term which is multiplied by a measure of the strength of the cationoxygen bond, summed over all the first-neighbour cations. To make the equation more general for structures containing distortions arising from highly strained tings, a correction term log(D) was introduced, where D is the distance between the Si and the nearest-neighbour cation; thus, ~O' - SI[(1 - 3cos20i)/3Ri3)(exp[(ro - ri)/0.37])(log(Di)]

(4.26)

where the term in COS20describes the interaction between the magnetic dipoles of the bond and the nucleus, and the term in ro - ri is a weighting factor to correct the dipole moment according to its bond valence (Sherriff et al. 1991). A good linear relationship has been found between ~ and 1)' for 124 Si sites in a wide variety of silicates: 6 = 701.6~' - 45.7

(4.27)

This relationship can accurately predict the chemical shifts of a wide range of silicates from a knowledge of their structures (Figure 4.14B), and has also been applied with reasonable success to a series of titanosilicates (Labouriau et al. 1998) but the introduction of higher levels of correction factors makes it difficult to use in the reverse sense to derive structural information from the chemical shift. Although simple correlations between the 29Si chemical shift and geometrical parameters can provide useful indications of structure for restricted groups of related compounds, the most precise structural information is certain to be provided in the future by comparing the NMR spectra with ab initio calculations of the NMR parameters made on the basis of assumed models. An indication of the potential of such an approach is provided by the ab initio calculations made by Bull et al. (2000) for a zeolite, siliceous ferrierite. The chemical shifts and intensities of the five Si sites in this compound were calculated for two differing published structures, and compared with the experimental NMR spectrum (Figure 4.15). This approach, which was also carried out for the 170 spectrum of siliceous ferrierite, clearly demonstrates the superiority of one of the postulated structures over the other (Bull et al. 2000). Calculations using density functional theory have also been successfully used to predict 298i chemical shifts in zeolites (Valerio et al. 1999) and in the SiO2 polymorphs coesite, low cristobalite and e~-quartz (Xue and Kanzaki 2000). Approaches to structure elucidation by NMR involving theoretical calculations will become more generally used as the calculation techniques improve.

225

29Si N M R

A

3

2

5

1

4

observed

B

3

2

5

1

4

calculated

Lewis model

C

M~176 i i. i

calculated

.

-108

....

'.

-114

4

.

-120

zgsi shift (ppm) w.r.t. TMS Figure 4.15. Schematic representation of A. observed and B.,C. calculated 29Si NMR spectra of the five Si sites in siliceous ferrierite zeolite. The ab initio calculations are based on two reported structural models, and indicate a clearly better fit to model B. From Bull et al. (2000), by permission of the American Chemical Society.

4.7. FIVE AND SIX-COORDINATED Si-O COMPOUNDS

The best-known compound containing Si(VI) is the high-pressure silica polymorph stishovite, which has a chemical shift of 191.3 ppm (Stebbins and Kanzaki 1991). Other high-pressure silicate phases also known to contain Si(VI) are shown in Table 4.4, together with their 298i chemical shifts. The natural abundance 29Si MAS NMR spectra of the two high-pressure hydrous magnesium silicates Phase B and superhydrous Phase B, obtained by cross-polarisation with 1H, show, in addition to tetrahedral resonances at - 64.0, - 75.8 and - 75.0 (Phase B) and at - 74.6 (superhydrous Phase B), the most deshielded Siw resonance positions reported for octahedral Si (Phillips et al. 1997). These unusually positive Si w shifts were attributed to the absence of oxygen sharing with other Si vt and the large degree of oxygen sharing with Mg octahedra (in both Phase B and superhydrous Phase B all 12 polyhedral edges are shared with MgO6). Attempts have been made to relate the Si(VI) NMR data to structural parameters using similar relationships to those for Si(IV) units. The more negative chemical shifts of Si(VI) can qualitatively be explained in terms of the number and field strength of the nearest-neighbour cations (Stebbins and Kanzaki 1991). The approach of Sherriff et al. (1991) (equation 4.26 without the term in log(D)) is found to give reasonably good predictions of chemical shifts, as long as the protons in the structure are not included in the calculation (Figure 4.16A). This results in a line of best fit to the Si(VI) data given by

226


Table 4.4. A selection of 2 9 8 i chemical shifts reported for compounds containing Si(VI), in ppm with respect to TMS.

29Si shift

Compound thaumasite Mg-Si ilmenite stishovite Na-Mg-Si pyroxene Mg-Si garnet wadeite Mg-Si perovskite Ca-Si perovskite CaSi205 Mg12Si4019(OH)2 (Phase B) Mg2oSi6H8036 (superhydrous Phase B)

-

.mq

B ~

-80 .~ -120

-120

[~

Si

r~ oH

o~Si

~ -16o

r~ -160

o,1)Si

-200

~

-200

,

-0.3

Grimmer (1980) Stebbins and Kanzaki (1991 ) Stebbins and Kanzaki (1991 ) Stebbins and Kanzaki (1991 ) Stebbins and Kanzaki (1991) Stebbins and Kanzaki (1991) Stebbins and Kanzaki (1991 ) Stebbins and Kanzaki (1991) Stebbins and Kanzaki (1991) Phillips et al. (1997) Phillips et al. (1997)

179.6 181.0 191.3 194.7 197.6 203.1 191.7 194.5 193.4 170.4 166.6

A

-80

Reference

-0.2

I

-0.1

,

,,

I

0

n

,

i

I

i

15

17.5

20

22.5

25

~EN

Figure 4.16. 298i chemical shifts ~ for 6-coordinated Si-O compounds plotted against: A. the structural parameter 1"~of Sherriff et al. ( 1991) (equation 4.26 without the term in log(D)), B. the group electronegativity parameter EN of Janes and Oldfield (1985). The lines for 4-coordinated Si-O compounds (without data points) are also included for both equations.

6 = 145.4gT -

173.2 (Stebbins and Kanzaki 1991)

(4.28)

Note, however, that this line does not coincide with that defined for Si(IV) c o m p o u n d s , also plotted in Figure 4.16A. T h e g r o u p e l e c t r o n e g a t i v i t y a p p r o a c h (Janes and O l d f i e l d 1985) also gives a reasonably good fit to the data (apart from that for Mg-Si perovskite), leading to the linear relationship for Si(VI) c o m p o u n d s 6 - - 13.48 , ~ E N + 108.7 (Stebbins and Kanzaki 1991)

(4.29)

29Si NMR

227

These data, plotted in Figure 4.16B, show significant scatter, although the line about which they lie represents a reasonable continuation of the line for Si(IV) compounds, and also passes through the region now known to be characteristic of Si(V). The literature contains considerable discussion of the existence of Si(V) in glassy systems, inferred from the presence of small 298i NMR peaks at about - 150 ppm in the spectra of potassium silicate glass (Stebbins and McMillan 1989, Stebbins 1991), alkali silicate glasses quenched from the liquid state at high pressures (Xue et al. 1991) and at ambient pressure (Stebbins and McMillan 1993), aluminosilicate glasses (Risbud et al. 1987, Sato et al. 1991) and "superquenched" calcium aluminosilicates (Sato et al. 1991 a). Since the structures of these amorphous materials could not be determined independently by X-ray crystallography, the existence of Si(V) and its assignment to the - 150 ppm resonance was made by inference from its position between Si(IV) and Si(VI). It was also supported by the observation that in some organic molecules the chemical shifts for Si(OR)5 groups are 43-51 ppm lower than the corresponding Si(OR)4 groups (Holmes 1990), and that quantum mechanical calculations for SiF molecules (Tossell and Lazaretti 1986) predict a similar effect accompanying a coordination change from four to five. Direct confirmation for this assignment has now come from a study of crystalline CaSi2Os, which contains a 298i NMR resonance at - 150 ppm (Stebbins and Poe 1999) (Figure 4.3) and is known from X-ray crystallography to contain Si(V) in addition to Si(IV) and Si(VI). 4.8. CROSS-POLARISATION (CPMAS) EXPERIMENTS

In cross-polarisation experiments two nuclides are excited at the correct frequencies to satisfy the Hartmann-Hahn condition (Chapter 2) so that the magnetisation of the more abundant spin system (typically 1H) is transferred to the system with the smaller signal (298i). The FID of the nuclide of interest (29Si) is acquired with a stronger signal and at the usually shorter T1 value of the protons, allowing the spectrum to be obtained more quickly and with an enhanced signal/noise level. 4.8.1 Cross-polarisation between

1H a n d 29Si

The Hartmann-Hahn condition in these experiments is usually established by using Q8M8(Si[(CH3)318Si8020) as the calibration compound which also has the advantage of providing a secondary chemical shift reference. Other reference compounds including kaolinite and sepiolite have been suggested, but the toxicity of the latter could prove to be a disadvantage. As well as providing improved sensitivity, cross-polarisation experiments between 1H and 29Si can also be used to provide additional structural information, since the Si atoms in closest proximity to protons are preferentially enhanced. The data from CPMAS experiments should, however, be regarded as qualitative rather than quantitative. If improved quantification is required, it is essential to ensure that the signal is not

228


saturated (i.e. all the 298i nuclei are allowed to recover their magnetisation completely between pulses). By taking strict precautions and using single-pulse experiments, Farnan et al. (1987) were able to identify the hydrated structural units in hydrous silica glass. The hydroxyl concentrations estimated from these experiments proved to be lower than previously indicated by other techniques. Similar work on sodium silicate glasses (Ktimmerlen et al. 1992) has indicated that H20 depolymerises the silicate network, and that both Si-OH and molecular water are present in these hydrous glasses. The improved sensitivity of 1H-29Si CP MAS has been exploited by Alma et al. (1984) to record the 298i spectra of synthetic mica and montmorillonite and by Kodama et al. (1989) in a study of ground kaolinite. In these studies, the non-CP and CP spectra were identical in shape and position, indicating that all the Si sites are in similar proximity to protons. Of greater interest from a structural point of view are examples where CP and non-CP spectra differ. In the case of the hydrated phases of pure silica (silica gel and fumed silica), the intensities of the Q2 and Q3 peaks which are bonded to hydroxyl groups should be significantly increased by comparison with the Q4 peak (unbonded silica). This was shown to be the case by Chuang and Maciel (1997) who used their 298i results to refine a model of the silanol bonding at the surface of silica gel (Figure 4.17). The technique was also used in a study of gel synthesis of albite glass (Schmelz and Stebbins 1993). Non-CP

CP

Silica ge

F u m e d s" "

!

-70

|_

!

.I

-100

i

_|_

|

-130

!

I

-70

a

I

I

-100

I

I

!

m

-130

29Si shift (ppm) w.r.t. T M S Figure 4.17. Use of cross-polarisation between ~H and 298i to investigate the surface hydration of silica gel and fumed silica. Cross-polarisation increases the intensity of the Si sites in proximity to hydroxyls, shown by the curve-fitted spectra to be the Q2 and Q3 sites. Adapted from Chuang and Maciel (1997).

29Si NMR

229

The discrimination of protonated Si sites by CP MAS was also used by Yang and Kirkpatrick (1989) in a study of the hydrothermal decomposition of albite and sodium aluminosilicate glass, and rhyolitic glass (Yang and Kirkpatrick 1990), and as a means of differentiating between the various sites in acid-treated montmorillonites (Tk~ic et al. 1994). CP MAS NMR has also been used to identify a Si site at - 100 ppm in a microporous material derived from acid-leached metakaolinite as the Q3 unit Si(OSi)3OH (Okada et al. 2000).

4.8.2 Cross-polarisation between 19F and 29Si Like 1H, 19F is another nuclide with abundant magnetisation which can be transferred to 29Si to improve its sensitivity in fluorinated Si compounds. The Hartmann-Hahn condition was established by using sodium hexafluorosilicate, for which the relaxation time (3.2 s) is rather long for convenience, but no more suitable compound has been identified (Hoffner et al. 1993). The technique has been used to study octadecasil, a siliceous clathrate compound containing fluoride ions, and a fluorinated siliceous MF1type zeolite prepared in a fluoride medium (Hoffner et al. 1993). The 19F to e9si experiment was found to be not as efficient as a 1H to e9si cross-polarisation because the dipolar interactions between F and Si are weaker due to the lower magnetogyric ratio of 19F by comparison with 1H. Nevertheless, this cross-polarisation experiment can be used to enhance the sensitivity of S i, especially for atoms directly bonded to fluorine, and was used to confirm the assignment of the two Si resonances in the octadecasil spectrum (Hoffner et al. 1993). The technique has also been used to study fluorinedoped aluminosilicate glasses (Sebald et al. 1992).

4.8.3 O t h e r cross-polarisation e x p e r i m e n t s with

29Si

Cross-polarisation to protons is only applicable to a restricted range of protonated inorganic materials (gels, some glasses and minerals), but the benefits of increased sensitivity have recently been demonstrated in non-protonated systems by cross-polarising to other nuclei such as quadrupolar eTA1 and e3Na (Shore et al. 1999). The technical difficulties arising from the use of a quadrupolar nucleus and the relatively small differences between the Larmor frequencies of 295i and eTA1 or e3Na can be overcome by optimising the efficiency of the spin-lock, which depends strongly on the stability of the spinning speed, the rf power levels and the quadrupolar coupling constants (Shore et al. 1999). The larger linewidth of the e3Na resonance makes it more difficult than eTA1 to spin-lock uniformly. Applying 27Al-e9si CP to a sample of crystalline albite (Figure 4.18A) increased the signal/noise by a factor of five, corresponding to a 25-fold reduction in the ~acquisition time. However, the two Si sites with one next-nearest neighbour A1 (at -96.1 and 103.9 ppm) are more intense than the site at - 91.8 which has two nearest-neighbour -

230


A Non-CP

23Na --~ z9Si CP

C 27A1 ~ 29Si CP ,,~

....

i ....

-90

| ....

i ....

-100

' w , ~'~ , i " "

-110

29Si shift (ppm) w.r.t. TMS Figure 4.18. Effect of cross-polarisation to 23Na and 27A1 on the 298i spectrum intensity of albite. Note that all three spectra have been scaled to be noise-equivalent. From Shore et al. (1999), by

permission of the American Chemical Society. A1 atoms; this is related to differences in the relaxation rate and the cross-polarisation rate of the - 9 8 . 1 ppm site (Shore et al. 1999), and illustrates the necessity for caution when drawing conclusions from CP intensities. A smaller but still useful signal/noise enhancement was found for this material with 23Na-29Si CP (Figure 4.18B), but the intensity ratios of the three Si sites are different again, reflecting differences in the relaxation time of the sites and the larger quadrupolar frequency of 23Na (Shore et al. 1999). Such cross-polarisation procedures to enhance the signal intensity of 298i are expected to be of use in making two-dimensional isotropic-anisotropic correlation spectroscopy more accessible to systems without protons.

4.9. GLASSES, GELS AND OTHER AMORPHOUS MATERIALS

Unlike X-ray diffraction techniques, solid-state NMR does not depend on the presence of long-range atomic periodicity in a structure, but is sensitive to short-to-medium range geometries and orderings within 10A of the observed nucleus. It is therefore particularly useful for investigating X-ray amorphous phases such as glasses, gels, and other amorphous materials. In most amorphous materials there is, generally, a much wider range in the parameters determining the local Si environments than in the corresponding crystalline form, leading to broad, overlapping peaks from which the information is extracted by peak fitting or deconvolution techniques. Since these procedures

29Si N M R

231

are critical to the extraction of data from broad overlapping 29Si NMR spectra they are considered in more depth in Section 4.9.2.

glasses 298i NMR allows the identification and quantification of the various types of structural

4.9.1 Silicate

units in silicate glasses, defined as Qn, where n is the number of Si-O-Si bridges. The presence of a range of Si-O-Si angles in each Qn group leads to broadening, which can be modelled by a variety of analytical functions, as reviewed by Dupree (1994). The peak maximum of the broad Si spectrum of pure fused silica occurs at about - 111.5 to - 1 1 2 ppm, and its width is consistent with a range of tetrahedral bond angles of about 130-179 ~ (Oestrike et al. 1987). 298i NMR measurements on amorphous silica densified at 50 kbar and 600~ (Devine et al. 1987) have been used to detect a pressure-induced shift in the mean Si-O-Si bond angle from 143 ~ to 138 ~ Analysis of the bond-angle distribution suggested that the compressive force facilitates formation of low-member ring structures by reducing the distance of closest approach of the second-nearest-neighbour oxygens. The structure of amorphous SiO powders has been invesigated by 298i NMR which indicates that by contrast to the random bonding present in evaporated SiO films, the powders consist of microscopic regions of Si greater than 20A in size and SiO2, together with a significant amount of interphase material (Dupree et al. 1984a). The addition of alkali or alkaline earth oxides to fused silica promotes the formation of non-bridging oxygens, and changes the distribution of the Qn units which have been ascertained from the 298i NMR spectra and compare well with theoretical predictions (Dupree et al. 1984). Resolution of the various Qn units in alkali silicate glasses by 298i NMR can be enhanced by exploiting differences in the chemical shift anisotropy (CSA) of the different units. Thus, Stebbins (1987) was able to distinguish a small amount of Q4 in the major Q3 unit of sodium silicate glasses using static (non-spun) spectra in which the Q4 sites are accentuated because of the small CSA of this more symmetrical unit (Figure 4.19). Differences in the CSA of various Q3 units of sodium silicate glasses have also been exploited by Duer et al. (1995) in a 2D experiment in which the normal MAS spectrum is obtained in one dimension and the chemical shift tensor associated with each Qn species is obtained in the second dimension. This technique has revealed the existence of two Q3 units of different CSA in sodium disilicate glass (Duer et al. 1995), and has potential application to many other glass systems. The situation in aluminosilicate glasses is complicated by the additional effects of next-nearest-neighbour A1 on the Si shifts. Although some Qn sites can be unambiguously assigned, others such as @(0A1) and Q4(3A1) occur in the same chemical shift range and cannot be differentiated on this basis. Systematic variations in the 298ipeak positions and widths of sodium aluminosilicate and calcium aluminosilicate glasses can

232


A

MAS

34% Na20

B

Unspun

34% N

37%Na~O~/[Q~Q ~ / ~ 37~176 "~'~"--'----41%Na20_~_,,, ~ 41%Na~/__~ , ~ -60

-80 -100 29Si shift

0

-100

-200

(ppm) w.r.t. TMS

Figure 4.19. 298iNMR spectra of sodium silicate glasses containing the indicated amounts of Na20 (mol%). A. MAS spectra, showing resolved Q2 and Q3 units. B. Unspun spectra allowing the Q4units (shaded) to become visible because of their smaller chemical shift anisotropy. From Stebbins (1987), by permission of MacMillan Magazines Ltd.

be related to the extent of Si-A1 ordering (Lee and Stebbins 1999). A downfield shift with increasing A1203 content in the 29Si spectra of rapidly-quenched sodium aluminosilicate glasses has been interpreted in terms of a lower degree of network polymerisation as the concentration of network modifiers increases (Schmticker et al. 1997). Provided appropriate fitting methods are used, NMR spectra provide quantitative information about the relative numbers of the various Q" units in binary silicate glasses which can be tested against predictions from various structural models (Eckert 1992). The distribution of Qn units in simple glasses depends on the composition, and could theoretically follow either a binary distribution of the two species present in the crystalline composition (Figure 4.20A), or a random statistical distribution (Figure 4.20B). 29Si NMR results for lithium silicate glasses are consistent with a binary distribution over the limited composition range 25-29 mol% Li20 (Figure 4.20C), with deviations above 33 mol% Li20 caused by disproportionation of Q3 to Q4 + Q2 (Dupree et al. 1990). A similar binary distribution of Q4/Q3 units has been found in lead silicate glasses below about 30 mol% PbO, but at higher PbO concentrations up to 65 mol%, the distribution of Q" units is more consistent with the random statistical model (Dupree et al. 1987). At 70 mol% PbO, the glass contains mainly isolated SiO44- groups in a lead-oxygen matrix. A 298i MAS NMR study of a series of sodium lead silicate glasses shows the presence of Q~ and Q2 units which interconnect the Pb-O-Pb network in compositions of > 50 mol% PbO. As the alkaline oxide concentration of the glass is increased, the microhardness decreases, probably due to the conversion of more rigid covalent Q3 and Q4 structural units to less-rigid Q1 and Q2 units (Shrikhande et al. 2001).

298i

233

29Si NMR A

--

B

~

C

Q3 QZQ,QO

'~176 It74AA

l,,,,,I 0

li.,,I 0

Qi 20 o

20 20

60

100

o

20

R+zO or 1/2+O (tool %)

60

/.~-'/"

""

~"...

60 " ~ .....

: 60[ ~Q3 zj

o

.

~ 100 o .

20

"..~i~...94 24

~"

QiS"

32

40

LizO (mol %)

Figure 4.20. Distributionof Qnunits in a binary silicate glass, predicted by: A. binary distribution model, and B. random statistical distribution model, from Dupree et al. (1987). C. Experimental data for lithium silicate glasses plotted against the line calculated from the binary distribution model (dashed). From Dupree et al. (1990), by permission of Elsevier Science.

298i MAS NMR together with 31p NMR has been used to study the changes occurring during thermal conversion of phosphosilicate gels to glasses (Clayden et al. 2001). Dried gels containing 10 and 30 mol% P205 have very similar siloxane frameworks containing silanol groups and trapped orthophosphoric and pyrophosphoric acid. However, the course of the subsequent structural evolution depends markedly on the P205 content, with the presence of 6-coordinated Si being observed in the glassy matrix of the higher P-content glass in which co-polymerisation of the silicate and phosphate tetrahedra occurs at lower temperatures than in the lower-P composition (Clayden et al. 2001). Octahedral SiO6 also occurs in alkaline earth phosphosilicate glasses as well as in the related crystalline phosphosilicates Si50(PO4)6, 8i3(PO4)4 and SiP207. Other glassy systems in which the distributions of the silicate units have been studied by 298i NMR include SiO2-PzOs-CaO-MgO phase-separated glasses (Oliveira et al. 2000), SiO2-LizO-P205 glasses (Holland et al. 1998), binary lead silicate glasses (Fayon et al. 1998) and SiO2-NazO-P2Os-B203 glasses (Yamashita et al. 1999). Some compositions within the latter system were found to contain 6-coordinated Si. Devitrification (formation of crystalline products) in lithium metasilicate glass has also been studied by 29Si NMR (Clayden et al. 1998), as has the devitrification of rapidly quenched glasses of cordierite composition (Okada et al. 1998). The 29Si NMR spectra of a series of sodium borosilicate glasses (Bunker et al. 1990) show that in compositions containing 30-40 mol% B203 the predominant structural unit is Q4 (~ of about - 1 l0 ppm), but at lower B203 contents the 29Si shift becomes less negative, reflecting an increase in the number of Q4(1B) ( - 105 ppm) and Q3(0B) units ( - 90 ppm). The latter component could also arise from the presence of Q4(3B)

234


units, but this possibility is ruled out by the glass stoichiometry (Bunker et al. 1990). Borosilicate glasses of various compositions have also been studied by Martin et al. (1995) and by Martens and Miiller-Warmuth (2000), who concluded on the basis of 29Si, liB and 23Na NMR spectroscopy that there is much better mixing of the silicate and borate units than previously assumed, and that the Na + is more uniformly distributed. Borate, borosilicate and boroaluminate melts have also been studied by 298i, liB and 27A1 NMR by Sen et al. (1998). Yamashita et al. (2000) have used a number of NMR nuclides including 298i in a study of alkaline earth phosphosilicate and aluminoborosilicate glasses. In some of these compositions, 29Si signals corresponding to 5 and 6-coordinated Si were detected, the intensities of which are related to the glass composition (Figure 4.21). Studies of the differences between the distribution of the silicate species in molten and quenched glasses have been facilitated by the development of high-temperature NMR probes capable of operating up to 2000~ (Taulelle et al. 1989). However, many silicate systems melt at much lower temperatures and do not require this type of laser-heated probe. Furthermore, suitably narrow spectra can be obtained from molten systems without the need for magic angle spinning, considerably simplifying the probe design (although MAS probes are now available for operation at temperatures of 600-700~ The narrow 298i NMR lines obtained from a variety of silicate liquids indicate the operation of a mechanism for rapid exchange between the various structural species known to be present in the glasses. The NMR results for K28i409 have allowed the process to be modelled and the exchange frequencies determined (Faman and Stebbins 1990).

o~)Si oV)si k ~Si i

Ca Sr

L

0

I

I

-100

.

I

-200

.

I

I

._

-300

29Si shift (ppm) w.r.t. TMS Figure 4.21. 29Sispectra of alkaline-earth phosphosilicate glasses of composition 0.3MO-0.05SiO2-0.65P2Os, showing evidence for Si in 6-fold and possibly 5-fold coordination. From Yamashita et al. (2000), by permission of Elsevier Science.

29Si NMR 0% N

i

. . . .

!

. . . .

!

235

2.3% N

. . . . .

!.

,.

i

-50 -100 -150

.

.~..

I

. . . . . . .

8. . . . . .

4.3% N

d ~

~

,

....

-50 -100 -150

!

. . . . . .

!

. . . . . . .

J_-~,

-50 -100 -150

29Si shift (ppm) w.r.t. T M S Figure 4.22. Curve-fitted 29Si spectra of Na-Si-O-N glasses showing the effect of increasing nitrogen (in atom %) on the Q1 and Q2 structural units. From Unuma et al. (1992), by permission of copyright owner.

An increasing interest in oxynitride compounds has led to NMR studies of the effect of incorporating nitrogen into glassy phases. In work on the Na-Si-O-N system, Unuma et al. (1992) have shown that increasing the N content from zero to 4.3 at % results in the apparent progressive increase in the number of Q1 and Q2 units and a decrease in the number of Q4 units (Figure 4.22). Taking into account the change in chemical shift resulting from the substitution of one N for one O in SiO4 (estimated as + 15 ppm) and the charge compensation by Na + accompanying each such substitution, it was concluded from the NMR results that about half the total nitrogen atoms are bonded to two silicons. Other studies of nitrogen-containing glasses and their recrystallisation products have been reported by Aujla et al. 1986 (Y-Si-A1-O-N glasses), Sato et al. 1990 (SiA1ON glass) and Nordmann et al. 1996 (Li-A1-Si-O-N glasses). 29Si NMR has also been used to investigate the incorporation of nitrogen into glasses in the silicon oxynitride system (Kohn et al. 1998), in which the presence of SiO4 _ xNx tetrahedra were detected (where x = 0,1,2,3,4). Glasses in the silica-rich portion of the system SiO2Si3N4-AlzO3-A1N have been shown by 29Si NMR to contain Si(O3N) tetrahedral units (McMillan et al. 1998).

4.9.2 D e c o n v o l u t i o n

of 29Si NMR

spectra

The most commonly used method for deconvoluting 29Si NMR spectra is to fit Gaussian peaks, the area of which is taken as a measure of the concentration of that particular species. Gaussian peaks describe a random distribution of parameters, a condition which may not necessarily be fulfilled in some glasses, in which regions of heterogeneity may exist. In an examination of this problem, Mahler and Sebald (1995) deconvoluted the 29Si spectra of sodium silicate glasses using Gaussian and several non-symmetric lineshapes, and found that the judgement of which gave the most acceptable fit depended on the goodness-of-fit criterion used. More importantly, the

236

Multinuclear Solid-State NMR of Inorganic Materials ~iso (ppm)

+

A

B

Q2

B

Q Q0

~, -90

,

~

Q3 . ~,

|

omq

~9 -70

,
'

~,~ -60 'i f!

~~ -20 .me ro~

~

.el

t

i

.,

I

i

I

,~

-80 -90 -I00 -ll0 29Si shift (ppm) w.r.t. TMS

Figure 4.25. 298idouble quantum NMR spectrum of 29Si-enriched 25Na20-75SiO2 glass. The diagonal peaks are marked by filled circles, and the off-diagonal peaks indicate the connectivities between the structural units (in this case between Q3 and Q4). From Glock et al. (1998), by permission of Elsevier Science. connections between the Q3 and Q4 units, but implications for the bond angles between the connected units arising from this earlier work have been questioned (Dupree 1991).

4.9.4 Chalcogenide glasses Silicon-containing chalcogenides are non-oxide compounds such as SiS2 and SiSe2 containing both comer and edge-sharing 8i84/2 or SiSe4/2 tetrahedral units, by contrast with oxide glasses, in which the connectivity mode is exclusively via corner-sharing tetrahedra. The resulting structures have given rise to their own nomenclature; E (~ denotes corner-shared SIS4/2 tetrahedra, E ~) denotes Si84/2 units sharing one common edge with an adjacent tetrahedron and linked to two other 8i84/2 units by comer sharing, and E 1100~ (Day et al. 1995) and synthetic hydrotalcite, Mg6Alz(OH)16CO3.4H20, a layer-lattice compound with useful catalytic properties in which A1 is substituted for Mg in the octahedral layers, with charge balance being achieved by the presence of the interlayer carbonate ions. A combination of 27A1 and 25Mg NMR indicates that on dehydroxylation this compound forms an assemblage of poorly crystalline MgO containing substituent A13+ and vacancies, and a spinel-type transition alumina which subsequently forms MgAI204 (MacKenzie et al. 1993). Aluminium titanate (tielite, AlzTiOs) has excellent thermal shock resistance but poor mechanical strength which can, however, be improved by reinforcing with whiskers of a related phase such as potassium hollandite (KzAlzTi6016). Such composites can be formed by thermal decomposition of sol-gel precursors, reaction sintering of the two phases or by thermal treatment of an appropriate glass-ceramic material. 27A1 MS NMR has been used to study the co-formation of these two phases during thermal treatment, and indicates that hollandite crystallises as whiskers within the tielite matrix (Kohn and Jansen 1998).

5.10. CEMENTS Portland cement and high-alumina cements contain, in addition to calcium silicate phases, calcium monoaluminate, CaAI204 (or CA in cement chemist's shorthand, where C = CaO and A = A1203). The 27A1 NMR spectra of this compound, in which the A1 is exclusively in tetrahedral coordination, and a number of other calcium aluminates have been determined (Mtiller et al. 1986), and more recently, using satellite transition spectroscopy (SATRAS) which has allowed the multiple tetrahedral sites in the various calcium aluminates to be distinguished (Skibsted et al. 1993). The NMR parameters for the synthetic aluminates and a number of their hydration products are shown in Table 5.4. The calcium aluminates, especially CaAl204 (CA) and Ca3A1206 (C3A) react readily with water, contributing to the hydraulic activity of the cement. The A1 in the hydrated phases is exclusively in six-fold coordination, making 27A1 NMR a convenient method for monitoring the progress of hydration of both the pure aluminate phases and alumina cements (Figure 5.32A). This technique has been used to study the

314

Multinuclear Solid-State NMR o f lnorganic Materials Table 5.4. 27A1 NMR parameters for calcium aluminates and their hydration products in Portland and high alumina cements, from Skibsted et al. (1993). Chemical shifts relative to Al(H20)63+. Compound

XQ (MHz)

"q

~(ppm)

CaA1407 (CA2)

6.25 9.55 2.50 2.60 2.60 3.32 3.37 4.30 9.7 3.8 8.69 9.30 2.4 0.705 1.8 0.36 1.7

0.88 0.82 0.2 0.75 0.95 0.53 0.39 0.47 0.4 0.7 0.32 0.54

75.5 69.5 81.9 83.8 86.2 82.7 81.6 81.2 85.9 80.2 79.5 78.3 10.2 12.36 10.2 13.1 11.8

CaA1204 (CA)

CaI2Al14033 (Cl2AT) Ca3AI206 (C3A) CaA12H2oO14 (CAHlo) Ca3A12HI2OI2 (C3AH6) Ca4A12H2602o (C4AHI3) Ca6AI2S3H64Oso (C6AS3H32) Ca4AI2SH24022 (C4ASHI2)

0.09 0.19

A

I

200

I

l

100

0

..,I

,

~

1.0

-100

1:1 water:cementN~ complete hydration

o

0.8

f

(b)

~ (a) % O

0.6 %% ~ ~ ..

I

I

I

200

100

0

.

I

-100

o~

~

0.4

o

0

4

8 12 T i m e (hr)

O

16

0

2o

27A1shift (ppm) w.r.t. AI(H2O)63+ Figure 5.32. A. 27A1NMR spectra of (top) unhydrated alumina cement (principally monocalcium aluminate), and (bottom) product of full hydration with demineralised water at a cement: water mass ratio of 1:1. Asterisks indicate spinning side bands. B. Change in the percentage of four-coordinated AI in alumina cement during hydration, as a function of time estimated by 27A1 MAS NMR. Open symbols (a): hydration with demineralised water, Filled symbols (b): hydration with 0.5 mass percent Li2CO3 solution. After Luong et al. (1989), by permission of the American Ceramic Society.

27 A 1 N M R

315

hydration of C3A (Lippmaa et al. 1982) and the temperature dependence of CA hydration (Mtiller et al. 1984, Rettel et al. 1985). 27A1 NMR has also been used to study the hydration of alumina cement and the effect on the reaction of lithium-containing setting accelerators (Luong et al. 1989). The lithium was shown by this means to eliminate the induction period of the hydration without changing the rate of hydration (Figure 5.32B). Cements often contain small amounts of heavy metals carried over from the raw materials from which they are produced. The location of these impurities in the hydrated cement phases may affect their subsequent leachability, and is therefore of environmental interest. A detailed analysis of the 27A1NMR sideband structure of C3A hydrated in the presence of chromium and zinc ions has proved useful in determining the way in which these species enter into the structure of the hydration products of C3A (Moulin et al. 2000). 27A1 NMR has also been used to study the environment of A1 as a guest ion in the two calcium silicates which constitute the other major phases of Portland cement, Ca3SiO5 (alite) and CazSiO4 (belite). Low levels of A1 (well below 1%) substituted for Si in both minerals have been detected (Skibsted et al. 1994). The 27A1NMR spectrum of Al-substituted belite (Figure 5.33A) indicates a single quadrupolar tetrahedral site with a value of g (96.1 ppm) representing the most deshielded chemical shift reported for A104. The octahedral resonance in this spectrum was attributed to an amorphous aluminate impurity phase (Skibsted et al. 1994). The 27A1 NMR spectrum of A1 substituted for Si in alite (Figure 5.33B) shows a principally tetrahedral A1 resonance, broadened due to the superposition of several lineshapes arising from A1 in different tetrahedral sites in alite (the monoclinic alite structure contains 18 different tetrahedral Si sites). A low-intensity A106 resonance was ascribed to substitution for octahedral Ca or vacancies in the alite structure (Skibsted et al. 1994).

A

B

simu ~ o b s.............................................................. e ~ ,.J., ~ :~.~= 120

60

0

-60

~ ~..,__,~] ~_...., 80... 40 300 100 -100 -300

27A1shift (ppm) w.r.t. AI(H20)63+ Figure 5.33. A. 9.4T 27A1NMR spectrum of A1 guest ions in Ca2SiO4 (lower) and (upper) simulation of the partly overlapping lineshapes using an AI(W~:A1(vI~intensity ratio of 58:42 with a single A1~ site and a Gaussian distribution of lineshapes for the A1(vI~site. B. 9.4T 27A1 NMR spectrum of A1 guest ions in Ca3SiO5 with (inset) the expanded central transition for the A1(~v)resonance. The asterisk marks the A1(vI) resonance corresponding to a total A1 intensity of 3%. From Skibsted et al. (1994), by permission of the Royal Society of Chemistry.

316


The substitution of A1 into poorly crystalline calcium silicate hydrate C-S-H has been studied using 27A1 MQ MAS NMR (Faucon et al. 1998). This reaction is facilitated by the presence of Na + which lodges in the interlayer spaces of the C-S-H structure and compensates the charge imbalance when the tetrahedral S i is substituted by A1. The 27A1 NMR spectra distinguished two substitution sites corresponding to the bridging and non-bridging positions in the tetrahedral chains of C-S-H, with the bridging sites being more favoured at higher aluminium concentrations (Faucon et al. 1998).

5.11. NITRIDE AND OXYNITRIDE COMPOUNDS

Aluminium nitride (A1N) has a wurtzite-type structure composed of A1N4 units in a hexagonal lattice. Its 27A1 spectrum consists of a single resonance at 114 ppm (Dupree et al. 1988). Analysis of the 27A1 satellite peaks in the spectra indicates a value for XQ and r I of 1.913 MHz and ca. 0 respectively, with an unusual positive temperature coefficient of • which may be related to the wurtzite structure (Bastow et al. 1998). A1N is commonly prepared by nitridation of A1 metal, and may contain oxide impurities which can be detected by 27A1 NMR with extremely high sensitivity (down to about 0.05 wt%) (Haase et al. 1989). Surface reactions of A1N with moisture form aluminium hydroxides of Which the AI(VI) can readily be detected by 27A1 NMR, and in ultrafine A1N powders can constitute the major feature of the A1 spectrum (Hayashi et al. 1987). Oxygen can dissolve in the A1N structure to form a defect spinel related to ~-A1203 but more thermally stable on account of the presence of the nitrogen. The region of phase stability occurs around 35.7 mol% A1N, corresponding to A123027N5, with the 27A1 NMR spectra of these compounds containing resonances corresponding to A1N4 (114 ppm), A104 (65 ppm) and A106 (12 ppm) structural units (Dupree et al. 1988). The 14.1 T 27A1NMR spectrum of an A1ON sample prepared with 50 mol percent A1N obtained at spinning speeds of 18-20 kHz (Figure 5.34) shows evidence of several overlapping resonances between 50 and 110 ppm which appear to arise from mixed A1-N-O units (Fitzgerald et al. 1994). Simulation of this spectrum has led to the suggested peak assignment: 114-117 ppm = A1N4, 106 ppm = A1N30, 96 ppm = A1N202 or A1NO3, 66 ppm - A104 (Fitzgerald et al. 1994). The 27A1 peak positions for A1-N, A1-O-N and Si-A1-O-N compounds are collected in Table 5.5. A new low-temperature method for producing ultrafine powders of A1N and A1ON involves the reaction of aluminium sec-butoxide and anhydrous hydrazine in acetonitrile at 80~ (Kim et al. 2000). The amorphous precursor is then heated in nitrogen, argon or ammonia and forms crystalline products < 1000~ 27A1 NMR has provided useful insights into the structural changes occurring during heating, revealing the presence of A104, A105 and A106 groups which give way to the single A1N resonance at ca. 113 ppm as nitridation progresses. The 27A1 NMR results suggest that the formation of

317

27 A 1 N M R

AINzO2 AINO3 IAIO~ AIN30 / I .

.

.

A'AI/A,O41{/: : ~ / components

.

..'./'Yl

.;..~5,,~..a

.

;

;-~

/i\~-i~-

150 50 -50 -150 27A1 shift (ppm) w.r.t. AI(H20)63+ 150

50

-50

-150

27A1shift (ppm) w.r.t. AI(H20) 3+ Figure 5.34. 27A1MAS NMR 14.1T spectrum of A1ON powder containing 50 tool % A1N. Observed spectrum (upper left), simulated spectrum (lower left) and peak components from deconvolution analysis (right). From Fitzgerald et al. (1994) by permission of the American Chemical Society. Table 5.5. 27A1 peak positions for A1-N, AI-O-N and SiA1ON compounds, relative to Al(H20)63+. Compound

8peak (ppm)

A1N 114 A1ON 14,67,106 15R polytypoid SiA1ON 10,93,112 21R polytypoid SiA1ON 10,112 [3-SiA1ON, z = 1 75,89,108 [3-SiA1ON, z = 4 8,75,89,108 O-SiA1ON, x -- 0.24 64.7 O-SiA1ON, x = 0.2 60 X-SiA1ON 2.8,59 X-SiA1ON 0.05-0.8,61.9-63

Reference Dupree et al. (1988) Fitzgerald et al. (1994) Smith (1992) Smith (1992) Dupree et al. (1988) Dupree et al. (1988) Sjoberg et al. (1992) Barris et al. (1997) Smith (1994) Sheppardet al. (1997)

A105 groups in the precursor results from the replacement of alkoxy groups by hydrazide species, and once formed, A1Os appears to facilitate the incorporation of nitrogen during heat treatment (Kim et al. 2000).

5.12. SIALON COMPOUNDS 5.12.1

Polytypoid sialons

Of the various known structures for the extensive series of compounds of Si, A1, O and N (the sialons), those most closely related to A1N are the polytypoid sialons (Si,A1)m(O,N)m+l. The non-metal atoms in excess of the 1:1 ratio required by the

318

M u l t i n u c l e a r Solid-State N M R o f l n o r g a n i c Materials

wurtzite structure are accommodated in the polytypoids by the incorporation of octahedrally coordinated layers in the structure and by half-occupancy of some of the tetrahedral layers. Earlier 27A1 NMR studies of the polytypoid sialons showed an A1N4 resonance at ca. 110 ppm and an A106 peak at ca. 10 ppm (Klinowski et al. 1984), sometimes with an underlying broad signal (Butler et al. 1984). Faster spinning speeds have revealed a second tetrahedral resonance at ca. 90 ppm (Smith 1992), assigned to tetrahedral A1N30 units on the basis of a model in which only A1 occupies the octrahedral layer and the A1 in the tetrahedral layers bond preferentially to O, by comparison with the Si (Figure 5.35A). Analysis of these 11.7 T polytypoid sialon spectra has led to the identification of the chemical shifts of the A1N30, A1N202 and A1NO3 resonances as occurring at ca. 93, 89 and 75 ppm respectively. The differences in these shift ranges from those found for A1ON (Fitzgerald et al. 1994) may reflect the fact that the sialon spectra were less well resolved than those of A1ON, possibly due to chemical shift dispersion effects arising from the presence of Si as well as A1 next-nearest neighbours.

5.12.2

fS-sialons

These compounds are the most commercially exploited sialons to date. They are isostructural with [3-Si3N4 and have a range of composition Si6-zAlzOzN8-z, where z can take values from zero (pure Si3N4) to ca. 4. From structural considerations, the A1 is expected to occur in a range of tetrahedral coordinations, but the 27A1 spectra are very field-dependent. At lower fields (4.7 T) reasonably narrow lines are seen at approximately 103 and 66 ppm, corresponding to A1N4 and A104 units respectively, but these are superimposed on a broader envelope and correspond to only about 10% of the

A

21Rpolytypoid sialon

B

13-sialon z --- 2.7

10A t

200

-I

O-sialon x = 0.24

D

x-sialon

6

If

I

C

3/0

.8

112

I

0

I

I

-200

1

!_

200

!

I

0

I

_1

-200

27A1 shift (ppm) w.r.t. AI(I-I20)63+

I

I

200

|-

I

0

l___J

-200

1__

200

i__

I

0

_l

_

I,L

-200

27A! s h i f t ( p p m ) w . r . t . A I ( H 2 0 ) 3+

Figure 5.35. Typical 27A1MAS NMR spectra of sialons. A. 21R polytypoid sialon. B. [3-sialon, z - 2.7, C. O-sialon, x = 0.24, D. X-sialon. Asterisks indicate spinning side bands. Spectra A-C from Sj6berg et al. (1992), by permission of the Royal Society of Chemistry. Spectrum D from Sheppard et al. (1997).

27 A l N M R

319

total A1 signal (Dupree et al. 1988). At higher fields (11.7 T) some detail emerges, but the spectra are still not well resolved (Figure 5.35B). The resonance at ca. 110 ppm is thought to be unlikely to arise from pure A1N, as local charge compensation requires at least one of the A1 nearest neighbours to be oxygen; evidence of a second peak at ca. 106 ppm reported in higher-A1 ~-sialon samples (Sj6berg et al. 1992) is consistent with more than one type of A1-N environment. The tetrahedra127A1 resonance at ca. 65 ppm in all [3-sialons is broad and probably corresponds to mixtures of overlapping bands from A1NxO4-x units. Small octahedral A1 resonances often found in the 27A1 spectra of [3-sialons are not expected from structural considerations and are normally ascribed to impurity phases. The 27A1 NMR spectra of the various [3-sialons appear to be independent of the synthesis method, being similar for samples prepared by sintering (Sj6berg et al. 1992), self-propagating high-temperature synthesis (Yue et al. 1996) or carbothermal synthesis from kaolinite (MacKenzie et al. 1994) or halloysite (Neal et al. 1994). Carbothermal synthesis and its variant, silicothermal synthesis, have proved attractive routes for preparing sialons from readily available clay mineral raw materials. The clay is mixed with fine carbon and/or silicon powder and reacted in a stream of purified nitrogen at > 1400~ The [3-sialon product carbothermally synthesised from kaolinite has the composition Si3A1303N5 (z = 3), controlled by the SIO2:A1203 ratio of the clay 3A12Si2Os(OH)4 + 15C + 5N2---> 2Si3A1303N5 + 6H20 1" + 15CO 1"

(5.20)

The complex sequence of reactions preceding the formation of the sialon has been studied by 27A1 and 29Si NMR (MacKenzie et al. 1994a, MacKenzie et al.~ 1996) as has sialon formation itself (Neal et al. 1994, MacKenzie et al. 1994). [3-sialons with lower z-values, which are of interest because of their Si3N4-1ike physical properties combined with sintering benefits due to the presence of the A1, can be carbothermally prepared from kaolinite with the necessary extra Si added either as SiO2 or elemental Si. 27A1NMR has been used to study the changes in the A1 environment during these reactions, and the effect of adding three mass% Y203 as a mineralising agent (MacKenzie et al. 1997). The formation of amorphous volatile products during carbothermal reduction processes can cause problems by condensing in the cooler parts of the furnace and altering the composition of the original mixture. 27A1 NMR shows that these amorphous phases all contain A1 solely in tetrahedral coordination, with chemical shifts consistent with the potassium feldspars microcline and sanidine (54-58 ppm). Thermodynamic analysis confirmed the feasibility of vapour-phase transport of both A1 and alkali metal impurities from the clay which can then condense to form short-range feldspar-like units (Ekstr6m et al. 1996).

320


Since [3-sialons can be difficult to sinter, metal oxides such as A1203, MgO and Y203 may be added to assist the process. 27A1 NMR has been used in conjunction with 29Si, 25Mg and 89y NMR to study the effect of these additives, both singly and in combination (MacKenzie and Meinhold 1996). The results indicate that Y203 by itself or in combination forms Y3A150~2 (yttrium aluminium garnet) which reacts at higher temperatures to form polytypoid sialon and Y-containing glass. MgO initially forms MgAI204, thereby removing some A1 from the sialon and lowering the z-value, but at higher temperatures a glass is formed, containing Mg in both octahedral and tetrahedral coordination. A1203 reacts with the [3-sialon, increasing its z-value (MacKenzie and Meinhold 1996). When placed in service at higher temperatures under oxidising conditions, sialons progressively degrade. The oxidation of carbothermal [3-sialon powder has been studied by 27A1 and 298i NMR (MacKenzie et al. 1997a). A progressive change in the A1 environment during oxidation (an increase in the octahedral A1 peak and a shift of the tetrahedral A1 resonance from ca. 67 ppm in the sialon to ca. 55 ppm) is related to the formation of mullite, while the appearance of a small amount of oL-A1203 at 14 ppm is consistent with the known stoichiometry of the oxidation reaction (MacKenzie et al. 1997a).

5.12.30-sialons

These sialons are structurally related to silicon oxynitride, Si2N20, and have the composition Si2-xAl~O~+~N2-x, where x varies from zero to ca. 0.4. Over the whole composition range the 27A1 spectra show a tetrahedral A104 resonance at 65 ppm, and in many cases a minor octahedral peak at ca. 2 ppm which is attributed to an impurity phase (Figure 5.35C) (Sj6berg et al. 1992). A new compound formed during a novel silicothermal synthesis of O-sialon from clay, silica and elemental Si was found to have similar 27A1 and 29Si NMR spectra to O-sialon. This NMR observation led to its subsequent identification as a low-temperature form which was previously unknown because other synthesis methods require higher temperatures, at which the low-temperature form is unstable, and conversion to the more usual high-temperature form has occurred (Barris et al. 1997). Because of their high SiO2 content, O-sialons show superior resistance to oxidation. The oxidation of O-sialon powder has been studied by 27A1 and 295i NMR, which indicates that the octahedral A1 thought to be present as an impurity phase in the unoxidised sialon is retained at all oxidation temperatures (MacKenzie et al. 1998). Some of this octahedral A1 is present as mullite which forms at 120(O1500~ but at higher temperatures, the oxidation is progressively hindered by the formation of a protective fused layer which is found by 27A1 NMR to contain an unexpectedly high proportion of octahedral A1 in the amorphous silica-rich material formed at 1600~ (MacKenzie et al. 1998).

27 A 1 N M R

321

5.12.4 X-sialons

This compound, which may be considered as a solid solution between Si3N4 and mullite, has a nominal composition of Si12Al18039Ns, and contains both tetrahedral and octahedral 27A1 NMR resonances. An earlier study reported that the A104 and A106 peaks occur at 66.9 and 0.8 ppm respectively (Klinowski et al. 1984), but more recently, the use of faster spinning speeds has revealed distinct 27A1 signals at 59 and 2.8 ppm (Smith 1994). Similar sharp 27A1 NMR peaks (at 62-63 ppm and 0.5-0.8 ppm) have been found in the spectra of X-sialons produced by carbothermal and silicothermal syntheses (Figure 5.35D) (Sheppard et al. 1997). 27A1 NMR spectroscopy has also been used to study the various stages in the silicothermal reaction sequence leading to the formation of X-sialon (Sheppard et al. 1997) and the effect of a number of metal oxide additives on the silicothermal formation and sintering of X-sialon (Sheppard and MacKenzie 1999). The silicothermal formation of X-sialon from clay, ~/-A1203 and Si proceeds via the formation of mullite 3AlzSi2Os(OH)4 + 6~/-A1203 ---->3A16Si2013 + 6H20 ]"

(5.21)

which then reacts with Si3N4 formed in situ by nitridation of the Si 3A16Si2013 + 6Si + 4N2 --~ Si12AllsO39N8

(5.22)

During this reaction sequence the tetrahedral A1 chemical shift should vary from about 65 ppm in ~-A1203 to about 59 ppm in mullite and back to about 63 ppm in X-sialons (Figure 5.36A), with a corresponding change in the tetrahedral:octahedral A1 ratio from about 0.5 in ~/-A1203 through about 1.1 in mullite to about 1.3 in X-sialons (Figure 5.36B). Measurements of the tetrahedral A1 shift and the tetrahedral:octahedral ratio as a function of temperature for reaction systems containing a number of different metal oxide additives have been used as an indicator of the way in which the various additives influence this complex reaction sequence (Sheppard and MacKenzie 1999), and indicate that the modification of the Si(A1)-O portions of the mullite structure to form X-sialons is assisted by all the common metal oxides except BaO, ZrO2 and Fe203 (Figure 5.36). 27A1 NMR has been used to monitor the oxidation of X-sialon powder (MacKenzie et al. 1998), and shows at about 1200~ an abrupt change in the relative amount of tetrahedral A1 from the typical value for X-sialon (ca. 61%) to the typical value for mullite, measured at the same field strength (ca. 52%) (Figure 5.37A). The accompanying change in the tetrahedral A1 chemical shift is more gradual, from the typical X-sialon value of ca. 62 ppm at 950~ to ca. 50 ppm at 1450~ (MacKenzie et al. 1998), the latter representing the mean of the two tetrahedral shifts of pure crystalline mullite (Figure 5.37B).

322


B

A l~ 66.5 ,~

64.5

.m

62.5 F , ~ / ~ / ~ ~ ~ . . 60.5

~9 3 ~

1.5

.* y-AlzO3

58.5 1200

,

y

/-"

~

X-sialon mullite

Ms

1.1

X-sialon

o

0.7

i~ 1300

. , , i_ ~mulfite 1400 1500 Temperature (*C)

y-Al203

1200

1300

1400

Temperature

1500 (~

Figure 5.36. Modification of the silicothermal synthesis of X-sialon by metal oxide additives monitored by A. changes in the tetrahedral 27A1shift as a function of synthesis temperature, and B. changes in the tetrahedral:octahedral 27A1ratio as a function of synthesis temperature. The heavy line in both graphs refers to the additive-free control sample. From Sheppard and MacKenzie (1999).

B

A

62

/ *.' 58 o o

60 .~ ,g:l

r

54 900

\o

r~

~ 52 1100

1300

Oxidation temperature

1500 (~

\

15'00 1100 1300 91P0 Oxidation temperature (~

Figure 5.37. Oxidation of X-sialon powder monitored by A. changes in the amount of tetrahedral 27A1 in the sample as a function of oxidation temperature, and B. changes in the tetrahedral 27A1 shift as a function of oxidation temperature. From MacKenzie et al. (1998).

5.12.5 a - s i a l o n s

These c o m p o u n d s have the structure of o~-Si3N4 but need to be stabilised by the presence of other cations (typically Y, Ca, Mg or the rare earths). 27A1 and 29Si N M R have been used to study the carbothermal and silicothermal synthesis of a series of Y-containing oL-sialons YxSile-4.sxAln.5xOi.5xNl6-1.Sx, where x = 0.3, 0.5 and 0.7 (Ekstr6m et al. 1998). W h e n hot-pressed at 1800~ the 27A1 N M R spectra of the powders produced in the original synthesis (essentially a mixture of e~ and [3-sialon) were considerably broadened (Figure 5.38). The strong octahedral peak at ca. 0.8 ppm arises from the development of polytypoid sialon, while the broadening and shift of the original A1N resonances at ca. 113 ppm to ca. 9 0 - 1 0 6 ppm suggests the development of a

323

27 A 1 N M R

fromSiO2 j 113

1800~ .

k....._A~___...... from Si /113

32 Mpa 2 hr 1~

92 ~ 0 " 7

J~ ~ 13 32 Mpa 2 hr ...... 120 40 -40 120 40 -40 27A1shift (ppm) w.r.t. AI(H20)63+

,----","-'r---f~',"r--:r-__

Figure 5.38. Effect of hot-pressing on the 27A1MAS NMR spectra of yttrium-oL-sialon synthesised by carbothermal and silicothermal synthesis using SiO2 as the silica source (top) and Si as the silica source (bottom). Asterisks denote spinning side bands. From Ekstr6m et al. (1998).

continuum of A1-O-N units (predominantly A1ON3, but with smaller numbers of A1OzN2 and A103N) (Ekstr6m et al. 1998). N-melilites are yttrium or rare-earth silicon oxynitrides of general formula RzSi303N4 which often form when rare earth oxides are used to assist sintering in Si3N4-based ceramics. N-melilites can also take A1 into their structure, but NMR studies of the resulting phases are often hampered by the large magnetic moments possessed by many of the rare earth ions which cause severe broadening of the NMR signals of nearby nuclei. Since these effects are smaller for samarium, it has been possible to study the incorporation of aluminium into samarium N-melilite by 27A1 and 298i NMR (Chee et al. 1995). The results indicate that Al-substituted samarium N-melilite contains predominantly A104 and SiOzN2 units, irrespective of the extent of A1 solubility (Chee et al. 1995).

5.12.6 Sialon glasses

The incorporation of nitrogen into aluminosilicate glass results in an improvement in the physical properties (increased hardness, refractive index, softening temperature, mechanical strength and resistance to chemical attack). Pressureless melting and quenching at a controlled rate results in the formation of dense sialon glasses with compositions ranging from almost pure SiO2 to Si3.8All.307.4N (McMillan et al. 1998). The structural units in a sialon glass of typical composition Si25A16.6055N have been studied by 27A1 and 29Si NMR (Sato et al. 1990), which indicate the presence of AI(IV), AI(V) and AI(VI) species coordinated to oxygen, by contrast with the corresponding nitrogen-flee aluminosilicate glass which contains only AI(IV) and AI(VI) (Sato et al. 1990, McMillan et al. 1998). The presence of trivalent nitrogen in the glass is thought

324


to lead to cross-linking in the anion sub-network, increasing the viscosity of the melt at the glass transition temperature and allowing the 5-coordinated A1 sites to be more readily quenched in (Sato et al. 1990). Yttrium-sialon glasses have also been investigated by 27A1 NMR (Jin et al. 1994). Although the interpretation of these results was limited by the slow spinning speeds used, the absence of A1-N bonding was noted, with the A1 occupying tetrahedral A104 sites within the glass structure.

REFERENCES

Alemany, L.B. & Kirker, G.W. (1986) J. Amer. Chem. Soc., 108, 6158. Alemany, L.B., Steuernagel, S., Amoureux, J-P., Callender, R.L. & Barron, A.R. (1999) Chem. Phys. Lett., 14, 1. Alemany, L.B., Steuernagel, S., Amoureux, J-P., Callender, R.L. & Barron, A.R. (1999a) Solid State Nucl. Mag. Reson., 14, 1. Barbosa, V.F.F., MacKenzie, K.J.D. & Thaumaturgo, C. (2000) Int. J. lnorg. Mater., 2, 309. Barrie, P.J. (1993) Spectroscopy of New Materials, Eds. Clark, R.J.H. & Hester, R.E., Wiley, NY, p. 151. Barris, G.C., Bowden, M.E., Sheppard, C.M. & MacKenzie, K.J.D. (1997) Proceedings of the IPENZ Annual Conference, Wellington, NZ, Vol. 1, p. 303. Bastow, T.J., Hall, J.S. & Smith, M.E. (1995) Flash Reaction Processes, Eds. Davies, T.J., Kluwer, The Netherlands, p. 123. Bastow, T.J., Massiot, D. & Coutures, J.P. (1998) Solid State Nucl. Mag. Reson., 10, 241. Bleam, W.F., Dec., S.F. & Frye, J.S. (1989) Phys. Chem. Minerals, 16, 817. Bodart, P.R., Parmentier, J., Harris, R.K. & Thompson, D.P. (1999) J. Phys. Chem. Solids, 60, 223. Bradley, S.M. & Hanna, J.V. (1994) J. Amer. Chem. Soc., 116, 7771. Brown, I.W.M., MacKenzie, K.J.D. & Meinhold, R.H. (1987) J. Mater. Sci., 22, 3265. Bryant, P.L., Harwell, C.R., Wu, K., Fronczek, F.R., Hall, R.W. & Butler, L.G. (1999) J. Phys. Chem. A, 103, 5246. Butler, N.D., Dupree, R. & Lewis, M.H. (1984) J. Mater. Sci. Lett., 3, 469. Carduner, K.R. (1989) J. Mag. Reson., 81, 312. Chee, K.S., Cheng, Y-B. & Smith, M.E. (1995) J. European Ceram. Soc., 15, 1213. Chen, T-H., Wouters, B.H. & Grobet, P.J. (2000) European J. Inorg. Chem., 281. Clayden, N.J., Esposito, S., Aronne, A. & Pernice, P. (1999) J. Non-Cryst. Solids, 258, 11.

Coster, D., Blumenfield, A.L. & Fripiat, J.J. (1994) J. Phys. Chem., 98, 6201. Cruickshank, M.C., Dent Glasser, L.A., Barri, S.A.I. & Poplett, I.J.F. (1986) J. Chem. Soc. Chem. Commun., 23. Day, R.A., Vance, E.R., Cassidy, D.J. & Hartman, J.S. (1995) J. Mater. Res., 10, 2693. Dec, S.F., Fitzgerald, J.J., Frye, J.S., Shatlock, M.P. & Maciel, G.E. (1991) J. Mag. Reson., 93, 403. Dirken, P.J., Jansen, J.B.H. & Schuiling, R.D. (1992) Amer. Mineralogist, 77, 718.

27A1NMR

325

Dirken, P.J., Kentgens, A.P.M., Nachtegaal, G.H., van der Eerden, A.M.J. & Jansen, J.B.H. (1995) Amer. Mineralogist, 80, 39. Drachman, S.R., Roch, G.E. & Smith, M.E. (1997) Solid State Nucl. Mag. Reson., 9, 257. Dupree, R., Lewis, M.H. & Smith, M.E. (1986) Philos. Mag. A, 53, L 17. Dupree, R., Lewis, M.H. & Smith, M.E. (1988) J. Appl. Cryst., 21, 109. Ekstr6m, T.C., MacKenzie, K.J.D., White, G.V., Brown, I.W.M. & Barris, G.C. (1996) J. Mater. Chem., 6, 1225. Ekstr6m, T., Shen, Z-J., MacKenzie, K.J.D., Brown, I.W.M. & White, G.V. (1998) J. Mater. Chem., 8, 977. Farnan, I., Dupree, R., Forty, A.J., Jeong, Y.S., Thompson, G.E. & Wood, G.C. (1989) Philos. Mag. Lett., 59, 189. Faman, I., Dupree, R., Jeong, Y., Thompson, G.E., Wood, G.C. & Forty, A.J. (1989a) Thin Solid Films, 173, 209. Faucon, P., Charpentier, T., Nonat, A. & Petit, J.C. (1998) J. Amer. Chem. Soc., 120, 12075. Fitzgerald, J.J., Dec, S.F. & Hamza, A.I. (1989) Amer. Mineralogist, 74, 1405. Fitzgerald, J.J., Kohl, S.D., Piedra, G., Dec, S.F. & Maciel, G.E. (1994) Chem. Mater., 6, 1915. Fletcher, R.J., Risbud, S.H. & Kirkpatrick, R.J. (1990) J. Mater. Res., 5, 835. Florian, P., Gervais, M., Douy, A., Massiot, D. & Coutures, J-P. (2001) J. Phys. Chem. B, 105, 379. Fyfe, C.A., Grondey, H., Mueller, K.T., Wong-Moon, K.C. & Markus, T. (1992) J. Amer. Chem. Soc., 114, 5876. Fyfe, C.A., Mueller, K.T., Grondey, H. & Wong-Moon, K.C. (1992a) Chem. Phys. Lett., 199, 198. Fyfe, C.A., Mueller, K.T., Grondey, H. & Wong-Moon, K.C. (1993) J. Phys. Chem., 97, 13484. Fyfe, C.A., Wong-Moon, K.C., Huang, Y. & Grondey, H. (1995) Microporous Mater., 5, 29. Fyfe, C.A., Wong-Moon, K.C. & Huang, Y. (1996) Zeolites, 16, 50. Fyfe, C.A., Bretherton, J.L. & Lam, L.Y. (2000) J. Chem. Soc. Chem. Commun., 1575. Fyfe, C.A., Bretherton, J.L. & Lam, L.Y. (2001) J. Amer. Chem. Soc., 123, 5285. Gervais, C., MacKenzie, K.J.D. & Smith, M.E. (2001) Mag. Reson. Chem., 39, 23. Ghose, S. & Tsang, T. (1973) Amer. Mineralogist, 58, 748. Gilson, J-P., Edwards, G.C., Peters, A.W., Rajagopalan, K., Wormsbecher, R.F., Roberie, T.G. & Shatlock, M.P. (1987) J. Chem. Soc. Chem. Commun., 91. Gobbi, G.C., Christoffersen, R., Otten, M.T., Miner, B., Buseck, P.R., Kennedy, G.J. & Fyfe, C.A. (1985) Chem. Lett., 771. Grey, C.P. & Vega, A.J. (1995) J. Amer. Chem. Soc., 117, 8232. Haase, J., Freude, D., Frohlich, T., Himpel, G., Kerke, F., Lippmaa, E., Pfeifer, H., Sarv, P., Schaefer, H. & Sieffert, B. (1989) Chem. Phys. Lett., 156, 328. Haddix, G.W., Narayana, M., Tang, S.C. & Wu, Y. (1993) J. Phys. Chem., 97, 4624. Hamilton, D.L., Chesworth, W., Kennedy, G. & Fyfe, C. (1986) Geochim. Cosmochim. Acta, 50, 123. Han, O.H., Lin, C.Y. &Haller, G.L. (1992) Catal. Lett., 14, 1. Hayashi, H., Hayamizu, K. & Yamamoto, O. (1987) Bull. Chem. Soc. Japan, 60, 761.

326


Herreros, B., Barr, T.L. & Klinowski, J. (1994) J. Phys. Chem., 98, 738. Hovis, G.L., Spearing, D.R., Stebbins, J.F., Roux, J. & Clare, A. (1992) Amer. Mineralogist, 77, 19. Hughes, A.E., Rajendran, S. & Smith, M.E. (1993) Mater. Lett., 17, 303. Ito, T., Yoshiasa, A., Yamanaka, T., Nakatsuka, A. & Maekawa, H. (2000) Z. Anorg. Allgemeine Chem., 626, 42. Iuga, D., Simon, S., de Boer, E. & Kentgens, A.P.M. (1999) J. Phys. Chem. B, 103, 7591. Jakobsen, H.J., Skibsted, J., Bilds~e, H. & Nielsen, N.C. (1989) J. Mag. Reson., 85. 173. Jansen, S.R., Hintzen, H.T., Metselaar, R., de Haan, J.W., van de Ven, L.J.M., Kentgens, A.P.M. & Nachtegaal, G.H. (1998) J. Phys. Chem. B, 102, 5969. Jelinek, R., Chmelka, B.F., Wu, Y., Grandinetti, P.J., Pines, A., Barrie, P.J. & Klinowski, J. (1991) J. Amer. Chem. Soc., 113, 4097. Jelinek, R., Chmelka, B.F., Wu, Y., Grandinetti, P.J., Pines, A., Barrie, P.J. & Klinowski, J. (1991) J. Amer. Chem. Soc., 113, 4097. Jin, J., Yoko, T., Miyaji, F., Sakka, S., Fukunaga, T. & Misawa, M. (1994) Philos. Mag. A, 70, 191. Jin, J., Sakida, S. Yoko, T. & Nogami, M. (2000) J. Non-Cryst. Solids, 262, 183. Karabulut, M., Metwalli, E. & Brow, R.K. (2001) J. Non-Cryst. Solids, 283, 211. Kashii, N., Maekawa, H. & Hinatsu, Y. (1999) J. Amer. Ceram. Soc., 82, 1844. Kerns, L., Weinberg, M.C., Myers, S. & Assink, R. (1998) J. Non-Cryst. Solids, 232-234, 86. Kim, J.Y., Kumta, P.N., Phillips, B.L. & Risbud, S.H. (2000) J. Phys. Chem. B, 104, 7895. Kinsey, R.A., Kirkpatrick, R.J., Hower, J., Smith, K.A. & Oldfield, E. (1985) Amer. Mineralogist, 70, 537. Kirkpatrick, R.J., Smith, K.A., Schramm, S., Turner, G. & Yang, W-H. (1985) Ann. Rev. Earth Planeta:~y Sci., 13, 29. Klinowski, J., Thomas, J.M., Thompson, D.P., Korgul, P., Jack, K.H., Fyfe, C.A. & Gobbi, G.C. (1984) Polyhedron, 2, 1267. Kodama, H., Kotlyar, L.S. & Ripmeester, J.A. (1989) Clays Clay Minerals, 37, 364. Kohn, S.C., Dupree, R., Mortuza, M.G. & Henderson, C.M.B. (1991) Amer. Mineralogist, 76, 309. Kohn, S.C., Henderson, C.M.B. & Dupree, R. (1997) Amer. Mineralogist, 82, 1133. Kohn, S. & Jansen, M. (1998)J. Mate:'. Chem., 8, 1593. Kunath, G., Losso, P., Steuernagel, S., Schneider, H. & J~iger, C. (1992) Solid State Nucl. Mag. Reson., 1,261. Kunath-Fandrei, G., Rehak, P., Steuernagel, S., Schneider, H. & J~iger, C. (1994) Solid State Nucl. Mag. Reson., 3, 241. Kunath-Fandrei, G., Bastow, T.J., Hall, J.S., J~iger, C. & Smith, M.E. (1995) J. Phys. Chem., 99, 15138. Kunath-Fandrei, G., Bastow, T.J., Jaeger, C. & Smith, M.E. (1995a) Chem. Phys. Lett., 234, 431. Lacassagne, V., Bessada, C., Massiot, D., Florian, P. & Coutures, J-P. (1998) J. Chim. Phys. Phys. Chem. Biolog., 95, 322. Lambert, J.F., Millman, W.S. & Fripiat, J.J. (1989)J. Amer. Chem. Soc., 111, 3517.

27A1NMR

327

Lippmaa, E., Magi, M., Tarmak, W., Wieker, W. & Grimmer, A.R. (1982) Cement Concrete Res., 12, 597. Lippmaa, E., Samoson, A. & M/igi, M. (1986) J. Amer. Chem. Soc., 108, 1730. Livage, J., Babonneau, F., Chatry, M. & Coury, L. (1997) Ceram. Int., 23, 13. Luong, T., Mayer, H., Eckert, H. & Novinson, T.I. (1989) J. Amer. Ceram. Soc., 72, 2136. Ma, D., Deng, F., Fu, R., Han, X. & Bao, X. (2001) J. Phys. Chem. B, 105, 1770. MacKenzie, K.J.D. (2000) Proceedings of the First International Conference on Advanced Materials Processing, Rotorua, New Zealand, November 2000, Eds. Zhang, D.J., Pickering, K.L. & Xiong, X.Y. Institute of Materials Engineering, Australasia, p. 45. MacKenzie, K.J.D., Brown, I.W.M., Meinhold, R.H. & Bowden, M.E. (1985) J. Amer. Ceram. Soc., 68, 293. MacKenzie, K.J.D., Brown, I.W.M., Cardile, C.M. & Meinhold, R.H. (1987) J. Mater. Sci., 22, 2645. MacKenzie, K.J.D., Bowden, M.E., Brown, I.W.M. & Meinhold, R.H. (1989) Clays Clay Minerals, 37, 317. MacKenzie, K.J.D., Bowden, M.E. & Meinhold, R.H. (1991) Clays Clay Minerals, 39, 337. MacKenzie, K.J.D., Meinhold, R.H., Sherriff, B.L. & Xu, Z. (1993) J. Mater. Chem., 3, 1263. MacKenzie, K.J.D., Meinhold, R.H., White, G.V., Sheppard, C.M. & Sherriff, B.L. (1994) J. Mater. Sci., 29, 2611. MacKenzie, K.J.D., Meinhold, R.H., Brown, I.W.M. & White, G.V. (1994a) J. Mater. Sci., 29, 5631. MacKenzie, K.J.D. & Meinhold, R.H. (1996) J. Mater. Chem., 6, 821. MacKenzie, K.J.D., Meinhold, R.H., Brown, I.W.M. & White, G.V. (1996) J. European Ceram. Soc., 16, 115. MacKenzie, K.J.D., Meinhold, R.H., Patterson, J.E., Schneider, H., Schmticker, M. & Voll, D. (1996a) J. European Ceram. Soc., 16, 1299. MacKenzie, K.J.D., Ekstr6m, T.C., White, G.V. & Hartman, J.S. (1997) J. Mater. Chem., 7, 1057. MacKenzie, K.J.D., Shimada, S. & Aoki, T. (1997a) J. Mater. Chem., 7, 527. MacKenzie, K.J.D., Sheppard, C.M., Barris, G.C., Mills, A.M., Shimada, S. & Kiyono, H. (1998) Thermochim. Acta, 318, 91. MacKenzie, K.J.D. & Kemmitt, T. (1999) Thermochim. Acta, 325, 13. MacKenzie, K.J.D. & Kemmitt, T. (1999a) Thermochim. Acta, 325, 5. MacKenzie, K.J.D., Schmticker, M. & Mayer, L. (1999) Thermochim. Acta, 335, 73. MacKenzie, K.J.D., Temuujin, J. & Okada, K. (1999a) Thermochim. Acta, 327, 103. MacKenzie, K.J.D., Schmticker, M., Smith, M.E., Poplett, I.J.F. & Kemmitt, T. (2000) Thermochim. Acta, 363, 181. MacKenzie, K.J.D., Temuujin, J., Smith, M.E., Angerer, P. & Kameshima, Y. (2000a) Thermochim. Acta, 359, 87. MacKenzie, K.J.D., Temuujin, J., Jadambaa, T.S., Smith, M.E. & Angerer, P. (2000b) J. Mater. Sci., 35, 5529. Maekawa, H., Kato, S., Kawamura, K. & Yokokawa, T. (1997) Amer. Mineralogist, 82, 1125.

328


Massiot, D., Mtiller, D., Htibert, Th., Schneider, M., Kentgens, A.P.M., Cot6, B., Coutures, J-P. & Gessner, W. (1995) Solid State Nucl. Mag. Reson., 5, 175. McManus, J., Ashbrook, S.E., MacKenzie, K.J.D. & Wimperis, S. (2001) J. Non-Cryst. Solids, 282, 278. McMillan, P.F., Petuskey, W.T., Cot6, B., Massiot, D., Landron, C. & Coutures, J-P. (1996) J. Non-Cryst. Solids, 195, 261. McMillan, P.F., Sato, R.K. & Poe, B.T. (1998) J. Non-Cryst. Solids, 224, 267. Meinhold, R.H. & Tapp, N.J. (1990) J. Chem. Soc. Chem. Commun., 219. Meinhold, R.H., Slade, R.C.T. & Newman, R.H. (1993) Appl. Mag. Reson., 4, 121. Meinhold, R.H., Slade, R.C.T. & Davies, T.W. (1993a) Appl. Mag. Reson., 4, 141. Merwin, L.H., Sebald, A., Rager, H. & Schneider, H. (1991) Phys. Chem. Minerals, 18, 47. Millard, R.L., Peterson, R.C. & Hunter, B.K. (1992) Amer. Mineralogist, 77, 44. Morris, H.D. & Ellis, P.D. (1989) J. Amer. Chem. Soc., 111, 6045. Moulin, I., Stone, W.E.E., Sanz, J., Bottero, J-Y., Mosnier, F. & Haehnel, C. (2000) J. Phys. Chem. B, 104, 9230. Mtiller, D., Rettel, A., Gessner, W. & Scheler, G. (1984) J. Mag. Reson., 57, 152. Miiller, D., Jahn, E., Fahlke, B., Ladwig, G. & Haubenreisser, U. (1985) Zeolites, 5, 53. Mtiller, D., Gessner, W., Samoson, A., Lippmaa, E. & Scheler, G. (1986) Polyhedron, 5, 779. Neal, G.S., Smith, M.E., Trigg, M.B. & Drennan, J. (1994) J. Mater. Chem., 4, 245. Oestrike, R., Yang, W-H., Kirkpatrick, R.J., Hervig, R.L., Mavrotsky, A. & Montez, B. (1987) Geochim. Cosmochim. Acta, 51, 2199. Oldfield, E., Kinsey, R.A., Smith, K.A., Nichols, J.A. & Kirkpatrick, R.J. (1983) J. Mag. Reson., 51,325. Pecharrom~in, C., Sobrados, I., Iglesias, J.E., Gonz~ilez-Carrefio, T. & Sanz, J. (1999) J. Phys. Chem. B, 103, 6160. Peeters, M.P.J. & Kentgens, A.P.M. (1997) Solid State Nucl. Mag. Reson., 9, 203. Phillips, B.L., Allen, F.M. & Kirkpatrick, R.J. (1987) Amer. Mineralogist, 72, 1190. Phillips, B.L., Kirkpatrick, R.J. & Putnis, A. (1989) Phys. Chem. Minerals, 16, 591. Phillips, B.L. & Kirkpatrick, R.J. (1994) Amer. Mineralogist, 79, 1025. Ray, G.J. & Samoson, A. (1993) Zeolites, 13, 410. Rehak, P., Kunath-Fandrei, G., Losso, P., Hildmann, B., Schneider, H. & J~iger, C. (1998) Amer. Mineralogist, 83, 1266. Rettel, A., Gessner, W., Mtiller, D. & Scheler, G. (1985) British Ceram. Trans. J., 84, 25. Risbud, S.H., Kirkpatrick, R.J., Taglialavore, A.P. & Montez, B. (1987) J. Amer. Ceram. Soc., 70, C10. Roch, G.E., Smith, M.E. & Drachman, S.R. (1998) Clays Clay Minerals, 46, 694. Rocha, J. (1999) J. Phys. Chem. B, 103, 9801. Rocha, J. & Klinowski, J. (1990) Phys. Chem. Minerals, 17, 179. Rocha, J., Klinowski, J. & Adams, J.M. (1991) J. Mater. Sci., 26, 3009. Sakida, S., Shojiya, M. & Kawamoto, Y. (2000) J. Fluorine Chem., 106, 127. Samoson, A., Sarv, P., van Braam Houckgeest, J.P. & Kraushaar-Czarnetzki, B. (1993) Appl. Mag. Reson., 4, 171. S~inchez-Soto, P.J., Sobrados, I., Sanz, J. & P6rez-Rodrfguez, J.L. (1993) J. Amer. Ceram. Soc., 76, 3024.

27A1NMR

329

S~inchez-Soto, P.J., P6rez-Rodriguez, J.L., Sobrados, I. & Sanz, J. (1997) Chem. Mater., 9, 677. Sanz, J., Serratosa, J.M., Moya, J.S. & Aza, S. (1988) J. Amer. Ceram. Soc., 71, C418. Sato, R.K., Bolvin, J. & McMillan, P.F. (1990) J. Amer. Ceram. Soc., 73, 2494. Sato, R.K., McMillan, P.F., Dennison, P. & Dupree, R. (1991) J. Phys. Chem., 95, 4483. Sato, R.K., McMillan, P.F., Dennison, P. & Dupree, R. (1991a) Phys. Chem. Glasses, 32, 149. Schmiicker, M. & Schneider, H. (1996) Berichte Bunsenges Phys. Chem., 100, 1550. Schmticker, M., MacKenzie, K.J.D., Schneider, H. & Meinhold, R.H. (1997) J. Non-Cryst. Solids, 217, 99. Schmticker, M., Schneider, H. & MacKenzie, K.J.D. (1998) J. Non-Cryst. Solids, 226, 99. Schmticker, M., Schneider, H., MacKenzie, K.J.D. & Okuno, M. (1999) J. European Ceram. Soc., 19, 99. Schneider, H., Merwin, L. & Sebald, A. (1992) J. Mater. Sci., 27, 805. Schneider, H., Voll, D., Saruhan, B., Schmticker, M., Schaller, T. & Sebald, A. (1994) J. European Ceram. Soc., 13, 441. Schurko, R.W., Wasylishen, R.E. & Phillips, A.D. (1998) J. Mag. Reson., 133, 388. Selvaraj, U., Komarneni, S. & Roy, R. (1990) J. Amer. Ceram Soc., 73, 3663. Selvaraj, U., Komarneni, S. & Roy, R. (1993) J. Solid State Chem., 106, 73. Sheppard, C.M., MacKenzie, K.J.D., Barris, G.C. & Meinhold, R.H. (1997) J. European Ceram. Soc., 17, 667. Sheppard, C.M. & MacKenzie, K.J.D. (1999) J. European Ceram. Soc., 19, 535. Sj6berg, J., Harris, R.K. & Apperley, D.C. (1992) J. Mater. Chem., 2, 433. Skibsted, J., Henderson, E. & Jakobsen, H.J. (1993) Inorg. Chem., 32, 1013. Skibsted, J., Jakobsen, H.J. & Hall, C. (1994) J. Chem. Soc. Faraday Trans., 90, 2095. Slade, R.C.T. & Davies, T.W. (1991) J. Mater. Chem., 1, 361. Slade, R.C.T., Southern, J.C. & Thompson, I.M. (1991) J. Mater. Chem., 1, 563. Slade, R.C.T., Southern, J.C. & Thompson, I.M. (1991a) J. Mater. Chem., 1, 875. Smith, M.E. (1992) J. Phys. Chem., 96, 1444. Smith, M.E. & Steuernagel, S. (1992) Solid State Nucl. Mag. Reson., 1, 175. Smith, M.E., Jaeger, C., Schoenhofer, R. & Steuernagel, S. (1994) Chem. Phys. Lett., 219, 75. Sohlberg, K., Pennycook, S.J. & Pantelides, S.T. (1999) J. Amer. Chem. Soc., 121, 10999. Sorar~, G.D., Mercadini, M. & Dal Maschio, R. (1993) J. Amer. Ceram. Soc., 76, 2595. Spearing, D.R., Stebbins, J.F. & Farnan, I. (1994) Phys. Chem. Minerals, 21,373. Stebbins, J.F. & Farnan, I. (1992) Science, 255, 586. Stebbins, J.F., Farnan, I., Dando, N. & Tzeng, S-Y. (1992a) J. Amer. Ceram. Soc., 75, 3001. Taylor, A. & Holland, D. (1993) J. Non-Cryst. Solids, 152, 1. Temuujin, J., Okada, K. & MacKenzie, K.J.D. (1998) J. European Ceram. Soc., 18, 831. Temuujin, J., Okada, K., MacKenzie, K.J.D. & Jadambaa, T.S. (1998a) J. European Ceram. Soc., 19, 105. Temuujin, J., Okada, K. & MacKenzie, K.J.D. (1999) Ceram. Int., 25, 85. Temuujin, J., MacKenzie, K.J.D., Jadambaa, T.S., Namjildorj, B., Olziburen, B., Smith, M.E. & Angerer, P. (2000) J. Mater. Chem., 10, 1019.

330


Temuujin, J., MacKenzie, K.J.D., Schmficker, M., Schneider, H., McManus, J. & Wimperis, S. (2000a) J. European Ceram. Soc., 20, 413. Toplis, M.J., Kohn, S.C., Smith, M.E. & Poplett, I.J.F. (2000) Amer. Mineralogist, 85, 1556. van Bokhoven, J.A., Roest, A.L., Koningsberger, D.C., Miller, J.T., Nachtegaal, G.H. & Kentgens, A.P.M. (2000) J. Phys. Chem. B, 104, 6743. van Eck, E.R.H., Kentgens, A.P.M., Kraus, H. & Prins, R. (1995) J. Phys. Chem., 99, 16080. Vosegaard, T. & Jakobsen, H.J. (1997) J. Mag. Reson., 128, 135. Weller, M.T., Brenchley, M.E., Apperley, D.C. & Davies, N.A. (1994) Solid State Nucl. Mag. Reson., 3, 103. Wilson, S.J. (1979) Proc. British Ceram. Soc., 28, 281. Wood, B.J., Kirkpatrick, R.J. & Montez, B. (1986) Amer. Mineralogist, 71,999. Wood, T.E., Siedle, A.R., Hill, J.R., Skarjune, R.P. & Goodbrake, C.J. (1990) Mater. Res. Soc. Symp. Proc., 180, 97. Wu, Y., Chmelka, B.F., Pines, A., Davis, M.E., Grobet, P.J. & Jacobs, P.A. (1990) Nature, 346, 550. Xu, Z. & Sherriff, B.L. (1993) Appl. Mag. Reson., 4, 203. Yang, X. (1995) J. Phys. Chem., 99, 1276. Yasumori, A., Iwasaki, M., Kawazoe, H., Yamane, M. & Nakamura, Y. (1990) Phys. Chem. Glasses, 31, 1. Youngman, R.E. & Aitken, B.G. (2001) J. Non-Cryst. Solids, 284, 9. Yue, Y., He, H., Klinowski, J., Wu, Y. & Zhuang, H. (1996) J. Mater. Chem., 6, 1391.

Chapter 6

170 NMR 6.1. Introduction 6.2. Background 6.2.1 Enrichment Schemes 6.2.2 Experimental NMR Methodology 6.2.3 Relationships between NMR Parameters and Structure 6.3. Binary Oxides 6.3.1 Crystalline Materials 6.3.2 Sol-Gel Produced Samples 6.4. Crystalline Ternary Ionic Systems 6.5. Silicates and Germanates 6.5.1 Crystalline Materials 6.5.1.1 Silica and Germania 6.5.1.2 Ternary Silicates 6.5.1.3 Silicates and Germanates of Zirconium and Titanium 6.5.2 Amorphous Materials 6.5.2.1 Silica and Germania 6.5.2.2 Metal Silicate and Germanate Glasses 6.5.2.3 Gel-Based Silicates 6.6. Aluminium- and Gallium-Containing Systems 6.6.1 Alumina and Aluminates 6.6.2 Crystalline Alumino- and Gallosilicates 6.6.3 Amorphous Aluminosilicates 6.7. Boron-Containing Systems 6.7.1 Borates 6.7.2 Ternary and Quaternary Systems 6.8. Other Systems 6.9. Hydrogen-Containing Samples 6.9.1 Crystalline Hydroxides and Other Hydrogen-Containing Materials 6.9.2 Hydrous Gels and Glasses 6.10. High Temperature Ceramic Superconductors References

333 334 334 337 346 349 349 352 355 359 359 359 361 365 366 366 367 369 372 372 375 379 381 381 382 384 386 386 387 388 390


Chapter 6

170 NMR 6.1. INTRODUCTION Oxygen is a ubiquitous component of the inorganic compounds of fundamental importance to materials technology (advanced, structural and functional ceramics, catalysts, etc.). This is hardly surprising since oxygen is the most abundant element in the earth's crust (~62.5 at%), 2.95 times more abundant than silicon and 9.67 times more plentiful than aluminium, the two next most abundant elements. This also clearly illustrates the important place of oxygen in mineralogy, and minerals are the source of many raw materials for advanced processing. Metals are usually refined from natural ores, many of which are oxides (e.g. bauxite, rutile). The production of aluminium by the Bayer process is an oxide-based industry with an annual production of almost 20 billion tonnes. Thus, oxygen NMR should have many important applications; since it is a technique sensitive to short-range interactions of the oxygen atoms located throughout the structure, it can provide information about the structure in its entirety (other nuclei are often further away from the centres of structural change). However, until the last few years, reported 170 NMR studies of solids have been relatively few. There are a number of reasons for this. Of the three stable oxygen isotopes (~60 with 99.76 % natural abundance, 170, with 0.037 % abundance and ~SO, with 0.2 % abundance), only ~70 has a nuclear spin (I - 5/2) and is accessible to NMR. Hence for routine observation isotopic enrichment is necessary, involving both cost and effort which hindered the initial development of ~70 NMR of solids. However, the importance of oxygen and in particular the oxygen sites to materials' properties led to a few early high-resolution solid-state 170 NMR studies, the first one published in 1983 (Schramm, Kirkpatrick and Oldfield 1983). A second problem with 170 is the significant broadening which can affect the resonances of this quadrupole nucleus, the quadrupole interaction being a strong function of the covalency of the M-O bond. Historically, the first group of compounds studied by ~70 NMR (silicates and zeolites) was unfortunately one of the most difficult. The ability to characterise the many types of oxygen sites in silicates and aluminosilicates would be extremely useful, but these sites have relatively large quadrupole interactions that can only partially be narrowed by MAS. Furthermore, these sites show a very small shift range (--~40 ppm) for ~70. These factors meant that the early 11.7 T 170 NMR MAS spectra of silicates and zeolites did not show much promise because of the lack of resolution, often detecting only Si-O-Si and Si-O-A1 rather than all the inequivalent sites expected from crystallography. Hindsight shows that the use of 170 333

334


NMR to solve such problems was correct, but required improvements in the technique which were not then available. Unfortunately these early experiments cast an unduly pessimistic perspective on the use of 170 NMR as a probe of silicate materials. The much smaller quadrupole interactions in more ionic oxides, and even in glassy materials, and the very large shift range for 170, allow the different sites readily to be distinguished. 170 is probably best characterised as a nucleus with a small quadrupole moment and a large shift range, making it attractive for solid state NMR. The 1990s brought great advances in the methodology for 170 NMR, including improved procedures for isotopic enrichment, much wider availability of fast MAS and high applied magnetic fields, and the development of specialised techniques for removing second-order quadrupole interactions, including DAS, DOR and MQ MAS. There now exist in the literature elegant examples of the application of all these techniques to 170, bringing impressive gains in resolution with as many as ten different oxygen sites being resolved. In the last few years the literature has seen an explosion in the number of papers using 170 solid state NMR and provided the cost of the isotope does not become prohibitive, this trend is likely to continue, assuring 170 NMR of an important role in materials characterisation.

6.2. BACKGROUND

6.2.1 Enrichment schemes A wide range of schemes for l Vo enrichment of samples is now available. As these techniques have become almost routine this is the first step in making 170 NMR a more widely used tool for analysis of oxides. The two readily available 170-labelled sources are O2 and H20, with typical 170-enrichment levels of 10-75 at %. The enrichment level of choice for any given experiment is a compromise between cost and the required sensitivity. Our experience shows that 20 at % enrichment of the precursor provides sufficient sensitivity for 1D spectroscopy and at current prices l g of the labelled oxide product typically costs --~s to manufacture. For MQ and DOR experiments, higher enrichments (35-45 at %) are desirable. A convenient route for producing certain oxides is by hydrolysis of the chloride. SiO2 can be prepared this way, as outlined by Bray et al. (1982). SIC14 is dissolved in ether and held at ice temperature under a dry nitrogen atmosphere. 170-labelled water is then added dropwise to this mixture while stirring. After addition of all the water the mixture is allowed to gradually warm to room temperature, again stirring. The resulting precipitate must be washed thoroughly several times to remove residual chloride and is then vacuum dried. The subsequent heat treatment depends on the target product but is typically 250~ for 1 day followed by calcination at 600~ for a few hours

170 NMR

335

to produce very clean, dry amorphous silica gel, or at 1500~ for 15 minutes to produce well-crystallised low cristobalite. Other oxides can also be enriched in this way but care must always be taken to control the temperature of the reaction as some chlorides hydrolyse vigorously. An alternative route becoming increasingly popular with the wider commercial availability of a variety of alkoxides is to start with a metal alkoxide (M(OR)x, where R is an alkyl chain). The alkoxide is dissolved in alcohol (our laboratory normally uses propanol) to act as a mutual solvent for the alkoxide and the water which is then added dropwise at room temperature while stirring to initiate a hydrolysis reaction (Brinker and Scherer 1990).

M(OR)x + xH20 ---->M(OH)~ + xROH

(6.1)

This reaction provides an efficient method for attaching the oxygen label to the metal. The hydroxyl groups on the metal then react with each other to cross-link the structure:

M(OH)x + M(OH)x --> (OH)x_ ~MOM(OH)~_~ + H 20

(6.2)

This results in the formation of a metal-oxygen-metal bond by a process which continues, resulting in the growth of the metal clusters and the removal of the hydroxyls. The nature of the liquid changes, first becoming a colloidal suspension, then a sol, and eventually turning into a gel. The alcohol and water produced must be removed by stirring at room temperature until the sample has dried to a crumbly solid. The sample is then powdered and vacuum dried. 13C NMR has revealed that even at this stage residual organic fragments are attached to the oxide, but these can be removed by gradually heating to 250~ and then 500~ The key is to heat gently to remove the carbon, producing an amorphous finely divided oxide, as the porous structure does not sinter which would seal in the carbon. Another route for the formation of the oxides of metals that have insoluble hydroxides is to start with a soluble salt such as CaC12 or Mg(NO3)2 and add a soluble hydroxide such as KOH in the presence of 170-labelled water. The insoluble hydroxide precipitate is washed prior to thermal dehydroxylation to form the oxide. Carbonates can also be enriched by sealing in a thick walled glass tube with ~70-labelled water and heating at 80-110~ for typically --~1-2 weeks. The exchange reaction can be facilitated by the addition of a small amount of NH4C1 flux (a typical mixture would be 1.23 g of CaCO3, 0.33g of water and 0.077g of NH4C1). The flux and any excess water can be removed by subsequent drying under nitrogen at 450~ for 1 hour. K2CO3 can be enriched this way and then used to prepare other sparingly soluble metal carbonates by an exchange reaction with the chloride of the required metal, the solubility of the KC1 facilitating its removal from the metal carbonate. Similar exchange reactions can be carried out with

336


hydroxides by sealing them with enriched water. This is a common route for preparing enriched alumina from AI(OH)3, or more commonly A1OOH, by sealing them in either a quartz or gold tube with enriched water and heating at 200-350~ for a few days. These syntheses readily produce enriched binary oxides from which more complex phases can be formed by mixing the appropriate oxides, hydroxides or carbonates and heating to a temperature where solid state reaction takes place. However 170 can be easily lost during this heat treatment, necessitating the use of different methods at temperatures higher than 1000~ Below 1000~ the powder can be heated in an open alumina boat or platinum crucible under a nitrogen atmosphere, but exchange and loss of oxygen becomes an increasing problem as 1000~ is approached. Above 500~ the use of a dry nitrogen atmosphere produces significant advantages over a normal atmosphere. Above 1000~ the loss of labelled oxygen becomes very rapid, and the best strategy is to seal the mixture in a platinum or gold foil capsule. An approach often used in the preparation of silicates is to mix and homogenise the oxides by forming a glass which is then crystallised to form the product. Even for sealed samples it is important to keep the time spent at temperatures in excess of 1000~ as short as possible. Another key to securing successful solid state reactions is to achieve intimate mixing between the different metal oxide components. In this respect, the alkoxide route is attractive, since the components are homogeneously mixed in the initial solution. Care must be taken to match the hydrolysis rates of the different metal alkoxides, but techniques have now been developed by sol-gel chemists for overcoming such problems (Brinker and Scherer 1990). Gas exchange can be used to enrich many oxides. The oxygen in high-temperature ceramic superconductor phases such as YBa2Cu3Ov_x is mobile and some can be removed by heating under vacuum (Oldfield et al. 1989). The system can then be back filled with 170-labelled 02 before cooling back down to room temperature. Heating under a 1702 atmosphere has been found to be a very widely applicable approach for enriching oxides (Yang et al. 1989). A scheme typically used with zeolites is to heat to 500-750~ and evacuate to 10-4-10 -5 Torr for 12-24 hours. 1702 is then introduced and the sample left for about another day before cooling back down to room temperature. Water can also be exchanged with zeolites. 170 NMR has been used to study in detail the kinetics of this process in both stilbite (Xu and Stebbins 1998) and analcime (Cheng et al. 2000). The powders were ground and the 44-75 txm fraction used for the reaction. The samples were initially enriched by sealing with 170-labelled water in a gold tube and heated to the appropriate temperature for 10 days to 6 weeks to ensure uniform labelling of the sites. In both stilbite and analcime it was found that the rate of reaction was determined by the exchange step and not by diffusion in these finegrained porous materials. The samples were then evacuated and backreacted with normal water for varying times at various temperatures, using the 170 NMR signal to monitor the kinetics of the equilibrium

170 NMR M-170- M' + 92160 ~ M - 1 6 0 - M' + 02170

337 (6.3)

where M,M' = Si,A1. For stilbite in the temperature range 150-200~ the Si-O-A1 fragments were shown to exchange faster than the Si-O-Si. For analcime at 400~ SiO-A1 exchanges faster than Si-O-Si but by 500~ the rates of exchange become comparable, suggesting that Si-O-Si exchange has a higher activation energy.

6.2.2 Experimental N M R methodology 170 NMR provides an ideal opportunity for the exploitation of the full range of solid-

state NMR methodology now available. The relative contributions of the interactions, particularly the chemical shift and quadrupole interaction, vary so widely that no single approach is correct for ~70 NMR of all samples. The most straightforward experiments are one-pulse acquisition and a spin-echo sequence, used with both static and MAS conditions. If only relatively narrow lines are present, as is the case for ionic systems, one pulse acquisition tends to be sufficient, although the moderate Larmor frequency of oxygen makes the use of a spin-echo sequence advisable at lower fields (-< 8.45 T) even with MAS. When echoes are combined with MAS, the echo spacing should be set to an integral number of rotor periods. In samples where there is significant quadrupole interaction, such as silicates and borates which contain more covalent links, broader lines are encountered and echo techniques are commonly used. 170 shows a wide variation in relaxation times, making it necessary to consider carefully the relaxation delay to be used. A delay of 0.25 s is often sufficient for gel-formed precursor materials, but much longer T1 times are encountered in more rigid structures where there are no efficient relaxation mechanisms. The relaxation time of BaHfO3 was found to be 32 s, a value typical of this type of material. Figure 6.1A shows the 170 NMR spectra of albite glass acquired at 7.05 T, in which the Si-O-Si and Si-O-A1 signals completely overlap under both static and MAS conditions. However, the static spectrum shows better-defined features (the singularities in the quadrupole lineshape), allowing the two contributions to be unambiguously separated. Combination of the 170 MAS NMR data for a crystalline titanodiphenylsiloxane acquired at various magnetic fields (Figure 6.1B) with the static NMR data (Figure 6.1C) has allowed the interactions in this material to be deduced (Gervais et al. 2000). In materials containing X-O-Y linkages with very different interaction parameters, the choice of an optimum magnetic field is not straightforward, particularly where there is a large variation in the quadrupole interaction at different sites and the material is amorphous. The magnetic field variation of an amorphous TiO2-SiO2 based material is shown in Figure 6.2. This material has a complex cross-linked structure giving five distinct 170 NMR signals, OTi3, OTi4, two types of Si-O-Ti and Si-O-Si (Gervais et al. 2001). The resonances arising from oxygen bonded only to titanium are

338


B

Staticecho

5.6T

~~/u ~_J

500

0

-500 MAS

C

~ated

, 17.6T ~ /~

0

-5O0

170 shift (ppm) w.r.t. H20

400 .......

observed

observed

_~_~~170 CP has also been demonstrated in oL-AI203, giving a large increase in the signal-to-noise ratio (Haase and Oldfield 1994).

A AI-O-AI Non-CP

obs

r~

AI-OH

=

.

~

/

~

~

k~ oJ 4

sim.late./ 0.1

r~

, ,

r

200

0

-200


0

.

.

.

.

.

.

.

.

0.3 1.5

Contact time (ms)

3.0

. . . .

1000

i

. . . .

,

0

. . . .

~

. . . .

,

. . . .

J

-1000


Figure 6.8. A. 170 MAS NMR spectra of boehmite, with and without CP. Asterisks denote spinning sidebands. Note the significant enhancement of the A1-OH signal under CP conditions. From Walter et al. (1988). B. 1H-170 cross-polarisation curve for the hydroxyl group singularity at 323 ppm in the spectrum of Mg(OH)• as a function of the contact time. The inset shows the experimental data of the first 300 txs and the best fit to the data (solid line). From van Eck and Smith (1998). C. Observed and simulated 1H-170 CP echo NMR spectrum of Mg(OH)x(OCH3)2-x without using proton decoupling during acquisition. From van Eck and Smith (1998). All figures used by permission of the copyright owners.

346


6.2.3 Relationships between N M R parameters and structure After acquisition, the NMR data must be interpreted in terms of the structure of the sample material with the aim of providing information about the local atomic environment. Much of the interpretation has been based on the semi-empirical Townes-Dailey approach which relates the NMR parameters to the covalency of the bonds by consideration of the p- and d-orbital occupancy (Townes and Dailey 1949). Early applications to 170 NMR of this approach were made for borates (Jellison et al. 1977), silicates (Geissberger and Bray 1983) and zeolites (Janes and Oldfield 1986). Other treatments have applied quantum mechanical calculations (Tossell and Lazzeretti 1988, Tossell 1990). Steinberg examined the 170 Ti-values of A-O-A bonds using a geometrical approach which made no assumptions about the electron distribution or the p-orbital occupancy since these effects cancel, although the electron distribution must have certain symmetry (Sternberg 1993). The work is applicable to Si-O-Si linkages, allowing the A-O-A bond angle (ct) to be deduced from its relationship with ~q, determined to be:

3(cos a + 1) 1/=- 3cosa-1

(6.7)

Comparison of this relationship with ab initio calculations showed the largest difference occurs as the Si-O-Si bond angle approaches 90 ~ Although the geometric formula gives slightly lower values, the same trends were found in both approaches. There is a need to increase the number of experimental data points to test these relationships and allow their refinement. One of the most elegant studies has been of the SiO2 polymorph coesite, containing five distinct oxygen sites with bond angles ranging from 137.22 ~ to 180~ (Grandinetti et al. 1995). A high quality 170 DAS spectrum of coesite was accumulated over a period of more than 15 days (Figure 6.9), for which the resonances were assigned on the basis of the general increase of XQ with bond angle and a consideration of the intensity of the different peaks. These data suggested the relationship Zo - ZQ (180 ~ 2 cos a cos a - 1

(6.8)

Ab initio calculations show that the x and z axes of the efg tensor lie in the plane of the Si-O-Si bond with the x-axis bisecting the Si-O-Si bond angle. Ab initio calculations have also been used to examine the effect of cations coordinated to Si-O-Si bonds by considering the (OH)3Si-O-Si(OH)3 unit with lithium and sodium coordinated to the Si-O-Si site. Both XQ and xl were found to have the same functional dependence on oLirrespective of the presence of the cations, although there was an offset depending on the number of coordinating cations and their field strength (Vermillion et al. 1998). The value of XQ was found to shift systematically to lower

170 N M R

347 observed

;~

/

-20

'" 9

.~ ~ ~

~

~

%~o

.

.

,mq

simulated

o.

~ m l ~ i ~ ~ : : ~ '

o

"0"

20

~

60

-60

0

~...,...,..~,.

......

60

Frequency (ppm)

0

,.~,...0...

T...,..=~...,

-60

....

60

0

.,.,-., .... ,...,...

-60


Figure 6.9. 2D 170 DAS NMR spectrum of coesite, with cross-sections taken parallel to the anisotropic dimension for all five sites in coesite and the best-fit simulations (extreme right). From Grandinetti et al. (1995) by permission of the American Chemical Society.

0 Li §

N -6

1.0

1 Li §

2 Li§

0.6 1="

CY-4 100

2 Li §

-4

0 Li § 140

100

180

Si-O-Si (~

140

0 Na + 1 Na +

2 Na +

cy-4 180

Si-O-Si (o)

0.2

0.6

~

ONa +

J 1

0.6

100

,Na 4t

2 Na § . . . . 140

Si-O-Si (~

1.0

11

1.0

0.2 140

180

Si-O-Si (o)

-6

100

1 Li +

0.2

0 Na +

180

0.2

0.6

1.0

q

Figure 6.10. Results of ab initio calculations of the 170 NMR interaction parameters XQ and ~q of the (OH)3Si-O-Si-(OH)3 unit in sodium and lithium silicates. From Vermillion et al. (1998) by permission of the copyright owner.

values as the number of cations and the field strength of the coordinating cation increased (Figure 6.10), as described by the relationship"

xQ a(l+cosCOS

348


On the other hand, TI shifts to much higher values when one cation is added but shows a much smaller effect when there are two coordinating cations. The value of ~q was described by the function: r / = b ( 12 cosC~ - 1 ] q+2gIN

(6.10)

A comparison made of the dependence of XQ and xI on the orientation of the oxygencation internuclear vector showed that for a single cation, the value of XQ is insensitive to this orientation whereas -q shows a degree of orientation dependence, making it a less reliable parameter for the estimation of e~ in these systems. These calculations predicted that for cations of smaller radius and high field strength (e.g. Mg 2+) the dependence of XQ on oLcan be significantly different. A difficulty of all this work is the relative lack of experimental measurements of systems containing bridging oxygens against which these relationships can be tested. Ab initio calculations were extended to a series of related clusters containing central M-O-M bonds. A linear relationship was found between XQ and both the bond distance and the cation group number. These parameters were suggested to be better for predicting XQ in systems containing bridging oxygens than arguments based on electronegativity (Clark and Grandinetti 2000). Sophisticated methods developed for calculating these parameters (Palmer and Blair-Fish 1994) have been extended by ab initio density functional theory using the WIEN code and applied to ~70 NMR calculations for fosterite (Winkler et al. 1996). In recent studies of J70 NMR data for zeolites (Bull et al. 1998, 2000) the efg tensor has been calculated using the Moloch module of the TURBOMOLE programme (Ahlrichs et al. 1989) and by using density functional theory within the GAUSSIAN programme (see references within Bull et al. 1998, 2000 for more details). The question of correlating the ~70 ~o,c~ values with the structure has also been addressed. Shift calculations use various approaches including the CPHF-GIAO method (H~iser et al. 1992). Examination of the sites within siliceous faujasite and ferrierite has led to the conclusion that there is no obvious correlation of the shift with the structure. The isotropic chemical shifts in coesite which span a range of 29 ppm are not consistent with the expected monotonic relationship between ~i~o,c~and oL, nor with the predicted shift range of 10 ppm (Grandinetti et al. 1995). However, in another series of zeolites the ~i~o,c~values were found to decrease as o~ increased (Freude et al. 2001), suggesting, for a restricted ranges of materials such as the sites in sodium A and LSX, the relationship cos a

5(ppm) = - 2 1 4 ~

cos a - 1

+ 136

(6.11)

Currently it appears that shift correlations may have only limited application to 170 NMR in groups of materials with closely related structures.

170 N M R

349

One of the most powerful applications of these correlations between NMR parameters and structure is to provide a better understanding of the structure of amorphous materials which are very difficult to study by other techniques. Silicate glasses have been studied by relating the giso,cs value of 298i to the bond angle distribution (Dupree and Pettifer 1984, Pettifer et al. 1988). In the case of 170, the parameter -q has been used to determine the Si-O-Si bond angle distribution (Farnan et al. 1992) using the relationship: D3(a ) - D4 (r]) ~

.

(6.12)

This has provided important structural information about glasses (Section 6.5.2).

6.3.

BINARY

OXIDES

6.3.1 Crystalline materials The large 170 NMR shift range in diamagnetic oxides (Table 6.2) indicates the sensitivity of 170 NMR to structural effects. Oxygen can take a large range of coordination numbers (typically 2-6), the 170 shift becoming more shielded with increasing coordination

Table 6.2. 170 NMR peak positions in diamagnetic binary oxides. Compound

Coordination

Chemical shift* (ppm)

Reference

BeO MgO

OBe4 OMg6

26 47 47 294 390 629 141 - 18 - 18.7 121 289 293 247 246 105 334 557 552 584 591 591

Turner et al. (1985) Turner et al. (1985) Bastow & Stuart (1990) Turner et al. (1985) Turner et al. (1985) Turner et al. (1985) Turner et al. (1985) Turner et al. (1985) Bastow & Stuart (1990) Turner et al. (1985) Oldfield et al. (1989) Bastow & Stuart (1990) Oldfield et al. (1989) Bastow & Stuart (1990) Oldfield et al. (1989) Oldfield et al. (1989) Oldfield et al. (1989) Bastow et al. (2000) Bastow et al. (2000) Oldfield et al. (1989) Bastow et al. (1993)

CaO SrO BaO CdO (200~

OCa6 OSr6

OBa6

ZnO

OZn4

Yellow-HgO PbO

OHg4 OPb6

SnO

OSn3

SnO2

OSn 3

BaO2 TiO2, anatase brookite, site 1 site 2 rutile

OTi3 OTi3 OTi3 OTi3

350


Table 6.2. (Continued) Compound

Coordination

Chemical shift* (ppm)

Reference

ZrO2, cubic Tetragonal Monoclinic, site 1 site 2 HfO2 Monoclinic, site 1 site 2 CeO2

OZr4 OZr4 OZr4 OZr3 OHf4 OHf3 OCe6

VO2,

OV3 OV3 OCu4

355 378 324 401 266 334 877 878 755 815 - 181 193 - 277 496 467 467 584 584 305 196 364 355 356 359 383 377 346 313 242 97 455 305

Bastow & Stuart (1990) Bastow & Stuart (1990) Bastow & Stuart (1990) Bastow & Stuart (1990) Bastow & Stuart (1990) Bastow & Stuart (1990) Oldfield et al. (1989) Bastow & Stuart (1990) Bastow & Stuart (1990) Bastow & Stuart (1990) Oldfield et al. (1989) Bastow & Stuart (1990) Bastow & Stuart (1990) Bastow & Stuart (1990) Bastow & Stuart (1990) Yang et al. (1992) Bastow & Stuart (1990) Yang et al. (1992) Yang et al. (1992) Oldfield et al. (1989) Oldfield et al. (1989) Oldfield et al. (1989) Oldfield et al. (1989) Florian et al. (1995) Florian et al. (1995) Florian et al. (1995) Florian et al. (1995) Florian et al. (1995) Florian et al. (1995) Oldfield et al. (1989) Pickup et al. (2000)

site 1 site 2

Cu20

-

Ag20 Ti203 La203,

OAg4 site 1

OLa6

site 2

OLa4

Lu203 Bi203 T1203 Sc203 C-Y203

OLu4 OBi4 OT14 OSc4 OY4

B-Y203, site 1 site 2 site 3 site 4 site 5 In203 Ta205, site 1 site 2

OY4 OY4 OY4 OY5 OY6 OIn4 OTa3 OTa2

9- all shiftsin Chapter6 are referencedto H20.

number. The sensitivity of 170 N M R to structure is well illustrated by ZrO2, which principally occurs in three polymorphic forms, cubic (OZr4), tetragonal, (OZr4) and monoclinic (OZr3, OZr4). The 170 N M R spectra of the tetragonal and monoclinic forms are readily observable even at natural abundance (Bastow and Stuart 1990) (Figure 6.11A). The monoclinic form shows two very sharp resonances separated by 77 ppm, with linewidths of --~ 2 ppm. The tetragonal phase shows a single peak --~10 ppm wide at an intermediate position. The cubic phase is not a stable form of pure ZrO2 at room temperature, but can be stabilised by the addition of a second oxide such as MgO or Y203. The

351

170 NMR A tetragonal

T(~

1A

6OO

mznol~nic [ [ t A

anatase 800

rutile

Mg-stabilised

1000

400

tA -200


A

rutile

650

550

450


800

400

0


Figure 6.11. A. 170 natural abundance MAS NMR spectra of zirconia polymorphs. The peak marked A is from oxygen in the alumina rotor. The asterisks denote spinning side bands. From Bastow and Stuart (1990) by permission of Elsevier Science. B. ~70 MAS NMR spectra of titania gel heated to various temperatures, showing the evolution of rutile at the expense of anatase. From Bastow et al. (1993) by permission of the Royal Society of Chemistry. C. Static and MAS 170 NMR spectra of cubic Y203. Adapted from Florian et al. (1995).

resulting solid solution contains a range of next-nearest neighbours resulting in atomic disorder which is reflected in the NMR spectrum as a shift from the resonance position of the tetragonal phase and a much increased linewidth (84 ppm) arising from chemical shift dispersion. Whereas XRD has difficulty distinguishing the tetragonal and cubic phases, this distinction can readily be made by 170 NMR. NMR also shows that additions of MgO to ZrO2 produce no MgO signal, indicating that the magnesium enters the network and does not form a separate phase. The effect of the nearest element on the 170 NMR spectra can be gauged by comparing the shifts of the isostructural compounds ZrO2 and HfO2, in which the corresponding sites show a shift difference of --~ 60 ppm. The various polymorphs of TiO2 differ in only the relative disposition of the TiO6 units but their 170 shift differences span a range of ---35 ppm. The solid state conversion of anatase to rutile can be readily followed by 170 NMR (Figure 6.11B). The more distorted structural units in rutile give rise to a noticeable field gradient (Figure 6.11B). The value of XQ has been determined from the MAS spectrum to be 1.50 +_ 0.03 MHz, with xl = 0.87 +_ 0.03 (Bastow e t al. 1996), agreeing accurately with an earlier single crystal study (Gabathuler e t al. 1973). Some of the Group II oxides with NaC1 structures show very narrow resonances arising from the cubic structure containing the

352


spherically symmetric 0 2 - ion. The value of XQ in CaO, SrO and BaO is close to 0 (Turner et al. 1985). The 170 shifts of the group IIA and liB oxides show correlations with the cation radius r (,~)"

~ ( p p m ) - 2571.3 + 11 and ~(ppm)- 150r 3 - 7 8

(6.13)

HgO, which is much more electronegative than the other metals, has a much larger XQ value (---7 MHz) (Turner et al. 1985). On the basis of a larger data set, a more general correlation of ~ with the cationic radius has been suggested:

~(ppm) = -439r + 3205

(6.14)

Although it is not obvious how to rationalise these correlations, they provide guidance to the variations expected in such compounds. In a similar way, XQ values for 170 have been correlated with the bond ionicity; as the bond becomes more ionic the oxide ion becomes more O2--like and the field gradient decreases according to

ZQ(MHz) = -0.2031(%) + 14.8

(6.15)

where I is the bond ionicity (Schramm and Oldfield 1984, Oldfield et al. 1989). The single oxygen site in cubic C-form of Y203 has a CSA (Figure 6.11 C) with a span of 115 ppm (Florian et al. 1995). In the monoclinic B-form there are five oxygen sites (three OY4 sites, one OY5 and one OY6). All five sites can be clearly resolved in the 170 MAS NMR spectrum (Florian et al. 1995) and have values of XQ which vary between 0.32 and 1.80 MHz. The OY5 site shows the greatest distortion with a hint of second-order quadrupolar structure in the MAS centrebands. A good correlation has been found between the 170 isotropic chemical shift and the Y-O bondlength for all the OY4 sites in these two compounds.

6.3.2 Sol-gel produced samples The sol-gel process has become one of the most important routes for 170-enriching binary oxides. One of the spin-offs is that the oxides formed by this process evolve into the final crystalline state via a series of amorphous structures that can be directly probed by 170 NMR, providing a great deal of structural detail which would be difficult to derive by other methods. An early example illustrating the utility of 170 NMR in such studies was an investigation of the sol-gel formation of ZrO2 (Bastow et al. 1992). The three peaks in the spectra indicated the presence in the amorphous material of predominantly monoclinic-like ZrO2 with some tetragonal-like regions. On crystallisation the spectrum significantly narrowed and indicated the presence of predominantly

170 N M R

353

tetragonal phase. The nature of the oxygen sites is unequivocally indicated by the 170 shifts, providing in this case a precise identification of tetragonal zirconia. This is an interesting result, since the stable phase in pure undoped ZrO2 at room temperature is monoclinic, except in the case of very fine, nanocrystalline ZrO2 in which the tetragonal phase is stabilised by a surface energy effect. A subsequent, much more detailed study of 170 NMR of ZrO2 combined with Zr K--edge EXAFS and XANES (Chadwick et al. 2001) compared nanocrystalline ZrO2 formed by the sol-gel and hydroxide precipitation methods. The 170 MAS NMR spectra (Figure 6.12A) show two peaks at 405 and 303 ppm with an intensity ratio ---1:1 at positions very close to crystalline monoclinic. These resonances persist to the point of crystallisation at 360~ of the tetragonal phase, which predominates until the particle growth above 500~ brings about a reversion to the monoclinic form. The data reveal that the amorphous precursor has a monoclinic-like local structure prior to crystallisation, thereafter becoming a complex mixture of tetragonal- and monoclinic-like sites (Chadwick et al. 2001). A

B

OTi3

T(~

T(~ ,[

OTi4 $

300

250

J^

'\

360 (15h)

200

o

____) ';oo'

in

\...e.to. ,,"~'

~lll,ll,lT

3;o

IY'| I

2;0


800

400

0

170 shift (ppm) w.r.t. H 2 0

Figure 6.12. A. 170 MAS NMR spectra of undoped ZrO2 gel heated to various temperatures. Note the change from the twin resonances of the monoclinic form, which are replaced by the single tetragonal resonance upon recrystallisation at 360-380~ but revert to the monoclinic form above 500~ From Chadwick et al. (2001) by permission of the American Chemical Society. B. 170 MAS NMR spectra of titania gel heated to various temperatures. Note the gradual loss of the OTi4 resonance on heating. From Bastow et al. (1993) by permission of the copyright owners.

354


Other examples of the use of 170 NMR to monitor the evolution of structure in sol-gel samples include a study of TiO2 (Bastow et al. 1993), V205 (Pozarnsky and McCormick 1994), La203 (Ali et al. 1996), HfO2 (Bastow et al. 1996), MgO (Chadwick et al. 1998) and Ta205 (Pickup et al. 2000). The results clearly indicate that there is no well-defined evolution route for the structure and each oxide system has its own peculiarities. HfO2 is chemically very similar to ZrO2, and shows a 170 NMR spectrum of the gel with two resonances close to the monoclinic position. The only difference between the spectra of these two compounds is that on crystallisation HfO2 shows very much narrower monoclinic peaks and no indication of the formation of the tetragonal phase (Bastow et al. 1996). The formation of La203 by the sol-gel process proceeds via LaO(OH), which is readily identified by the ~70 peak corresponding to the OLa4 site at 546 ppm (Ali et al. 1996). TiO2 is a very important system with many technological applications which have encouraged much research, especially into the formation of nanocrystalline forms with increased surface area and higher activity. Two 170 NMR resonances are observed in sol-gel produced TiO2 at 514 and 368 ppm which have been assigned by careful work carried out on 170-enriched molecular titanium-oxo-organo clusters (Day et al. 1991, 1992, 1993, Scolan et al. 1999). These results indicate that the OTix coordinations have shifts in the range 650-850 ppm for x = 2,450-650 ppm for x = 3,250--450 ppm for x = 4 and < 250 ppm for x - 5. Variable field NMR clearly indicates that for all sites bonded only to titanium, the spectra are dominated by chemical shift effects, with almost no contribution from quadrupole effects (Bastow et al. 1993, Scolan et al. 1999). The OTi2 sites are characterised by an extremely large CSA with a typical span o f - 650 ppm (Scolan et al. 1999). On heating there is a gradual decrease in OTi4 until only OTi3 remains at crystallisation (Figure 6.12B, Bastow et al. 1993). Nanoparticles of TiO2 showed a range of OTix environments, one of the OTi3 environments perhaps showing evidence of a contribution from second-order quadrupole effects (Scolan et al. 1999). The nanoparticles are characterised by a bulk OTi3 signal, surface signals from OTi2, OTi3 and a signal from species coordinated to acetylacetone. ~70 NMR has also been used to study structure development in titania gels where the reactions are controlled by regulating the hydrolysis ratio and by the use of complexing agents (Blanchard et al. 2000). The nature of the ligand was found to change the local structural units in the titania core. 170 MAS NMR provided a direct quantitative measure of the different OTix units present, providing an insight into the structure. 170 NMR gives a direct indication of the changes in the proportion of the different oxygen environments in amorphous gels during heating as well as other information about subtle changes in the structure which are provided by observing other common trends in the 170 NMR data. For example, the linewidths of the resonances in the initial gel tend to increase up to the point of crystallisation. Since the linewidth is largely determined by chemical shift dispersion, this indicates a wider range of environments

170 NMR

355

and hence a more disordered structure. Furthermore, the 170 resonances of the initial gel tend to show a monotonic increase in shift up to the point of crystallisation, as illustrated by the OZr, and Hf4 resonances in zirconia and hafnia gels respectively; the former resonance shifts from 303 to 321 ppm and the latter from 245 to 258 ppm. The chemical shift reflects the electron density in the bonds, and is determined by factors such as coordination number, hybridisation and bond length. The changes in the shifts of these gels can be interpreted in terms of a decrease in the mean bond length, illustrating the use of NMR as a detailed structural probe of the amorphous state.

6.4. CRYSTALLINETERNARY IONIC SYSTEMS Inspection of the 170 MAS NMR spectra of ternary oxygen-containing materials reveals a large variation in linewidth, with some spectra showing extremely high resolution and revealing minor differences in the site crystallography. A selection of typical spectra of titanates and zirconates are shown in Figure 6.13 and those of the corresponding hafnates are shown in Figure 6.14. Comparison of the shift ranges of titanates (372-564 ppm), zirconates (298-376 ppm) and hafnates (237-332 ppm) indicates that in compounds of the form AxBOy, the B ions principally determine the 170 shift range. This is illustrated by a comparison of isostructural zirconates and

SrTiO3

~ __._t__

CaTiO3 ,

,

5

SrZr03

~

Li2TiO3 .

0

~_~

CaZr03

Li2Zr03

340 320

600

400

200

600

400

200

600

400

200

170 shift (ppm) w.r.t. H20 Figure 6.13. A selection of typical 170 MAS NMR spectra of metal titanates and zirconates. From Bastow et al. (1996) by permission of the copyright owner.

356


BaHfO 3 t

,,

i

400

!

|

200

!

!

1

0

170 shift (ppm) w.r.t. H20 Figure 6.14. A selection of 170 MAS NMR spectra of metal hafnates. From Bastow et al. (1996b) by permission of the copyright owner.

hafnates for which the ratio of the isotropic shifts (Hf/Zr) falls in the very narrow range 0.84--0.88 (Bastow et al. 1996b). The average value for zirconates and titanates (Zr/Ti) is 0.74 + 0.03, very close to the ratio of the polarising powers of these two elements (0.7) (Bastow et al. 1996). Alternatively, the ratio of the bond lengths in these compounds may be considered. The symmetry of a structure is reflected by the number of oxygen sites it contains. For example, the cubic perovskites (SrTiO3, BaZrO3) show only a single sharp oxygen resonance, reflecting the equivalence of all the oxygen sites. In BaTiO3 there is a slight distortion of the structure, in which the titanium and barium are slightly displaced from their ideal positions, giving rise to two inequivalent oxygen sites with a ratio of 2:1. 170 MAS NMR clearly resolves these two sites at room temperature but as the temperature is raised the resonances move together. The cubic phase formed in the tetragonal-to-cubic transition at 130~ shows only a single peak at 538 ppm (Spearing and Stebbins 1994). The splitting of these two resonances in materials such as CaTiO3 with the GdFeO3-structure is typically 5 ppm. In monoclinic materials of lower symmetry (e.g. Li2TiO3) there are still only two signals but these show greater differences in shift (--~20 ppm) (Bastow et al. 1996).

170 N M R

357

Table 6.3. 170 NMR data for crystalline ternary oxide compounds. Compound

Peak position (ppm)

Reference

Li2TiO3 K2Ti409 KzTi6013 MgTiO3 CaTiO3 SrTiO3 SrzTiO4 BaTiO3

405.8,372.3 250, 350, 660 320, 440, 590 398 448.0, 443,4 465 426, 407 564, 523 553,530 303 298 301 301,273 420 465 298.4, 280 308.9, 286.0 336, 329 343.7,340.2 376.0 375 237.8, 249.2 239.8, 258.4 292.7,284.1 296, 298 331.9 85 423 213, 198 504 350 160", 215 375 1197 422, 429, 437

Bastow et al. (1996) Bunker et al. (1997) Bunker et al. (1997) Millard et al. (1995) Bastow et al. (1996) Bastow et al. (1996) Bastow et al. (1996) Anuradhi (1990) Spearing & Stebbins (1994) Millard et al. (1995) Millard et al. (1995) Millard et al. (1995) Millard et al. (1995) Bunker et al. (1997) Bunker et al. (1997) Bastow et al. (1996) Bastow et al. (1994) Bastow et al. (1996) Bastow et al. (1996) Bastow et al. (1996) Bunker et al. (1997) Bastow et al. (1996b) Bastow et al. (1996b) Bastow et al. (1996b) Bastow et al. (1996b) Bastow et al. (1996b) Bastow et al. (1996) Bastow et al. (1996) Bastow et al. (1996) Bastow et al. (1996) Bunker et al. (1997) Adler et al. (1994) Oldfield et al. (1989) Schramm & Oldfield (1984) Schramm & Oldfield (1984)

Cubic MgzTiO4 Tetragonal MgzTiO4 Cubic ZnzTiO4 Tetragonal ZnzTiO4 BazTiO4 PbTiO3 Li2ZrO3 NazZrO3 CaZrO3 SrZrO3 BaZrO3 PbZrO3 LizHfO3 Na2HfO3 CaHfO3 SrHfO3 BaHfO3 LiSnO3 SrSnO3 Sr2SnO4 LiNbO3 PbNb206 Ba2In205 BaBiO3 KMnO4 K2WO4

* - isotropic chemical shift In ionic materials, where broadening of the central transition is dominated by CSA, the chemical shift tensor can be characterised. The 170 CSA values for such compounds are summarised in Table 6.4. The chemical shift tensors in the titanates tend to have larger spans than the zirconates, reflecting the influence of the B cation on the CS tensor. Many oxides have properties such as ionic conductivity, making them suitable for use as solid electrolytes and oxygen sensors. These properties depend on ionic motion in the material and can be studied by lVO NMR. Such a study has been made between room temperature and 1200~ of BazIn205 which contains three inequivalent oxygen sites

358

M u l t i n u c l e a r Solid-State N M R o f I n o r g a n i c M a t e r i a l s

Table 6.4. 170 chemical shift tensor components of ternary oxide compounds. Compound BaZrO3 SrTiO3 SrZrO3 CaTiO3 CaZrO3 LiNbO3 KzWO4, site 1 site 2 site 3

8iso,cs(ppm) Span(ppm) 367.2 468.1 340.1 443.9 329.3 444.7 437 429 422

337 473 332 388 356 610 347 365 353

Skew

Reference

-0.49 - 0.73 -0.67 - 0.60 - 0.42 - 0.34 -0.80 - 0.73 - 0.64

Bastow et al. (1996) Bastow et al. (1996) Bastow et al. (1996) Bastow et al. (1996) Bastow et al. (1996) Bastow et al. (1996) Schramm & Oldfield (1984)

within the orthorhombic structure (Adler et al. 1994). The spectra show two oxygens with similar shifts of --~160 ppm and large XQ value, and a third site characterised by a Gaussian line at 215 ppm. The spectra show an order-disorder transition at 925~ above which temperature the population of mobile anions increases. At --~1075~ the structure becomes cubic and at 1200~ a single Lorentzian resonance is seen at 210 ppm (Adler et al. 1994). Similar high temperature studies have been carried out on the highly atomically disordered perovskites Ba(Ino.67Zro.33)Oy and Ba(Ino.67Ceo.33)Oy. DAS measurements showed that the oxygen is displaced from the position of cubic symmetry and that there are relatively few mobile oxygen atoms below 800~ (Adler et al. 1994a). Defect sites can have profound effects on the properties of materials. BaFBr and BaFC1 are modem storage X-ray phosphor materials. Since both compounds scavenge oxygen from their environment they usually contain oxygen impurities. Detailed knowledge of the oxygen location in the structure is necessary to understand its substitutional preference and any tendency to form oxygen interstitials. A complex lVo MAS NMR spectrum has been observed for oxygen-doped BaFBr, containing four sharp lines at 630, 610, 597 and 587 ppm, together with a much broader resonance at 197 ppm (Bastow et al. 1994a). The corresponding spectrum for BaFC1 contained signals at 181 and 161 ppm. The lines at about 600 ppm were assigned to O 2- substitution on the anion sites with a preference for fluoride displacement, but some substitution also occurs on the bromine site. The splitting of these resonances is probably due to next-nearest neighbour and vacancy displacement effects. The other peak which shows a large shift is probably from a peroxide anion 022-. The BaFC1 spectra show no evidence of O 2-, but suggest the presence of 022- in two different sites; the one corresponding to the resonance at 181 ppm is probably a substitutional site and the other, at 161 ppm, arises from an interstitial anion. The solid state chemistry of mixed oxide systems can be directly probed by 170 NMR. The formation of nanocrystalline TiO2 has been studied by doping the system

170 N M R

359

with SnO2 which has the rutile structure. A 170 N M R study of the sol-gel reaction in this system showed two peaks at 595 and 375 ppm. Since the latter remained even after extensive heat treatment, it was not an O(Ti,Sn)4 resonance such as seen in T i Q gels (Bastow et al. 1995). Neither do the peaks match the 170 N M R spectrum of SnO2. The major peak is therefore in the position of OTi3 in rutile and the other peak probably arises from OX3, where X is a combination of titanium and tin. The most striking aspect of this result is the presence in the initial gel prior to heat treatment of oxygen in a rutile-like OTi3 configuration rather than the anatase-like configuration found in a pure TiO2 gel. The ~70 N M R spectra of a series of solid solutions in the PbZrxTi~-xO3 system showed distinct peaks arising from nearest-neighbour zirconium and titanium environments, lVo N M R was used to monitor the formation of stable phases resulting from changes in the composition of the system at room temperature (Klemperer and Richard 1998).

6.5. SILICATES AND GERMANATES

materials and germania. Because of the special place occupied by SiOa as the

6.5.1 C r y s t a l l i n e 6.5.1.1 S i l i c a

archetypal network glass former, there have been extensive efforts using many experimental techniques to understand its structural details. The crystalline polymorphs used to calibrate these experimental probes have been studied in detail by ~70 NMR. Their parameters are summarised in Table 6.5. The N M R data for N M R coesite were derived from the anisotropic dimension of a DAS data set (Figure 6.9, Grandinetti et al. 1995). The N M R parameters for all the other samples were taken from 1D static and MAS data. The static measurements of the SiO2 Table 6.5. 170 NMR parameters for crystalline SiO2 and GeO2 polymorphs. Sample

~i.... s(ppm)

XQ (MHz)

~l

Si-O-Si bond angle

Cristobalite

40 + 2 44 38 + 2 29 + 1 41 53 + 1 57 + 1 58 + 1 109 + 1 70 + 5 160 + 5

5.3 _+ 0.1 5.3 5.25 + 0.1 6.05 + 0.05 5.43 +_ 0.05 5.52 _+ 0.05 5.45 _+ 0.05 5.16 _+ 0.05 6.5 _+ 0.1 7.3 _+ 0.1 7.5 _+ 0.1

0.125 _ 0.005 0 0.18 _+ 0.02 0.000 + 0.005 0.166 _+ 0.005 0.169 +_ 0.005 0.168 _+ 0.005 0.292 + 0.005 0.125 _+ 0.05 0.48 + 0.05 0.10 + 0.05

146.4~

Quartz Coesite, site 1 site 2 site 3 site 4 site 5 Stishovite Quartz-GeO2 Rutile-GeO2

Reference

Spearing et al. (1994) Timken et al. (1986) Dupree (2000) 180.00~ Grandinetti et al. (1995) 142.56~ 149.53~ 144.46~ 137.22~ OSi3 Xue et al. (1994) 133.0~ Hussin et al. (1999) OGe3 Hussin et al. (1999)

360


polymorphs require the inclusion in the fitting of a CSA contribution. The characteristic span and skew of the tensors have been determined to be 80 ppm and --~ 0.4 for quartz (Dupree 2000), 70 _+ 5 ppm and 1 for cristobalite (Spearing et al. 1992) and 94 ppm and --~ 0.45 for stishovite (Xue et al. 1994). In going from the SiO4 units in quartz, through cristobalite to coesite and to the SiO6 units in stishovite, both the isotropic chemical shift and • markedly increase. The two forms of GeO2 show only a very small difference in • but the two coordinations can be distinguished by a change of--~ 90 ppm in the isotropic shift. If these experimental data are then plotted to test the functions in Section 6.2.3, ~q shows good agreement with Equation 6.7 (Figure 6.15), providing a strong basis for the use of NMR data to provide information about the structure of amorphous materials. The experimental data have also been used to test the reliability of ab initio molecular orbital calculations which were found to accurately reproduce the results, suggesting they could also be used to elucidate the structure of amorphous silicates (Xue and Kanzaki 2000). Variable temperature 170 NMR has been used to examine the c~[3phase transition in cristobalite at 254~ The T1 value decreases from 150 s at room temperature to ---29 s just below the transition temperature, dropping still further to 1.5 s at the transition point. Analysis of the 170 NMR lineshape showed that ~q decreased to zero (within the accuracy of the measurements) at the transition temperature. These results imply that the transition is related to reorientational fluctuations of the SiO4 groups and that the [3-phase is characterised by dynamical averaging of twin domains on the unit cell scale (Spearing et al. 1992).

0.4 [-

0.2

0 I

I

140

I

!

c.

. . .I .

160

t

1

L

I

180

Si-O-Si angle (o) Figure 6.15. Relationship between the 170 asymmetry parameter "q and the Si-O-Si bond angle for the silica polymorphs cristobalite, coesite and quartz. The solid line corresponds to Equation 6.7. From Dupree (2000) by permission of John Wiley and Sons Ltd.

170 NMR

361

6.5.1.2 Ternary Silicates. The ternary silicates were amongst the first inorganic

systems to which 170 MAS NMR was applied. Such materials contain several oxygen sites, and although the MAS spectra showed features from each site, there was overlap between the resonances (Figure 6.16A). However, spectral simulation was able to provide values of the interactions for each site. These materials were ideal candidates for high resolution techniques such as DOR, DAS and MQ MAS, which allowed much higher resolution and were able to distinguish the individual resonances. In the 1D MAS NMR spectrum of forsterite the resonances of the three non-bridging oxygens (nbo) all overlap, but the corresponding 3Q data resolve the three separate lines and allow the interaction parameters to be determined from the anisotropic dimension (Figure 6.16B,C) (Ashbrook et al. 1999). In crystalline materials with many different inequivalent oxygen sites the 1D data can only be crudely apportioned to different sites whereas DOR and DAS show all the inequivalent sites, (six in clinoenstatite and nine in wollastonite (Mueller et al. 1991, 1992). Table 6.6 summarises the 170 NMR interaction parameters from such crystalline materials.

B

A

20 i

i

i

,

i

f

| , ,

i , ,

80

i

i , ,

!

40

z,

~,~

0

~

_

60 . . . . . . . . . 80 40

F2 (ppm) components

J

,,,

~~~,~ t

'

1

80

'

J

s

I

'

~

'

I

40

'

'

'

I

'

'

'

J

0

t----I

1 kHz

170 shift (ppm) w.r.t. H20 Figure 6.16. A. Observed and simulated 170 MAS NMR centre band spectrum of crystalline diopside (CaMgSi206) showing the individual fitted components. From Timken et al. (1987) by permission of the American Chemical Society. B. Three-quantum 170 MAS NMR spectrum of forsterite showing resolution of the three non-bridging oxygen sites. C. Cross-sections parallel to the F2 axis of the MQMAS spectrum in B from which the • and xl values were derived by computer simulation of these peak shapes. From Ashbrook et al. (1999) by permission of the Mineralogical Society of America.

362


Table 6.6.

170 NMR

parameters of crystalline silicates and germanates.

Sample

Site

~i.... s(ppm)

XQ (MHz)

~q

Reference

Li2Si205

bo(1) bo(2) nbo(1) bo(1) bo(2) nbo(1) bo(1) bo(2) nbo(1) nbo bo bo(1) bo(2) nbo(1) bo(1) bo(2)* bo bo(1) bo(2) nbo(1) bo(1) bo(1) nbo(2) nbo(1) nbo(2) nbo(3) nbo(1) nbo(2) nbo(3) nbo(1) nbo(2) nbo(3) nbo(1) nbo(2) nbo(3) nbo(4) nbo(1) nbo(2) nbo(3) nbo(4) nbo(5) bo(1) nbo(1) nbo(2) nbo(1) nbo(2) nbo(3) nbo(4)

108 _+ 10 35 + 10 38 +_ 2 55 + 5 55 + 2 34 + 1 52 + 10 74 + 10 36 + 2 45 + 5 42.6 114 _+ 10 69 _+ 10 72 _+ 2 62.5 _+ 1 97 _+ 1 51 124 _+ 10 59 _+ 10 93 _+ 2 62 _+ 2 60 + 2 42 + 2 61 _+ 2 62 _+ 2 47 _+ 2 ND ND ND 64 _+ 1 61 + 1 48 _+ 1 63 + 1 60 _+ 2 59 + 2 52 _+ 1 64 _+ 1 61 _+ 1 60 + 1 52 _+ 1 49 _+ 1 75 + 2 94 + 2 91 _+ 2 134 _+ 2 128 + 2 122 _+ 2 122 _+ 2

5.60 + 0.3 4.05 _+ 0.3 2.45 + 0.1 5.7 + 0.3 4.7 _+ 0.2 2.35 + 0.1 5.74 + 0.2 4.67 + 0.2 2.40 + 0.1 ND 5.10 5.1 _+ 0.4 4.7 _+ 0.4 2.1 _+ 0.2 4.45 _+ 0.05 4.90 _+ 0.05 4.95 4.4 + 0.4 4.7 _+ 0.4 1.9 + 0.2 5.1 _+ 0.2 3.2 _+ 0.2 3.2 _ 0.2 2.35 + 0.2 2.35 _+ 0.2 2.70 _+ 0.2 2.53 2.42 2.77 2.5 + 0.1 2.5 _+ 0.1 2.9 + 0.1 2.5 + 0.1 2.3 _+ 0.1 2.3 _+ 0.1 2.7 _+ 0.1 2.5 _+ 0.1 2.4 _+ 0.1 2.4 _+ 0.1 2.8 _+ 0.1 2.7 _+ 0.1 3.8 _+ 0.2 2.1 +_ 0.2 2.3 _+ 0.1 2.9 + 0.2* 2.7 _+ 0.2* 2.5 + 0.2* 2.8 + 0.2~f

0.55 + 0.2 0.0 +_ 0.1 0.1 _+ 0.1 0.0 + 0.2 0.25 _+ 0.2 0.1 + 0.1 0.2 + 0.1 0.3 + 0.1 0.2 + 0.1 ND 0 0.1 + 0.1 0.2 _+ 0.1 0.5 _+ 0.1 0.35 _+ 0.05 0.20 + 0.05 0.1 0.1 _+ 0.1 0.5 _+ 0.1 0.5. _+ 0.1 0.3 _+ 0.1 0.0 _+ 0.1 0.0 _+ 0.1 1.0 + 0.1 0.2 _+ 0.1 0.3 _+ 0.1 0.39 0.18 0.28 0.4 + 0.1 0.2 _+ 0.1 0.3 _+ 0.1 0.3 + 0.1 0.2 _+ 0.1 0.3 _+ 0.1 0.2 _+ 0.1 0.3 _+ 0.1 0.2 + 0.1 0.3 _+ 0.1 0.2 _+ 0.1 0.3 _+ 0.1 0.2 _+ 0.1 0.1 +_ 0.1 0.1 _+ 0.1 ND ND ND ND

Maekawa et al. (1996)

e~-Na2Si205

e-NazSi205 Sodium ilerite K2Si205

K2Si409 (wadeite) KHSi205 Rb2Si205

MgSiO3 (clinoenstatite) Mg2SiO4 (forsterite)

chondrodite

clinohumite

e~-CaSiO3 (wollastonite) CaeSiO4 (larnite)

Xue et al. (1994) Maekawa et al. (1996)

Xue et al. (1994) Brenn et al. (2000) Maekawa et al. (1996)

Xue et al. (1994) Oglesby et al. (2001) Maekawa et al. (1996)

Timken et al. (1987)

Schramm & Oldfield (1984) Fritsche et al. (1986)

Ashbrook et al. (1999)




Mueller et al. (1992)

170 N M R

363

Table 6.6. (Continued) Sample

Site

oL-SrSiO3

bo(1) nbo(1) nbo(2) bo(1) nbo(1) nbo(2) bo(1) nbo(1) nbo(2) bo(1) nbo(1) nbo(2) bo(1) bo(2) bo(3) bo(4) bo(1) bo(2) bo(3) bo(4) bo(5) bo(6) bo(7) bo(8) bo(9) bo(10) bo nbo

BaSiO3

CaMgSi206 (diopside)

faujasite

ferrierite

NazGeO4

8iso,c~(ppm) 80 • 2 108 ___2 105 +__2 87 ___2 169 • 2 159 ___2 69 ___2 84 ___2 63 __+2 69 • 2 86 ___2 64 ___2 47.3 ,,, 0.3 42.3 ___0.3 37.2 • 0.3 34.8 ___0.3 43.1 ___0.3 41.6 _ 0.3 40.7 ___0.3 39.6 __+0.3 39.0 ___0.3 37.0 +__0.3 37.0 __+0.3 35.9 ___0.3 34.8 ___0.3 28.0 _+ 0.3 70 _+ 5 47 • 5

XQ (MHz)

Xl

4.1 _ 0.2 0.4 ___0.1 2.1 ___0.2 0.1 ___0.1 2.2 +__0.2 0.1 +__0.1 3.7 ___0.2 0.4 ,,, 0.1 2.1 • 0.2 0.1 ___0.1 1.6 ___0.2 0.1 ___0.1 4.4 _ 0.2 0.2 ___0.1 2.7 __+0.2 0.0 • 0.1 2.7 _ 0.2 0.1 ___0.1 4.39 +__0.05 0.36 • 0.05 2.83 _ 0.05 0.13 ___0.10 2.74 + 0.05 0.00 --- 0.10 5.14 ___0.03 0.1 ___0.05 5.10 ___0.03 0.3 ___0.05 5.39 _+ 0.03 0.2 ___0.05 5.28 ___0.03 0.2 ___0.05 5.62 ... 0,03 t 5.22 ... 0.03 t 5.35 • 0.03 t 5.29 ___0.03 t 5.38 • 0.03 t 5.27 ... 0.03 t 5.32 __+0.03 ~ 5.46 ... 0.03* 5.64 ___0.03 t 5.57 + 0.03 t 5.2 + 0.5 0.5 _+ 0.1 2.5 + 0.5 0.5 + 0.1

Reference Timken et al. (1987)



Mueller et al. (1992)

Bull et al. (1998)

Bull et al. (2000)

Hussin et al. (1998)

* is a bridging oxygenis betweena tetrahedraland an octahedralunit, and t refers to the quadrupoleproductnot XQ"

It is i m m e d i a t e l y a p p a r e n t f r o m T a b l e 6.6 that bo a n d n b o c a n be r e a d i l y d i s t i n g u i s h e d b y their XQ v a l u e s w h i c h fall into d i s t i n c t r a n g e s ( 3 . 7 - 5 . 8 M H z for b o a n d 1 . 6 - 3 . 2 M H z for nbo). XQ is related to the differences in the ionicity o f the c a t i o n - o x y g e n b o n d s . T h e n b o are m o r e i o n i c b o n d s c o n t a i n i n g less p - o r b i t a l c o n t r i b u t i o n , a n d h e n c e XQ is s m a l l e r . T h u s , the XQ v a l u e s o f b o t h the b o a n d n b o units c a n be c o r r e l a t e d w i t h the c a t i o n e l e c t r o n e g a t i v i t y for g r o u p s o f r e l a t e d s i l i c a t e s (e.g. t h e a l k a l i n e - e a r t h c o m p o u n d s ) ( F i g u r e 6 . 1 7 A , T i m k e n et al. 1987), a n d the giso,cs v a l u e s are also r e l a t e d to the c a t i o n r a d i u s ( F i g u r e 6 . 1 7 B , T i m k e n et al. 1987). T h e c a t i o n t h u s p l a y s a c e n t r a l r o l e in d e t e r m i n i n g the ~70 N M R i n t e r a c t i o n p a r a m e t e r s for b o t h the b o a n d n b o g r o u p s , a l t h o u g h the e f f e c t is s t r o n g e r for the n b o units. T h e i m p o r t a n c e o f t h e s e e x p e r i m e n t a l d a t a is t h a t t h e y a l l o w the c o r r e l a t i o n function for the N M R p a r a m e t e r s to b e t e s t e d b e f o r e b e i n g u s e d to p r o b e the s t r u c t u r e o f

364


bridging N

160

:I~ 4.0 M g , C ~ t--

2.0

n o n ~

80

oo

Sr C a / . 1 4 r ~ Ba ~ non-bridging

Q

-

O

Mg,Ca

-

t~ ,..r

40

Mg I_

t

0.9

1.1

i

_

I

1.0

1.3

,

I

1.2

~

1.4

I

1.6

Cation radius (.~)

Cation electronegativity

Figure 6.17. A. Relationship between the 170 nuclear quadrupole coupling constant • and the cation electronegativity for the bridging and non-bridging oxygens in the alkaline earth metasilicates. B. Relation between the logarithm of the 170 chemical shift and the cation radius for bridging and non-bridging oxygens in the alkaline earth metasilicates. From Timken et al. (1987) by permission of the American Chemical Society.

6.0 6.0 N

N T

4.0

O

4.0

n

•

El

•

9

100

l

i .

.

140

I

,

I

I

180

-1.0

.

Si-O-Si (o)

I

,

I

,

I

-0.8

,

!

-0.6

cos(Si-O-Si)

Figure 6.18. Relationship between the nuclear quadrupole coupling constant XQ of a number of silicates and the bridging Si-O-Si angle (left) and cos(Si-O-Si) (fight). From Maekawa et al. (1996) by permission of the American Chemical Society. glasses. S o m e of the initial functions used by Farnan et al. 1992 and Grandinetti et al. 1995 for XQ did not fit the data (Figure 6.18), leading to the suggestion of an alternative function which gave a m u c h i m p r o v e d fit: ZQ ( M H z ) - - 5 . 8 9 2 c o s ( S / - O - Si) + 0.248

(6.16)

170 N M R

365

In the case of siliceous zeolites with complex structures, increasing the number of data points did not result in a strong correlation of the interaction parameters (Bull et al. 1998, 2000). The ab initio calculations used in this work provided a very useful understanding of the spectra. Two structural models for the compound ferrerite were used to calculate the N M R parameters, which were then compared with the experimental data. The sensitivity of the N M R parameters to the structure constituted a means of testing of the limitations of the crystallography, thus providing genuine crystallographic input to elucidate the correct structure.

All three oxygen atoms in Na4Zr2Si3012 show 170 XQ values in the range for nbo (Bastow et al. 1996) indicating that zirconium acts as a network modifier in silicates. Since only two of the oxygen sites in Na4Zr2Si3012 are inequivalent, one of the three observed peaks must arise from a secondary phase. The ~70 N M R spectrum of Na2ZrSiO5 also showed two signals at 120 and 30 ppm, with evidence of some structure in the spectrum. Since materials containing bonds between titanium and silicon have a number of important technological applications, the ability to characterise them directly is of great practical interest. Studies have shown that the XQ value in these systems is larger than in Zr-O-Si. Titanium is an intermediate oxide which can act as both a network

6.5.1.3 S i l i c a t e s a n d g e r m a n a t e s o f z i r c o n i u m a n d t i t a n i u m .

Table 6.7. 170 NMR interaction parameters for silicates containing zirconium and titanium. Sample

Site

~i..... (ppm)

Na4Zr2Si3012

ZrOSi(1) ZrOSi(2) ZrOSi(3) ZrOSi Si-O-Ti Si-O-Si Ti-O-Si Apical O Si-O(1)-Si Si-O(2)-Si SiO(1)-Si Si-O(2)-Si

169.5 118.0 126.0 160* 190" ND 157 741" 333 +_ 1 363 • 1 305 _+ 1 370 • 1

Si-O-Ti Si-O(1)-Si Si-O(2)-Si Ti-O-Ge Apical

295 • 1 69 +_ 1 54 • 1 148 749*

ZrSiO4 Ba2TiSi208 (fresnoite) LiTiOSiO4 [Ti(acac)20]2 [OSi(C6H5)212 [Ti(acac)O1.5]2 [OSi(C6H5)213. 2C4H802 TiOz[O4Si4(C6H5)812 LiTiOGeO4

* denotesthe peakpositionratherthanthe isotropicchemicalshift.

•

(MHz)

"q

Reference

2.68 2.75 2.80 ND ND 3 3.05 --~0 3.7 _+ 0.1 3.3 • 0.1 3.3 • 0.1 3.0 • 0.1

0.0 0.1 0.2 ND ND --~0 0.35 ND 0.1 + 0.05 0.1 _ 0.05 0.0 • 0.05 0.0 • 0.05

Bastow et al. (1996)

3.0 • 0.1 4.7 • 0.1 5.3 • 0.1 4.80 --~0

0.0 • 0.05 Gervais et al. (2000) 0.0 -+ 0.05 0.0 • 0.05 0.22 Bastow et al. (1999) ND

Dirken et al. (1995) Bastow et al. (1999) Gervais et al. (2000) Gervais et al. (2000)

366

Multinuclear Solid-State NMR of Inorganic Materials bridging O Li2TiOGeO4

rutile apical O

gO Li2TiOSiO4

"~utile ,

t

1000

,

.i

,

I

600

,

I

,

|

200

I

.

170 shift (parts in 106) w.r.t. H20 Figure 6.19. 170 MAS NMR spectra of Li2TiOSiO4 and Li2TiOGeO4 showing the resonances from the apical and bridging oxygen atoms. The asterisks denote spinning side bands from the apican oxygen resonances. From Bastow et al. (1999) by permission of the copyright owner.

former, with typically 4-fold Ti coordination, or as a network modifier, with Ti in 6-fold coordination. Titanium can also adopt 5-fold coordination. It is important to know the sensitivity of the 170 NMR interaction parameters to the coordination of the titanium, but the data set presently available is too small to be definitive. Important insight into this question has been provided by a series of crystalline cyclic diphenylsiloxanes, which, although essentially large organic molecules, have titanosilicate cores. Their XQ values lie in the range 3-3.5 MHz, not very different from those of Si-O-TiO3 and Si-O-TiOs. Although it is possible that these units might be discriminated, Si-O-TiO3 has a smaller shift (Gervais et al. 2000); more data points are required to test this possibility. LiTiOSiO4 contains TiO5 units in a square pyramid configuration in which the apical oxygen shows a very distinct 170 resonance. In this case the value of • is too small to be determined but the spectrum contains spinning sidebands due to CSA spread over --~1000 ppm (Figure 6.19) (Bastow et al. 1999).

6.5.2 Amorphous materials 6.5.2.1 Silica a n d germania. The values of XQ, T] and the 170 shift for silica glass are --~5 MHz, --~0.2 and 37 ___ 3 ppm respectively (Geissberger and Bray 1983, J~iger et al. 1993), indicating that the chemical environment in glass is very similar to

170 NMR

367

the environments in the crystalline silica polymorphs quartz and cristobalite. The relationships between the structural parameters and the NMR interactions for these crystalline modifications (Equations 6.8, 6.9 and 6.12) allow the bond angle distribution to be deduced in glass. A similar process has been followed for germania. The resulting distribution of angles was fitted by a Gaussian function with a mean Ge-O-Ge bond angle of 133 ~ and a width of 4 ~ (Hussin et al. 1999). This distribution is much narrower than the values found for both SiOa and GeO2 glasses from a combination of highenergy photon data and neutron diffraction data (Johnson et al. 1983 and Grimley, Wright and Sinclair 1990) which gave values of 148.3 ~ and 7.5 ~ for SiO2 and 133 ~ and 8.3 ~ for GeO2 (Neuefind and Liss 1996). The NMR interaction parameters XQ = 5.3 MHz, Xl = 0 and giso,cs = 42 ppm have been determined for Si-O-Si in a silica gel (van Eck et al. 1999). 6.5.2.2 M e t a l silicate a n d germanate glasses. Although the subtle crystallographic

distinction between the sites is lost in a glass because of the distribution of environments, 170 NMR can provide information about the structure of glasses by being able to distinguish gross features, including bridging and non-bridging sites and the effect of cation coordination. Both MAS and static NMR spectra can be used, the more distinct lineshapes in the latter being sometimes easier to simulate (Figure 6.20). The spectra of a series of disilicate glasses were modelled by a Gaussian distribution of parameters (although not using the physical basis described in Section 6.2.3). This then allowed the mean Si-O-Si bond angle and the angular distribution to be determined. The maximum of the distribution profile shifted from 143 ~ in lithium silicate glasses to 136~ for caesium. Potassium silicate glasses showed the narrowest distribution (Maekawa et al. 1996). In a series of glasses quenched at ambient and high pressure (> 6 GPa), both the nbo and bo in potassium tetrasilicate glass were shifted to higher frequency in the higher pressure sample by ~ 3 ppm, the nbo peak becoming ~20% broader. This shift is consistent with an increase in the mean Si-O-Si bond angle, and may indicate the formation of new species such as O3Si-O-SiO5 with the presence of tricoordinate sites (Xue et al. 1994). In metal silicate glasses the intermediate range order is determined by both the Si-O-Si bond angle distribution and the ordering of the cations, both of which factors can be elucidated by 170 DAS. Cation ordering has profound implications for many properties of a glass such as the configurational entropy of the system and transport characteristics. The bond angle distribution in K2Si409 derived from 170 DAS data showed a maximum at 143 ~ and had a halfwidth of 21 ~ (Faman et al. 1992). Studies of crystalline materials have shown that the 170 shift of both oxygen sites, but especially the nbo site, is sensitive to the local coordination of the metal. Comparison of the isotropic dimension of the spectra of K2Si409 and KMgo.sSi409 glasses showed no significant increase in width in the mixed cation glass, but the peak position was shifted

368

Multinuclear

Solid-State NMR

of lnorganic

Materials

red

_-.. . . . = . . . .

~

a

t

e

d non-bridgingoxygen component bridgingoxygen nent

~ ,

I

I

,

1

1000 0 -1000 170 shift (ppm) w.r.t. H20 Figure 6.20. Observed and simulated static 170 NMR spectrum of Li2Si205 glass showing the bridging and non-bridging oxygen components of the simulation. From Maekawa et al. (1996) by permission of the American Chemical Society.

Table 6.8.

170 NMR parameters for metal silicate and germanate glasses.

Sample

Site

Li2Si205

bo nbo bo nbo bo nbo bo nbo bo nbo bo nbo bo nbo bo nbo bo nbo

NazSi205

Na2Si307 NazSi409

K2Si205 K28i409

Cs28i205

~iso.c~(ppm) 68 + 2 42 + 2 65 _+ 5 40 +_ 2 61 _+ 2 39 _+ 2 60 + 5 39 _+ 2 50 + 4 36 _+ 2 51 _+ 2 38_+2 60 +_ 2 84 _+ 2 52 + 4 76 + 2 68 _+ 2 145 + 2

XQ (MHz)

rl

Reference

5.0 _+ 0.1 2.55 + 0.1 5.0 _+ 0.3 2.3 +_ 0.1 4.9 + 0.1 2.35 _+ 0.1 5.0 _+ 0.3 2.3 + 0.1 5.0 _+ 0.3 2.3 _+ 0.1 5.1 _+ 0.1 2.4_+0.1 4.7 _+ 0.1 2.5 _+ 0.1 4.9 + 0.2 2.3 _+ 0.1 4.55 + 0.1 3.1 _+ 0.1

0.15 _+ 0.1 0.2 _+ 0.1 0* 0* 0.1 _+ 0.1 0.2 _+ 0.1 0* 0* 0* 0* 0.0 _+ 0.1 0.25_+0.10 0.25 _+ 0.1 0.45 _+ 0.1 0* 0* 0.3 + 0.1 0.55 _+ 0.1

Maekawa et al. (1996) Xue et al. (1994) Maekawa et al. (1996)

Xue et al. (1994) Xue et al. (1994) Maekawa et al. (1998)

Maekawa et al. (1996) Maekawa et al. (1996) Maekawa et al. (1996)

170 NMR Table 6.8. (Continued)

170

369

NMR parameters for metal silicate and germanate glasses.

Sample

Site

~isoxs(ppm)

XQ(MHz)

xl

Reference

CaO.SiO2 (Ca/Si = 0.7) NazGe9019 Na4Ge9020

bo nbo bo bo

104-112 60-75 165 _+ 5 80 + 5

4.6 + 1 2.1 + 1 7.0 + 0.5 6.0 + 0.5

0.0 +_ 0.5 0.2 + 0.5 0.5 + 0.1 0.5 + 0.1

Cong & Kirkpatrick (1996) Hussin et al. (1998) Hussin et al. (1998)

* - value assumed in the fitting procedure.

from the position in the pure potassium glass. This suggested a high degree of cation ordering with the non-bridging oxygen (nbo) atoms probably coordinated by two potassiums and one magnesium ion. The isotropic dimension of the 170 DAS spectra of a series of KxNal- xSi205 glasses showed resonances from both the bo and the nbo units which appeared at 16.1 and 68.7 ppm respectively in the glass with x = 1, and at 11.8 and 27.0 ppm respectively in the glass with x = 0 (Florian et al. 1996). The shift of the nbo peak could be modelled if the nbo unit is assumed to be coordinated by four alkali cations occupying these sites randomly. The isotropic dimension of an MQ MAS spectrum can provide the same information. Comparison of the 1 7 0 3Q MAS spectra of a series of calcium and magnesium silicate glasses indicated that the nbo units are coordinated by three randomly mixed cations (Stebbins et al. 1997). 170 NMR can also provide information about the cations coordinated to the bo units. No 170 NMR evidence was found for nbo units in two sodium germanate glasses containing _< 18.2 mol % Na20. In the glass with 10 mol % Na20, 170 MAS NMR suggests that the dominant coordination is GeO6 but in the 18.2 mol % Na20 glass it is GeO4 (Hussin et al. 1998). 6.5.2.3 G e l - b a s e d silicates. A 1 7 0 MAS NMR study of hydrothermally synthesised crystalline calcium silicate hydrates has identified the sites within amorphous samples (Cong and Kirkpatrick 1996). Hydrated calcium silicate gels are important products formed in the hydration reactions of Portland cements. Complex 170 1D MAS NMR spectra were obtained, many of which showed 6 peaks, 2 from nbo units, 1 from a bo unit, and others from the H20 and hydroxyl groups associated with the calcium and silicon. Cong and Kirkpatrick (1993, 1996a) also used lVO NMR to determine the evolution time of the various sites. The chemical shifts of both the nbo and bo resonances decreased with increasing calcium/silicon ratio in the sample, reflecting the decreasing polymerisation of the network. The 170 NMR spectra of the bo units suggest that an increase in the calcium/silicon ratio results in a decrease of the average Si-O-Si bond angle. All the 170 NMR data could be interpreted in terms of the defect tobermorite model of hydrated calcium silicate gels.

370


Sol-gel reactions provide a means of producing oxides mixed at the atomic level. NMR can be used to monitor the reaction from the stage of the initial solution through to the final dense solid. In the case of silica, the presence of small amounts of a second oxide can change the properties such as the refractive index or catalytic behaviour, making an understanding of the mixing homogeneity of vital importance. Where small amounts of TiO2 are added to SiO2, it is important to know whether the titanium is atomically dispersed, what sites it occupies and whether there are discrete nanoparticles present. Another important consideration is to match the rates of hydrolysis of the components in the initial reaction to maximise the number of cross-linked M-O-Si groups. The extensive work of Babonneau has shown that solution-state 170 NMR provides a direct measure of this cross-linking and information about its variation with time in the initial sol. One of the important factors in the process is the change of reactivity when the functional groups are changed. The solution-state NMR studies of the SiO2-TiO2 and SiO2-ZrO2 systems have been summarised by Delattre and Babonneau (1997) and the work on siloxane-oxide hybrid materials by Babonneau and Maquet (2000). Once formed and dried, the amorphous gel is a porous solid which changes in structure when heated, eventually recrystallising from the amorphous state. In silicate gels such as MO-SiO2, a key question is the dispersion of the metal, which can be monitored by ~70 NMR measurements of the relative number of M-O-M, M-O-Si and SiO-Si links. The large shift range of the 170 resonances allows all these different fragments in the TiO2-SiO2 system to be readily resolved in the 1D MAS NMR spectra. The sensitivity of this method makes it applicable even when only small amounts of TiO2 are added. The NMR data can clearly show the presence of nanoscale phase separation, since some samples show only Ti-O-Si linkages, whereas additional Ti-OTi resonances occur in other samples (Smith and Whitfield 1994). A more detailed study showed that gels containing 7-41 tool % TiO2 remain amorphous up to 600~ according to X-ray diffraction. Preparation of a 7 tool % TiO2-SiO2 gel by both a oneand two-step process showed an absence of Ti-O-Ti linkages in the latter, indicating atomic-scale mixing of the metals in that case (Dirken et al. 1995). The technical importance of the TiO2-SiO2 system has led to a number of detailed studies in which NMR was combined with EXAFS, XANES and diffraction. To improve the catalytic activity, the surface area of these materials can be increased to 450 m 2 g - 1 by washing with heptane. ~70 MAS NMR has been used to demonstrate the complete mixing of titanium with the silica in all these samples (Holland et al. 2000). The reactivity of the titanium alkoxide can also be improved by complexing with acetylacetone to encourage a greater degree of Ti-O-Si crosslinking. Even samples initially containing 18 tool % TiO2 showed no sign of TiO2 phase separation, the 170 NMR spectra indicating that almost all the titanium remained as Ti-O-Si even after heating at 750~ (Figure 6.21A) (Pickup et al. 1999). The atomic dispersion can be further improved by

170 NMR

371

A T(~C) *~~J~l .

'

750

500

~

ed ,,

5O0 170

0

-500

shift (ppm) w.r.t. H20

500

0

-500


Figure 6.21. A. 170 MAS NMR spectra of (TiO2)o.18(SiO2)o.s2 xerogel after heating at various temperatures. The asterisks denote spinning side bands. From Pickup et al. (1999). B. 170 MAS NMR spectra of (Ta205)o.25(SiO2)o.75 xerogel after heating to various temperatures. From Pickup et al. (2000). All spectra used by permission of the Royal Society of Chemistry.

functionalising the precursor (e.g. R'xSi(OR)4-x) in addition to using acetylacetone. The ~70 NMR spectra of all such gels are complex, showing up to five resonances (Figure 6.2) (Gervais et al. 2001). 170 NMR showed that samples of the type Si(OR)4 (x = 0) give the highest (Si-O-Ti)/(Ti-O-Ti) ratio, indicating that these are the most favourable for titanium incorporation. The addition of CH3 groups increases the hydrophobic properties of the system, allowing the Si-O-Ti bonds to be more readily broken. Samples in which x = 0 show two 170 Ti-O-Si NMR resonances, one at --- 250 ppm and the other at 180-110 ppm. It is likely that 250 ppm resonance corresponds to Si-O-TiO3 framework sites, with the other resonance probably due to 4-coordinate Ti attached but in a much less thermally stable unit. Other systems studied by 170 NMR include ZrO2-SiO2. Since zirconia acts as a network modifier, it is retained in much higher concentrations in the system than TiO2. Both Zr-O-Zr and Zr-O-Si bonds are clearly observed (Dirken et al. 1995a). The 170 NMR spectrum of a sample containing 10% ZrO2 showed, in addition to the main Si-O-Si peak, a Zr-O-Si resonance at ---150 ppm. At higher zirconia concentrations phase separation occurs (Pickup et al. 1999a). The same behaviour was found in

372


Ta2Os-SiO2 gels in which even at high Ta205 concentrations (--~25 mol%) where significant phase separation has occurred, appreciable Ta-O-Si bonding is detected (Figure 6.21B, Pickup et al. 2000). 170 MAS NMR spectra have been reported in more complex nanocomposite materials from spirooxazine and spiropyran-doped matrices which contain polysiloxane and zirconium oxopolymer domains. Although the MAS speeds were quite low (5 kHz), the Zr-O-Si bonding between the distinct nanodomains could be detected as a peak at 160 ppm (Schaudel et al. 1997). As polydimethlysiloxane-zirconium oxo nanocomposite samples were dried, changes in the intensity of the different 170 signals showed that complex reorganisations of the structure were occurring (Guermeur et al. 1999). 170 MAS NMR has also shown that in the sol-gel reaction of HfO2 with SiO2 and GeO2 there is a greater tendency for HfO2 to form a solid solution with GeO2 (Bastow et al. 1996). By selectively enriching the siloxane oxygen in elastomeric polydimethylsiloxane-vanadate hybrid materials, a sharp 170 NMR signal was observed at 70 ppm above Tg, indicating that there is sufficient motion to average the interaction. Below the glass transition temperature (Tg) the 170 parameters were found to be XQ = 4.9 _+ 0.1 MHz and ~1 - 0.15 _+ 0.05, these values as expected for a Si-O-Si linkage. The intensities of the quadrupole echoes were sensitive to the motional correlation times occurring in the region of the glass transition. The estimate of Tg made from the 170 NMR spectrum at 184 _+ 5 K was consistent with the calorimetrically determined value of 177 _+ 1 K (Alonso et al. 2000).

6.6. ALUMINIUM-AND GALLIUM-CONTAININGSYSTEMS 6.6.1 A l u m i n a a n d aluminates

Walter and Oldfield (1989) reported the 170 MAS NMR spectra from o~-A1203, transitional aluminas and hydrous aluminas. Most oxygens atoms were in OA14 configurations, but a signal in 0-A1203 with a slightly higher shift and a significantly larger XQ (Table 6.9) was assigned to OA13. The values for the quadrupole parameters of o~-A1203 agree very well with a single crystal NMR study of ruby (Brunet al. 1970). In all the transitional aluminas the presence of cation vacancies result in the formation of OA13 sites, showing up as a broader line underlying the much narrower OA14 resonance. However, only in the more ordered transitional alumina 0-A1203 could sufficient structure be observed for XQ to be deduced. In the other transitional aluminas the distribution of parameters results in blurred lineshapes. The identification of OA13 with a site of larger quadrupole interaction has been confirmed by calculations of the 170 NMR interaction parameters for isostructural [3-Ga203 (Walter and Oldfield 1989). Although all the values in the gallium system were larger, reflecting the relative electronegativity of gallium and aluminium, the OGa3 sites showed larger XQ values.

170 N M R

373

Table 6.9. 170 NMR interaction parameters from aluminas and aluminates. Sample

Site

oL-A1203

OA14

~/-A1203 xI-A1203 ~-A1203 0-A1203

OA14 OA14 OAI4 OA14 OA13 OA14 OA12H OA12H

A10(OH) (boehmite) AI(OH)3 (bayerite) LaA103 NaA102 CaA1204

OA12La2 OA12 OA12,site 1 site 2 site 3 site 4 site 5 site 6 site 7 site 8 CaA1407 OA12,site 1 site 2 site 3 OA13 Y3A150~2 OA12Y2 YA103, site 1 OA12Y3 site 2 OA12Y2

8i~o,cs(ppm) Xe (MHz)

Xl

Reference

ND 75 _+ 2 72 ___ 1 73 _+ 1 73 _+ 1 72 _+ 1 72 _ 1 79 + 5 70 _ 1 40 ___5 40 _ 5

2.167 2.17 _ 0.05 2.13 _ 0.03 1.8 _ 0.2* 1.6 _+ 0.2* 1.6 _+ 0.2* 1.2 _ 0.2* 4.0 ___0.2 1.20 ___0.05 5.0 + 0.2 6.0 ___0.2

0.517 0.55 _+ 0.05 0.50 _ 0.05 ND ND ND ND 0.6 __+0.1 0.1 _ 0.1 0.5 _ 0.1 0.3 ___0.1

Brunet al. (1970) Walter & Oldfield (1989) Florian et al. (2001) Walter & Oldfield (1989) Walter & Oldfield (1989) Walter & Oldfield (1989) Walter & Oldfield (1989)

170.2t 30.9 ___0.5 86.8 _+ 1 80.7 __+ 1 76.2 ___ 1 72.4 _ 1 69.1 ___ 1 66.7 ___ 1 61.8 _ 1 57.3 ___ 1 71.6 _+ 2 61.5 _+ 2 56.8 + 2 40.6 _+ 2 14 _+ 1 143 _+ 1 165 + 1

ND 1.81 _ 0.1" 1.5 _ 0.2* 1.6 +_ 0.2* 1.6 _ 0.2* 1.6 _ 0.2* 1.3 ___0.2* 1.6 ___0.2* 1.4 _ 0.2* 1.4 +__.02* 1.9 + 0.2 1.8 + 0.2 2.1 + 0.2 2.5 _+ 0.2 1.49 _+ 0.03 1.57 + 0.03 1.65 + 0.03

ND ND ND ND ND ND ND ND ND ND 0.7 _+ 0.3 0.5 + 0.3 0.5 + 0.3 0.4 + 0.3 0.99 _+ 0.05 1.00 _+ 0.05 0.35 + 0.05

Bastow et al. (1996) Stebbins et al. (1999) Stebbins et al. (1999)

Walter & Oldfield (1989) Walter & Oldfield (1989)

Stebbins et al. (2001)

Florian et al. (2001) Florian et al. (2001)

* -determinedfromthe quadrupoleshift so representsthe quadrupoleproductPQand t is the peakpositionnot the isotropic chemical shift.

P r o t o n a t e d sites in t h e s e c o m p o u n d s h a v e m u c h h i g h e r XQ v a l u e s , w h i c h c a n be e x p l a i n e d in t e r m s o f the i n c r e a s e d c o v a l e n c y o f the bonds. A k e y m i n e r a l o g i c a l p r o b l e m w i t h a l u m i n a t e s is the q u e s t i o n o f cation d i s o r d e r i n g in spinels. N u m e r o u s 27A1 M A S N M R studies h a v e b e e n carried out on MgA1204. O n e ~70 M A S N M R study o f spinels q u e n c h e d f r o m b e t w e e n 7 0 0 and 1400~ i n c r e a s e in d i s o r d e r w i t h t e m p e r a t u r e . B o t h the

170 a n d

s h o w e d an

27A1 M A S N M R s p e c t r a

s h o w e d a s i m i l a r i n c r e a s e in d i s o r d e r but the v a l u e d e t e r m i n e d f r o m ~70 N M R was s y s t e m a t i c a l l y l o w e r ( M i l l a r d et al. 1992). ~70 M A S has b e e n u s e d to m o n i t o r the solid state N M R r e a c t i o n o f s o l - g e l f o r m e d L a A 1 0 3 ( F i g u r e 6.22). H e a t i n g to 4 5 0 ~ p r o d u c e d 170 signals f r o m L a O ( O H ) and an a l u m i n i u m o x y h y d r o x i d e , but p e a k s f r o m

374

Multinuclear Solid-State NMR of Inorganic Materials LaOOH

T(~ $ OAI4

La~O~~

450

I La~O~

LaAIO3 ]

I

800

1

I

!

I

400

,

!

t

I

I

,

!

!

0

~70 shift (ppm) w.r.t. H20 Figure 6.22 170 MAS NMR spectra of LaA103 gel heated to various temperatures. From Bastow et al. (1996a) by permission of the copyright owners. La203 and A1203 were seen at 800~ resulting from the formation of a very finely divided mixture of the unreacted individual oxides. At 950~ some unreacted oxides remained but there was evidence of significant formation of LaA103. An extensive multinuclear study has been made of the Y203-A1203 system. In the structurally simpler compounds Y3A15012 and YA103, one and two inequivalent oxygen sites respectively are observed (Table 6.9). Y2A1409 has nine inequivalent sites (OY2A12, OY3A1, OYzA12 and OY4 coordinations). Seven lines were observed in the 170 MAS NMR spectrum of this phase, two with approximately twice the intensity of the others. The shifts of these resonances span a range of 146 ppm. The data show a decrease in the 170 shift with increasing oxygen coordination number, correlated with the sum of the inverse bond lengths (Florian et al. 2001). The crystallisation of YA103 and the structure of vitreous Y3A15OI2 have also been investigated by 170 NMR. An MQ MAS spectrum showed five 170 lines, three of which were readily observable. The linewidths of the oxygen data were dominated by chemical shift dispersion. No simple model of the local oxygen coordination could be proposed to explain the relative intensities of the different peaks. Stebbins et al. (1999) have carried out important 170 MAS NMR studies of the calcium aluminates including CaA1204, which, like NaA102 has a stuffed tridymite

170 NMR

375

structure containing continuous A104 networks with well-defined A1-O-A1 links. The chemical shift and quadrupole parameters deduced for these materials (Table 6.9) indicate that the interactions of the A1-O-A1 are quite different from those of S i-O-A1 and Si-O-Si units in aluminosilicates (Stebbins et al. 1999). Analysis of the NMR data for CaA1407 took into consideration the presence of both A1-O-A1 linkages and welldefined A103 tricluster units. The position of the tricluster resonance was found to be shifted by --- 20 ppm and it has a significantly larger • value (Table 6.9), which should facilitate its identification in glassy materials (Stebbins et al. 2001).

6.6.2 Crystalline a l u m i n o - a n d gallosilicates Aluminosilicates have importance in mineralogy and in materials technology, as ceramics and microporous catalysts. One of the most important questions is the ordering of silicon and aluminium within the structure. The resolution of the various Qn(mA1) resonances in the 29Si MAS NMR spectra has provided insights into these crystalline materials. 170 NMR provides an alternative view of this ordering. Early work combining static and MAS measurements showed that the 170 resonances from Si-O-Si and Si-O-A1 can be separated, although resolution of small crystallographic differences was not possible (Timken et al. 1986, 1986a). The static measurements were necessary since the MAS often did not resolve the different fragments, although the spectra could be fitted on the basis of parameters derived from the static measurements. The 170 XQ values estimated from the static measurements were found to be larger than from the MAS spectra, and the • values from static measurements at lower fields were smaller again, indicating the presence of CSA effects which broadened the lines. The work of Timken et al. (1986) showed that the less covalent Si-O-A1 fragments have significantly smaller • values and slightly smaller shifts (Table 6.10). These trends have been correctly predicted both by the Townes-Dailey approach and by the empirical considerations of Schramm and Oldfield (1984) since both these treatments are based on the hybridisation/ionicity of the bonds. Zeolites are ideal materials on which to test structural relationships as they consist of TO4 units with only bridging T-O-T bonds and often without the complications associated with coordinating cations. For simple bridging sites (i.e. no coordinating cations) Timken et al. (1986a) developed a relationship for these materials between their XQ values and the average ionicity difference between the T components (Figure 6.23), assuming that the degree of w-back-bonding, the bonding angle and the hybridisation of the cr-orbitals remains constant. This approach correctly predicted that the • values for Si-O-Ga links should be ---20% larger than for Si-O-A1 (Table 6.10). Consideration of similar zeolites (e.g. Y) (Table 6.10) shows that the cations exert an influence at a secondary level on the NMR interaction parameters; for example, large Group II cations shift the 170 resonance to more deshielded values.

376


Table 6.10. 170 NMR interaction parameters in crystalline alumino- and gallosilicates. Sample

Site

~i ..... (ppm)

XQ (MHz)

xl

Reference

Na-A, R = 1 Na-Y, R = 2.74

Si-O-A1 Si-O-Si Si-O-A1 Si-O-Si Si-O-A1 Si-O-Si Si-O-A1 Si-O-Si Si-O-A1 Si-O-Si Si-O-A1 Si-O-Si

32 44 31 48 31 51 31 50 34 52 40 45

3.2 4.6 3.1 5.0 3.2 5.0 3.2 5.0 3.2 5.1 3.4 5.2

0.2 0.1 0.2 0.1 0.2 0.1 0.2 0.1 0.2 0.15 0.4 0.2

Timken et al. (1986) Timken et al. (1986)

Si-O-Si Si-O-Ga Si-O-AI(1) Si-O-AI(2) Si-O-AI(3) Si-O-Si Si-O-AI Si-O-Si Si-O-A1 Si-O-AI(1) Si-O-AI(2) Si-O-AI(3) Si-O-AI(4) Si-O-AI(1) Si-O-AI(2) Si-O-AI(3) Si-O-AI(4) Si-O-A1(1) Si-O-AI(2) Si-O-AI(3) Si-O-Si Si-O-Ga Si-O-Si Si-O-Ga

49 29 42.5 29.7 39.5 43 33 40 30 50.3 41.7 45.0 36.9 50.6 45.2 42.1 36.8 60.7 53.4 75.5 51 29 51 29

5.0 4.0 3.4 3.4 3.4 5.1 3.5 5.3 3.5 3.2 3.3 3.4 3.6 3.3t 3.4* 3.3* 3.6* 3.3 t 3.6t 3.2t 5.1 4.0 5.1 4.0

0.0 0.3 0 0 0.25 0.18 0.28 0.12 0.29 0.4 0.3 0.3 0.15 0.3 0.3 0.2 0.15 0.15 0.05 0.2 0.0 0.3 0.0 0.3


NH4-Y, R = 2.92 NHa-Y, R = 4.98 NHa-Y, R = 7 . 5 1 Ba,Na-Y, R = 2.74 Dealuminated, Na-Y, R > 25 Ga-X, R = 1 . 6 3 Na-A, R = 1

Stilbite NHa-ZSM5, R = 19 Na-LSX, R = 1

Na,K-LSX, R = 1

T1-A, R = 1

Ga-sodalite Ba,Na Ga-sodalite

Timken et al. (1986) Timken et al. (1986) Timken et al. (1986) Timken et al. (1986) Timken et al. (1986)

Pingel et al. (1998)

Xu & Stebbins (1998) Amoureux et al. (1998) Pingel et al. (1998)

Freude et al. (2001)

Freude et al. (2001)

Timken et al. (1986a) Timken et al. (1986a)

t - value is the quadrupole product PQ based on D O R field variation measurements.

T h e h i g h e r - r e s o l u t i o n t e c h n i q u e s D A S , D O R and M Q - M A S

have provided

additional resolution of these sites, allowing the different S-O-A1 sites to be distinguished in zeolite structures (Pingel et al. 1998, Freude et al. 2001). A combination of D O R and 3 Q - M A S (Figure 6.24) has allowed the relevant N M R interactions to be deduced. For the zeolites N a - A and N a - L S X , a direct correlation b e t w e e n ~iso,cs and the Si-O-A1 b o n d angle has been p r o p o s e d based on this limited range of c o m p o u n d s

170 NMR 00

--~

/

..... / . . . .

7 MHz /

40

.NI

r

o~,,4

I

/

/

/'

I

6 MHz /

..... /

377 ,'

/

/

J

//

'

' / .....

5 MHz

/'"

/

/

/

!

I

/

-/1

/

/1

,,"/ """/ "" / '" I ////;///

20

t '

I

I

I i m

0

J

/ i I

,

/ i ~

I I

. I

.

J s I.

I

/ / I

/ I i

I I

i m

!

45

I

t

/ / I

I

! I

..

i

/

a t I

.

/

i

i

3M.z / / / I

.I

55

65

Average ionicity (%) Figure 6.23. Expected dependence of the 170 XQ values (shown in MHz on the curves) on the ionicity of the tetrahedral cations in the T-O-T' linkage. From Timken et al. (1986a) by permission of the American Chemical Society.

5iso(ppm) 35.2

~ 20

43.9

___.___~ 60

40

47.8 20

40

W 60 i

60

40

82 (ppm)

20

i

i

.......

i

60 40 20 0 170 shift ( p p m ) w.r.t. H 2 0

Figure 6.24. 170 MQMAS NMR spectrum of hydrated zeolite Na-A showing at right anisotropic slices of the 2D spectrum with their corresponding 8iso values. The MAS spectrum at lower right was fitted in accordance with the values derived from the 2D data. From Pingel et al. (1998) by permission of Elsevier Science.

378


(Pingel et al. 1998). This relationship was subsequently superseded by a correlation with the hybridisation parameter (Eq. 6.11) (Freude et al. 2001). On dehydration of the zeolite, typical shifts of --- 8ppm are observed, resulting from a combination of a change in the bond angle at the sites and the polarisation of the framework by the water molecule. As the cation is varied, the 170 NMR chemical shift becomes more positive with increasing cation radius (Freude et al. 2001). The silicon-aluminium ordering of aluminosilicate structures appears to obey Lowenstein's rule stating that A104 units avoid sharing comers with each other. In some zeolites a small excess of Si-O-Si arising from the presence of A1-O-A1 would be expected if Lownstein' s rule is not strictly obeyed. The increased resolution of 170 MQMAS can now be used to monitor such deviations. The 3Q NMR data for stilbite have provided unequivocal evidence for the presence of A1-O-A1 units, amounting to about 3% of the oxygen in this configuration (Figure 6.25) (Stebbins et al. 1999). The presence of such small amounts of A1-O-A1 is extremely important for the detailed thermodynamic modelling of these systems. Back-reacting a ~70-enriched stilbite sample showed that the A1-O-A1 bonds react preferentially with moisture (Stebbins et al. 1999). However, no A1-O-A1 could be detected in the zeolites 4A, 13X and natural analcime (Zhao et al. 2001). Variable amounts of A1-O-A1 have been detected in analcime, depending on the temperature at which the sample was synthesised (cf. Cheng et al. 2000, Zhao et al. 2001).

Si-O-AI .~AI-O-Al

Si-O-Si

/ /~ AI-O-Si

//"~ original

-, 9 20

Si-O-Si

ID

I( ili:li j .,

o j,,,~

ra~
~(ppm)

XQ (MHz)

~q

Reference

Ba(C104)2 Ba(C104)2.3H20 Ba(C|O4)z.6H20 NH4C104 (CH3)4NC104

983.6 994.6 998.4 991.5 1003.3

2.256 0.383 0.328 0.6949 0.307

0.58 0.00 0.00 0.755 0.00

Skibsted & Jakobsen (1999) Skibsted & Jakobsen (1999) Skinsted & Jakobsen (1999) Bastow & Stuart (1989) Skibsted & Jakobsen (1999)

* - chemical shift with respect to aqueous NaC1.

A 35C1 NMR study of the X-ray storage phosphor material BaFC1 has revealed a well defined second-order quadrupolar lineshape from which the quadrupolar parameters were deduced (Bastow et al. 1994). 35C1 NMR has also been used to study the anion dynamics, interlayer structure and phase transitions in the mixed-metal layered hydroxide compounds hydrotalcite [Mgo.764Alo.z36(OH)z](CO3)o.ooyClo.zzl.rlH20 and hydrocalumite CazAI(OH)6C1.2H20 (Kirkpatrick et al. 1999). Hydrocalumite shows a well-defined 35C1 lineshape over a range of temperatures and relative humidities, whereas hydrotalcite spectra are more poorly defined. Differences in the atomic ordering of the interlayers of these two materials are reflected in the 3SC1NMR evidence for an order-disorder structural phase transition, which occurs over a much wider temperature range in hydrotalcite (Kirkpatrick et al. ! 999). Materials with useful semiconducting properties can be produced by encapsulating sodium silver halides in the pore structure of zeolites such as sodalite (Si6A16012). Typically these compounds are prepared by progressively exchanging silver for sodium in the sodalite cavities, leading to the eventual formation of an expanded Ag4C1 supralattice. 35C1 NMR has been used to study this process, initially showing a sharp resonance at - 22 ppm from Na4C1 clusters (Figure 8.21A) (Jelinek et al. 1993). On the addition of silver, a weak line at - 6 ppm corresponding to surface AgC1 appears (Figure 8.21B), but at very high silver exchange levels a new resonance appears at - 3 1 0 ppm (Figure 8.21D), attributed to Ag4C1 clusters. Changes in the position of the 35C1NMR resonance observed when Iis exchanged into the system provide evidence for interactions between the clusters (Jelinek et al. 1993). 8.3.5 39K N M R

With its natural abundance of 93.1% and a relative receptivity of its central transition comparable to that of 13C, 39K appears to be a suitable nucleus for solid state NMR. These factors are to some extent offset by its small magnetic moment and quadrupolar characteristics which have limited the number of solid-state NMR studies to date. An early study demonstrated the success of the nuclear quadrupole double resonance

496


A

[

NaCI

-44 -122

B

-6

C D

-310

-100 -2 0 -300 35C1 shift (ppm) w.r.t. NaCI soln.

Figure 8.21. 35C1MAS NMR spectra monitoring the progressive replacement of Na + by Ag + in the cavities of sodalite. A. Sodalite containing NasC12 groups. Inset-resonances of bulk NaC1 and AgC1. B. After treatment with Ag to form Na7AgCI2 groups. The small resonance at - 6 ppm is from surface AgC1. C. After formation of Na4Ag4C12groups. D. Fully-exchanged Ag8C12sodalite. Note the characteristic tails to the negative sides of the resonances arising from a distribution of XQ values. From Jelinek et al. (1993), by permission of the American Chemical Society.

technique in determining the 39K resonance from 11 potassium salts at room temperature (Poplett and Smith 1981). More recently, the static 39K NMR spectra of 16 potassium compounds have been obtained by Bastow (1991) using a solid pulse echo sequence. Most of these compounds have relatively small dipolar coupling, resulting in sharp powder pattern features of the observed central transition. Accurate simulations allowed the values of XQ and TI to be determined and also indicated very small CSA contributions. Most of these model compounds contain only 1 potassium site, but even those compounds with 2 inequivalent K sites could be sufficiently resolved to allow accurate and unambiguous simulation (Bastow 1991). The 39K NMR interaction parameters for potassium compounds are collected in Table 8.8. Potassium superoxide KO2 exists in 2 polymorphic forms (cubic and tetragonal) which co-exist over the temperature range 353-423 K. The 39K MAS NMR spectra of these polymorphs (Figure 8.22A) show shifts which vary with temperature (Figure 8.22B), due to simple paramagnetism which is reflected in a temperaturedependent Curie-like susceptibility (Krawietz et al. 1998). The 39K NMR spectrum of KOH shows a typical quadrupolar powder pattern which changes with temperature as the compound is cooled through the temperature at which an antiferroelectric structural transition occurs (Figure 8.23A). Simulation of these

497

NMR of Low- y Nuclides

Table 8.8. 39K NMR interaction parameters of potassium compounds. Compound

giso* (ppm)

XQ(MHz)

~

Reference

KF KC1 KBr KI KOH

22.6 47.8 55.1 59.3 N.D.

N.D. N.D. N.D. N.D. 1.680, 1.682

N.D. N.D. N.D. N.D. 0.104

KNO3

N.D.

1.322, 1.326

0.173

N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. - 100 - 17 N.D. N.D. 90, - 30

0.056 0.958, 0.864 0.614,1.220 1.06 1.45 1.536, 1.546 1.490 0.978, 0.995 0.952, 0.968 0.738 0.107, 0.300 1.956 0.904 1.694, 1.680 1.148 N.D. N.D. 0.780 0.518 N.D.

--~0 0.043, 0.90 0.356, - 1 0.58 0.85 0.860, 0.63 0.239 0.689 0 0.274 0 0 0 0 0 N.D. N.D. 0.69 0 N.D.

Hayashi & Hayamizu (1990) Hayashi & Hayamizu (1990) Hayashi & Hayamizu (1990) Hayashi & Hayamizu (1990) Bastow (199 lb), Bastow et al. ( 1991) Bastow & Stuart (1989), Bastow (1991b) Bastow (199lb) Bastow (1991b) Bastow (1991b) Lim et al. (2001) Lira et al. (2001) Bastow (1991b) Bastow (1991b) Bastow (1991b) Bastow (1991b) Bastow (1991b) Segel (1981), Bastow (1991b) Bastow (1991b) Bastow (1991b) Bastow (199 lb) Segel (1978) Markgrabe & Engelhardt (1999) Markgrabe & Engelhardt (1999) Bastow (1991b) Bastow (199lb) Alloul et al. (1994)

KNO2 K2804 KHSO4 KHSO4 (site 1) (site 2) KCO3 KHCO3 KC103 KBrO3 KC104 KIO4 KAI(SO4)2.12H20 KH2AsO4 KH2PO4 KReO4 KNiF3 K2SiF6 KBF4 KN3 K3C6o -

* shiftwithrespectto diluteaqueousKC1.

powder patterns indicates that the values of XQ changes only slowly as the structure passes through the transition temperature, but ~q changes very rapidly (Figure 8.23B), consistent with the first-order nature of the transition (Bastow et al. 1991). The potassium halides show static 39K N M R spectra with linewidths of 8.4-14.7 ppm and isotropic shifts which vary by about 37 ppm throughout the series (Hayashi and Hayamizu 1990). A single crystal 39K N M R study of KNO3 has confirmed that the CSA in this compound is very small and that the Xe value decreases by ~--10% over the temperature range 2 9 5 - 3 7 5 K (Bastow and Stuart 1990, 1991). Other potassium compounds investigated by 39K N M R include KNO2, in which relaxation in the low-temperature plastic crystal was found to be due to the motion of the NO2(Kenmotsu et al. 1994), and KSCN which undergoes an order-disorder antiferromagnetic phase transition at 415K, the temperature of a change from orthorhombic to tetragonal

498


A

o 450 ' -,,~Tet

ragonal (13)

..........

~d 250

~

~Cubic

s0 800 400

0 -400-800

,,,l:l

% qB. 2.0

39K shift (ppm) w.r.t. KCI soln.

(~)

~u,t

.....

I''~"

I ....

3.0

I .... " " l "

4.0

'~''

5.0

Reciprocal temperature (1000/K)

Figure 8.22. A. 39K MAS NMR spectra of KO2 at temperatures in the vicinity of the phase transition between the c~ (cubic) and 13(tetragonal) forms. The a-form is solely present below 388K, but between 353 and 423K the two polymorphs coexist. Above 423 K only the [3-form is present. B. Temperature dependence of the 39K chemical shifts of the e~ and 13-forms of KO2. From Krawietz et al. (1998), by permission of the American Chemical Society. A

B

1050

A

~

~A

1.0

A A

N

o

=-,=~

950 9 9

24~__J ~ ~~_ ..........

40

, ........

0

-40

Frequency (ldtz)

0.5 1"1

9

85o

A A

750 150

tinp OooO

200

9

250

o

o 300

Temperature (K)

Figure 8.23. A. Typical 39K NMR powder patterns of KOH at temperatures near the antiferroelectric transition. B. Variation with temperature of the 39K nuclear quadrupole coupling constant XQ and the asymmetry parameter TI for KOH. Note the sharp change in ~1at the transition temperature by contrast with the sluggish change in XQ. From Bastow et al. (1991) by permission of Elsevier Science.

symmetry. The homogeneous and inhomogeneous contributions to the 39K N M R spectrum were separated by a two-dimensional N M R experiment (Figure 8.24), showing the line broadening at about the temperature of the transition is purely homogeneous, and that the disordering process is thus a dynamic one (Blinc et al. 1995).

499

N M R of Low- 7 Nuclides A

T(K)

4o6

-4000

L__

0

~ ~ ~ . , ~ . 419 - 4000

~ ~Ll

i

2000

,

!

'-2000 ~..... O)fl2~: ( H z )

i

:6000

4 i

_l

2 i

1 L

_~A. ~

=.

2 -2 -6 4 f02/2~ (kHz) r inhomogeneous

~ i

=:l

~

L

0 -4 (kHz)

homogeneous

Figure 8.24. A. Two dimensional 39K spectrum of KSCN. The to1 and o~2dimensions represent the homogeneous and inhomogeneous interactions respectively. The three peaks in the 002 dimension are due to the partially overlapping site and domain splittings. B. The separate inhomogeneous and homogeneous 39K spectra at various temperatures about the temperature of the order-disorder transition of KSCN showing the gradual merging of the three-line homogeneous spectra as the temperature is lowered to Tc and the broadening of the homogeneous spectra below To. From Blinc et al. (1995), by permission of Elsevier Science.

Solid solutions of potassium and rubidium bromides or iodides have been studied by 39K NMR. In the course of this work, the chemical shifts of all the well-defined compounds in the system (e.g. K3RbI4, K2Rb2Br4) were documented, allowing the atomic distribution in mixed crystals of other compositions to be deduced. Values of XQ estimated from the residual MAS linewidths of these compounds fall in the range 0.35-0.53 MHz (Endo et al. 1996). The 39K MAS NMR spectrum of KNiF3 has been measured over the temperature range 180-450 K (Markgraber and Engelhardt 1999). The changes in the shift of this compound with temperature show a correlation with the magnetic susceptibility, indicating that the isotropic hyperfine interaction is non-zero, contrary to previous predictions from molecular orbital (MO) theory. This compound undergoes a transition from antiferromagnetic to paramagnetic at 180 K. The 39K NMR signal which can be observed in the antiferromagnetic state rapidly broadens beyond detection below the transition temperature (Markgraber and Engelhardt 1999). A 39K NMR study of single-crystal KHSO4 has resolved two sets of crystallographically inequivalent K § ions and allowed their quadrupole interaction parameters to be determined (Lim et al. 2001). In both types of site the K is surrounded by nine

500


oxygen atoms, but the symmetry of site 2 is lower than that of site 1, as evidenced by the larger XQ value of the former (Table 8.8). The geological and agronomical significance of potassium suggests that greater future use could be made of 39K NMR by mineralogists, even though the spectra of minerals tend to be broad and rather featureless. By measuring the position of the centre-of-gravity (cog) of the 39K resonances, Lambert et al. (1992) have shown that the tectosilicate orthoclase (--~205 ppm) can be distinguished from the phyllosilicates in which the cog position is below 40 ppm (Figure 8.25A). 39K has also been used to distinguish between structural and exchangeable potassium in the clay mineral montmorillonite as it undergoes wetting and drying. Dry montmorillonite shows only a broad resonance, but when the material is wetted, a superimposed narrow resonance appears, attributed to the exchangeable potassium that becomes mobile on hydration (Figure 8.25B). The integrated area of the narrow residual 39K signal remaining after subtraction of the spectra of a wetted and dry sample correlates with the exchangeable K determined by chemical analysis (Lambert et al. 1992). Geopolymers are useful inorganic aluminosilicate framework compounds which form and harden at room temperature. Both the aluminium and silicon atoms in their structure occupy tetrahedral sites, with charge balance achieved by the presence of hydrated monovalent ions. 39K NMR has been used to study the changes occurring when a potassium sialate geopolymer is heated to high temperatures. The cog positions

A

B montmor~

?~t,,',-~'v-v.,m~

orthoc

montmorillonitewet

vermiculite ~

100%

~ : / # . , a ~ 1000

-1000

2000

-2000

39K shift (ppm) w.r.t. KCI soln. Figure 8.25. A. 39KNMR spectra of nominally single crystal orthoclase (upper), muscovite (middle) and vermiculite (lower). B. Use of 39KNMR to estimate the amount of exchangeable potassium in montmorillonite by subtracting the spectra of the dry and wetted material to give the narrow residual spectrum corresponding to the exchangeable K. From Lambert et al. (1992), by permission of the copyright owner.

N M R o f Low- 7 Nuclides

501

A Unheated

.

.

-4o

& ~

t,.

i

I

1000

i

.

i

0

I,

i

i

......

i

-1000

39K shift (ppm) w.r.t. KCI soln.

,~ @ ~,4

-80

~.,

-120

~

r~

,

0

!

500

.

_

_

|

1000

. . . . . .

i

1500

Temperature (~

Figure 8.26. A. 39K NMR spectra of a potassium sialate geopolymer after curing at room temperature (upper) and after heating at 1300~ (lower). B. Change of the 39K peak position of potassium polysialate geopolymer as a function of heating temperature. Note the progressive shift in the peak position towards that of the anhydrous potassium aluminosilicates. From Barbosa and MacKenzie (unpublished). of the spectra shift progressively from - 47 ppm (more typical of a hydrated potassium phase) in the unheated geopolymer, to - 120 ppm in the material heated at 1300~ (Figure 8.26). The latter cog position is approaching that of an anhydrous feldspar, consistent with the known formation of leucite (KA1Si206) and kalsilite (KA1SiO4) in these heated geopolymers (Barbosa and MacKenzie, unpublished). The discovery that buckminsterfullerine (C6o) containing K or Rb in the composition K3C6o shows several structural phase changes and becomes superconducting at lower temperatures has prompted several 39K NMR studies of this phenomenon (Alloul et al. 1994, Yoshinari et al. 1996, Apostol 1996, Apostol et al. 1996, Sasaki et al. 1998). The potassium ions which occupy the two tetrahedral and one octahedral void per C6o molecule can be distinguished by 39K NMR, and occur in the expected intensity ratio of 2"1 (Figure 8.27A). These peaks are symmetrical and show no evidence of quadrupolar structure. At 210 K, K3C6o undergoes a phase transition, accompanied by the appearance of a second tetrahedral 39K NMR peak. This splitting of the tetrahedral resonance has been analysed in terms of alkali cation-vacancy interactions, providing a satisfactory explanation of the temperature dependence of the tetrahedral splitting (Apostol 1996). The temperature dependencies of the tetrahedral and octahedral shifts (Figure 8.27B) have been shown to be similar to that of the 13C shift in this compound, and estimates of the spin relaxation time suggest that the conduction electron density at the potassium site is very similar to that of the carbon site indicating a uniform Pauli susceptibility (Sasaki et al. 1998).

502


A 100 oetahedral (0)

tetrahedral (T)

~'

300K

,~

te

T' site

0

o~

230K / ' ~ tetrahedral (T')

..... -200

J ..... -100

t . 0

.

.

39K shift ~ p m ) w.r.t. KCI soln.

~ -100

. . 100

.

200 0

100

200

300

Temperature (K)

Figure 8.27. A. 39K spectra of K3C6oshowing the splitting of the tetrahedral peak as the temperature is lowered. From Yoshinari et al. (1996), by permission of the copyright owner. B. Temperature dependence of the octahedral and tetrahedral 39K shifts in K3C6o. From Sasaki et al. (1998), by permission of Elsevier Science. 8.3.6

43CaN M R

Calcium is an important element in the chemistry of many compounds (minerals, cements, bone, etc.) but NMR studies of calcium compounds have been severely hampered by the low natural abundance of this nucleus (0.14%) and the high cost of isotopically-enriched 43Ca compounds. In spite of these drawbacks, most of the more recent 43Ca NMR work has been on isotopically-enriched samples. IH ~-> 43Ca cross-polarisation (CP) has also been investigated as a means of increasing the sensitivity, using a 46 kHz spin-locking field on the protons, matched to the central transition of the Ca nucleus. Measurements made on 50 %-enriched 43Ca acetate indicate an optimum contact time of 20-30 ms, placing severe strain on the rf circuits of the spectrometer. Two signals were detected, one at about 60 ppm with a narrow linewidth (24 Hz), and a broader line covering the range - 50 to l0 ppm. It is unclear whether the elements of structure which may be present in the broad line are from a second-order quadrupolar lineshape or from overlapping lines (Bryant et al. 1987). The relatively ionic nature of the Ca-O bond results in generally small values of the nuclear quadrupolar coupling constant XQ, making it possible to narrow the NMR signal by the application of even relatively modest magic angle spinning rates. The problem of low sensitivity may then be addressed by using large diameter sample rotors (9.514 mm) spun at 1-2 kHz. Under these conditions it has proved possible to acquire natural abundance 43Ca MAS NMR spectra of 12 inorganic compounds including silicates, carbonates and sulphates, with reasonable signal/noise characteristics in about

N M R o f Low- y Nuclides

A

B

503

t

calcite

g

dd~,,,

JL,.J,,k.,~

,

.

,

.

J

.

,

.

i

.

40 0 -40 shift (ppm) w.r.t. CaCI2

43Ca la

C

C ~ j ~-simulated . . . , ,

200

1O0

0

-100

. . . . . . . . . .

100

,

. . . .

0

,

. . . .

, , ~ - . ~

-100

43Ca shift (ppm) w.r.t. CaCl 2 soln.

43Ca (CaAlz(OH)z[SizO7]H20,

Figure 8.28. A selection of natural abundance MAS NMR spectra obtained using a 14 mm rotor with a sample volume of 2.1 ml. and MAS speeds of about 2 kHz. A. Spectra from bottom: CaO, pectolite (CazNaH[SiO3]3), lawsonite apatite (CasPO4)3(OH,F)) and gypsum B. MAS spectra of the two forms of CaCO3, Upper spectrum calcite, observed and simulated, lower, aragonite. From Dupree et al. (1997), by permission of Elsevier Science. C. MAS spectra of calcium hexaluminate (upper) and calcium dialuminate, observed and simulated (lower). From MacKenzie et al. (2000a), by permission of the copyright owner.

(CaSO4.2H20).

24 hours (Figure 8.28A) (Dupree et al. 1997). These compounds show a range of shifts of about 160 ppm, but at a field strength of 8.45 T the resolution was not sufficiently good to distinguish between sites in samples containing more than 1 Ca environment. The 43Ca NMR interaction parameters of the various Ca compounds are collected in Table 8.9. The 2 polymorphs of CaCO3, aragonite and calcite, can readily be distinguished on the basis of their 43Ca MAS spectra (Figure 8.28B). Calcite shows a characteristic second-order quadrupolar lineshape from which the NMR parameters can be extracted by spectral simulation. The narrower 43Ca MAS resonance from aragonite shows no discernible structure, but the corresponding static aragonite spectrum is about 20 times broader than under MAS conditions. This suggests that CSA is a major contributor to the static linewidth, which is confirmed by satisfactory simulation of the spectrum

504


Table 8.9.

43Ca

NMR interaction parameters in calcium compounds.

Compound

~p*(ppm)

Av (Hz)**

Reference

CaO CaCO3 (calcite) CaCO3 (aragonite) CaSiO3 Ca silicate gel CaSOa.2H20 CaTiO3

128 14i - 34 -9 50 - 28 13 45.7 i - 52.6 - 21 45 7 - 6 - 22 - 2 60, - 20

20 250 80 1400 2-3* kHz 500 1000 1300 80 700 500 200 470 900 360 24,600

Dupree et al. (1997) Dupree et al. (1997) Dupree et al. (1997) Dupree et al. (1997) Nieto et al. (1995) Dupree et al. (1997) Padro et al. (2000) MacKenzieet al. (2000a) MacKenzieet al. (2000a) Dupree et al. (1997) Dupree et al. (1997) Dupree et al. (1997) Dupree et al. (1997) Dupree et al. (1997) Dupree et al. (1997) Bryant et al. (1987)

CaA1407 CaAll2019

CazMgSizO7 CazNaH(SiO3)3 KFCaa(SigOz0).8HzO CaAIz(OH)z(SizOv).H20 Ca5(PO4)3.(OH,F) Ca6[Al(OH)612(SO4)3.26H20

Ca(CH3COO)2

* MAS peak position with respect to saturated aqueous CaC12 solution, except i which indicates an isotropic chemical shift. ** MAS linewidth, except $ - which represents the static linewidth.

using CSA parameters alone (Dupree et al. 1997). The chemical shifts of the various types of calcium compounds in this study show linear relationships with the Ca-O distance in the first coordination sphere (Figure 8.29). The lines for the groups of oxides, silicates and carbonates all show a similar slope of 280 ppm/A (Dupree et al. 1997). The natural abundance 43Ca NMR spectra of calcium hexaluminate, CaAll2019, and calcium dialuminate, CaA1407, have also been reported (MacKenzie et al. 2000a). The resonance line of the hexaluminate (Figure 8.28C) is much narrower than that of the dialuminate, which shows quadrupolar features allowing it to be simulated. The 43Ca shifts for both these aluminates lie closer to the silicate line in Figure 8.29 than, as would be expected, to the oxide line. This suggests either that aluminium exerts a similar effect to silicon on the 43Ca isotropic chemical shift, or that the influence of the next-nearest neighbour in these compounds is less than structural or geometrical factors of the Ca coordination polyhedron (MacKenzie et al. 2000a). The 43Ca MAS NMR spectra of model calcium compounds related to cement and concrete formation (CaO, CaCO3, Ca(OH)z) have also been reported by Zanni et al. (1996). 43Ca NMR has been used in a study of calcium silicate formation by sol-gel synthesis (Nieto et al. 1995). The initial sol shows a broad symmetric peak at about 50 ppm which, immediately after gelling, becomes too broad to observe. Aging of the gel causes the reappearance of a broad 43Ca peak near the initial position. The disappearance of the 43Ca peak on gelling and its reappearance in the same position on aging suggest that the Ca environment remains the same throughout the process, but the

N M R o f Low- y Nuclides 150 I I ~ I

505

+' I I I I I+"'"I I I I I I I I

100 r~

r~

5t1

_

_

0

-511 2.3

2.5

2.7

Mean Ca-O distance (,~)

Figure 8.29. Relationship between the 43Cashifts of some inorganic calcium compounds and the mean Ca-O distance of the coordination polyhedron. Full circles denote oxides, full squares denote silicates and full diamonds denote carbonates. The open symbols denote sulphates and phosphate. From Dupree et al. (1997), by permission of Elsevier Science. NMR parameters suffer a large distribution on gelling which is slowly reduced on aging (Nieto et al. 1995). Calcium compounds such as (Bi,Pb)2Sr2CaCu2Os+• (Bi,Pb)zSr2CazCu3Olo+x and (Cao.sLao.5)(Bal.zsLao.75)Cu3Oy have attracted considerable interest as hightemperature superconductors. 43Ca NMR of isotopically-enriched samples has been used to monitor the effect of temperature and doping levels on the spin susceptibility at the Ca site, which is usually located between the CuO2 planes (Trokiner et al. 1994, 1994a, 1994b, Bellot et al. 1998). The 43Ca NMR linewidth also provides information about the magnetic field distribution below the transition temperature caused by the vortex lattice (Bellot et al. 1998). The 43Ca quadrupolar and shift parameters have been deduced for the main Ca site in the (Bi,Pb) compounds (Trokiner et al. 1994b), while the relatively high symmetry of the 43Ca resonance in the Ca-La-Ba cuprates has been taken as evidence of a relatively symmetrical cation environment in the central plane (Trokiner et al. 1994a).

8.3.7

47Ti a n d

49Ti N M R

Titanium is an element of considerable interest in materials science because of its role in technical electroceramics such as barium titanate and lead zirconium titanate, and in engineering ceramics (TIN, TiC) and glasses. Titanium compounds also play a role in establishing catalytic activity in microporous materials. Despite the practical interest in Ti compounds, there have been relatively few NMR studies of this nucleus because of experimental difficulties, some of which are associated with the properties of its two NMR-active nuclei, which both have moderately large quadrupole moments Q in the

506


ratio 49Q/47Q = 0.8179. B e c a u s e the 2 nuclei h a v e very similar values of ~/, their resonance frequencies are similar, differing by only about 9 k H z at the high magnetic field of 14.1 T. Normally, the s e c o n d - o r d e r - b r o a d e n e d central ( 1 / 2 , - 1/2) transition is observed, for which the relative broadening Al147/Al) 49 is 3.522. The result of this combination of circumstances is that most Ti spectra will consist of the completely overlapping b r o a d e n e d resonances from the two isotopes, except in the case of Ti in very symmetrical sites. The conventional standard for Ti N M R is liquid TIC14, but a convenient solid secondary standard is cubic SrTiO3 which produces a sharp M A S N M R line shifted by - 843 p p m against 49Ti in the liquid primary standard (Dec e t al. 1993). Hence, although the 49Ti N M R data for titanium c o m p o u n d s collected in Table 8.10 are referenced to SrTiO3 they can readily be related to the primary shift reference.

Table 8.10. 49Ti NMR interaction parameters for titanium compounds. Compound

~i~o* (ppm)

TiO2 (anatase)

- 195

TiO2 (rutile) TiO2 (brookite) Ti203 SrTiO3 FeTiO3 CaTiO3 CdTiO3 (ilmenite) CdTiO3 (perovskite) MgTiO3 PbTiO3 BaTiO3 YzTi207 LiTi204 Ti metal TiAg Ti3A1 TiAI TiA12 TiA13 TiB2 TiN TiC Till2 (cubic)

•

(MHz)

xI

Reference

4.6, 4.7, 4.79

0

--~0 - 100 1100 0 4500 - 10.5 + 1

13.9 6.04 N.D. 0 N.D. 2.75, < 3.7

0.19 0.55 0 0 > 0 0.70

30 40 - 150 153 112 - 30 - 300 2750 3155 3600 5000 3300 27500 2750 - 748** - 317** 3293, 2600 +

11.1 4.07 15.52 9.98 3.7 24.0 17.6 9.25 --~0 13.9 14.0 8.5 14.39 12.35 0 0 N.D.

0.10 0.40 0.0 0.0 0.0 0.0 --~0 0 --- 0 0.1 0 0.70 0 0 0 0 N.D.

Kanert & Kolem (1998), Labouriau & Earl (1997), Bastow et al. (1998a) Bastow et al. (1998a) Bastow et al. (2000) Bastow et al. (1998a) Dec et al. (1993) Bastow et al. (1998a) Padro et al. (2000), Bastow et al. (1998a) Padro et al. (2000) Padro et al. (2000) Padro et al. (2000) Padro et al. (2000) Padro et al. (2000) Padro et al. (unpublished) Tunstall et al. (1994) Bastow et al. (1998a) Bastow et al. (1998a) Bastow et al. (1998a) Bastow et al. (1998a) Bastow et al. (1998a) Bastow et al. (1998a) Bastow et al. (1998a) MacKenzie et al. (1995) MacKenzie et al. (1995) Nowak et al. (1992), Frisch & Forman (1968)

* chemicalshifts quotedrelativeto solidSrTiO3, except+ whichwas measureddirectlyas a magneticfield. ** with respectto TiCla

507

N M R o f L o w - 7 Nuclides

TiO2 is a commercially important material, with uses as a white pigment, an opacifier in ceramic glazes and as paper filler. The 3 TiO2 polymorphs anatase, rutile and brookite all contain TiO6 structural units but are distinguished by different connectivities and orientations of these octahedral units. The 49Ti NMR spectrum of anatase obtained from a static sample shows that the Ti site in this polymorph is the most symmetric, with the smallest XQ valHe. These anatase spectra contain sufficient detail for the lineshapes from both isotopes to be distinguished and accurately recorded (Figure 8.30A) (Bastow et al. 1998a, Labouriau and Earl 1997). MAS has allowed the 49Ti lineshape of anatase to be clearly seen, but the spinning speeds used (4-6 kHz) were not sufficient to disentangle the 47Ti centreband from the 49Ti sidebands (Dec et al. 1993). The static 49Ti resonance of rutile has been recorded and simulated (Figure 8.30B) (Bastow et al. 1998a), giving parameters which agree with a single crystal rutile study (Kanert and Kolem 1998). The 49Ti shift and quadrupolar parameters of brookite (Figure 8.30C) have values intermediate to those of the other 2 polymorphs (Bastow et al. 1998a, Labouriau and Earl 1997, Bastow et al. 2000, Bastow 2000). The resonance widths of natural and synthetic brookite differ considerably, the broader linewidth of the natural sample being due to the

A

B

Anatase

C

Rutile

Brookite

1

simulated

/

synthetic

simulated

lOO -lOO -3oo Frequency (kHz) observed

. . 40. . .

0

Frequency (kHz)

80

D

200 0 -200 Frequency (ldtz)

'' '200 ' {) '-20O'Frequency (kHz)

Figure 8.30. Static 47'49TiNMR spectra of titanium oxides. A. TiO2 (anatase), showing the resolved quadrupolar lineshape of 49Ti (inner) enclosed by the 47Ti lineshape for which a simulation is also shown. B. TiO2 (rutile), observed and simulated. Note the significantly broader lineshape corresponding to the 49Ti spectrum only. C. TiO2 (brookite) showing a superposition of the unresolved 47Ti and 49Ti lineshapes of the single Ti site in the synthetic material (lower) and additional broadening of the natural mineral (upper). D. Ti203 consisting of superposed 47Ti and 49Ti central transition lineshapes. Spectra A, B and D from Bastow et al. (1998a), by permission of the copyright owner. Spectrum C from Bastow et al. (2000), by permission of the American Chemical Society.

508


presence of paramagnetic impurities. The Ti NMR spectrum of the suboxide Ti203 (Figure 8.30D) reveals no well-defined lineshape from either isotope and its large paramagnetic shift (1100 ppm) arises from the presence of Ti 3+ (Bastow et al. 1998a). Static 47'49TiNMR has been used to study the thermal evolution of rutile from TiO2 gels formed by hydrolysis of titanium isopropoxide (Bastow and Whitfield 1999).The amorphous gel heated at 200~ shows a surprisingly narrow resonance, suggesting either that even at this early stage the TiO6 units are quite symmetric, or that partial averaging of the quadrupolar parameters is occurring. The spectra of samples heated at 500 and 600~ contain complex lineshapes, which can be shown to contain both anatase and rutile resonances. The coexistence of these 2 polymorphs over a temperature range of at least 100~ has been taken to indicate that the development of crystallinity in these gels is spatially inhomogeneous, while the presence of crystalline anatase and rutile NMR lineshapes in samples showing little X-ray crystallinity implies that small but well crystallised TiO2 particles are already present at these low temperatures (Bastow and Whitfield 1999). BaTiO3, an important electroceramic with useful piezoelectric and ferroelectric properties, has been extensively studied by Ti NMR spectroscopy. The static 47'49Ti NMR spectrum of the cubic phase above the Curie point shows narrow resonances from both isotopes which disappear when the tetragonal phase is formed on cooling below the Curie point (Forbes et al. 1987). Three static 49Ti NMR studies of singlecrystal BaTiO3 have given values for 49XQof 3.65, 3.78 and 4.03 MHz (Sommer et al. 1990, Bastow 1989, Kanert et al. 1994), in agreement with a static powder study of three different BaTiO3 samples which all showed spectra with XQ = 3.7 + 0.1 MHz (Padro et al. 2000). The rotation pattern of Bastow (1989) also indicated a CSA contribution of about 40 ppm in that BaTiO3 sample. Solid state titanium NMR could be much more usefully applied to a range of technologically significant problems if it had the ability to distinguish between different local coordination states of titanium (e.g. TiO4, TiOs, TiO6). Although this is not yet possible, it is the goal of much ongoing research effort. The effect of the second cation on the 49Ti NMR parameters has been studied by Dec et al. (1993) who showed the existence of such an influence on the peak positions in a limited range of complex oxides and related compounds. A more extensive static and MAS study at 14.1 T of a series of ATiO3 titanates with perovskite and ilmenite structure has examined this matter in greater detail (Padro et al. 2000). The NMR interaction parameters were accurately deduced from the very clean lineshapes recorded for these compounds, and show a linear relationship between • and the shear strain t~ as defined in equation (8.3). This relationship, shown in Figure 8.31A, has the equation

XQ (MHz) = 7.28qJ + 1.53

(8.5)

509

NMR of Low-~[ Nuclides

A 2O

5/

150

100 ~

~1o

~"

50

om~

~"

0 . . . . . . . . . . . . .

0

.

0

.

.

.

i

.

.

.

.

.

i

.

.

.

.

.

1.0

i

,

9

-

-

Mean shear strain

i

.......

I

I__A

-

J

2.0

1.;6

1.98

2.00

2.02

Mean Ti-O distance (/l.)

Figure 8.31. A. Relationship between the 49Ti nuclear quadrupole coupling constants of ATiO3-type titanates and their structural shear strain q~ defined in equation (8.3). B. Relationship between the 49Ti isotropic chemical shifts (ppm) and the mean Ti-O bond lengths (A) for a series of ATiO3 compounds with the perovskite structure. Note that compounds with the ilmenite structure do not fit this relationship and are not included here. From Padro et al. (2000), by permission of the copyright owner.

The perovskites also show a reasonable linear correlation between ~iso (in ppm) and the mean Ti-O bondlength (in A) (Figure 8.31B) given by the equation (3iso = 2370(Ti-O) - 4632

(8.6)

but the compounds with ilmenite structure do not fit well to this line (Padro et al. 2000). The compound FeTiO3 is paramagnetic at room temperature, with a broad featureless Ti resonance and a large positive shift due to the transferred hyperfine field from the Fe 3+ (Bastow et al. 1998a). Another titanate of interest because of its superconducting properties is the spinel Lil +xTi2-xO4 which has a critical temperature of about 12 K for the composition range 0 < x < 0.10. The static 47'49Ti NMR spectra of stoichiometric LiTi204 have been recorded as a function of temperature and as a function of composition of the non-stoichiometric compounds. A negative Knight shift indicates the probable dominance of core polarisation in these compounds, in which Xe decreases monotonically with increasing temperature (Tunstall et al. 1994). The NMR parameters for titanium metal deduced in earlier studies by field sweeping techniques (Narath 1967, Ebert et al. 1986) have been confirmed by more recent room temperature FT NMR (Bastow et al. 1998a). The value of XQ deduced from the (+_ 3/2, +_ 1/2) satellite transitions was used in an accurate simulation of the central transition, which required an axial Knight shift of 70 + 10 ppm. The Ti NMR spectra of a number of titanium aluminide alloys and TiAg have also been reported

510


(Bastow et al. 1998a). These alloys show Knight shifts of 2750-5000 ppm and have values of XQ up to 14.4 MHz (Table 8.10). The axial component of the Knight shift in TiA13 has a value of 800 + 50 ppm. The electric field gradient at the Ti site in TiAg is unexpectedly small for this tetragonal structure, giving a Ti spectrum of 2 narrow lines separated by 6 kHz (Figure 8.32A) corresponding to the resonances of 47Ti and 49Ti as seen in perovskite and other compounds in which the Ti is located in much more symmetrical sites (Bastow et al. 1998a). Other titanium compounds which have been studied by 47'49Ti NMR include TiB2, a hexagonal metal in which the 47Ti and 49Ti lineshapes can be observed (Figure 8.32B). The reason for the very small Knight shift in this compound is probably the fortuitous cancellation of positive and negative shift contributions (Bastow et al. 1998a). The cubic and tetragonal phases of Till2, has also been studied by 47'49TiNMR and the temperature variation of the Ti Knight shift measured (Nowak et al. 1992, Frisch and Forman 1968). A series of Ti-V alloys has also been studied by 4749Ti NMR, which shows a decrease in the Knight shift with increasing vanadium content (Nowak et al. 1992). The 47'49Ti spectra of the cubic phases TiN and TiC (Figure 8.32C) show peaks from both isotopes, with very little change in the resonance positions measured at field

A

TiAg

C

Ti-C-N

TiC "~..__._._.~-860 50 2oo' '5o' Frequency (kHz)

~-~1160

/

9

'260''

0

L

i

i

,

500

-200

Frequency (kHz)

|

-11o0

Ti N ~

-500 -1500 shi~ (ppm) w.r.t. 49TIC]4

Figure 8.32. Static 47'49TiNMR spectra of A. TiAg, showing the narrow resonances of 47Tiand 49Ti separated by 6 kHz, B. TiB2, showing the lineshape of the central transition of 49Ti (lower) with the 47Ticentral transition lineshape superimposed (upper). From Bastow et al. (1998a), by permission of the copyright owner. C. TiC and TiN, showing resonances from both 47Tiand 49Tiin the single cubic sites, and a series of titanium carbonitride solid solutions formed between these two compounds. From MacKenzie et al. (1995), by permission of the copyright owner.


511

strengths of 11.7 T and 14.1 T reflecting the small electric field gradients in these compounds (MacKenzie et al. 1995). The formation of cubic carbonitride solid solutions between TiN and TiC results in considerable broadening and loss of detail from the 47'49Ti spectra, which, however, retain characteristics of the nitride and carbide end-members (Figure 8.32C) (MacKenzie et al. 1995).

8.3.8 67Zn NMR Zinc is of interest to a range of practical applications from electroceramics to metalloproteins, for which 67ZnNMR studies could potentially provide valuable information. However, 67ZnNMR suffers from the various problems associated with its low natural abundance and low magnetic moment, in addition to its nuclear quadropole interaction which presents linewidth problems for samples containing Zn in a non-cubic environment. Consequently, the number of reported 67Zn studies of solids is limited. The primary reference for 67Zn NMR is a dilute aqueous Zn 2+ solution, but cubic ZnSe would make a useful solid secondary reference material since its MAS linewidth is only 16 Hz (Wu 1998). ZnO occurs in a wurtzite-type crystal structure with hexagonal symmetry containing tetrahedral ZnO4 with the zinc site axially symmetric, giving a 67Zn MAS NMR spectrum with a quadrupolar lineshape and qq = 0 (Figure 8.33A) (Dec et al. 1993). The rotation pattern of single-crystal ZnO has been recorded as a function of temperature over the range 250-400 K (Bastow and Stuart 1988). The static spectrum of ZnO is determined by both the quadrupolar and CSA effects. Since MAS removes the CSA but leaves the quadrupolar lineshape, the 67ZnMAS NMR spectrum of ZnO has been used to determine the quadrupolar parameters, which were then combined with a CSA contribution to simulate the static spectrum (Wu 1998). The value of the chemical shift parameters derived in this indirect manner agree well with the results of an earlier single crystal determination. Heavily doped ZnO is of technical importance as a transparent electrical conductor with applications in advanced optical displays. The effect of adding 0.03-3 at % A1 or Ga metal to ZnO has been studied by monitoring the changes in the 67Zn NMR spectrum (Roberts et al. 1998). The addition of Ga produces a blurring of the distinct second-order quadrupolar lineshape of the ZnO spectrum (Figure 8.33B) due to the appearance of a distribution of nuclear quadrupole coupling constants (but with little change in the linewidth), and the 67Zn shift also increases with increasing dopant concentration (Roberts et al. 1998). ZnS occurs in 2 polymorphic forms, cubic wurtzite and hexagonal zincblende, which often occur together in commercial ZnS powders. The 67Zn NMR resonances from the 2 forms have different shifts (Figure 8.33C) (Bastow and Stuart 1988), both of which can be resolved in the 67Zn MAS NMR spectra of commercial powders

512


C A

ZnO

B

ZnS

ZnO

MAS

observed

600

400

200

si

0%~Ga

1, . . . . . .

!

_

T ~ 7 - - - _ .J_

400 200 0 67Zn shift (ppm) w.r.t. ZnCI2 soln.

al,

-10

,

,

0

,I,

10

i

I

20

Frequency (kHz)

__••.•exagonal i

~

i

l

i

i __J...._....l_......~

4 0 360 3OO 67Zn shift (ppm) w.r.t.

ZnSO 4 soln.

Figure 8.33. A. Observed and simulated 67ZnMAS NMR 11.7 T spectrum of ZnO, from Dec et al. (1993), by permission of the American Chemical Society. B. Effect of additions of Ga on the 67Zn NMR spectrum of ZnO, adapted from Roberts et al. (1998). C. 67ZnMAS NMR spectrum of commercial ZnS (upper) showing the Zn resonance from the cubic and hexagonal polymorphs, from Dec et al. (1993). Below: static 67ZnNMR spectra of the two ZnS polymorphs, adapted from Bastow and Stuart (1988).

(Dec et al. 1993, Wu 1998). The 67Zn NMR spectra of all the zinc chalcogenides (ZnS, ZnSe and ZnTe) have been determined, showing that static 67Zn lineshapes of the hexagonal forms contain a CSA contribution (Bastow and Stuart 1988). All the NMR interaction parameters for zinc metal have been determined from the central and satellite transitions of the 67Zn spectrum which indicates a value of - 124 ppm for the axial component of the Knight shift. The isotropic component of the Knight shift and the nuclear quadrupole coupling constant have also been determined as a funtion of temperature over the range 149-432 K (Bastow 1996). Other zinc compounds for which the 67Zn NMR spectra have been reported include ZnSO4, in which the zinc is in octahedral coordination with oxygen. Comparison of this spectrum with that of tetrahedral Zn in ZnO indicates that the two coordination states are separated by about 200 ppm (Wu 1998). The 67Zn NMR spectra of zinc acetate and its hydrate show that on hydration the zinc coordination changes from four to six, with a large change in the isotropic chemical shift. (Figure 8.34). Distortions in the octahedral units of the hydrate are reflected by a large increase in the XQ value

513

NMR of Low- y Nuclides -'--

ZnS4 9 ZnC4, ZnN 4 -

ZnSe 4 --

ZnO 4 -

ZnTe4

ZnO 6 I

400

I

,I

I

300

I

200

67Zn i s o t r o p i c

I

I

,,,

I

100

I

_

O

shift (ppm) w.r.t. Zn(NO3)2 soln.

Figure 8.34. Range of 67Znisotropic shifts in various zinc compounds, from data of Sham and Wu (1999).

(Wu et al. 1998). A detailed single-crystal 67Zn study of zinc acetate has indicated that although the CSA effects in this compound are small, they significantly affect the value of XQ (Vosegaard et al. 1999). The 2 inequivalent octahedral Zn sites in zinc formate dihydrate have been studied by 67Zn NMR, which indicates that although their isotropic chemical shifts differ by only 10 ppm, their XQ values differ by more than 50% (Larsen et al. 1999). The 67Zn MAS NMR spectrum of Kz[Zn(CN)4] has also been recorded as part of a study of potassium tetracyanometallates in which the CN ligand was partially enriched in 13C (Wu et al. 1995). Information about the 67Zn NMR characteristics of ZnN4 and ZnS4 groups (Figure 8.34) has been provided by a study of zinc complexes with imidazole and thiourea respectively (Sham and Wu 1999). The XQ values of the compounds in which the zinc is tetrahedrally coordinated to a variety of ligands show no systematic relationship with the distortion index of the tetrahedral bond angles (the degree of deviation from the ideal tetrahedral angle of 109.5~ indicating that the nature of the coordinating ligand is exercising as powerful an effect on XQ as the structural geometrical factors, and must be taken into account. For the limited number of compounds investigated containing octahedrally coordinated Zn-O (Sham and Wu 1999) there is evidence of some degree of correlation between XQ and the octahedral distortion index DI which can be expressed by

XQ(MHz) = 0.229DI + 0.0542 The 67Zn interaction parameters for zinc compounds are collected in Table 8.11.

(8.7)

514

M u l t i n u c l e a r Solid-State N M R o f I n o r g a n i c M a t e r i a l s

Table 8.11. 67Zn interaction parameters in zinc compounds. Compound

~iso~ (ppm)

ZnO

238,240.1,240

ZnS (cubic) (hexagonal) ZnSe

Reference

XQ (MHz)

Dec et al. (1993), Bastow & Stuart (1988b), Wu (1998) -,0,0 Haller et al. (1980), Bastow & Stuart (1988b), Wu(1998) Bastow & Stuart (1988b), Wu (1998) Bastow & Stuart (1988b) 0 Bastow (1996) N.D. Bastow (1996) 0.2 Wu (1998) 0, 0.1 Wu (1998), Vosegaard et al. (1999) Wu (1998) 0.87, 0.819 Larsen et al. (1999) 0.99 0.39 Wu et al. (1995) N.D. Wu et al. (1995) 0.4 Wu et al. (1995) 1.0 Wu et al. (1995)

2.4, 2.4065, 2.40, 2.38 378, 380.5, 381.9 --~0 360, 365 --~0, < 0.5, < 0.4 276.3,276 --~0

ZnTe Zn CUl.O1Zno.99 ZnSOn.7H20 Zn(CH3COO)2

87.6 1776 1879 10 260, 245

--~0 11.983 N.D. 1.70 2.42, 2.42

Zn(CH3COO)2.2H20 Zn(OOCH)2.2H20 I II Zn(C104)2.6H20 Zn[ImH]4(C104)2 Zn[SC(NH2)2]4(NO3)2 K2[Zn(CN)4]

0, - 123+ ----10 0 - 3 291 359 291

5.3, 5.34 6.05 9.52 < 0.2 2.80 3.15 0

0, 0

* shifts relativeto diluteaqueousZn2+, exceptvaluemarkedt whichis relativeto 1MZnC12.

8 . 3 . 9 9~Z r N M R

Zirconium plays an important role in a number of materials applications including engineering ceramics, toughened ceramic materials, fuel cells, and as an additive in the synthesis and sintering of non-oxide ceramics. Although narrow 91Zr N M R resonances can be obtained from a limited number of materials such as cubic BaZrO3 in which the Zr is in an extremely symmetrical site (Dec et al. 1993, Hartman et al. 1991), the majority of Zr-containing materials of practical interest have very broad 91Zr N M R spectra, as in ZrSiO4 (zircon) in which the width of the central transition is about 350 kHz at 9.4 T (Bastow 1990). Unfortunately this width is much greater than can be narrowed by any accessible MAS speed, and is also too great to be recorded without distortion by direct pulse methods, including echo techniques. For this reason stepped frequency measurements have been most commonly used for solid materials with this nucleus, but the t i m e - c o n s u m i n g and laborious nature of these experiments has severely limited the number of reported 91Zr N M R studies of solid materials. 91Zr N M R spectra are normally referenced to a saturated solution of bis(cyclopentadienyl)zirconium dibromide in tetrahydrofuran, but the narrow line of BaZrO3 makes it an excellent solid secondary reference resonating at 208.1 ppm from the primary liquid reference (Hartman et al. 1991). The pointwise stepped frequency approach has been used to determine the undistorted 91Zr lineshapes in the various forms of ZrO2 (Figure 8.35A) (Bastow and Smith 1992).

515

N M R of Low- y Nuclides

Pure zirconia exhibits a monoclinic-to-tetragonal phase transition at 1000~ which involves a large volume change and makes it impractical as an engineering material. The addition of elements such as Ca, Mg or Y forms stable cubic solid solutions, the technically important stabilised zirconias. Analysis of the stepped-frequency 91Zr lineshapes of the tetragonal, monoclinic, orthorhombic and cubic forms of ZrO2 indicates that they cover a remarkably small is 9 shift range (--~ 10 ppm), but the quadrupolar parameters provide a means of discriminating between the phases and estimating the phase compositions in transformation-toughened zirconias (Bastow and Smith 1992). The tetragonal and cubic phases of ZrO2 are only stable at room temperature when stabilised by Ca, Mg or Y, the highest level of doping being required for cubic ZrO2. This is reflected in the featureless 91Zr lineshape of cubic ZrO2 (Figure 8.35A) in which atomic disorder produces a range of electric field gradients. A 91Zr NMR single crystal study of ZrSiO4 (zircon) showed a considerable variation of the linewidth as the crystal was rotated, possibly resulting from a range of electric field gradients associated with defects in the natural mineral sample. The fitted rotation pattern revealed a sizeable axial CSA of about 183 ppm (Bastow 1990).

D A

Zr02

Zr metal

B simulated

-. tetragonai

p

BaZrO3 [

"""'"" "

~

~~,

observed

/

J

"~-.

SrZr03

/ "~,.~

%

9

"

monoclinic " 9

9

9

9149

cubic

9

9

9 9 9

o 9149

2010 . . . . 0'. . . . -200' ~-91Zr shift (ppm) w.r.t.BaZrO3

9

9

400

i

I

_0

i

I

-4o0

E AI3Zr

..

.

t

o

Frequency (ir~z)

Na2ZrSiOs .: "" "... 9 9

"

~ _ ~ . .

460

i

l

i.

-400

Frequency (kHz)

i

,

:

:

~

,,

9 .'

. 9

200 -100 -400

Frequency (kltz)

9

L

. . I. . .

t

I

0 -50 Frequency (kHz) 50

Figure 8.35. A selection of 91Zr NMR spectra. A. Stepped-frequency spin-echo spectra of 3 phases of ZrO2, adapted from Bastow (1994). B. 14.1 T MAS NMR spectra of Ba and Sr zirconates. Asterisks denote spinning side bands. Note the broader lineshape with possible quadrupolar structure of the more distorted Zr site in SrZrO3. From Dec et al. (1993), by permission of the American Chemical Society. C. Static stepped-frequency NMR spectrum of NazZrSiOs. 1996. D. Stepped-frequency NMR spectrum of Zr metal (lower) with simulated spectrum (upper). 1992. D. Static 91Zr NMR spectrum of the central transition of A13Zr. From Bastow et al. (1992, 1996, 1998b), by permission of the copyright owners.

516


Although the 91Zr resonance of cubic BaZrO3 (Figure 8.35B) is sufficiently narrow to be detected by MAS NMR, the zirconium site in the orthorhombic analogue SrZrO3 is not at the centre of the octahedron defined by the six nearest-neighbour oxygen atoms. This greater distortion is reflected in its broader MAS NMR resonance (Figure 8.35B) which shows signs of second-order quadrupolar structure (Dec et al. 1993). The stepped frequency method has been used to determine the broad central transitions of several more complex zirconium compounds including NaZrO3 (Bastow et al. 1994a) and Na2ZrSiOs, (Figure 8.35C) in which the two crystallographically inequivalent Zr sites could not be distinguished by 91Zr NMR (Bastow et al. 1996). A stepped echo approach has been used to determine the 91Zr NMR spectra of a range of zirconium-containing phosphate and fluoride compounds for which the values of XQ are much smaller, typically 1-2 MHz (Table 8.12) (Hartmann and Scheler 1995). Although the 91Zrcentral transition of ZrF4 extends over a range of 1.5 MHz, its static lineshape has been detemined by the stepped frequency method (Bastow 1994). Early 91Zr NMR studies of Zr metal reported its Knight shift at room temperature (Yamada and Asanuma 1965) and at 4 K (Hioki et al. 1975). More recently the values of XQ and xl for zirconium metal have been determined from simulation of the roomtemperature resonance lineshape recorded by the stepped-frequency method (Figure 8.35D) (Bastow et al. 1992). 91Zr NMR has been used to study a series of amorphous Zr-Cu alloys (Abart et al. 1982), to examine the internal field in ZrFe2 (Dumelow and Riedi 1987) and to determine the Knight shifts of the isostructural metallic compounds ZrV2, ZrC2 and ZrMo2 (Torgeson and Barnes 1967). The static 91Zr NMR spectrum of the central transition of A13Zr (Figure 8.35E) shows a Knight shift of 40 ppm, compared with the value of 3500 ppm for zirconium metal (Bastow et al. 1998b). This small Knight shift is thought to result from a fortuitous cancellation of the positive s-contact and d-orbital terms by the negative d-core polarisation term. Relatively narrow 91Zr NMR resonances (11-16 kHz) have been reported in cubic ZrCo and ZrC and tetragonal ZrH2 (Bastow et al. 1992). The spectra of the carbide and hydride show evidence of weak subsidiary structure which is probably due to structural defects. The values of the Knight shift and T1 for ZrH2 have been used in an analysis of the density of states in that material (Zogal et al. 1991). Reported values of the solid state 91ZrNMR interaction parameters for zirconium compounds are collected in Table 8.12.

8.3.10

95Moa n d 97MoNMR

The resonance frequencies of the isotopes 95Mo and 97Mo are very similar, differing by only 2%, but the sensitivity of 95M0 and its significantly smaller second-order quadrupolar broadening make it the preferred Mo NMR nucleus even though 97M0 has the advantage of relaxing much faster in cases where quadrupole relaxation is dominant.

NMR of Low- 7 Nuclides

517

Table 8.12. 91ZrNMR interaction parameters of zirconium compounds.

Compound

~iso* (ppm)

XQ (MHz)

"q

Reference

ZrO2 (monoclinic) ZrO2 (tetragonal) ZrO2 (orthorhombic) BaZrO3

7 12 N.D. 0

23.1 19.1 17 ~ 0-0.05

--~0.1 0 0.8 ~ 0

SrZrO3

- 30*, - 12 - 29 --~0 - 285 -- 260 --~0 - 374 - 341 - 375 - 359 N.D. - 399 - 454 3292 3377 40 127 2262, 2550 t

N.D., 0.678 N.D. 14.6 1.288 20.47 11.3 29.4 0.318 1.591 0.637 0.848 53.7 2.682 1.481 18.2 0 7.3 0 --~0

N.D., 0.6 N.D. 0.15 0.5 0 0 0.70 0.9 0.2 0 0 0.30 0 0 0 0 -

Bastow & Smith (1992) Bastow & Smith (1992) Bastow & Smith (1992) Dec et al. (1993), Hartman et al. ( 1991) Dec et al. (1993), Hartman et al. (1991) Hartman et al. (1991) Bastow et al. (1994a) Hartmann & Scheler (1995) Bastow (1990) Bastow (pers. comm). Bastow et al. (1996) Hartmann & Scheler (1995) Hartmann & Scheler (1995) Hartmann & Scheler (1995) Hartmann & Scheler (1995) Bastow (1994) Hartmann & Scheler (1995) Hartmann & Scheler (1995) Bastow et al. (1992) Bastow et al. (1992) Bastow et al. (1998b) Bastow et al. (1992) Bastow et al. (1992), Zogal et al. (1991)

Ba2ZrO4 NaZrO3 CaZrO3 ZrSiO4 Zr(WO4)2 Na2ZrSiO3 CuZrz(PO4)3 RbZrz(PO4)3 AgZr2(PO4)3 NaZr2(PO4)3 ZrF4 Cs2ZrF6 Li2ZrF4 Zr metal ZrCo A13Zr ZrC ZrH2

* shifts withrespectto BaZrO3whichis 208.1ppm fromCp2ZrBr2,* indicatesthe peakposition, , referencedto RbC1solutionand scaledfromthe magneticmomentof 85Rb.

This is borne out by relaxation m e a s u r e m e n t s which have been m a d e for both isotopes in solid Na2MoO4. Q u a d r u p o l e r e l a x a t i o n is d o m i n a n t in this c o m p o u n d , and the values of T1 for 95Mo and 97Mo are 132 s and 1.1 s respectively (Bastow 1998). This was also borne out in an early solid state N M R study of anhydrous cubic NazMoO4 for which the spectra of both 95Mo and 97Mo were collected. The similarity in the spectra indicated that the quadrupole effects are small, but the 97Mo data took a p p r o x i m a t e l y 6 times longer to acquire to a c o m p a r a b l e signal/noise level (Lynch and Segel 1972). The chemical shift reference for M o N M R is normally aqueous Na2MoO4 solution, but the sharp resonance from Mo(CO)6 w h i c h can be o b s e r v e d in a single scan w o u l d m a k e this a g o o d secondary reference c o m p o u n d for 95Mo. The static and M A S 95Mo N M R spectra of MoO3 have been reported by Bastow (1998) and by Edwards et al. (1990) who used an isotopically-enriched sample. The shape of the spectrum determined by Bastow indicates that C S A dominates over the q u a d r u p o l e interaction, while the M A S spectrum, f r o m w h i c h the C S A has b e e n

518


eliminated, shows a clear second-order quadrupolar lineshape capable of direct simulation (Figure 8.36A). The resulting quadrupolar parameters are different from those of Edwards et al. (1990), which were derived from a simulation of the static powder pattern, for which the principal values of the CSA tensor and the angular orientation parameters had to be supplied. This discrepancy illustrates some of the difficulties which can arise with spectral simulations. The static 95Mo NMR spectra of a number of inorganic molybdates (Mastikhin et al. 1988) show that the XQ values of the alkali molybdates are generally approximately zero, but that over the whole series of compounds, • increases with increasing structural distortion. However, it should be noted that in determining XQ from the lineshapes, Mastikhin et al. (1988) generally did not take account of the CSA which can be very large, as has subsequently been shown. In compounds with small quadrupole interactions for which ~iso could be most precisely determined, the 95Mo shifts tend to become more positive with increasing Mo-O bondlength. The three polymorphs of K2MoO4 can readily be distinguished on the basis of their • and 8iso values (Table 8.13). A 95Mo MAS NMR study of Na2MoO4.2H20 has shown that the dipolar coupling can be removed under MAS conditions, giving a well-defined second-order quadrupolar lineshape from which XQ and -q can be deduced (Eichele et al. 1997). Knowledge of these parameters then allows the span of the CSA (about 200 ppm) to be estimated from the static spectrum.

A

MoO3

B

C

M02C

MoSe2 ~

/~Jl simulated

imulated

J 5

,,

0 -5 -10 Frequency (kttz)

t

I

i

I

,

'

100 0 -100 Frequency (kttz)

f-

I

~served i

.i

,

I

,l

'

200 0 -200 Frequency (kttz)

Figure 8.36. A selection of 95MoNMR spectra of molybdenum compounds. A. Observed MAS spectrum of MoO3 (lower) with simulation (upper). B. Powder lineshapes of the central transition of MoSe2 (upper) and MoS2 (lower). C. Observed powder lineshape of the central transition of Mo2C (lower) with the simulation (upper). From Bastow (1998), by permission of Elsevier Science.

519


Table 8.13. 95Mo NMR interaction parameters of molybdenum compounds. Compound

~iso*(ppm)

XQ (MHz)

qq

Reference

MoO3

- 1 5 0 , - 114 *, - 30 +

LizMoO4 NazMoO4

- 72 - 35, 0 +, - 32.9

N.D., 3.49, 2.85 "~ 0 --~ 0

N.D., 0.99, 0.32 N.D. N.D.

NazMoO4.2H20

4 t, 8

ot-KzMoO4 ~-KzMoO4 ~-KzMoO4

- 24 12 2 - 25 - 122 35 - 45 108, 151 82, 8

N.D., 1.15 1.5 1.3 --~0 --~0 --~ 0 2.7 1.6 1.9, 2.05 --~0

N.D., 0.82 N.D. N.D. 0 N.D. N.D. N.D. N.D. N.D., 0.20 N.D.

Mastikhin et al. (1988), Edwards et al. (1990), Bastow (1998) Mastikhin et al. (1988) Mastikhin et al. (1988), Lynch & Segal (1972). Machida & Eckert (1998) Eichele et al. (1997), Mastikhin et al. (1988) Mastikhin et al. (1988) Mastikhin et al. (1988) Mastikhin et al. (1988) Mastikhin et al. (1988) Mastikhin et al. (1988) Mastikhin et al. (1988) Mastikhin et al. (1988) Mastikhin et al. (1988) Machida & Eckert (1998), Kautt et al. (1976) Edwards et al. (1990) Machida & Eckert (1998) Machida & Eckert (1998)

Cs2MoO4

CsLiMoO4 CaMoO4 BaMoO4 PbMoO4 AgzMoO4 A12(MoO4)3 Ag2Mo207 NazMo207 Td Oh (NH4)zMo207 Td Oh (NH4)6Mo7Oza.4H20

H3PMo1204o.xH20 Mo(CO)6

- 300 t 63 -74 - 177 N.D. N.D. 4t

N.D. N.D. N.D. N.D. 2.44 3.41 (2.98, 5.76, 3.73) 100, (468, - 29) N.D. - 1857, - 1852, N.D. 1854 0.141. 0.091, 0.0893 -

MoS2 MoSe2 Mo2Se4 MoSi MozC

- 940 + - 1000 + 5885 + - 2100 + 900 +

6.2 5.7 --~0 2.928 6.1

N.D. N.D. N.D. N.D. 0.47 0.07 (0.73, 0.42, 1.00) N.D. N.D., N.D., N.D., < 0.1, 0.142, 0.151

0 0 N.D. 0 0.98

Edwards et al. (1990) Edwards et al. (1990) Mastikhin et al. (1988) Eichele et al. (1997), Mastikhin et al. (1988), Edwards et al. (1990), Nolle (1977), Shirley (1987), Vosegaard et al. (1999a) Bastow (1998) Bastow (1998) Bastow (1998) Bastow (1998) Bastow (1998)

* shift with respect to aqueous NazMoO4 solution, except values marked + for which cubic anhydrous NazMoO4 was taken as zero. t denotes the peak position, $ denotes the position of the most intense singularity. Parenthesis indicate multiple sites.

A n important consideration in applying 95M0 N M R to practical p r o b l e m s is the ability o f the t e c h n i q u e to d i s t i n g u i s h b e t w e e n M o O 4 and M o O 6 units. 95M0 N M R studies of a series o f i s o t o p i c a l l y e n r i c h e d p o l y o x o m o l y b d a t e c o m p o u n d s similar to t h o s e u s e d as h y d r o d e s u l p h u r i s a t i o n catalysts s h o w e d that at a r e l a t i v e l y l o w field (9.4 T) and l o w

520


MAS speeds (3-4 kHz) unambiguous deconvolution is extremely difficult where the spectra consist of strongly overlapping resonances from different sites (Edwards et al. 1990). The corresponding static spectra are hampered by the comparable magnitude of the second-order quadrupole and CSA effects. Extension of this work to other polyoxomolybdate compounds suggests that where both octahedral and tetrahedral Mo-O species are present they can be distinguished by 95Mo NMR, since the tetrahedral sites give much narrower resonances than the octahedral (Edwards et al. 1990a). The complex lineshapes arising from the presence of inequivalent octahedral sites can be fitted by multiple components which often reveal differences in the quadrupole and asymmetry parameters for the different sites related to the structural distortion of those sites (Edwards et al. 1990a). Cross polarisation from ~H to 95Mo has been shown to produce enhancements of 66-86% of the theoretical maximum and reliable second-order powder lineshapes in isotopically-enriched compounds such as (NH4)6MovOz4.4H20 and (Bu4N)zMo207 which contain suitable proton sources (Edwards and Ellis 1990). The optimum contact times were found to be 20-30 ms and the value ofT~ 0 for molybdenum is long by comparison with the other relaxation processes, making this a suitable candidate for CP. An important application of molybdenum as a surface species in heterogeneous catalysis has led to a number of 95Mo NMR studies of model molybdenum catalyst compounds (Edwards et al. 1990, 1990a) and compounds in the series MoOx-Al203 (Edwards et al. 1990). 95Mo NMR suggests that freshly prepared uncalcined Mo-A1 catalysts contain both tetrahedral and octahedral Mo species adsorbed on the alumina surface, with [Mo7024] 6+ and AI2(MoO4)3 possibly also occurring at higher molybdenum concentrations. Calcination of the catalyst produces a "MoO3-1ike phase" consisting of polymerised tetrahedral and octahedral polyoxomolybdenum species formed by condensation reactions with the surface hydroxyl groups (Edwards et al. 1990). 95Mo solid-state NMR has also been used to study the effects of cobalt, caesium and potassium on MoO3-A1203 catalysts (Edwards and Ellis 1991). The molybdenum carbonyl Mo(CO)6 has been extensively studied by solid state 95Mo NMR (Eichele et al. 1997, Mastikhin et al. 1988, Edwards et al. 1990, Nolle 1977, Shirley 1987, Vosegaard et al. 1999). Because the quadrupolar interaction in this compound is so small, all the transitions are readily observed in a one-pulse experiment, allowing accurate determination of all the interaction parameters from the central and satellite transitions and making use of both static and MAS spectra. The principal components of the CSA tensor ( - 1843, - 1855 and - 1865 ppm) have been derived from the static central transition, while the satellite transitions have provided information about the quadrupolar parameters. The MAS spectra of this compound also show weak peaks from a 68 Hz ~J-coupling between 95Mo and 13C (Eichele et al. 1997). Even more precise information about the relative tensor orientation in Mo(CO)6 has been provided by a single-crystal study using a two-axis goniometer probe (Vosegaard et al. 1999a).

521

NMR of Low-7 Nuclides

Other Mo compounds studied by 95Mo NMR include MoSe2 and Mo3Se4 which can be distinguished from each other in mixtures of the 2 phases, since Mo3Se4 shows a large shift and relatively rapid relaxation (< 1 s) due to conduction electron effects (Bastow 1998). The compound MoS2 shows a static 95Mo lineshape with a similar shift to MoSe2 but dominated by quadrupolar effects (Figure 8.36B), as is also the case with the 95Mo NMR spectrum of MoC2 (Figure 8.36C) which is not narrowed by magic angle spinning (Bastow 1998). The satellite transitions in the 95Mo spectrum of MoSi have been used to deduce the value of • The small negative Knight shift in this compound by comparison with that of Mo metal (+ 6100 ppm) has been explained in terms of the possible cancellation of the conventional positive Knight shift by a negative shift arising from core-polarisation effects (Bastow 1998). 95Mo NMR has been used to study a series of ionically-conducting glasses in the system AgI-Ag20-MoO3. The spectra suggest that only tetrahedral monomeric orthomolybdate anions MoO42-, evidenced by their single sharp peak at about 70 ppm, are present in glasses of composition Ag20/MoO3 = 1, whereas glasses with compositions Ag20/MoO3 < 1 show an additional broader 95Mo resonance at about - 100 to - 120 ppm (Figure 8.37B) interpreted as a polymeric species containing linked MoO4 tetrahedra and MoO6 octahedra probably similar to the chain units present in crystalline Na2Mo207 (Figure 8.37A) (Machida and Eckert 1998). A

B

Ag2MoO4

(e)

Na2MoO4 @ ~0

(a) t ....

2000

I ....

I ....

i ....

I''

"' I '"'

'1 ....

I ' ;-~ i

| ....

'/"' ' ' ' i ' '

""1 ....

I'

~ ' ~'"|"' ' ' ' i . . . .

0 -2000 2000 0 95Mo shift ~ p m ) w.r.t. Na2MoO4 soln.

I' ;";i

-2000

Figure 8.37. 95MoMAS NMR spectra of A. a series of model molybdenum compounds containing Mo in various environments and B. a series of glasses in the system AgI-Ag20-MoO3 with Ag20/MoO3 < 1. Note the growth of the broad feature to the right of the sharp tetrahedral MoO4 resonance with increasing MoO3 content, possibly due to a polymeric chain unit as in Na2Mo207. From Machida and Eckert (1998), by permission of Elsevier Science.

522 8.3.11


135Baand 137BaN M R

Although under the definition of low- 7 nuclei adopted in this chapter only one of the Ba nuclei (135Ba) falls into this category, both will be discussed here. Both nuclei are spin I - q/2 and have small gyromagnetic ratios (see Chapter 1, Table 1.2). Although both nuclei have been used in NMR studies of isotopically-enriched samples, the slightly greater natural abundance of 137Ba (11.32% by comparison with 6.6% for ~35Ba) makes it more favourable for studying unenriched samples. Narrow 137Ba spectra have been recorded for compounds in which the Ba is in a highly symmetrical environment. Thus, the Ba site in cubic BaO has a local octahedral symmetry and is unaffected by second-order quadrupole effects and its MAS NMR spectrum shows a single sharp resonance (Figure 8.38) (Dec et al. 1993, MacKenzie and Meinhold 2000). The 137Ba MAS NMR spectrum of cubic BaZrO3 shows a similarly sharp resonance at 279 ppm with respect to aqueous BaCI2, arising from its symmetrical Ba site (Figure 8.38), suggesting that this compound would be a useful solid secondary reference material (Dec et al. 1993, MacKenzie and Meinhold 2000). By contrast, the Ba ion in BaTiO3 is displaced from the centre of its coordination polyhedron resulting in a relatively broadened 137Ba MAS NMR spectrum containing a resolvable quadrupolar lineshape (Figure 8.38). Static 137BaNMR spectra of BaTiO3 recorded over a temperature range (118-133~ show that as the temperature is lowered to the onset of the cubic-to-tetragonal phase transition at the Curie point, the intensity of the 137Ba NMR signal decays abruptly to zero. The lack of observable NMR intensity below the Curie point reflects the greater electric field gradient (efg) associated with increased Ba site distortions in the tetragonal phase (Forbes et al. 1987). A single-crystal ~37Ba study of BaTiO3 has yielded values of Xo and the axially anisotropic component of the chemical shift tensor. The change in the second-order shift with temperature up to the Curie point has also been shown to relate to the temperature-dependence of the spontaneous polarisation (Bastow 1989). Analysis of single-crystal 135Ba and 137Ba NMR data for tetragonal BaTiO3 in terms of a polarisable point multipole model has proved unsatisfactory, possibly because the quadrupole moment was not taken into account (Sommer et al. 1990). More recently the 137Ba NMR interaction parameters of the non-cubic phases of BaTiO3 have been determined for multidomain crystals (Taye et al. 1999) and strain-free polycrystalline specimens (Bastow and Whitfield 2001). Between 278 and 393 K the stable phase of BaTiO3 has tetragonal symmetry, becoming orthorhombic between 193 and 278 K. Below 193 K the stable phase is ferroelectric with rhombohedral symmetry. The recent measurements of the NMR parameters of these phases in multidomain crystals and polycrystalline samples (Table 8.14) are in reasonable agreement and clearly reflect the changes in symmetry in the various phases. In most of the other inorganic Ba compounds of interest as ceramic materials, the Ba occurs in irregular coordination polyhedra giving very broad lineshapes which are

523


Table 8.14. 137Bainteraction parameters of barium compounds. Compound

~iso*

BaO

748, 760** 279,279**

BaZrO3

XQ (MHz)

BaTiO3

394** 395,417

2.829

BaTiO3 (tetrag)

414"*

2.85, 2.86

(orthorhombic)

409**

2.30, 2.28

(rhombohedral)

409**

2.15, 2.06

Ba(OH)2.8H20 BaCO3 BaA1204 BazSiO4

N.D. 11.5 7.8 N.D.

xl

Reference

MacKenzie & Meinhold (2000), Dec et al. (1993) MacKenzie & Meinhold (2000), Dec et al. (1993) 0.3 Dec et al. (1993), Forbes et al. (1987), Bastow (1989), MacKenzie & Meinhold (2000) 0,0 Bastow & Whitfield (2001), Taye et al. (2000) 0.85, 0.98 Bastow & Whitfield (2001), Taye et al. (2000) 0,0 Bastow & Whitfield (2001), Taye et al. (2000) N.D. MacKenzie & Meinhold (2000) 0.3 MacKenzie & Meinhold (2000) 0.4 MacKenzie & Meinhold (2000) N.D. MacKenzie & Meinhold (2000)

BaAlzSi208 YBazCu307

- 260, - 7 82 1070 306 - 100, - 420 82 702, 401 82 2563

N.D. 14

N.D. 0.94

Ba acetate BaFC1 BaFBr

- 100 82 -

N.D. 1.7 17.0

N.D. 0 0

MacKenzie & Meinhold (2000) MacKenzie & Meinhold (2000), Shore et al. (! 992) MacKenzie & Meinhold (2000) Bastow & Stuart (1996) Bastow & Stuart (1996)

9shiftwithrespectto aqueousBaC12 9* originallyreportedwithrespectto BaZrO3 82 approximatepositionof the singularitiesof a possiblequadrupolarlineshape

in some cases too featureless to be simulated (as in Ba(OH)2, barium acetate, BaSi204 and BaAlzSi2Os). In other compounds (BaCO3, BaAI204), sufficient detail has been resolved to allow a spectral simulation to be made (Figure 8.38) (MacKenzie and Meinhold 2000). An insufficient number of values of XQ have been reported in Ba compounds to determine whether a relationship exists with geometrical parameters such as bond length or bond angle, but an apparent relationship has been observed in a limited number of cases between the centre-of-gravity (cog) of the Ba resonance and the coordination n u m b e r of the Ba polyhedron, reflecting the shielding at the Ba nucleus (MacKenzie and Meinhold 2000). 137Ba N M R has been used in conjunction with 27A1 and 29Si N M R to study the thermal evolution of crystalline celsian, BaAlzSi2Os, from gel precursors. The results indicate that although the characteristic elements of the tetrahedral aluminosilicate feldspar framework begin to form at quite low temperatures, migration of the larger Ba ion into the celsian sites is slower and requires higher temperatures (MacKenzie and Kemmitt 1999a).

524


B a ~ _ ~

BaAI204

BaTiO3 observed

rved

~~/

mulated

simulated , L

,

i

600

i

400

l

t_

!

2000

200

_

i

'

0

9

-2000

BaCO3/~A rved

.aa !

2000

t

0

j !..

~

i

I

9

,

t.

2000 -2000 2000 0 -2000 137Ba shift (ppm) w.r.t. BaCI2 soln.

ulatd 0

-2000

Figure 8.38. A selection of 137BaMAS NMR spectra of inorganic compounds, illustrating Ba in highly symmetric sites (as in BaO, BaZrO3 and BaTiO3) and in more distorted sites, some of which show sufficient second-order quadrupolar lineshape to be simulated (as in BaAI204 and BaCO3). From MacKenzie and Meinhold (2000), by permission of the copyright owner.

High-Tc superconducting ceramics such as YBa2Cu307 have been the subject of intense investigation since their discovery. Solid-state NMR studies of these compounds include determinations of the 135Ba and 137Ba NMR spectra of isotopicallyenriched samples, which have proved to be dominated by quadrupolar effects. The 135Ba and 137Ba spin-echo lineshapes of magnetically-aligned YBazCu307 in both the normal and superconducting states have been determined as a function of temperature and field dependence (Shore et al. 1992). In the superconducting state, the resonances broaden and shift to lower frequency and the 137Ba relaxation rates decrease abruptly, suggesting that the barium sites more closely reflect the behaviour of the plane rather than the chain copper sites (Shore et al. 1992). The temperature dependences of the 135Ba and 137Ba relaxation times have also been measured at lower fields for isotopically-enriched samples of differing oxygen content. Unlike the temperature dependence of the Y and O atoms in these compounds which show Korringa behaviour (relaxation rate increasing linearly with temperature), the temperature dependence of the Ba shows non-Korringa behaviour in the normal state, and is similar to that of


525

the Cu(2) sites but two orders of magnitude smaller (Yakubowskii et al. 1992). The quadrupole frequency determined for YBazCu307 by Shore et al. (1992) and confirmed by the NQR results of Yakubowskii et al. (1992) has been successfully used to simulate a reasonable approximation of the MAS lineshape (MacKenzie and Meinhold 2000). The barium fluorohalides BaFC1 and BaFBr are of practical interest as matrices for X-ray storage phosphors. The 137Ba NMR spectra of both compounds have been reported and their NMR interaction parameters determined (Bastow and Stuart 1996). The room temperature • value of BaFC1 is relatively small but increases rapidly with temperature, possibly due to variations in the thermally averaged structure about the Ba nucleus. The room temperature XQ value of BaFBr is larger by a factor of 10 than that of BaFC1, and its 137Ba linewidth was such that the spectrum had to be determined by the stepped-frequency method, by contrast with the static BaFC1 spectrum which was sufficiently narrow that the whole transition could be excited by ordinary pulse spectroscopy (Bastow and Stuart 1996).

8.3.12 Other miscellaneous low-), nuclei A small number of reports have appeared of the NMR spectra of other low-~/nuclei. These include 53Cr NMR which has been used in a study of the ferromagnetic materials CdCrzS4 and CuCr2Se4. Information about the magnetic hyperfine interactions in these compounds was derived from the echo formed by multiple quantum effects (Abelyashev et al. 1988). 61Ni NMR has been used to examine the approximately equiatomic alloy Ni49.6A15o.4, which showed a Knight shift of 1220 ppm at 9.4 T (Bastow et al. 1997), compared with a value determined at the lower field of 1.6 T of 1890 ppm (Drain and West 1965). The discrepancy between these two values illustrates the difficulty of making such measurements at lower fields and the critical dependence of the calculated diamagnetic position on the value assumed for the magnetic moment. 73Ge NMR is subject to great difficulty, and is seldom studied. Recently reported spectra from single crystal germanium with differing isotope contents show quadrupolar lineshapes arising from local lattice distortion related to isotopic disorder (Verkhovskii et al. 2000). 99Ru and l~ NMR spectra have been reported in RtlO2, SrRuO3 and SrzRuO4 which exhibit a variety of magnetic and superconducting properties. The NMR spectrum of ruthenium metal shows all the transitions, from which a value of 6700 ppm was deduced for the isotropic Knight shift. Values of XQ of 1.93 and 11.2 MHz were determined for 99Ruand l~ respectively in ruthenium metal, with -q = 0 for both nuclei (Mukuda et al. 1999). The ratio of these 2 XQvalues agrees exactly with the ratio of the quadrupole moments of the 2 isotopes. Only the central transition of 99Ru could be

526


detected in RuO2, from which it was determined that XQ = 21.1 MHz and xl = 0.74 (Mukuda et al. 1999).

REFERENCES

Abart, J., Socher, W. & Voitlander, J. (1982) Z. Naturforsch. A, 37, 1030. Abelyashev, G.N., Berzhanskij, V.N., Sergeev, N.A. & Fedotov, Y.V. (1988) Phys. Lett. A, 133,263. Akai, T., Martin, S.W. & Borsa, F. (2000) Phys. Rev. B, 63, 024303. Alloul, H., Mendels, P., Collin, G. & Monod, P. (1988) Phys. Rev. Lett., 61,746. Alloul, H., Mendels, P., Casalta, H., Marucco, J.F. & Arabski, J. (1991) Phys. Rev. Lett., 67, 3140. Alloul, H., Holczer, K., Yoshinari, Y. & Klein, O. (1994) Physica C, 235-240, 2509. Apostol, M. (1996) J. Phys. Chem., 100, 3175. Apostol, M., Goze, C., Rachdi, F., Mehring, M. & Fischer, J.E. (1996) Solid State Commun., 98, 253. Auler, T., Horvatik, M., Gillet, J.A., Berthier, C., Berthier, Y., Carretta, P., Kitaoka, Y., Segransan, P. & Henry, J.Y. (1999) Physica C, 313, 255. Ba, Y., Kao, H-M., Grey, C.P., Chopin, L. & Gullion, T. (1998) J. Mag. Reson., 133, 104. Bailey, W.C. & Story, H.S. (1974) J. Chem. Phys., 60, 1952. Balakrishnan, G., Dupree, R., Farnan, I., Paul, D.McK. & Smith, M.E. (1988) J. Phys. C, 21, L847. Balakrishnan, G., Caves, L.W.J., Dupree, R., Paul, D.McK. & Smith, M.E. (1989) Physica C, 161, 9. Barnes, R.G., Torgeson, D.R., Bastow, T.J.M., West, G.W., Seymour, E.F.W. & Smith, M.E. (1989) Z. Phys. Chem. Neue Folge, 164, $867. Bastow, T.J. & Stuart, S.N. (1988) Physica Status Solidi (b), 145, 719. Bastow, T.J. (1989) J. Phys. Condensed Matter, 1, 4985. Bastow, T.J. & Stuart, S.N. (1989) J. Phys. Condensed Matter, 1, 4649. Bastow, T.J. (1990) J. Phys. Condensed Matter, 2, 6327. Bastow, T.J. & Stuart, S.N. (1990) Z. Naturforsch. A, 45, 459. Bastow, T.J. (1991) Solid State Commun., 77, 547. Bastow, T.J. (1991a) J. Phys. Condensed Matter, 3, 753. Bastow, T.J. (1991b) J. Chem. Soc. Faraday Trans., 87, 2453. Bastow, T.J., Segel, S.L. & Jeffrey, K.R. (1991) Solid State Commun., 78, 565. Bastow, T.J. & Stuart, S.N. (1991) Chem. Phys. Lett., 180, 305. Bastow, T.J. & Smith, M.E. (1992) Solid State Nucl. Mag. Reson., 1, 165. Bastow, T.J., Smith, M.E. & Stuart, S.N. (1992) Chem. Phys. Lett., 191, 125. Bastow, T.J. (1994) Z. Natmforsch., 49a, 320. Bastow, T.J., Stuart, S.N., McDugle, W.G., Eachus, R.S. & Spaeth, J.M. (1994) J. Phys. Condensed Matter, 6, 8633. Bastow, T.J., Hobday, M.E., Smith, M.E. & Whitfield, H.J. (1994a) Solid State Nucl. Mag. Reson., 3, 49.


527

Bastow, T.J. & Smith, M.E. (1995) J. Phys. Condensed Matter, 7, 4929. Bastow, T.J. (1996) J. Phys. Condensed Matter, 8, 11309. Bastow, T.J. & Stuart, S.N. (1996) Solid State Nucl. Mag. Reson., 6, 95. Bastow, T.J., Hobday, M.E., Smith, M.E. & Whitfield, H.J. (1996) Solid State Nucl. Mag. Reson., 5, 293. Bastow, T.J., Smith, M.E. & West, G.W. (1997) J. Phys. Condensed Matter, 9, 6085. Bastow, T.J. (1998) Solid State Nucl. Mag. Resort., 12, 191. Bastow, T.J., Massiot, D. & Coutures, J.P. (1998) Solid State Nucl. Mag. Reson., 10, 241. Bastow, T.J., Gibson, M.A. & Forwood, C.T. (1998a) Solid State Nucl. Mag. Reson., 12, 201. Bastow, T.J., Forwood, C.T., Gibson, M.A. & Smith, M.E. (1998b) Phys. Rev. B, 58, 2988. Bastow, T.J. & Celotto, S. (1999) Solid State Commun., 110, 271. Bastow, T.J. & Whitfield, H.J. (1999) Chem. Mater., 11, 3518. Bastow, T.J. (2000) Z. Naturforsch. A, 55, 291. Bastow, T.J., Doran, G. & Whitfield, H.J. (2000) Chem. Mater., 12, 436. Bastow, T.J. & Whitfield, H.J. (2001) Solid State Commun., 117, 483. Batchelder, L.S. & Ragle, J.L. (1980) J. Mag. Reson., 37,469. Battle, P.D., Montez, B. & Oldfield, E. (1988) J. Chem. Soc. Chem. Commun., 584. Becker, K.D. (1978) J. Chem. Phys., 68, 3785. Becker, K.D. &von Goldammer, E. (1990) Chem. Phys., 48, 193. Bellot, P.V., Trokiner, A., Zhdanov, Y. & Yabukovskii, A. (1998) J. Chem. Phys. PhysicoChem. Biol., 95, 280. Blinc, R., Seliger, J., Zager, V., Apih, T., Dolinsek, J., Warharek, H., Fuith, A. & Schranz, W. (1990) Phys. Rev. B, 42, 8125. Blinc, R., Dolinsek, J., Apih, J., Schranz, W., Fuith, A. & Warharek, H. (1995) Solid State Commun., 93, 609. Borsa, F., Carretta, P., Costi, M., Nehrke, K., Rigamonti, A. & Tabak, F. (1997) Nouve Cimento D, 19, 1199. Brom, H.B. & Alloul, H. (1991) Physica C, 185, 1789. Bryant, R.G., Ganapathy, S. & Kennedy, S.D. (1987) J. Mag. Resort., 72, 376. Carretta, P. & Costi, M. (1992) Phys. Rev. Lett., 68, 1236. Chadwick, A.V., Poplett, I.J.F., Maitland, D.T.S. & Smith, M.E. (1998) Chem. Mater., 10, 864. Chung, S.H., Jeffrey, K.R., Stevens, J.R. & Borjesson, L. (1990) Phys. Rev. B, 41, 6154. Daunch, W.A. & Rinaldi, P.L. (1996) J. Mag. Reson. A, 123, 219. Dec, S.F., Davis, M.F., Maciel, G.E., Bronnimann, C.E., Fitzgerald, J.J. &Han, S. (1993) Inorg. Chem., 32, 955. Derighetti, B., Hafner, S., Marxer, H. & Rager, H. (1978) Phys. Lett., 66A, 150. Dickson, E.M. & Seymour, E.F.W. (1970) J. Phys. C, 3, 666. Differt, K. & Messer, R. (1980) J. Phys. C, 13, 717. Dougan, P.D., Sharma, S.N. & Williams, D.L. (1969) Can. J. Phys., 47, 1047. Drain, L.E. & West, G.W. (1965) Philos. Mag., 11, 1061. Dumelow, T. & Riedi, P.C. (1987) Hyperfine Interact., 34, 407. Dupree, R. & Smith, M.E. (1988) Chem. Phys. Lett., 148, 41. Dupree, R. & Smith, M.E. (1988a) J. Chem. Soc. Chem. Commun., 1483. Dupree, R., Gencten, A. & Paul, D.McK. (1992) Physica C, 193, 81. Dupree, R., Howes, A.P. & Kohn, S.C. (1997) Chem. Phys. Lett., 276, 399.

528


Ebert, H., Abart, J. & Voitlander, J. (1986) J. Phys. F, 16, 1287. Eckert, H. & Yesinowski, J.P. (1986) J. Amer. Chem. Soc., 108, 2140. Edwards, J.C., Adams, R.D. & Ellis, P.D. (1990) J. Amer. Chem. Soc., 112, 8349. Edwards, J.C., Zubieta, J., Shaikh, S.N., Chen, Q., Bank, S. & Ellis, P.D. (1990a) Inorg. Chem., 29, 3381. Edwards, J.C. & Ellis, P.D. (1990) Mag. Reson. Chem., 28, $59. Edwards, J.C. & Ellis, P.D. (1991) Langmuir, 7, 2117. Eichele, K., Wasylishen, R.E. & Nelson, J.H. (1997) J. Phys. Chem. A, 101, 5463. Ekstr6m, T.C., MacKenzie, K.J.D., Ryan, M.J., Brown, I.W.M. & White, G.V. (1997) J. Mater. Chem., 7, 505. Endo, K., Kenmotsu, M., Frye, J., Hondo, K., Kitagawa, S. & Deguchi, K. (1996) J. Phys. Chem. Solids, 57, 1609. Ermolaev, K. & Fung, B.M. (1999) J. Chem. Phys., 110, 7977. Fijolek, H.G., Oriskovich, T.A., Benesi, A.J., Gonzalez-Duarte, P. & Natan, M.J. (1996) Inorg. Chem., 35, 797. Fiske, P.S. & Stebbins, J.F. (1994) Amer. Mineralogist, 79, 848. Fiske, P.S., Stebbins, J.F. & Farnan, I. (1994) Phys. Chem. Minerals, 20, 587. Florian, P., Massiot, D., Humbert, G. & Coutures, J-P. (1995) Comptes Rendus Acad. Sci. Paris Sdrie lib, 320, 99. Florian, P., Gervais, M., Douy, A., Massiot, D. & Coutures, J-P. (2001) J. Phys. Chem. B, 105, 379. Forbes, C.E., Harwood, W.B., Cipollini, N.E. & Lynch, J.F. (1987) J. Chem. Soc. Chem. Commun., 433. Frisch, R.C. & Forman, R.A. (1968) J. Chem. Phys., 48, 5187. George, A.M. & Stebbins, J.F. (1998) Amer. Mineralogist, 83, 1022. Grey, C.P., Smith, M.E., Cheetham, A.K., Dobson, C.M. & Dupree, R. (1990) J. Amer. Chem. Soc., 112, 4670. Grey, C.P., Eijkelenboom, A.P.A.M. & Veeman, W.S. (1995) Solid State Nucl. Mag. Reson., 4, 113. Haller, M., Herler, W.E., Lutz, O. & Nolle, A. (1980) Solid State Commun., 33, 1051. Harazono, T. & Watanabe, T. (1997) Bull. Chem. Soc. Japan, 70, 2383. Harazono, T., Yokota, E., Uchida, H. & Watanabe, T. (1998) Bull. Chem. Soc. Japan, 71,825. Harazono, T., Yokota, E., Uchida, H. & Watanabe, T. (1998a) Bull. Chem. Soc. Japan, 71, 2796. Harazono, T., Adachi, R., Kijima, N. & Watanabe, T. (1999) Bull. Chem. Soc. Japan, 72, 2655. Harazono, T., Adachi, R., Shimomura, Y. & Watanabe, T. (2001) Phys. Chem. Chem. Phys., 3, 2943. Hartman, J.S., Koffyberg, F.P. & Ripmeester, J.A. (1991) J. Mag. Reson., 91,400. Hartmann, P. & Scheler, G. (1995) Z. Naturforsch., 50a, 90. Hayashi, S. & Hayamizu, K. (1990) Bull. Chem. Soc. Japan, 63, 913. Helluy, X., Kummerlen, J., Sebald, A. & Zogal, O.J. (1999) Solid State Nucl. Mag. Reson., 14, 225. Hill, E.A. & Yesinowski, J.P. (1996) J. Amer. Chem. Soc., 118, 6798. Hioki, T., Kotani, M. & Masuda, Y. (1975) J. Phys. Soc. Japan, 39, 958. Hiyama, Y., Woyciesjes, P.M., Brown, T.L. & Torchia, D.A. (1987) J. Mag. Reson., 72, 1.


529

Ida, T., Endo, K., Suhara, M., Honda, K., Kitagawa, S. & Kawabe, H. (1999) Bull. Chem. Soc. Japan, 72, 2061. Jakobsen, H.J., Bildsoe, H., Skibsted, J. & Giavani, T. (2001) J. Amer. Chem. Soc., 123, 5098. Jansen, M. (1987) Ang. Chem. Int. Ed., 99, 1098. Jelinek, R., Stein, A. & Ozin, G.A. (1993) J. Amer. Chem. Soc., 115, 2390. Jeschke, G. & Jansen, M. (1998) Ang. Chem. Int. Ed., 37, 1282. Jeschke, G., Hoffbauer, W. & Jansen, M. (1998) Solid State Nucl. Mag. Reson., 12, 1. Jurga, S., Harbison, G.S., Blumich, B., Spiess, H.W., Fujara, F. & Olinger, A. (1986) Berichte der Bunsen-Gesellschaft Phys. Chem. Chem. Phys., 90, 1153. Kanert, O. & Kolem, H. (1988) J. Phys. C, 21, 3909. Kanert, O., Schulz, H. & Albers, J. (1994) Solid State Commun., 91,465. Kenmotsu, M., Hondo, H., Ohki, H., Ikeda, R., Erata, T., Tasaki, A. & Furukawa, Y. (1994) Z. Naturforsch. A, 49, 247. Karr, C. & Schultz, H.D. (1968) Spectrosc. Lett., 1, 205. Kautt, W.D., Kruger, H., Lutz, O., Maier, H. & Nolle, A. (1976) Z. Naturforsch. A, 31, 351. Khitrin, A.K. & Fung, B.M. (1999) J. Chem. Phys., 111, 8963. Kirkpatrick, R.J., Yu, P., Hou, X. & Kim, Y. (1999) Amer. Mineralogist, 84, 1186. Knight, C.T.G., Turner, G.L., Kirkpatrick, R.J. & Oldfield, E. (1986) J. Amer. Chem. Soc., 108, 7426. Krawietz, T.R., Murray, D.K. & Haw, J.F. (1998) J. Phys. Chem. A, 102, 8779. Kremer, R.K., Mattausch, H.J., Simon, A., Steuemagel, S. & Smith, M.E. (1992) J. Solid State Chem., 96, 237. Labouriau, A. & Earl, W.L. (1997) Chem. Phys. Lett., 270, 278. Lambert, J-F., Prost, R. & Smith, M.E. (1992) Clays Clay Minerals, 40, 253. Larsen, F.H., Lipton, A.S., Jakobsen, H.J., Nielsen, N.C. & Ellis, P.D. (1999) J. Amer. Chem. Soc., 121, 3783. Lee, K. (1968) Phys. Rev., 172, 284. Lim, A.R., Jeong, D.Y. & Park, H-M. (2001) J. Phys. Condensed Matter, 13, 3511. Looser, H. & Brinkmann, D. (1985) J. Mag. Reson., 64, 76. Lucken, E.A.C. (1969) Nuclear Quadrupole Coupling Constants, Academic Press, London. Lutz, O. (1983) NATO ASI Series C, 103, 389. Lynch, G.F. & Segel, S.L. (1972) Can. J. Phys., g0, 567. Machida, N. & Eckert, H. (1998) Solid State lonics, 107, 255. MacKenzie, K.J.D., Meinho|d, R.H., Sherriff, B.L. & Xu, Z. (1993) J. Mater. Chem., 3, 1263. MacKenzie, K.J.D. & Meinhold, R.H. (1993) Thermochim. Acta, 230, 339. MacKenzie, K.J.D. & Meinhold, R.H. (1993a) J. Mater. Sci. Lett., 12, 1696. MacKenzie, K.J.D. & Meinhold, R.H. (1993b) Thermochim. Acta, 230, 331. MacKenzie, K.J.D. & Meinhold, R.H. (1994) J. Mater. Chem., 4, 1595. MacKenzie, K.J.D. & Meinhold, R.H. (1994a) Amer. Mineralogist, 79, 250. MacKenzie, K.J.D. & Meinhold, R.H. (1994b) Amer. Mineralogist, 79, 43. MacKenzie, K.J.D. & Meinhold, R.H. (1994c) Thermochim. Acta, 244, 195. MacKenzie, K.J.D. & Meinhold, R.H. (1994d) Thermochim. Acta, 232, 85.

530


MacKenzie, K.J.D., Meinhold, R.H., McGavin, D.G., Ripmeester, J.A. & Moudrakovski, I. (1995) Solid State Nucl. Mag. Reson., 4, 193. MacKenzie, K.J.D. & Meinhold, R.H. (1996)J. Mater. Chem., 6, 821. MacKenzie, K.J.D. & Meinhold, R.H. (1997) Amer. Mineralogist, 82, 479. MacKenzie, K.J.D. & Kemmitt, T. (1999) Thermochim. Acta, 325, 13. MacKenzie, K.J.D. & Kemmitt, T. (1999a) Thermochim. Acta, 325, 5. MacKenzie, K.J.D. & Meinhold, R.H. (2000) Ceram. Int., 26, 87. MacKenzie, K.J.D., Sheppard, C.M. & McCammon, C. (2000) J. European Ceram. Soc., 20, 1975. MacKenzie, K.J.D., Schmticker, M., Smith, M.E., Poplett, I.J.F. & Kemmitt, T. (2000a) Thermochim. Acta, 363, 181. Markert, J.T., Noh, T.W., Russek, S.E. & Cotts, R.M. (1987) Solid State Commun., 63, 847. Markgrabe, D. & Engelhardt, G. (1999) Chem. Phys. Lett., 300, 701. Mastikhin, V.M., Lapina, O.B. & Maximovskaya, R.I. (1988) Chem. Phys. Lett., 148, 413. Meinhold, R.H. & MacKenzie, K.J.D. (1995) Solid State Nucl. Mag. Reson., 5, 151. Merwin, L.H. & Sebald, A. (1990) J. Mag. Reson., 88, 167. Merwin, L.H. & Sebald, A. (1992) J. Mag. Reson., 97, 628. Merwin, L.H. & Sebald, A. (1992a) Solid State Nucl. Mag. Reson., 1, 45. Mukuda, H., Ishida, K., Kitaoka, Y., Asayama, K., Kanno, R. & Takano, M. (1999) Phys. Rev. B, 60, 12279. Mustarelli, P., Tomasi, C., Quartarone, E., Magistris, A., Cutroni, M. & Mandanici, A. (1998) Phys. Rev. B, 58, 9054. Mustarelli, P., Tomasi, C., Magistris, A. & Cutroni, M. (1998a) J. Non-Cryst. Solids, 232-234, 532. Narath, A. (1967) Phys. Rev., 162, 320. Nieto, P., Dron, R., Thouvenot, R., Zanni, H. & Brivot, F. (1995) Comptes Rendu Acad. Sci. Serie H, 320, 485. Nolle, A. (1977) Z. Phys. A, 280, 231. Nowak, B., Zogal, O.J. & Niedzwiedz, K. (1992) J. Alloys Compounds, 189, 141. Ohno, T., Alloul, H., Mendels, P., Collin, G. & Marucco, J.F. (1990) J. Mag. Mag. Mater., 90-91, 657. Ohno, T., Mizuno, K., Kanashiro, T. & Alloul, H. (1991) Physica C, 185, 1067. Olsen, K.K. & Zwanziger, J.W. (1995) Solid State Nucl. Mag. Reson., 5, 123. Ono, H., Ishimaru, S., Ikeda, R. & Ishida, H. (1997) Chem. Phys. Lett., 275, 485. Ono, H., Ishimaru, S., Ikeda, R. & Ishida, H. (1999) Bull. Chem. Soc. Japan, 72, 2049. Padro, D., Howes, A.P., Smith, M.E. & Dupree, R. (2000) Solid State Nucl. Mag. Reson., 15,231. Pecoul, N., Bourbigot, S. & Revel, B. (1997) Macromol. Symp., 119, 309. Plishke, J.K., Benesi, A.J. & Vannice, M.A. (1992) J. Phys. Chem., 96, 3799. Poplett, I.J.F. & Smith, J.A.S. (1981) J. Chem. Soc. Faraday Trans. 2, 77, 1155. Poplett, I.J.F. & Smith, M.E. (1998) Solid State Nucl. Mag. Reson., 11, 211. Retcofsky, H.L. & Friedel, R.A. (1972) J. Amer. Chem. Soc., 94, 6579. Rigamonti, A., Borsa, F. & Carretta, P. (1998) Rep. Prog. Phys., 61, 1367. Riseman, T.M., Alloul, H., Mahajan, A.V., Mendels, P., Collin, G. & Marucco, J.F. (1994) Physica C, 235, 1593.

NMR of Low- T Nuclides

531

Roberts, N., Wang, R-P., Sleight, A.W. & Warren, W.W. (1998) Phys. Rev. B, 57, 5734. Roos, J., Brinkmann, D., Mali, M., Pradel, A. & Ribes, M. (1988) Solid State Ionics, 28--30, 710. Santos, R.A., Tang, P., Chien, W-J., Kwan, S. & Harbison, G.S. (1990) J. Phys. Chem., 94, 2717. Sanz, J., Herrero, P., Rojas, J.M., Rossignol, S., Reau, J.M. & Tanguy, B. (1995) Solid State Ionics, 82, 129. Sasaki, S., Matsuda, A. & Chu, C.W. (1998) Physica C, 302, 319. Sebald, A. (1994) NMR Basic Princ. Prog., 31, 92. Segel,S.L. (1978)J. Chem. Phys., 68, 330. Segel, S.L. (1981) J. Chem. Phys., 75, 4746. Sham, S. & Wu, G. (1999) Can. J. Chem., 77, 1782. Sham, S. & Wu, G. (2000) Inorg. Chem., 39, 4. Shore, J., Yang, S., Haase, J., Schwartz, D. & Oldfield, E. (1992) Phys. Rev. B, 46, 595. Shirley, W.M. (1987) Z. Phys. Chem., 152, 41. Skibsted, J. & Jakobsen, H.J. (1999) Inorg. Chem., 38, 1806. Sommer, R., Maglione, M. & van der Klink, J.J. (1990) Ferroelectrics, 107, 307. Stebbins, J.F. (1996) Amer. Mineralogist, 81, 1315. Such, K.P. & Lehmann, G. (1988) Chem. Phys. Lett., 143, 463. Sub, J., Torgeson, D.R. & Borsa, F. (1993) Phys. Rev. Lett., 71, 3011. Suzuki, H., Komaru, T., Hihara, T. & Koi, Y. (1971) J. Phys. Soc. Japan, 30, 288 Tansho, M., Wada, H., Ishii, M. & Onoda, Y. (1996) Solid State Ionics, 86-88, 155. Taye, A., Klotzche, G., Michel, D., Mulla-Osman, S. & B6ttcher, R. (1999) J. Phys. Condensed Matter, 11,871. Thompson, A.R. & Oldfield, E. (1987) J. Chem. Soc., Chem. Commun., 27. Torgeson, D.R. & Barnes, R.G. (1967) Bull. Amer. Phys. Soc., 12, 313. Trokiner, A., Lenoc, L., Mikhalev, K., Yakubovskii, A., Lutgemeier, H., Heinmaa, I., Gippius, A., Verkhovskii, S., Goldschmidt, D. & Eckstein, Y. (1994) Physica C, 226, 43. Trokiner, A., Noc, L.L., Yakubovskii, A., Mykhalyov, K.N. & Verkhovskii, S.V. (1994a) J. Chim. Phys. Physico-Chem. Biol., 91, 862. Trokiner, A., Lenoc, L., Yakubovskii, A., Mykhalyov, K.N. & Verkhovskii, S.V. (1994b) Z. Naturforsch. A, 49, 373. Tunstall, D.P., Todd, J.R.M., Arumugam, S., Dai, G., Dalton, M. & Edwards, P.P. (1994) Phys. Rev. B, 50, 16541. Tunstall, D.P. & Webster, W.J. (1991) Supercond. Sci. Technol., 4, $406. Tycko, R. & Opella, S.J. (1987) J. Chem. Phys., 86, 1761. Verkhovskii, S.V., Malkin, B.Z., Trokiner, A., Yakubovskii, A., Haller, E., Ananyev, A., Gerashenko, A., Piskunov, Y., Saikin, S., Tikhomirov, A. & Ozhogin, V. (2000) Z. Naturforsch. A, J. Phys. Sci., 55, 105. Villa, M., Chiodelli, G., Magistris, A. & Licheri, G. (1986) J. Chem. Phys., 85, 2392. Vosegaard, Y., Andersen, U. & Jakobsen, H.J. (1999) J. Amer. Chem. Soc., 121, 1970. Vosegaard, T., Skibsted, J. & Jakobsen, H.J. (1999a) J. Phys. Chem. A, 103, 9144. Weeding, T.L. & Veeman, W.S. (1989) J. Chem. Soc. Chem. Commun., 946. Weiden, N. & Weiss, A. (1974) 18th AMPERE Congress, p.257. Williams, G.V.M., Tallon, J.L., Michalak, R. & Dupree, R. (1998) Phys. Rev. B, 57, 8696.

532


Williams, G.V.M., Tallon, J.L., Quilty, J.W., Trodahl, H.J. & Flower, N.E. (1998a) Phys. Rev. Lett., 80, 377. Wu, J., Boyle, T.J., Shreeve, J.L., Ziller, J.W. & Evans, W.J. (1993) Inorg. Chem., 32, 1130. Wu, G., Kroeker, S. & Wasylishen, R.E. (1995) Inorg. Chem., 34, 1595. Wu, G. (1998) Chem. Phys. Lett., 298, 375. Yakubowskii, A., Egorov, A. & Ltitgemeier, H. (1992) Appl. Mag. Reson., 3,665. Yamada, T. & Asanuma, M. (1965) Phys. Rev. Lett., 15, 695. Yesinowski, J.P. & Hill, E.A. (1999) Solid State NMR Spectroscopy of lnorganic Materials American Chemical Society Symposium Series 717, Ed. Fitzgerald, J.J., p.358. Ylinen, E.E., Kaikkonen, A. & Punkkinen, M. (1997) Solid State Nucl. Mag. Reson., 10, 25. Yoshinari, Y., Alloul, H., Brouet, V., Kriza, G., Holczer, K. & Forro, L. (1996) Phys. Rev. B, 54, 6155. Zogal, O.J., Nowak, B. & Niedzwiedz, K. (1991) Solid State Commun., 80, 601.

Chapter 9

NMR of Other Spin- 1/2 Nuclei 9.1. 9.2.

9.3.

Introduction Abundant High-~/Nuclei 9.2.1 1HNMR 9.2.1.1 Background to Proton Studies in Inorganic Materials 9.2.1.2 Studies of Stoichiometric Protons in Crystalline Materials 9.2.1.3 Non-Stoichiometric Proton Environments in Crystalline and Glassy Materials 9.2.1.4 ~H NMR of Hydrous Glasses 9.2.1.5 Biomineral-Related Materials 9.2.2 ~9FNMR 9.2.2.1 Introduction 9.2.2.2 Simple Inorganic Fluorides 9.2.2.3 More Complex Fluorides 9.2.2.4 Applications to Fluoroapatite Studies 9.2.2.5 Fluorine in Aluminosilicate Minerals and Related Materials 9.2.2.6 Surface Interaction of Fluorine with Silica- and Alumina-Based Materials 9.2.2.7 Fluorine in Alumino- and Gallophosphates 9.2.2.8 Fluorine in Oxygen-Containing Glasses 9.2.2.9 Fluoride Glasses 9.2.2.10 Fluorine in Other Materials 9.2.2.11 Fluorine as a Source of Cross-Polarisation 9.2.2.12 Summary of 19F Shift Trends and Other NMR Properties Dilute or Medium-~ Nuclei 9.3.1 13C NMR 9.3.1.1 13C NMR of Elemental Carbon 9.3.1.2 Silicon Carbide 9.3.1.3 Other Binary Carbides 9.3.1.4 Ternary and Quaternary Carbides 9.3.1.5 Carbonates 9.3.2 lSN NMR

535 536 536 536 539 542 545 550 550 550 551 554 555 556 557 559 559 560 562 562 562 563 563 563 568 570 572 572 574

9.3.3 9.3.4 9.3.5

9.3.2.1 Nitrides 9.3.2.2 Silicon Aluminium Oxynitride Ceramics and Glasses 9.3.2.3 Nitride Ceramics from Polymeric Precursors 9.3.2.4 Nitrates and Nitrites 778e NMR l llCd and l l3Cd NMR l l5Sn, l lVSn and l l9Sn NMR

9.3.5.1 Crystalline Oxygen-Containing Materials 9.3.5.2 Oxide Solid Solutions and Glasses 9.3.5.3 Non-oxide Materials 9.3.6 J23Te and 125Te NMR 9.3.6.1 Crystalline Tellurides 9.3.6.2 Crystalline Tellurites and Tellurates 9.3.6.3 Glassy Tellurium-Containing Materials 9.3.7 129Xe NMR 9.3.8 195pt NMR 9.3.9 199Hg NMR 9.3.10 2~ and 2~ NMR NMR 9.3.11 2~ 9.3.11.1 Correlations between 2~ Chemical Shifts and Structure 9.3.11.2 2~ NMR of Crystalline Lead Compounds 9.3.11.3 2~ NMR of Lead-Containing Glasses 9.3.11.4 2~ in Sol-Gel Prepared Ceramics References

575 576 579 582 583 587 591 591 594 595 598 598 599 601 601 603 604 604 607 607 609 613 615 616

Chapter 9

NMR of Other Spin- 1/2 Nuclei 9.1. INTRODUCTION The most important spin-1/2 nucleus for studies of inorganic materials is undoubtedly 29Si, which is dealt with in Chapter 4. Spin-1/2 nuclei with magnetic moments below 15N were discussed in Chapter 8. In the present chapter solid state NMR studies of other spin-1/2 nuclei that are not dealt with elsewhere are discussed. Spin-l/2 nuclei can be usefully divided into two groups: (i) abundant high-~/nuclei and (ii) dilute or medium-~/nuclei. The group into which a nucleus falls often determines the experimental approach to be employed and also the information that can be extracted from the NMR spectrum. The abundant high-~/nuclei are 1H, 19F and also 31p (dealt with in Chapter 7). These nuclei usually have strong homonuclear dipolar coupling. The extent to which this dipolar coupling is homogeneous varies between the different nuclei. In 1H the small chemical shift range and large coupling often makes the interaction homogeneous. Significant homogeneous interaction can also exist for fluorine although this is often less strongly the case since fluorine experiences a larger chemical shift interaction. For 31p the interactions are rarely strongly homogeneous. The implication of these different interactions is that often MAS at moderate speeds (e.g. 20 kHz) is ineffective for 1H and 19F and a CRAMPS approach can be usefully applied. Although examples of 31p CRAMPS exist, the faster MAS rates now widely available and the typical dipolar couplings experienced by 31p has reduced the need for 3~p CRAMPS. Although the other 2 nuclei experience extremely strong dipolar couplings, these occur more often in organic polymeric materials which are outside the scope of this book. In inorganic materials the protons are usually relatively spatially dilute, decreasing the ~H-1H interaction to the extent that moderate MAS is often sufficient. This is also usually true for 19F although the 19F-19F distance can be quite short in some pure inorganic fluorides, requiring fast MAS for efficient narrowing. As well as cases where spatial dilution reduces the dipolar coupling, MAS narrowing will also be effective for isolated spin pairs and linear chains of spins. The signals from these nuclei are very strong, resulting in high sensitivity. The presence of a strong dipolar coupling also means that information about internuclear distances can potentially be extracted. The dilute or medium-~/spin-l/2 nuclei have relatively small dipolar coupling (especially homonuclear coupling) because of spatial dilution or the small magnetic moment. Thus, even slow MAS is often sufficient to narrow these resonances. The factors which reduce the dipolar coupling unfortunately also lie behind the low 535

536


sensitivity of these nuclei. It is often also true that in many such systems the relaxation is weak, classic examples being diamond and silicon carbide where the T~ can be many hours. For such nuclei the magnetisation often shows stretched exponential relaxation behaviour (Hartman et al. 1994).

9.2. ABUNDANT HIGH-~ NUCLEI

9.2.1 1HNMR 9.2.1.1 Background to proton studies in inorganic materials. An important problem in many hydrous solids is the determination of the speciation of the protons within the structure. As water is added it can (i) remain intact as a water molecule (as structural hydrate water, liquid inclusions or surface-adsorbed species), (ii) form hydroxyl groups, or (iii) take the form of structural acid protons. Water dissolution is a very important scientific problem in a number of branches of science but especially in mineralogy since it is the dominant low density fluid in the Earth's crust, playing a central role in geochemistry. For so-called "nominally anhydrous minerals" (NAMs) the presence of even minor amounts of water can have a significant influence on many of the properties and behaviour of these minerals. The interaction of water with other inorganic solids such as ceramics, glasses and catalysts is often a key scientific and technological problem. Hydrogen species in such cases are invariably bonded to oxygen and can occur in many different forms. Protons can be present in a crystal structure as stoichiometric OH groups, water molecules, or hydronium ions. Hydroxyl groups can also play an important role in substitutions such as OH- ~ F-,

A104- + H + ~ SiO4, H404 6--) SiO4 (hydrogarnet)

(9.1)

Water molecules can occupy vacant cation and anion sites, zeolitic sites, or interlayer regions. Water molecules can also be present on mineral surfaces, in microscopic cracks in the crystal, or in macroscopic fluid inclusions. If the proton content is significant and stoichiometric, diffraction, especially by neutrons, can elucidate the position of hydrogen. However when the hydrogen does not occur regularly throughout the structure and its content is low, diffraction does not usually provide much information. Infra-red (IR) spectroscopy can qualitatively distinguish the different sites but the question of quantitative integrity is often raised. 1H MAS NMR offers an alternative to these approaches. The main drawback anticipated with 1H MAS NMR is the strength of the homogeneous dipolar interaction between the protons, which would render it difficult to achieve significant line narrowing. An approach to overcoming this is to physically dilute the protons by deuterating the materials. It is fortunate, however, that in inorganic

537

NMR of Other Spin -1/2 Nuclei

materials the low hydrogen content often results in the hydrogen-bearing species being relatively isolated from each other so that inhomogeneous line broadening mechanisms dominate. An isolated water molecule can be considered as a special case of an isolated two-spin system where the homonuclear dipolar interaction between the two protons is inhomogeneous (Maricq and Waugh 1979). The interaction between two protons can only be regarded as truly inhomogeneous if each of the dipolar-coupled protons has the same chemical shift tensor in the same orientation. In rigid water molecules the chemical shift tensor orientations will be different for each proton but rapid (on the NMR timescale) 180 ~ flips about the bisector axis render these tensors the same and hence the dipolar coupling inhomogeneous (Yesinowski and Eckert 1987). Modest MAS can then average the interaction even if the residual sidebands are quite broad. A series of studies by Yesinowski and Eckert demonstrated that for crystalline hydrated minerals high-resolution 1H MAS-NMR spectra could be obtained using MAS of 8 kHz with results compared at 200 and 500 MHz. With OH groups there was a steady decrease in the linewidth of the centreband as the spinning speed increased from 2 to 8 kHz (Figure 9.1), suggesting that the residual linewidth has a significant homonuclear dipolar contribution. It was suggested that the density of protons was a crude measure of the proton homonuclear dipolar coupling, and shown that at a field of 4.7 T with MAS spinning speeds of 7-8 kHz, linewidths of -< 1.5 kHz could be obtained for proton densities < 15 atoms/nm 3. By contrast, for diaspore (A1OOH) in which the proton density is 33.9 atoms/nm 3 no narrow signal could be observed (Yesinowski et al. 1988).

N

800

D

D

rl

H20 []

.P....l

[]

600 9J,..l O

~

O

4o0

z

OH O O

2OO

I

0

I

2

I

I

4

I

I

6

I

I

8

Spinning speed (kHz) Figure 9.1. Influence of the MAS speed on the 1H NMR linewidth from OH groups and molecular water in hydroxyapatite. Note the marked effect on the OH protons reflecting the homonuclear dipolar contribution. After Yesinowski and Eckert (1987), by permission of the American Chemical Society.

538


Where the analysis of proton spectra includes wideline interactions, the contribution of the CSA should not be underestimated, especially where the work is carried out at the higher applied magnetic fields now available. Previous studies have determined the 1H CSA values for H20 in deuterium-diluted ice as 34 ppm (Emsley and Pines 1994) and --~ 10 ppm for BaC104.H20 (Tekely et al. 1994). Broadband homonuclear dipolar coupling using the MSHOT-3 sequence combined with both sample rotation and heteronuclear 31p decoupling was used to study the 1H CSA tensor in polycrystalline KHzPO4 (Rasmussen et al. 1999). The chemical shift tensor was characterised by ~iso = 14.9 ppm, a span of 40.8 ppm and a skew of -0.66. The system provides an isolated 31p-1H-31p fragment which can be analysed to determine the absolute orientation of the tensor in the molecular frame even in a powder sample. The large anisotropy reflects strong hydrogen bonding. Another important area studied by solid state ~H NMR is the determination of the surface speciation of adsorbed protons. For example, a number of peaks are observed in the 1H NMR spectra of the various forms of A1203 (Mastikhin et al. 1987). The strong peak observed at 3.4 ppm from o~-A1203 is believed to arise from bulk OH while a smaller peak at - 0 . 2 ppm corresponds to surface hydroxyls. Other peaks observed from the transition aluminas indicate the effect of the differing coordinations of the attached aluminium neighbours. Similar studies have been carried out for silica where the parent aerosil SiO2 gave 3 1H MAS NMR signals; 1, at 1.4 ppm is from isolated SiOH, while those at 2.5 and 3.2-3.5 ppm represent SiOH groups hydrogen-bonded to differing degrees (Mastikhin et al. 1995). It is the isolated and more weakly hydrogen-bonded OH groups that appear to interact with the supported metal oxide. The importance of protonation sites in microporous catalytic materials has led to 1H NMR studies of zeolites (Pfeifer et al. 1985, Pfeifer 1988). The 1H data provide information about the Br~nsted acidity (Pfeifer et al. 1991) and enable different hydroxyl groups readily to be distinguished (Freude et al. 1987). Detailed consideration has been given to the linebroadening mechanisms of the 1H spectra of zeolitic materials. In siliceous materials the resolution limit was reached at the relatively modest magnetic field of 7.05 T (Brunner 1990). However, for zeolites with higher aluminiumcontent this limit was reached only at a higher magnetic field of 11.7 T (Brunner 1993). The NMR data have been correlated with the stretching frequency of the hydroxyl groups in solids such as zeolites, corroborating results from vibrational spectroscopy (Brunner et al. 1992). Hunger (1996) has extensively reviewed the use of 1H NMR as a probe of the hydroxyl sites in microporous materials where there is a distinction between BrCnsted acid sites (e.g. between Si-O-A1) and silanol (SiOH) groups located at the surface. Table 2 in Hunger's review gives precise assignments to a range of different hydroxyl environments. Direct proton NMR not only reveals the different proton species present but can also be used to determine the characteristics of the attached framework site. For example,

539

NMR of Other Spin -lIe Nuclei

the quadrupole interaction has been suggested to increase on protonation of the oxygen attached to an aluminium atom. By using a 1H-detected TRAPDOR experiment (Section 3.8.3) the Xe of the 27A1 can be estimated. Examples of this approach include a study of the Br0nsted acid site in dehydrated HY (Grey and Vega 1995), the Lewis acid site in undehydrated HY (Kao and Grey 1996) and the dealuminated zeolites ultrastable HY, HZSM-5 and mordenite (Deng et al. 1998). Values of 27A1XQ as high as 15.3 MHz have been estimated using this approach.

9.2.1.2 Studies o f stoichiometric protons in crystalline materials. Studies of crystalline materials have examined well-defined hydrogen environments to determine the spectral characteristics of the different proton species. The shift range of protons is --~20 ppm with strong overlap between the resonances of hydroxyl groups and molecular water. The most obvious spectral distinction between OH and H20 is the extent of the spinning sideband manifolds. Hydroxyls tend to have only 1 or 2 pairs of sidebands whereas water molecules display a large manifold of intense spinning sidebands extending over a range of --~ 100 kHz (Figure 9.2). Even where H20 and OH groups cannot be distinguished on the basis of their chemical shifts, the important H20/OH ratio can readily be determined either from the sidebands or from the static spectrum. The very different dipolar couplings of OH and H20 groups give rise to static spectra containing a comparatively narrow OH resonance superimposed on a broader H20 resonance. Table 9.1 lists some typical proton shifts in inorganic materials.

A

B

tremolite

.

analcite

.

.

.

A

_..jk_.J ~. ~

,._..) I

100 0 -100 1H shift (ppm) w.r.t. TMS

200

i

I

-200 0 IH shift (ppm) w.r.t. TMS

Figure 9.2. 1H MAS NMR spectra of proton-containing minerals, illustrating the difference between the spectral characteristics of OH groups as in tremolite, Ca2MgsSisO22(OH)2 (spectrum A) and water molecules as in analcite, NaA1Si206.H20 (spectrum B). Note the typical manifold of spinning side bands associated with H20 but not OH. After Eckert et al. (1988), by permission of the American Chemical Society.

540


Table 9.1. Characteristic proton shifts in some inorganic materials containing stoichiometric OH and H20. Compound

1H ~iso(ppm)*

Nature of site

Reference

pyrophyllite tremolite analcite gypsum talc topaz elbaite datolite pectolite hydroxyapatite monetite brushite ilerite Mg5SizO8(OH)2 Hydrous Mg silicate B Superhydrous Mg silicate B MgvSizOs(OH)6 hydroxysodalite hydrate hyalite

2.3 0.7 3.1 5.3 1.1 3.0 4.7 4.3 15.8 0.2 13.6-16.2 6.4, 10.4 3.8, 16.3 1.1 4.7, 3.3 5.0, 3.4

OH OH H20 H20 OH OH OH OH OH OH Acid H H20, acid H, H20, acid H OH OH OH

Eckert et al. (1988) Eckert et al. (1988) Eckert et al. (1988) Eckert et al. (1988) Yesinowski et al. (1988) Yesinowski et al. (1988) Yesinowski et al. (1988) Yesinowski et al. (1988) Yesinowski et al. (1988) Yesinowski & Eckert (1987) Yesinowski & Eckert (1987) Yesinowski & Eckert (1987) Brenn et al. (2000) Phillips et al. (1997) Phillips et al. (1997) Phillips et al. (1997)

5, 3.7 16, - 1 7.1, 5.6, 3.9

KHSi205 NH4HzPO4 KH2PO4 NH4HSO4 LizSOa.H20 KAI3(SO4)z(OH)6 with H z O / H 3 0 + 6--) K + sorensenite makatite octosilicate phosphoellenbergerite holtedahlite Mg(OH)2 pargasite

15.6 7.2, 14.8 14.9 6.9, 11.7 5.6 4.5, 7.0, 11.4 5.1 5.8 3.6, 16.0 1.2, 4.5, 11.0 1.2, 4.5 0.5 1.2

OH Acid H, OH H20(1), H20(2), OH Acid H NH4 +, acid H acid H NH4 +, acid H H20 OH, H20, H30 + H20 OH/H20 H20, OH OH OH OH OH

Kagi et al. (2000) Engelhardt et al. (1992) Graetsch & Topalovicdierdorf (1996) Deng et al. (1995) Ratcliffe et al. (1985) Rasmussen et al. (1999) Ratcliffe et al. (1985) Ratcliffe et al. (1985) Ratcliffe et al. (1985) Sebald et al. (1990) Almond et al. (1997) Almond et al. (1997) Brunet & Schaller (1996) Brunet & Schaller (1996) Sears et al. (1988) Welch et al. (1994)

* chemicalshiftsquotedwithrespectto tetramethylsilane(TMS)

The work of Yesinowski and Eckert (1987) showed that high-resolution 1H MAS N M R spectra could be obtained from minerals containing stoichiometric hydroxyl groups as the sole proton species provided the proton density in the sample is _< 15 atoms/nm 3. This conclusion agrees with a detailed ~H N M R study of the dehydration processes of silica gel surfaces (Bronnimann et al. 1988). Models of structurally isolated water molecules provided by analcite, N a A 1 S i 2 0 6 . H 2 0 , and gypsum, CaSO4.2H20, yield characteristic 1H MAS N M R spectra with numerous spinning


541

sidebands reflecting the strong, largely inhomogeneous character of the homonuclear dipolar coupling. Tremolite (CazMgsSigOz2(OH)2)and pyrophyllite (AlzSi401o(OH)2) have similarly been used to model the OH groups. Lawsonite and hemimorphite, minerals containing stoichiometric amounts of both OH and H20 groups, yield spectra with numerous intense spinning sidebands from which it is difficult to discriminate between OH and H20. It has been suggested that hydroxyapatite, with a shift of 0.2 ppm would be a good secondary 1H reference. The number of model compounds containing stoichiometric H20 and OH groups was greatly extended in a study by Yesinowski et al. (1988). The 1H shifts in such units correlate well with the hydrogen bond strength which is also reflected by the distance d(O-H-O) (Berglund and Vaughan 1980). The experimental data (Yesinowski et al. 1988) suggest the relationship 6iso(ppm) = 79.05 - 0.255d(O-H-O)(pm)

(9.2)

By using this relationship, 1H shifts can reveal subtle differences between proton sites. A recent detailed review of the application of solid state NMR to probe hydrogenbonding by Brunner and Sternberg (1998) includes 1H data. Protons can occur in other environments such as NH4 groups, as in the aluminosilicate buddingtonite in which the ~H resonance occurs at 6.8 ppm (Yesinowski et al. 1988). Two hydrous magnesium phosphates give principal aH signals arising from OH groups, some of which contain protons attached to the apical oxygen, with others containing protons attached to the octahedral units of double chains. The different sites show very different proton couplings and are reflected in the sideband patterns. An additional signal in phosphoellenbergerite arises from protons associated with magnesium vacancies in the single chains of the structure (Brunet and Schaller 1996). Phillips et al. (1997) carried out an extensive ~H MAS NMR investigation of high pressure magnesium silicate phases. Of particular interest was the hydrated phase B and the closely related superhydrous phase B. The B phase showed 2 distinct peaks at 4.7 and 3.3 ppm from the paired but inequivalent hydroxyl pairs occurring in the structure. A proton separation of 1.86~ deduced from the powder pattern was different from the distance determined by X-ray methods, which are, however, not ideal for locating poorly scattering protons. The spin-echo data from the superhydrous B phase gave a slightly shorter H-H distance (1.83 A). In samples containing ~-MgzSiO4 coexisting with these hydrous phases, 1H-298i CP indicated that this nominally anhydrous phase contains a significant proton content. In these experiments it was noted that CSA effects had to be included in order to simulate the asymmetry of the static pattern and that their inclusion had an effect on the estimated H-H distance. General expressions have been derived for the frequency (and hence the lineshape) under a combination of CSA and dipolar effects (Zilm and Grant 1981, Harris et al. 1985). Based on these general

542


expressions an explicit and convenient formulation was derived by Phillips et al. (1997) for the particular case of pairs of coupled rigid hydroxyl groups. It was also noted that when there is strong coupling between nuclei the peak position does not correspond identically with 8iso. In the superhydrous B phase, as Vr increased from 11 to 14 kHz the peak separation increased by ---0.4 ppm. A detailed study of the effect of structure on the 1H chemical shift of both the hydroxyls and water molecules in smectite clay minerals has recently been made by Alba et al. (2000). Significant differences were found in the ~H shift of hydroxyls in trioctahedral clays (--~0.5 ppm) and dioctahedral clays (--- 2.0 ppm). This shift difference is related to the orientation of the hydroxyls relative to the layers, with more hydroxyl interaction possible in dioctahedral smectites. The greater variation possible in the dioctahedral clays gives rise to broader ~H resonances than in the trioctahedral minerals. Furthermore, within each group there is another level of variation related to the layer charge, which in the octahedral layer tends to increase the proton shielding but has a negligible effect in the tetrahedral layer. Substitution within layers causes broadening of the 1H resonance. Interlayer cations have a negligible effect on the hydroxyl resonance, but the same is not true for interlayer water. As the charge increases the proton acidity increases, changing the ~H shift from --- 4.57 through 4.36 to 4.10 ppm on going from trivalent to monovalent interlayer cations (Alba et al. 2000). The static 1H spectrum of an NH4+-exchanged mica after heat treatment showed a single sharp 1H resonance, which was taken to indicate rapid proton motion (Noma et al. 1997). ~H NMR signals have been detected in sepiolite, arising from Mg-OH (0.4 ppm) and interlayer water (4.4 ppm) (Aramendfa et al. 1997). On removal of water an Si-OH signal appeared at 1.9 ppm. Treatment with acid and heating caused the sepiolite structure to breakdown to form a fibrous silica retaining an Si-OH signal at 1.9 ppm.

9.2.1.3 Non-stoichiometric proton environments in crystalline and glassy materials. Non-stoichiometric hydrogen in nominally anhydrous minerals (e.g. feldspars, nepheline, quartz, and grossular garnet) is found to occur in a variety of forms including mobile H20 in fluid inclusions, anisotropically constrained isolated He0 molecules and clustered species such as H404. Even in such materials where the proton content is low, usable NMR signals can be obtained in a relatively short timescale, providing directly quantitative information and allowing fluid-like inclusions readily to be distinguished by their very narrow lines. 1H signals were detected in feldspars, arising both from fluid water inclusions undergoing rapid isotropic motion and from structural isolated water molecules, with the ratio between the different water environments varying significantly for different feldspar samples. Fluid inclusions in quartz were also observed as a ~H resonance at 4.7 ppm. The water molecules in nepheline show 2 distinct proton environments with peaks at 4.6 and 3.2 ppm. The clusters of 4 O H replacing SiO4- in garnets produced only a relatively broad line at MAS speeds

NMR of Other Spin -~/2 Nuclei

543

of 7.7 kHz (Yesinowski et al. 1988). In the 3 nominally anhydrous minerals clinopyroxene, enstatite and forsterite, fluid-like water inclusions are immediately apparent from the peak at 4.8 ppm (Kohn 1996) (Figure 9.3). The peak at 1.5 ppm in this spectrum typically arises from organic contamination and is common in such relatively weak spectra. These spectra of nominally anhydrous minerals contain at least 2 peaks arising from structural protons; the broader peak typically at 4 ppm is associated with clustered structural hydroxyls and the narrower peak typically at higher shifts arises from non-clustered hydroxyls (e.g. point defects). Peaks with particularly large shifts (5.9 and 7.9 ppm) in the spectrum of enstatite suggest the presence of sites with moderately strong hydrogen-bonding. The solubility of water in these minerals was estimated by Kohn (1996) by examining the NMR data in the light of several different partitioning schemes. The results indicate water solubility levels in enstatite down to 0.024 wt %. Hydrogen can be forced at high pressure into the structure of silica-germania glasses. It has been proposed that the photosensitivity of these materials is related to the formation of Si-OH, so that in defect-free glasses the photoactivity depends on the presence of hydrogen. The 1H NMR characteristics of the different hydrogen centres

clinopyroxene

clinopyroxene

f~

clinopyroxe .

.

cnstatite (x4).i..,.. 30 Figure 9.3.

.

.

.

J/ ]/ ,~3 ~ ' ~

10 -10 1H shift (ppm) w.r.t. TMS

1H MAS NMR spectra of the nominally anhydrous minerals enstatite, forsterite and three synthetic clinopyroxenes. Note the sharp resonance at 4.8 ppm from fluid-like water inclusions. The peak at about 1.5 ppm is attributed to organic contamination. From Kohn (1996), by permission of the Mineralogical Society of America.

544


have been determined (Zeng et al. 1999a). These include H20 (4.8 ppm), SiOH (2.7 ppm), GeOH (3.3 ppm), Sill (4.5 ppm) and GeH (6.7 ppm). The NMR data were used to calibrate the infra-red spectra of these materials. Proton spectra have proved informative for studying the hydration kinetics of important cement phases (Rassem 1993) and distinguishing the different hydrates. The 1H MAS NMR spectra of polycrystalline paratungstates have been used to characterise the non-acid protons, and were able to distinguish the OH and water molecules (Fait et al. 1999). Incorporation of hydrogen into materials can have a profound effect on their optical and electrical properties, and is thus of considerable technological importance. The influence of the hydrogen on these properties is intimately associated with its distribution in the material. The ~H NMR spectra of materials such as amorphous hydrogenated silicon contain both broad and narrow components. A detailed understanding of the exact distribution of these components is of great importance. One of the most elegant approaches to solving this problem has been the application of multiple quantum NMR to the protons to determine the sizes of the clusters (Baum et al. 1986). This study concluded that in device-quality materials (and 1 sample not of device-quality) clusters of 6 hydrogens are formed, the clusters becoming physically closer as the hydrogen concentration is increased. This behaviour is in contrast to a polymeric sample in which a uniform proton distribution was observed. By modelling the hydrogen distribution it was concluded that the transition to a device-quality material occurs when the separation of the clusters matches that of the dilute monohydride group (Baum et al. 1986). Details of the growth of the 1H MQ coherence on the spatial distribution have been elucidated by comparing results from various phases where this distribution is well known, including calcium hydride, sodium bicarbonate and adamantane (Levy and Gleason 1992). The effect of the processing temperature on the detailed hydrogen distribution was investigated by the same methods, indicating that large hydrogen clusters tend to form in amorphous films at lower processing temperatures (Gleason et al. 1987). A strong relationship exists between the properties of the materials and the details of the hydrogen structure, rather than simply the hydrogen content. In porous silicon the hydrogen content has been determined by ~H NMR (Chang et al. 1996). The detection of low proton contents in materials demands careful probe design. A detailed description has been given of a probe from which polymeric materials have been eliminated and which is purged with dry nitrogen to remove ambient moisture (Levy and Gleason 1993). Techniques for cleaning the copper probe components to remove proton-containing contaminants are also described, and it is demonstrated that even at low proton contents the response of the system is highly linear, allowing accurate quantification. Proton studies using MQ techniques have been made to determine the proton distribution in systems such as hydrogenated amorphous silica films (Levy and Gleason 1993a) and silicon carbide (Petrich et al. 1987). Hydrogen doping of carbon-based


545

materials is also of considerable technological interest. ~H studies have included amorphous hydrogenated carbon (J~iger et al. 1994) and a series of studies on diamond (Levy and Gleason 1992, Mitra and Gleason 1993, McNamara et al. 1992, McNamara and Gleason 1994). Optimisation of the processing conditions for these materials has been greatly facilitated by the ability of ~H NMR to probe in detail the siting of the protons during the various stages of synthesis.

9.2.1.4 1H N M R of hydrous glasses. The effect of water in silicate and aluminosilicate glasses and the identification and quantification of the hydrous species present is a subject of long-standing interest to many fields, as far back as 1965 (Mtiller-Warmuth et al. 1965). Most NMR work has looked at glasses with comparatively high water content (> 10 mol%) but a recent study (Storek et al. 2000) of 3 alkali calcium silicate glasses containing 0.03 mols per litre showed 3 1H resonances at 6.3, 12.5 and 15.6 ppm. The relative population of the differently hydrogen-bonded sites changed with the nature of the alkali cation. The static 1H spectra of 2 essentially silicate glasses with low aluminium content, but with much higher water contents, revealed the presence of both hydroxyl and water, leading to the conclusion that significant alkali ion hydration produces mainly H20, with only a limited number of hydroxyls formed in these glasses (Bartholomew and Schreurs 1980). Another static 1H NMR study of hydrous glass has been made by Bray and Holubka (1984). The H20/OH ratios in a series of albite and orthoclase glasses were determined by fitting the spinning sidebands using model crystalline compounds as a template for the sideband intensity distribution (Figure 9.4) (Eckert et al. 1988). A solid-echo pulse sequence was used to overcome the effects of deadtime but care had to be taken to correct for differences in T2between the different species. Correcting for relaxation effects produced closer agreement between NMR and IR data but this agreement was not perfect. The residual broadening in glasses under MAS has been largely attributed to chemical shift dispersion. More recent work (Riemer et al. 2000) has concentrated on hydrous albite glasses using static 1H NMR to determine the HzO/OHratio. This study illustrates the need to use a probe (i) with a low proton background, (ii) which can generate hard rf pulses, and most crucially (iii) with very rapid recovery (-< 2 Ixs). This allows direct 1 pulse spectra to be acquired using short pulses without the use of echoes, thus eliminating the question of the differential T2 associated with echo techniques. It has also been pointed out that careful simulation of both components is necessary; in particular the Pake pattern from the water molecules requires correct simulation of the CSA. Furthermore, the spectrum in this case could only be simulated accurately using static interactions if the sample was cooled to 140 K. Multiple field studies on the hydrous glasses indicate the CSA for H20 ill these materials to be --~30 ppm. The HzO/OHratio has also been calculated for SiO2 glass with H20 contents varying from 0.12 to 8.7 wt % from their MAS spectra acquired using 1-pulse sequences with

546


t

_

~

w[:Oii~20

_

~" 21)0 '

0

' -21)0

~H shift (ppm) w.r.t. TMS Figure 9.4. 1H MAS NMR spectra of glasses with different water contents. Upper and middle spectra are of orthoclase glass, lower spectum is of anorthite-silica-wollastonite glass. Note the change in the intensity of the sidebands which is used to determine the H20/OH ratio. From Eckert et al. (1988), by permission of the American Chemical Society. relatively short deadtime delays (Figure 9.5) (Kohn et al. 1989). At low concentrations the 2 contributions can be simulated using 2 Gaussian peaks, 1 for H20 and 1 for OH. Initially the dissolution principally produces hydroxyls but at higher water contents more H20 begins to appear, as predicted by most dissolution models. In the 1H NMR spectrum of albite glass the 2 peaks corresponding to OH and H20 could not be distinguished in the centreband at 3.5-3.8 ppm, but the intensity distribution of the spinning sidebands indicated the presence of the 2 species. In SiO2 containing the highest concentration of water, 2 very narrow resonances, with an intensity ratio of 2:1, were attributed to an extremely well defined O H - : H 2 0 geometry, in the absence of a motional explanation capable of explaining the narrowness of the resonance and the lack of a narrow line in the static spectra. Two resonances have been observed in the depolymerised hydrous silicate glasses Na2Si2Os, BaSi205 and SrSi2Os. The resonances in the spectra of the sodium and barium are at --~ 4 ppm (arising from H20) and at 12 ppm (from a quite strongly hydrogen-bonded O H - group). In the strontium disilicate glass the hydroxyl was even more strongly hydrogen-bonded, as reflected by the resonance at 17 ppm, probably resulting from a Si-O- Si-OH interaction (Kohn et al. 1989).

N M R of Other Spin -1/2 Nuclei

A

547

wt% H 2 0

0.12

j

11

O

j

a

simulated

b 8.7

50

o

-50

~H shift (ppm) w.r.t. TMS

;o

1H shift (ppm) w.r.t. TMS

Figure 9.5. 1H MAS NMR spectra of hydrous silica glasses with different water contents. D is the simulation of spectrum C showing 4 separate proton resonances. Fitted peaks a and c are thought to be due to Si-OH groups in 2 different environments while peaks b and d are due to molecular water in 2 different environments. From Kohn et al. (1989), by permission of the copyright owner.

Other 1H NMR studies of glasses include those by Kummerlen et al. (1992) and Maekawa et al. (1998) employing increasingly sophisticated techniques for 1H observation. Kummerlen et al. (1992) applied CRAMPS to a hydrous Na2Si409 glass and resolved 2 lines separated by 7.5 ppm, taken to indicate quite strong hydrogen bonding at 1 of the sites. CRAMPS has the advantage that background signals from the probe are largely eliminated, and it removes broad sideband manifolds arising from the dipolar interaction. However, problems with the frequency offset dependence of CRAMPS data mean that although the sidebands are eliminated the quantitative certainty of the data is still questionable. It has also been shown that the CRAMPS linewidth is determined by the chemical shift dispersion. Simple MAS of a NazSi409 glass with 0.7 H20 per formula unit gave 2 well resolved signals at 4.5 and 13 ppm corresponding to molecular water and OH respectively (Schaller and Sebald 1995). This is in distinct contrast to aluminosilicate glasses in which resolution of the different sites cannot normally be achieved. The lines were also strongly asymmetric, probably due to the range of environments present with differing degrees of hydrogen-bonding. A non-rotor synchronised 2D ~H EXSY experiment was carried out to investigate

548


exchange between the different proton sites. Suppressing the homonuclear dipolar coupling removed the cross peaks, indicating that the exchange is a spin diffusion process resulting from intermolecular dipolar coupling. Common T1 proton relaxation and spin diffusion data provided an insight into the spatial arrangement and strongly suggest mixing of H20 and OH within the glass. Another 2D CRAMPS-MAS sequence was able to separate completely the sideband manifolds from the different sites (Schaller and Sebald 1995). The data (Figure 9.6) show the slices from the complete 2D data set, clearly revealing the different sideband distributions associated with OH and nonrigid water. 1H MAS, CRAMPS and static echo spectroscopy were employed to reveal relatively immobile SiOH and water molecules in several binary and ternary silicate electrode glasses (Herzog et al. 1994). This study showed systematic variations in the determinations of the OH/H20 ratio by the 3 NMR approaches. More recent work by Reimer et al. (2000) has revealed some of the reasons for these variations (see above). Zeng et al. (1999) employed IH-ZYA1 and IH-Z3Na TRAPDOR in combination with 1H MAS to investigate the proton distribution in 4 hydrous aluminosilicate glasses. Peaks at --~6 and 2.9 ppm correspond to 2 different water environments while others at 5-6, 3.5 and 1.5 ppm arise from 3 different hydroxyls. Rapid exchange occurs at room temperature between the 2 water environments. The data reveal the complexity of the proton speciation in these materials. All the resonances show a TRAPDOR effect to some extent. One of the groups identified is A1Q~3)-OH, implying that the aluminosilicate framework is depolymerised by the water. This conclusion does not agree B

A

o

00

00 18.0

,

,

,

,

90

,

i

,

,

,

,

.'',

,

v

.

.

30

.

.

'-30 ppm

,

,

.

._~

_

_

"

-'90

120 0 -120 ~H shift (ppm) w.r.t. TMS

Figure 9.6. A. ~H CRAMPS-MAS correlation spectrum of hydrated sodium disilicate glass showing projections in both dimensions. B. Slices through the CRAMPS dimension of spectrum (A) showing the separate spectra from the H20 resonance at 4.0 ppm (upper) and the OH resonance at 14.0 ppm (lower). Note that the different sideband distributions from the 2 protonated groups are clearly distinguishable. From Schaller and Sebald (1995), by permission of the copyright owner.

NMR of Other Spin -lIe Nuclei

549

with the work of Kohn et al. (1989) and is difficult to rationalise in terms of recent 170 NMR data (see Chapter 6). 1H NMR studies of water dissolution continue to hold strong interest; very recent work combined 1D spectroscopy with multiple quantum double resonance and 2D heteronuclear correlation experiments on dry and hydrous Na20.4SiO2 and the aluminosilicate phonolite (Robert et al. 2001). The contrast in resolution between proton environments in the pure silicate and aluminosilicate glasses was marked, 2 peaks being clearly observed in the silicate but only 1 peak in the aluminosilicate. Dipolar dephasing was used to improve the resolution by selectively dephasing some components of the line, allowing at least three distinct sites to be observed in phonolite with 1 site showing a strongly hydrogen-bonded environment. Since only double quantum coherence could be generated efficiently, significant proton clustering was ruled out. The position of the protons in relation to the other nuclei present was investigated by a combination of double resonance techniques including CP to 29Si, 1H-29SiHETCOR and {1H }-X REDOR to the other nuclei. Similar changes observed in the behaviour of the magnetisation accompanying hydration of sodium silicate and phonolite imply that both glasses depolymerise on hydration (Robert et al. 2001). It should be noted that both these glasses are already partially depolymerised differs from the findings for albite, which is initially fully polymerised. Interaction of water with inorganic materials at high pressure can result in sub-microscopic fluid water inclusions. In mineralogy it is important to know the pressure of the water to be able to determine its equation of state. If it is assumed that the molar magnetic susceptibility is a constant, the susceptibility is a function of the density (P) and hence the isotropic 1H chemical shift of the fluid in the inclusions can be used as an extremely accurate measure of its density (Withers et al. 2000). The relationship between these 2 parameters has been determined as p = 0.4921~iso - 1.340

(9.3)

This approach has greatly extended the pressure range that can be determined in fluid inclusions. Similar studies can be carried out on phosphate glasses. The presence of hydroxyls in ultraphosphate glasses strongly affect the thermal and optical properties and the high reactivity of the branching phosphate groups indicates that hydroxyls are nearly always present. Comparison of the spectrum of a zinc ultraphosphate glass with that of hydrated P205 showed that a resonance at 13 ppm is associated with QZ(H)-QZ(H) groups, with a more strongly hydrogen-bonded environment at 17 ppm due to QZ(H)-QZ(Zn) (Mercier et al. 1998). A similar assignment was made for lead-barium ultraphosphate glasses (Hosono et al. 1992). Glasses prepared by sol-gel methods may be regarded as examples of hydrous glasses since they can retain significant proton levels when the gel is dried and vitrified.

550


The intensities of the hydrogen-bonded and silanol groups can be monitored, providing insight into the dehydration process of the gel (Yang and Woo 1996). Since 1H NMR can be made quantitative it is very useful for determining the absolute proton content of sol-gel materials, providing important information needed for the analysis of scattering data (especially of neutrons) from such gels. 9.2.1.5 Biomineral-related materials. ~H NMR has been applied to studies of biomineralisation processes in the calcium phosphates apatite and fluoroapatite using crystalline calcium phosphates as standards. Several 1H peaks observed as fluorine was added to the system were attributed to changes in the hydrogen-bonding at different sites resulting from the presence of fluorine. The change in intensity of the peaks suggests that the O H - / F - mix is statistical rather than one which maximises the hydrogen-bonding between the different species (Yesinowski and Eckert 1987). In some of these phases a resonance from a surface-adsorbed water layer was clearly observed. It is possible for the motion in molecular water to cause significant averaging. Proton exchange can also average the IH-IH dipolar coupling but this is not sufficient to average the CSA; to accomplish this, the motion must change the orientation of the CSA tensor. 1H CRAMPS has also been used to examine model biomineral materials and some bone minerals (Santos et al. 1994). The ~H spectrum of actual bone mineral can be obscured by the contributions from the organic content of the attached collagen. To remove this unwanted part of the spectrum the collagen itself could be removed chemically, but this may change the nature of the sample. Alternatively IH-31p HETCOR can be used, in which the relatively large chemical shift range of the 31p provides a means of separating those ~H signals associated with the phosphate and therefore arising from the bone mineral phase.

9.2.2 19F NMR 9.2.2.1 Introduction. Two extremely comprehensive review articles summarise high resolution solid state 19F NMR, the first covering work up to 1990 (Harris and Jackson 1991) and the second extending this to 1996 (Miller 1996). 19F MAS studies can be classified according to the spinning speed below which only relatively dilute species can be observed; Harris and Jackson take this threshold MAS speed to be 7 kHz. The effect of MAS speed on 19F NMR spectra is illustrated by work on mixtures of fluorohydroxyapatite (FAP) and CaF2 (Kreinbrink et al. 1990). The effect of MAS on the linewidths differs greatly between dilute systems such as FAP and the more concentrated fluorides, the response being quite flat for FAP but decreasing much more steeply with increasing spinning speed in the simple inorganic fluorides (Figure 9.7A). Thus, at 15 kHz the residual linewidth of FAP is < 500 Hz, by comparison with linewidths of--~ 1.5 kHz for NaF and CaF2. Below 7 kHz only the relatively spatially

551

NMR of Other Spin -//2 Nuclei

B

A

,~, 4000 N ,I~ 3000 2000 ell ~ 1000 .p,,~

2.0

~

CaF2 FAP ~

MAS (kHz) $ 15.5

_

13.5

~

CaF2 i ~ ~

~ ~"~"~e.~

NaF

_ ~

FAP O ~ I ~ O - - - - - - O ~ 9 -O-O-O 6.0 10.0 14.0 Spinning speed (kHz) -

~

-

~

,

~

9

I

______~.~~~_~

_ 9"3 7.2

4.9

150 50 -5O 19F shift (ppm) w.r.t. C6F6

Figure 9.7. A. Effect of MAS spinning speed on the 19Fspectral linewidth of the spatially dilute compound fluorohydroxyapatite (FAP) and the more concentrated fluorides NaF and CaF2. B. 19F MAS NMR spectra of a binary FAP and CaF2 mixture showing acceptable resolution of the 2 phases at spinning speeds which are sufficiently fast to narrow the CaF2 signal. From Kreinbrink et al. (1990), by permission of the copyright owner. dilute FAP phase can be observed but as the speed increases the CaF2 signal narrows until spinning at > 15 kHz produces an acceptably quantitative spectrum reflecting the phase distribution (Figure 9.7B). The static linewidths of various simple fluorides are typically in the range 15-48 kHz (Aujla et al. 1987). Since fluorine is a nucleus for which various shift references have been used, care must be taken when comparing results from different workers. The standard reference is CFC13, the resonance of which is taken as 0 ppm, but other commonly used references are PTFE, C6F6 and 1 M NaF aqueous solution which have resonances at - 123.2 ppm, - 163.0 ppm and - 120 ppm respectively compared with CFC13. Spectra are usually readily obtained from fluorine-containing species but it may often be difficult to make completely unambiguous assignments of the spectra (Miller 1996).

9.2.2.2 Simple inorganic fluorides. There have been a number of reports of 19F MAS NMR of alkali metal fluorides (see Table 9.2). The analysis of these 19F shifts by Hayashi and Hayamizu (1990) revealed no simple relationship apart from a general trend downfield as the radius of the counterion increased. The advent of faster spinning has provided good quality high resolution NMR spectra from a wider range of inorganic fluorides, although Kreinbrink et al. (1990) found a wide variation in linewidths, suggesting the possibility that some of the broader observed resonances could be narrowed still further as the available spinning speed increases. In CaF2, which has

552

Multinuclear

Solid-State NMR

of Inorganic Materials

Table 9.2. Characteristic 19F shifts in some simple inorganic compounds. Compound LiF NaF

~iso(ppm)* 204 22 l, - 224 -

-

KF

- 130, - 130.2, - 123

RbF

-

CsF

-

88, - 9 0 80, - 79

KF.2H20 RbF.H20 CsF.2H20 KF-A1203 KF-SiO2 RbF-AI203 RbF-SiO2 CsF-CaF2 CsF-AI203 RbF-mont** CsF-mont** CaF2

- 133 - 113 - 97 - 159, - 115 - 129 - 109 - 122 - 79 - 116, - 88 - 123 - 113 - 104.8, - 107.7, - 107

SrF2 BaF2 CdF2 Hg2F2 HgF2 SnF2 SnF4 ot-PbF2

- 84.1 - 13 - 192.1 - 95.8 - 196.4 - 110.4 - 146.9 - 20.5, - 57.7 ( - 39.0)

CuF2-SiO2 ZnF2-SiO2 CdF2-SiO2 LaF3 A1F3

- 149 - 124 - 124 - 23.5, 17.1, 24.9 - 174, - 172

Reference Hayashi & Hayamizu (1990) Hayashi & Hayamizu (1990), Schaller et al. (1992) Hayashi & Hayamizu (1990), Kreinbrink et al. (1990), Clark et al. (1986) Hayashi & Hayamizu (1990), Clark et al. (1986) Hayashi & Hayamizu (1990), Clark et al. (1986) Schaller et al. (1992) Clark et al. (1986) Clark et al. (1986) Clark et al. 1986, Duke et al. (1990) Duke et al. (1990a) Duke et al. (1990) Asseid et al. (1990) Clark et al. (1986) Clark et al. 1986, Duke et al. (1990) Asseid et al. (1990) Asseid et al. (1990) Hayashi & Hayamizu (1990), Chan & Eckert (2001) Kreinbrink et al. (1990) C h a n & Eckert (2001) Kreinbrink et al. (1990) Kreinbrink et al. (1990) Kreinbrink et al. (1990) Kreinbrink et al. (1990) Kreinbrink et al. (1990) Wang & Grey (1995) (Sites F1, F2 and the exchange peak) Asseid et al. (1992) Asseid et al. (1992) Asseid et al. (1992) Wang & Grey (1997) Schaller et al. (1992), Chan & Eckert (2001)

* chemical shifts quoted with respect to CFCI3 ** adsorbed on to montmorillonite.

b e e n m u c h s t u d i e d b e c a u s e o f its i m p o r t a n c e in d e n t a l s c i e n c e , a s i g n i f i c a n t p a r t o f t h e l i n e w i d t h is d u e to h o m o n u c l e a r CRAMPS

dipolar coupling, necessitating fast spinning or

( S m i t h a n d B u r u m 1989).

19F N M R

h a s b e e n u s e d to s t u d y t h e i o n i c m o t i o n in e~-PbF2 in w h i c h t h e 2

c r y s t a l l o g r a p h i c a l l y i n e q u i v a l e n t f l u o r i n e sites h a v e shifts o f - 20.5 a n d - 5 7 . 7 p p m

553


corresponding to FPb4 and FPb5 coordinations respectively (Figure 9.8A) (Wang and Grey 1995). Bondlength data suggest that FPb4, with its shorter average bondlength, should show a larger 2~ J-coupling to the Pb. Lead decoupling removes the fine structure (Figure 9.8B). A peak with a shift of - 39.0 ppm, exactly half way between the above 2, is due to jumping of mobile fluorine atoms between the 2 possible sites. The data clearly show the existence of 2 populations of fluorine atoms in intermediate and slow motion regimes. The tysonite structure of LaF3 has 3 distinct fluorine sites, providing a mechanism for good conductivity via fluorine motion. Static 19F studies have demonstrated significant motion on the F1 sublattice at room temperature, with exchange occurring between the F1 and F2/F3 sublattices above - 247~ (Denecke et al. 1992). Doping with strontium lowers the temperature at which ionic mobility becomes significant. More recently fast MAS (23 kHz) has been applied to LaF3 and a sample doped with 1% strontium (Wang and Grey 1997). The improved resolution of the MAS spectrum allowed the different sites to be clearly resolved and the mobility of each site to be determined. The activation energies for fluoride hopping along each of the distinct pathways were shown to be very different, increasing in the order Fll < F13 < F12 (Wang and Grey 1997). High resolution 19F NMR spectra of samples from the solid solution Cal-xYxF2+• (0.03 --< x TeO3+ ~ ~ edge sharing TeO4 i> comer sharing TeO4), but the strong overlap of the shifts corresponding to these units makes their unambiguous identification difficult. However, different TeOx coordinations present in a given compound display different resonances, the higher coordination state showing a smaller shift (Table 9.10). Additional information from the CSA tensor has been invoked to help identify the different structural units. A composite plot of-q vs. IASI identified broad regions associated with the different structural units although again there was overlap between the different coordinations (Sakida et al. 1999a).

600


MAS

1722 Li2TeO3 h * 1669

~ M

1669

~

3

t

.i_l,

~

,

+

I

units

I

corner-sharing TeO4 units

1569 e 0

~ ,

TeO~units

1463

ot-TeO2

~

isolated TeO3units

.

,

,

,

I

,

i

edge-sharing units

4 ,

,

l

,

,

,

,

I

,

2500 1500 500 12STe shift (ppm) w.r.t (CHj)2Te Figure 9.33. ~25TeMAS NMR spectra of model crystalline compounds containing the representative Te-O units occurring in tellurium materials. The asterisks mark the positions of the 125Te isotropic shift. From Sakida et al. (1999), by permission of Elsevier Science.

Table 9.10. J25TeNMR parameters of crystalline tellurites and tellurates. Compound Li2TeO3 Na2TeO3 K2TeO3 Ag2TeO3 PbTeO3 BaTeO3 ZnTeO3 Sr8(A102) I2(TeO3)2 Cs2Te205 Te2V209 Te3Nb2Oll, site 1 site 2 MgTe205 a-LizTe2Os, site 1 site 2 [3-Li2Te205, site 1 site 2

Unit i-TeO3 i-TeO3 i-TeO3 i-TeO3 i-TeO3 i-TeO3 i-TeO3 i-TeO3 t-TeO3 t-TeO3 t-TeO3 c-TeO4 TeO3+l TeO3+l TeO3 + 1 TeO3+l TeO3 + 1

~iso

~'~

(ppm)*

(ppm)

1722 1787 1732 1658 1690 1712 1728 1742 1887 1669 1756 1557 1669 1599 1613 1632 1618

580 681 ND 573 759 658 882 ND 910 1068 1053 ND 1238 1285 ND ND ND

Skew

Reference

0.86 0.81 ND 0.75 0.42 0.41 0.31 ND 0.74 0.75 0.63 ND 0.65 0.17 ND ND ND

Sakida et al. (1999) Sakida et al. (1999) Sakida et al. (1999) Sakida et al. (1999) Sakida et al. (1999) Sakida et al. (1999) Sakida et al. (1999) Dann & Weller (1996) Sakida et al. (1999) Sakida et al. (1999) Sakida et al. (1999) Sakida et al. (1999) Sakida et al. (1999) Sakida et al. (1999)

601

N M R o f Other S p i n J / e N u c l e i

Table 9.10. (Continued)

Compound Zn2Te308, Mg2Te308,

Unit site 1 site 2 site 1 site 2

ot-Te02 TiTe308 ZrTe308 HfTe308 SnTe308 [3-TeO2 NaVTeO5

KVTeO5

c-TeO4 TeO3+l

c-TeO4 TeO3+l c-TeO4 c-TeO4

c-TeO4 c-TeO4 c-TeO4

e-TeO4 e-TeO4 e-TeO4

~iso

~'~

(ppm)*

(ppm)

1545 1679 1517 1620 1463 1461 1439 1493 1489 1536 1569 1630 1725

1540 ND 1517 ND 1509 1295 1533 1486 1541 1579 1509 1687 1688

Skew

Reference

0.24 ND 0.31 ND 0.23 0.27 0.17 0.29 0.29 0.18 0.29 0.32 0.33

Sakidaet al. (1999) Sakidaet al. (1999) Sakidaet al. (1999) Orionet al. (1997) Sakidaet al. (1999) Sakidaet al. (1999) Sakidaet al. (1999) Sakidaet al. (1999) Sakidaet al. (1999) Sakidaet al. (1999) Sakidaet al. (1999)

* chemical shifts quoted with respect to (CH3)aTe * ND - not determined, i - isolated, t - terminal, c - comer-sharing, e - edge-sharing.

m a t e r i a l s . Sakida and colleagues have carried out a series of 125Te NMR studies of binary tellurite glasses, in which they identified the different units by comparison with their extensive data collected for crystalline materials (Table 9.10). The static 125Te NMR spectra were fitted to 2 CSA powder patterns corresponding to TeO3 and TeO4 units with a difference of --~ 300 ppm in their isotropic shifts. Typical simulations are shown in Figure 9.34A. 125Te NMR studies have been made of the systems M20-TeO2 where M = Li, Na, K, Rb, Cs (Sakida et al. 1999a), MO-TeO2 where M = Mg, Zn, Sr, Ba, Pb (Sakida et al. 1999c), MzO3-TeO2 where M = A1, Ga (Sakida et al. 2001) and g 2 0 5 - Y e O 2 (Sakida et al. 2000). Where possible the NMR spectra of the other nuclei (e.g. 27A1, 51V) were also obtained, giving new insight into the constitution of these glasses and allowing new structural models to be proposed. An increase in the content of the second metal oxide led to a general tendency for the number of TeO3 units to increase at the expense of TeO4 units (Figure 9.34B). The nature of the glass network and the rate at which the different tellurium coordinations change was found to depend on the glass-forming tendency of the second metal oxide. 9.3.6.3 G l a s s y t e l l u r i u m - c o n t a i n i n g

9.3.7

129XeN M R

1 2 9 X e is a nucleus with a natural abundance of 26.4% and good relative sensitivity (greater than that of 2 9 8 i and t3C). In samples containing pure Xe gas, ~29Xe NMR suffers from the drawback of long T1 relaxation times, but these are significantly shortened by the presence of a small amount of paramagnetic oxygen.

602


A

composition x 100

-9, 1 , 1 . , 1

....

i,,,,l'r,,,|.J..|

....

| ....

i~-,!

~,,,

.Jl

~ 60 rJl

~

20 ,....i....|.u

0

20

40

100

.a,.~

~ 60 ~_

16 ~

'l~_l

tl

I I It

4000

Ilil,

t i ~ [ ~lllll,

2000

I,J,,J

~ I , ill

0

12STe shift (ppm) w.r.t. (CH3)~Te

20

VO4 VO4chain I ~ ~ V O s zigzag

oli

,.. ch.a!.,

0

20

,..

40

V20 s (mole %)

Figure 9.34. A. 125Testatic NMR spectra of xGa203(100 - x)TeO2 glasses with fitted peaks corresponding to simulated TeO4 and TeO3 lineshapes. From Sakida et al. (2001), by permission of the American Ceramic Society. B. Change in the content of tellurate and vanadate species in V2OsTeO2 glasses as a function of V203 content, determined from ~25Teand 5~V NMR data. From Sakida et al. (2000), by permission of the Institute of Physics.

In view of the chemistry of this inert element, the main application of Xe N M R is as a surface probe for studying meso and microporous solids and the free volume in polymers. The relaxation time for Xe adsorbed in solids is typically 10 ms to a few seconds. The use of 129Xe NMR as a probe for studying microporous solids has been extensively reviewed by Barrie and Klinowski (1992). A more recent example of the use of ]29Xe NMR to study surface interactions is provided by a study of borosilicalites with the ZSM-5 structure (Ngokoli-Kekele et al. 1998). The 129Xe shift of adsorbed xenon (referred to the shift of the pure gas extrapolated to zero pressure) was found to change regularly with boron content, with a discontinuity at a boron content of about one atom per unit cell ascribed to a change in the distribution of boron atoms in the lattice. A similar correlation between the 129Xe NMR shift and the aluminium content has been reported for the zeolite ZSM-5, in which the discontinuity occurred at about 2 A1 atoms per unit cell (Chen et al. 1992). One and two-dimensional 129XeNMR data have been obtained for the adsorption of Xe in NaY zeolite (Labouriau et al. 1999). Although the use of 129Xe N M R to gain


603

accurate information about the energetics or locations of the Xe atoms in the zeolite pores is hampered by a present systematic lack of understanding of the Xe shift behaviour, the two-dimensional data suggest that the Xe atoms spend most of their time in the oL-cages and, at lower temperatures, near the cage walls (Labouriau et al. 1999). A 129XeNMR study has also been made of 3 mesoporous silica materials synthesised by a sol-gel process (Pietrass et al. 1999). Measurements of the temperaturedependence of the chemical shift and the spin-spin lattice relaxation times suggest that the Xe does not penetrate the largely disordered silica structure. The relaxation data were confirmed by 2D NMR exchange experiments which indicate rapid exchange of Xe between the adsorbed and gas phase (Pietrass et al. 1999).

9.3.8 t9Spt NMR The considerable practical importance of elemental platinum in catalytic applications is reflected by the fact that of the few reported solid-state 195pt NMR studies, most are of catalyst systems. The principal importance of NMR to such studies arises from the observation that the ~95pt parameters are essentially independent of the nature of the support material but are sensitive to the surface conditions, allowing the local densities of states at the Fermi energy on "typical" metal surface sites (and even on sub-surface sites) to be deduced. Since these properties are considerably altered by the chemisorption of gas species such as hydrogen (Bucher et al. 1989), the NMR data can provide a sensitive probe of the surface interactions occurring under catalysis conditions. These principles have been used to interpret the results of a detailed 195pt NMR study of a platinum catalyst supported on anatase in which the. spectra were determined by a pointto-point spin-echo method and fitted with a number of Gaussian peaks, each describing the signal resulting from atoms at a particular depth from the surface. The integrated area of each fitted Gaussian peak is regarded as being proportional to the number of atoms in the layer. The results revealed in detail the effect of hydrogen chemisorption on the surface platinum atoms (Tong and van der K/ink 1994). 195pt NMR was also used to determine the effect on the platinum surface of electropositive additives such as alkalies which are commonly used as catalyst promoters (Tong and van der Klink 1994a). The results clearly indicate that alkali impregnation increases the local densities of states at the Fermi energy on the surface, in sharp contrast to the effect of chemisorbed hydrogen which diminishes the local densities of states at the surface. Since supported platinum catalysts are commonly prepared by deposition of the platinum species from aqueous solutions onto the supporting oxide, followed by thermal activation, a 195pt NMR study has been made of the initial stages of the deposition process on A1203 (Shelimov et al. 1999). Although sharp 195pt NMR signals were determined in the wet preparations and showed an increase in chemical shift with increasing pH of the solution, the signals were irreversibly lost after drying at 90~

604


overnight. This result was suggested to indicate the modification of the octahedral symmetry of the original Pt complexes to species with lower symmetry and greater CSA which are undetectable by static NMR experiments (Shelimov et al. 1999). 195pt NMR has been used to study platinum particles embedded in a zeolite and to compare their characteristics with the more common oxide-supported platinum catalysts (Tong et al. 1993). Spin-lattice relaxation measurements indicated that a measurable fraction of the platinum in a zeolite-Y sample is not in a metallic environment, but it was not clear whether the loss of metallic signal reflected very small particle sizes or was due to interactions with the framework and its counter-ions. The latter possibility was supported by the observation that part of the metallic signal was restored by chemisorption of hydrogen (Tong et al. 1993). A brief review of the 195ptCP NMR literature (coveting mostly organometallics and complexes) has been given by Sebald (1994).

9.3.9 199HgNMR 199Hg has a natural abundance of 16.84%. To date 199Hg NMR has been exploited mainly in studies of organomercury compounds which are outside the scope of this book, and only a few solid state studies of inorganic materials have been reported, mainly of semiconductor alloy compounds in the system Hg-Cd-Te. The earliest 199Hg NMR study of crystalline HgxCdl-xTe with varying values of x showed that the technique is capable of monitoring changes in the bulk composition and suggested a relationship with the bonding properties in the alloy (Willig et al. 1976). Subsequent single-crystal 199Hgwork on Hgo.78Cdo.22Te revealed only 1 Hg environment, and double-resonance Hg-Te and Hg-Cd experiments enabled assignments of the 125Te resonances to distinct chemical environments (Zax et al. 1993). Measurements of the temperature dependence of the 199Hg NMR spectra of these alloys in the composition range x = 0.204).28 have revealed the intrinsic Knight shifts in these compounds and provided a measure of the Hg orbital contributions to the conduction electron states (Shi et al. 1993). The results indicate a consistently strong average contribution from the Hg s-orbitals.

9.3.10 2~

and 2~

NMR

Of the 2 spin I = 1/2 thallium nuclei, 2~ is the most abundant (70.5 % by comparison with 29.5 % for 2~ The receptivity of 2~ is also high (it is the fourth most sensitive spin-l/2 nucleus), making this the preferred T1 isotope for NMR study. The chemical shift, coupling constants and relaxation times of the T1 NMR spectra are extremely sensitive to the chemical environment in which the nucleus is placed. The preferred oxidation states of T1 are + 1 and + 3, both of which show large chemical

605

N M R o f O t h e r S p i n -1/2 N u c l e i

shift ranges (about 7000 ppm for TI(III) and 3400 ppm for TI(I)). The origin of this large shift range lies in the paramagnetic term of the nuclear screening constant which is responsible for about 95% of the variation observed in thallium chemical shifts. Thallium in covalent systems can exhibit significantly large CSA effects, but in highly ionic T1 solids the ions do not experience strong directional electronic interactions and show only limited CSA. The T1 chemical shifts are normally referenced to an aqueous solution of T1NO3, but solid T1C1 is a potentially useful secondary reference since the cubic symmetry of its T1 site yields a readily detected narrow line at 383 ppm. The earliest 2~ N M R solid state studies were of the common thallium salts in both the solid phase and the melt, and of thallium silicate, borate and chalcogenide glasses. The 2~ N M R literature up to 1988 has been extensively reviewed by Hinton et al. (1988). The 2~ chemical shift data for a number of thallium compounds are presented in Table 9.11. In view of the large CSA values which can occur in T1 compounds, distortion of the lineshapes can occur by loss of signal during the probe deadtime. Some of the earlier 2~ N M R data may be corrupted in this way, and should be treated with caution. Table 9.11. 2~

shifts of thallium compounds.

Compound

~*(ppm)

T1C13 T1C13.4H20 KT1CI4 Cs2T1C15.H20 Na2T1C15.4H20 Na3T1C16.12H20 K3T1C16.2H20

2485 2051, 2960 2716 2022 1984 1972 2007 1926 1262 - 1271 - 1194 540 1965 1098 2960 2220 2220 1630, 2320, 1913, 1845 1100, 1070, 843, 820, 753, 325,600, 393, 383, 383 780, 1300, 680 1963, 1963 - 108, - 66, 1110 80, -- 45, -- 107, -- 107, -- 110 2 7 0 , - 135,- 135,- 147,- 147 - 525, - 544 (isotropic)

Cs3T12C19

KT1Bra.2H20 [Co(NH3)6]T1Br6

Cs3T12Br9 Tlo.3WO3 T1(C104)3

T1Br3.4H20 Zn(T1CI4)4 (NH4)3T1C16 K3T1C16 TII T1Br T1C1 T1F T13PO4

T12CO3 T12SO4

T1NO3 T1C104

Reference** Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al.

(1988) (1988) (1988) (1988) (1988) (1988) (1988)

(1988) (1988) (1988)

(1988) (1988) (1988) (1988) (1988) (1988) (1988)

(1988) (1988) (1988) (1988) (1988) (1988) (1988) (1988) (1988)

606

M u l t i n u c l e a r Solid-State N M R o f l n o r g a n i c M a t e r i a l s

Table 9.11. (Continued)

Compound

~*(ppm)

Reference**

T12Ba2CuO6

3397 3108 2917 3490 3151

Winzek et al. (1990) Winzek et al. (1990) Winzek et al. (1990) Panich et al. (2001) Panich et al. (2001)

T12Ba2CaCu208 T12Ba2Ca2Cu301o (Tlo.5Pbo.5)(Bao.1Sro.9)zCaCuzOy (Tlo.5Pbo.5)(Bao.2Sro.8)2Ca2Cu3Oy

* Chemical shifts quoted with respect to aqueous T1NO3 solution ** Shifts taken from Hinton et al. (1988) which should be consulted for the original references.

A 2~ NMR study has been reported of hydrated and dehydrated zeolites A, X and Y in which thallium was exchanged for sodium (Groombridge et al. 1993). The 2~ NMR lineshapes of these compounds were generally dominated by large CSA, necessitating the stepwise acquisition of the spectra by a frequency-stepped spin-echo method. MAS did not produce significant narrowing of the spectra. The 3-fold coordinated T1 sites in zeolite A adjacent to 6-membered tings were assigned to a resonance with a ~iiso value of 1183 ppm and a large CSA of - 1842 ppm, while the resonance corresponding to the remaining T1 site was isotropic, with ~iso = 300 ppm. The spectrum of dehydrated zeolite X shows an isotropic peak at 854 ppm associated with TI(I) in the double-ring site and a broader aniosotropic resonance possibly associated with TI(III) in the supercage. The NMR data suggest that the cation distribution in dehydrated zeolite Y is disordered (Groombridge et al. 1993). Semiconducting compounds such as T1Se have a chain structure and are of interest for their potential applications in optoacoustic and optoelectronic devices. A combination of 2~ and 2~ NMR has been used to study the indirect nuclear exchange coupling, electronic structure and wave-function overlap in single-crystal T1Se (Panich and Gasanly 2001). The NMR study indicates the existence of strong exchange coupling between the spins of the TI(I) and TI(III) ions in this compound arising from an overlap of the T1+ and T13+ electron wave-functions across the intermediate Se atom. This wave function overlap determines the electronic structure and properties of T1Se since it provides the dominant mechanism for the formation of the uppermost valence bands and the lower conduction bands (Panich and Gasanly 2001). The compound T12Te3 is a p-type semiconductor in which the thallium atoms are located in channels between puckered Te layers. A 2~ and 2~ NMR study of this material has revealed that significant indirect exchange coupling occurs between the nuclei by overlap of the thallium electron wave functions, mainly across the Te atoms. Good agreement was found between the NMR conclusions and the calculated electronic structure and density of states in this semiconductor (Panich and Doert 2000). Thallium occurs in a class of high-Tc superconducting compounds of general type Tl(Ba,Sr)2Can_lCunOy, of which the phase Tl(Ba,Sr)zCa2Cu3Oy has a Tc well above


607

110K, yields high critical currents and shows a relatively high irreversibility field. This phase can be more readily fabricated if the Ba is partially substituted by Sr, and doped with PbO and/or Bi203. Pb-doped T1 superconductors such as (Tlo.sPbo.5)(Bao.zSro.8)2 Ca2Cu3Oy and (Tlo.5Pbo.5)(Bao.lSro.9)zCaCu2Oy have proved to be excellent candidates for study by 2~ 2~ and 2~ NMR, yielding information about the valence states of these cations and the flux line dynamics in the vortex lattice (Panich et al. 2001). The 2~ spin-lattice relaxation times were determined at temperatures between 68 and 300 K, and the chemical shifts of the spectra indicated that the T1 is in the +3 valence state. A linear relationship was also observed between the 2~ shifts and the Tc values, with a similar relationship also operating for the 2~ shifts (Panich et al. 2001). Although the T1 atoms are located in the insulating T1-O layer of these superconductors, a positive Knight shift contribution has been reported, attributed to superexchange interactions resulting from the overlap of the T1 orbitals with the Cu 2+ dZz orbitals via the Pz orbital of the bridging apex oxygen (Winzek et al. 1990). Measurements of the temperature-dependence of the 2~ NMR linewidth have provided information about the vortex mobility, melting point and phase transitions of the vortex lattice, indicating anisotropic melting of the vortex state in the strontium and lead-substituted T1 superconductors (Panich et al. 2001).

9.3.11 207 P b N M R

2~ is a potentially useful nucleus for NMR spectroscopy, having a favourable gyromagnetic ratio (5.5968 • 10 -7 rad T-l), a reasonably high resonance frequency and a natural abundance of 22.6%. Its main drawback as a practical NMR nucleus is its large number of electrons whose polarisability can lead to a large CSA which in turn produces broad static NMR lineshapes and complicated MAS spinning sideband patterns even at spinning speeds of 15 kHz, resulting in phasing difficulties and ambiguities in identifying the isotropic bands. Where there are even small deviations from spherical symmetry of the Pb environment, the CSA can exceed several thousand ppm with isotropic chemical shifts coveting a range of 16,000 ppm. However, despite these drawbacks, the practical importance of lead compounds as environmental contaminants as well as ferroelectric and piezoelectric materials has maintained a high degree of interest in 2~ NMR and led to improved structural correlations with the chemical shift, allowing useful structural information to be extracted from 2~ NMR spectra. The chemical shifts of 2~ spectra are normally referenced to Pb(CH3)4, but crystalline Pb(NO3)2 with a shift of - 3473.6 ppm or aqueous Pb(NO3)2 solution at - 2941 ppm can be used as secondary reference compounds. 9.3.11.1 Correlations between Z~ chemical shifts and structure. On the basis of the 2~ NMR spectra of a number of lead oxides and silicates, Fayon et al. (1997) have deduced empirical relationships between ~iso and structural parameters. Provided


608

care is taken to correctly analyse the sometimes complex overlapping sideband patterns, the ~iso values for ionic compounds show good linear correlations with both the coordination number (CN) and the mean bond length in nm (Figure 9.35A). These relationships are described respectively by

~iso (ppm) = 622.8 - 349.7CN

(9.7)

6iso ( p p m ) = 2 0 8 5 4 - 86689.5(Pb-0)

(9.8)

For more covalent compounds, the best empirical correlation was obtained by taking into account the degree of oxygen s-p hybridization p, given by (9.9)

p = (cos 0)/(cos 0 - 1)

where 0 is the Pb-O-X bond angle. The correlation for covalent compounds also takes into account the next-nearest neighbour electronegativity (AN) in defining a parameter P P = p + AN

(9.10)

which shows a satisfactory linear correlation with the isotropic chemical shifts of a number of covalent lead compounds (Figure 9.35B) (Fayon et al. 1997) defined by the equation (9.11)

~iso (ppm) = 2076.2 - 2180.2P A

B o

t~

1ooo

o

o

~

15oo

-1000

.

t,o o

o

-3000

-500 t

0.22

500

I

0.24 0.26 0.28 Mean Pb-O length (nm)

-.

o:

0.4

0.8

1.2

P

Figure 9.35. A. Relationship between the Z~ isotropic chemical shift and the mean Pb-O bond length in lead compounds. Filled symbols denote sites with CN < 7, open symbols denote CN > 7. B. Relation between the 2~ isotropic chemical shift in lead compounds and P, a parameter defined in Equation 9.10 taking into account the degree of oxygen hybridisation and the next-nearest neighbour electronegativity. From Fayon et al. (1997), by permission of the American Chemical Society.


609

Attempts to establish relationships between the 2~ CSA values of lead compounds and the average deviation of the bond angles or bond lengths from those of an ideal polyhedron have proved less successful (Fayon et al. 1997). 9.3.11.2 Z~

N M R o f crystalline lead compounds. PbO occurs in 2 forms, the

tetragonal ~-form (red) having an oxygen-lead layer structure with interlayer Pb-Pb bonding, and the orthorhombic B-form (yellow) consisting of chain units with interchain Pb-Pb bonding. The 2~ NMR spectra of both oxides show significant CSA effects (Gabuda et al. 1999, Zhao et al. 1999) of - 2 2 1 2 ppm (red) and - 2 5 7 6 ppm (yellow) and isotropic chemical shifts which are paramagnetically shifted as a result of the electron density of the Pb sites being considerably distorted from spherical symmetry by the presence of a lone electron pair. The larger CSA of yellow PbO is consistent with the lower symmetry of the Pb site in this oxide (Zhao et al. 1999). The oxide Pb304 contains both Pb(IV) in a pseudo-octahedral environment and Pb(II) with 4 nearest neighbour oxygen atoms. The 2~ NMR spectrum contains 2 overlapping resonances corresponding to the 2 Pb environments (Zhao et al. 1999). The symmetric environment of the Pb(IV) site gives rise to a small CSA (98 ppm) whereas the Pb(II) site is significantly less symmetrical and has a CSA of - 1910 ppm. The 207 Pb NMR spectra suggest that scalar coupling occurs between the Pb(IV) and 2 of the 4 surrounding Pb(II) atoms, rather than between magnetically equivalent Pb(IV) sites as proposed by Fayon et al. (1997). The chemical shift of the Pb(II) site has been attributed to a Pb-Pb contribution to the shielding arising from a strong PbZ+-Pb 2+ exchange interaction (Gabuda et al. 1999a). Crystalline solid solutions formed between the nitrates of lead and strontium have been investigated by 2~ NMR which shows up to 13 lines arising from Pb sites with up to 12 nearest neighbour cations replaced by Sr (Figure 9.36) (Kye and Harbison 1998). The presence of the Sr produces an average shift in the 2~ resonance of 21.8 ppm per Sr2+, with similar but smaller effects being observed in lead-barium nitrate solid solutions. The observed shift in the Pb resonance, which is thought to be due to differences in polarisability of the 2 ions rather than size differences, provides a convenient method of investigating the microenergetics of solid solutions in these and similar systems (Kye and Harbison 1998). 2~ NMR has also been used to study a series of solid solutions in the system Ba• utilising the sensitivity of NMR towards nearest neighbour Pb 2+ and Ba 2§ distributions to re-evaluate the validity of Vegard's Law of solid solutions which was originally deduced from a study of this system (Crundwell et al. 1999). Vegard's Law is an empirical principle stating that for some miscible solids the lattice constant varies linearly with composition, but many exceptions to this simple rule are known. The intensities of the peaks in the 2~ NMR spectra and their deviation from a simple binomial dependence suggest that the Ba and Pb are incorporated in the bulk crystals in a non-statistical manner. A linear

610


Sr c o n t e n t (mole%) ,

*

9

**~,,

9

r

100 2~

0

4.6 _

_

~

',

1

-100

. . . .

,

-300

shift (ppm) w.r.t. Pb(NO3)2

Figure 9.36. 2~ MAS NMR spectra of (Pb,Sr)(NO3)2 solid solutions of various compositions. The asterisks denote spinning side bands. Note the shift in the 2~ resonance with increasing Sr-content, and the appearance of up to 13 lines arising from the Pb sites with up to 12 nearest neighbours replaced by Sr. From Kye and Harbison (1998), by permission of the American Chemical Society. relationship was found between the 2~ isotropic chemical shift and the number of Ba ions in the first coordination sphere, confirming Vegard's additivity relationship on a local scale but indicating that the inhomogeneities in bulk crystals are responsible for the documented violations of the rule (Crundwell et al. 1999). Lead compounds such as PbTiO3, PbZrO3 and the lead zirconium titanates form the basis of many important electronic materials. The 2~ NMR spectrum of PbTiO3 (Figure 9.37A) shows a single Pb(II) site in an environment of reduced symmetry giving rise to a CSA of - 772 ppm (Zhao et al. 1999). The isotropic chemical shift shows only a weak temperature dependence probably arising from changes in the average P b - O distance, but the CSA increases markedly at lower temperatures and shows a strong correspondence with the square of the tetragonal distortion parameter (Bussian and Harbison 2000), given by CSA (ppm) = 188,000(c/a - 1)2

(9.12)

where c and a are the lattice parameters. PbZrO3 contains 2 Pb(II) sites which are apparent from the static 2~ NMR spectrum (Figure 9.37B). Both sites have 12 nearest neighbour oxygen atoms, but the

611

N M R of Other Spin -lIe Nuclei

Pb environments are of reduced symmetry with 3 shorter Pb-O distances giving rise to CSA values of - 838 and - 546 ppm (Zhao et al. 1999). The larger CSA is associated with the Pb(II) site containing the shortest Pb-O bond length (2.26A). The 2~ MAS NMR spectrum of PbZrO3 contains complex overlapping spinning sideband manifolds which can be simplified by using a two-dimensional Phase-Adjusted Spinning Sideband (PASS) experiment (Vogt et al. 2000). This yields an isotropic/anisotropic correlation diagram which when sheared gives effectively an isotropic spectrum with no sidebands (Figure 9.37C). The normal 2D PASS experiment works best at lower MAS speeds (1-5 kHz), but since lead compounds generally have large CSA values requiting high MAS speeds, a variation of the PASS experiment using multiple MAS rotor cycles has been developed to avoid pulse overlap (Vogt et al. 2000) giving excellent results for PbZrO3. Other lead compounds of importance as electronic materials which have been investigated by 2~ NMR include PbNb206, lead zirconium titanate and lead magnesium niobate. All these compounds show very broad NMR resonances, necessitating their acquisition by the point-to-point incremental frequency method (Zhao et al. 1999). Measurements of the temperature dependence of the 2~ NMR spectra of PbMgo.33Nbo.6603 between 450 and 15 K reflect the existence of polar nanoclusters in this relaxor material and can be described by a spherical random-bond random-field model (Blinc et al. 2000). A

B

PbTiO3

C

PbZrO3

PbZrO3

,Static

MAS

-1000

j

tt. I MAS

-2000

-1000 2~

-2000

9~ ' , ' l , , , ' l , , i , I

''

-1000

PASS

'[

. . . .

I ' '

-2000

shift (ppm) w.r.t. (CH3)4Pb

Figure 9.37. A. Static and MAS 2~ NMR spectra of PbTiO3,B. Static and MAS 2~ NMR spectra of PbZrO3. From Zhao et al. (1999), by permission of the American Chemical Society. C. 2~ NMR spectra of PbZrO3 obtained under MAS conditions (upper) and from a two-rotor-cycle 2D PASS experiment (lower) showing the simplification obtained by this method giving essentially an isotropic spectrum with no sidebands. From Vogt et al. (2000), by permission of the copyright owner.

612


The lead silicates PbSiO3 and the high-temperature form of Pb2SiO4 contain multiple Pb sites giving rise to complex overlapping sets of spinning sidebands in their 2~ MAS NMR spectra (Figure 9.38). PbSiO3 contains 3 inequivalent lead sites, 2 of which are located in PbO4 square pyramids in the lead-oxygen spiral chains of the structure and the third Pb site is in PbO3 trigonal pyramids. The 2 isotropic peaks in the NMR spectrum at - 366 and - 166 ppm correspond to the PbO4 units and the remaining isotropic peak at 93 ppm arises from the PbO3 unit (Fayon et al. 1997). The high-temperature form of Pb2SiO4 contains 4 inequivalent Pb sites, giving rise to an even more complex sideband pattern (Figure 9.38). The isotropic peaks in this spectrum at 1382 and 1344 ppm have been assigned from a consideration of the Pb-O bond lengths to the 2 sites containing Pb-O-Pb linkages, the 2 other isotropic peaks at 329 and 634 ppm then being assigned to PbO3 units containing Pb-O-Si bonds. Two other low-intensity isotropic peaks at 744 and 710 ppm in this sample were attributed to the presence of a small amount of another Pb2SiO4 polymorph (Fayon et al. 1997). The location of the lead cations in a series of lead-containing A, X and Y-zeolites and their mobility on exposure to water have been studied by a combination of 2~ NMR and 27AI-2~ double resonance (SEDOR) experiments (Niessen et al. 2001). Systematic changes in the 2~ relaxation time and the average chemical shift with water content suggest that the chemical environment of the lead cations is strongly affected by the sorption of water and supports a 2-site exchange model for the formation of Pb 2+ O H - entities (Niessen et al. 2001). PbSiO 3

*

.

.

I IJ

N 1000

0

Pb2SiO4

**

-1000 .

-2000

qr

I

,~ [.[,~ | I t , I t . ' , i

2000

1000 2~

shift (ppm)

i

i

i

0

simulated i

-1000

i

i

-2000

w.r.t. (CH3)4Pb

F i g u r e 9.38. Observed and simulated 2~ MAS NMR spectra of PbSiO3 and the hightemperature form of Pb2SiO4. Note the complex sideband patterns arising from the 3 Pb sites in PbSiO3 and the 4 inequivalent Pb atoms in Pb2SiO4. The isotropic lines are denoted by asterisks. From Fayon et al. (1997), by permission of the American Chemical Society.

613


The 2~ MAS NMR spectra of a number of crystalline and glassy lead fluorides have been acquired by cross-polarising with the 19F nuclei (Bureau et al. 1999). The resulting spectra (Figure 9.39A) show a wide range of isotropic chemical shifts, from - 2 6 6 6 ppm in et-PbF2 to - 3 2 1 5 ppm in PbGaF5 (these shifts quoted with respect to Pb(CH3)4 rather than Pb(NO3)2 as in the original paper). The 2~ isotropic chemical shifts in these crystalline fluorides and related lead fluoride glasses show a linear correlation with the number of next-nearest-neighbour Pb 2+ atoms (n) (Bureau et al. 1999) (Figure 9.39B), described by (9.13)

t~iso (ppm) : -- 1077 + 116n The 2~ Table 9.12.

NMR chemical shifts for a number of lead compounds are collected in

9.3.11.3 2~ N M R of lead-containing glasses. Lead silicate, borate and phosphate glasses have a number of important optical and electronic applications by virtue of their high refractive indices and low melting temperatures. A 2~ MAS NMR study of lead silicate, borate and gallate glasses containing high lead-contents has indicated extremely broad resonances at about 3000-6000 ppm with respect to crystalline Pb(NO3)2, consistent with the presence of the lead in a wide variety of sites (Yoko et al. 1992). Comparison of the chemical shifts with those of crystalline lead compounds has led to the suggestion that the majority of the lead atoms occur as trigonal PbO3 or square pyramidal PbO4 environments. A

B glass iI

~ _ ~ . ~ _ _ J ~Pb3_Ga_z-"-_ ~ " ' ' t

....

i

FI~

f

Pb9Ga2F2 4 Pb~ZnF6

~u -2940 .~ ~

>,..o~.

~-PbF~ ~....

-2340 2~

-2940

13-PbF2

-3540

-4141

shift (ppm) w.r.t. (CH3)4Pb

J

-3540

/: 8

12

No. of nnn Pb 2+

Figure 9.39. A. 2~ CP-MAS NMR spectra of crystalline and glassy Pb-F compounds. Note the large range of the isotropic chemical shifts in these compounds. B. 2~ isotropic chemical shifts of crystalline and glassy Pb fluorides as a function of the mean number of next-nearest-neighbour Pb2+ in the compounds. Filled squares denote crystalline compounds, filled and open circles denote glasses. From Bureau et al. (1999), by permission of the copyright owner.

614


Table 9.12. 2~ Compound

NMR shifts of lead compounds. giso (ppm)*

Fayon et al. (1997), Gabuda et al. (1999), Zhao et al. (1999) 1515, 1525, 1536 Fayon et al. (1997), Gabuda et al. (1999), [3-PbO (yellow) Zhao et al. (1999) - 1105, - 1091, - 1101, - 1112 Fayon et al. (1997), Gabuda et al. (1999a), Pb304(Pb 4+) Zhao et al. (1999), Yoko et al. (1992) (Pb 2+) Fayon et al. (1997), Gabuda et al. (1999a), 795,804,808, Zhao et al. (1999) Fayon et al. (1997) PbSiO3 site 1 93 Fayon et al. (1997) site 2 166 Fayon et al. (1997) site 3 - 366 Fayon et al. (1997) 329 Pb2SiO4 site 1 Fayon et al. (1997) site 2 634, Fayon et al. (1997) 1344 site 3 Fayon et al. (1997) site 4 1382 Fayon et al. (1997), Yoko et al. (1992) - 1 7 1 7 , - 1728 PbC12 Fayon et al. (1997), Neue et al. (1994), - 2 6 6 7 , - 2 6 6 8 , - 2666 oL-PbF2 Bureau et al. (1999) Bureau et al. (1999) [3-PbF2 - 2793 Bureau et al. (1999) - 2786 Pb2ZnF6 Bureau et al. (1999) - 3661 PbGaF5 site 1 Bureau et al. (1999) site 2 - 3601 Bureau et al. (1999) site 3 - 3521 Bureau et al. (1999) site 4 - 3481 Bureau et al. (1999) - 3331 Pb3Ga2F12 site 1 Bureau et al. (1999) site 2 - 3221 Bureau et al. (1999) - 2941 Pb9Ga2F24 site 1 Bureau et al. (1999) site 2 - 2821 Bureau et al. (1999) site 3 - 2691 Fayon et al. (1997), Neue et al. (1996), - 3 4 9 4 , - 3 4 9 1 . 6 , - 3494 Pb(NO3)2 Zhao et al. (1999) Fayon et al. (1997), Neue et al. (1996), - 2630, - 2622.4, PbCO3 Nolle (1977), Yoko et al. (1992) - 2 6 4 1 , - 2638 Fayon et al. (1997), Neue et al. (1996), - 3 6 1 3 , - 3 5 0 5 , - 3611 PbSO4 Zhao et al. (1999) Fayon et al. (1997), Neue et al. (1996), - 2 0 0 9 , - 2 0 0 4 . 9 , - 1989 PbMoO4 Lauterbur & Burke (1965) Nolle (1977) PbCrO4 - 2292 Nolle (1977) - 2003 PbWO4 Zhao et al. (1999) 1419 PbTiO3 Zhao et al. (1999), Vogt et al. (2000) site 1 PbZrO3 - 1 3 6 3 , - 1349 Zhao et al. (1999), Vogt et al. (2000) site 2 1 0 1 7 , 1000 Zhao et al. (1999) site 1 134 PbNb206 Zhao et al. (1999) site 2 - 2829 Yoko et al. (1992) - 2581 PbSb206 Fayon et al. (1997) 2886 Pb3(PO4)2 site 1 Fayon et al. (1997) site 2 - 2016 ~-PbO (red)

1939, 1930, 1878

Reference

615

N M R o f Other Spin -~/2 Nuclei Table 9.12. (Continued)

Compound Pb(P03)2 Pb3P4013

Pb2P207

~iso (ppm)* site 1 site 2 site 1 site 2 site 3 site 1 site 2 site 3 site 4

PbC204

-

3023 2962 2926 2906 2533 2680 2635 2571 2412 1642

Reference Fayon et al. (1999) Fayon et al. (1999) Fayon et al. (1999) Fayon et al. (1999) Fayon et al. (1999) Fayon et al. (1999) Fayon et al. (1999) Fayon et al. (1999) Fayon et al. (1999) Zhao et al. (1999)

* chemical shifts quoted with respect to Pb(CH3)4.

The presence of covalent PbO3 and PbO4 units has been confirmed in binary lead silicate glasses by 2~ NMR which has shown the existence of a lead oxide-based glass network at PbO concentrations > 60 mol % (Fayon et al. 1998). The results suggest that at higher lead contents the silicate network is destroyed and polymeric interconnected PbOn chains predominate. The increased connectivity between the PbOn units at higher lead contents leads to a very wide distribution of 2~ chemical shifts and broad N M R lineshapes, which are further broadened by disorder in the second coordination sphere resulting from Pb for Si substitution, and bond length and bond angle distributions (Fayon et al. 1999a). Similar problems of broadening occur with the 2~ NMR spectra of lead phosphate glasses, in which the broad lineshapes remain unresolved by MAS and are identical to the corresponding static spectra. The isotropic and anisotropic components of the chemical shift tensors of these glasses have been resolved by using a shifted-echo version of the PASS experiment (Fayon et al. 1999) which allows a spectrum free of spinning sidebands (corresponding to an infinite spinning rate) to be deduced. The results show a continuous distribution of the lead environment in a phosphate glass with bonding ranging from more ionic (characterised by longer Pb-O bond lengths and high anisotropy) to more covalent (shorter Pb-O bond lengths and a more pronounced lone pair effect with high anisotropy) (Fayon et al. 1999).

9 . 3 . 1 1 . 4 2~

in s o l - g e l

prepared ceramics. Pyrolysis of sol-gel-derived precursors

of lead zirconate titanate ceramics and their subsequent crystallisation has been followed using 2~ MAS NMR (Brieger et al. 1998). Since the spinning rate was not fast enough to narrow the resonance, its width must be due to chemical shift dispersion, which appears however to be reduced on recrystallisation, since this process is

616


accompanied by significant narrowing of the spectra. The widths of the 2~ NMR lines of the crystalline product were also found to be strongly dependent on the preparation method, even though X-ray diffraction indicated that all the products were similarly crystalline. This is a further illustration of the ability of NMR to detect subtleties in the short-range atomic ordering that are not observed by X-ray diffraction.

REFERENCES

Alba, M.D., Becerro, A.I., Castro, M.A. & Perdig6n, A.C. (2000) Chem. Commun., 37. Almond, G.G., Harris, R.K. & Franklin, K.R. (1997) J. Mater. Chem., 7, 681. Anderson-Altman, K.L. & Grant, D.M. (1993) J. Phys. Chem., 97, 11096. Andreev, Y.G., Lundstrrm, T., Harris, R.K., Oh, S.-E., Apperley, D.C. & Thompson, D.P. (1995) J. Alloys Compounds, 227, 102. Apperley, D.C., Harris, R.K., Marshall, G.L. & Thompson, D.P. (1991) J. Amer. Ceram. Soc., 74, 777. Aramendfa, M.A., Borau, V., Jim6nez, C., Marinas, J.M. & Ruiz, J.R. (1997) Solid State Nucl. Mag. Reson., 8, 251. Asseid, F.M., Duke, C.V.A. & Miller, J.M. (1990) Can. J. Chem., 68, 1420. Asseid, F.M., Miller, J.M. & Clark, J.H. (1992) Can. J. Chem., 70, 2398. Aujla, R.S., Dupree, R., Holland, D. & Kemp, A.P. (1987) Mater. Sci. Forum, 1%20, 147. Babonneau, F., Bois, L., Yang, C.-Y. & Interrante L.V. (1994) Chem. Mater., 6, 51. Balz, R., Haller, M., Hertler, W.E., Lutz, O., Nolle, A. & Schafitel, R. (1980) J. Mag. Reson., 40, 9. Bank, S., Bank, J.F. & Ellis, P.D. (1989) J. Phys. Chem., 93, 4847. Bartholomew, R.F. & Scheurs, J.W.H. (1980) J. Non-Cryst. Solids, 38--39, 679. Barrett, S.E. & Tycko, R. (1992) Phys. Rev. Lett., 69, 3754. Barrie, P.J. & Klinowski, J. (1992) Prog. NMR Spectrosc., 24, 91. Barrie, P.J., Groombridge, C.J., Mason, J. & Moore, E.A. (1994) Chem. Phys. Lett., 219, 491. Bastow, T.J. & Stuart, S.N. (1988) Physica Status Solidi (B), 145, 719. Bastow, T.J., Murgaski, L., Smith, M.E. & Whitfield, H.J. (1995) Mater. Lett., 23, 117. Baum, J., Gleason, K.K., Pines, A., Garroway, A.N. & Reimer, J.A. (1986) Phys. Rev. Len., 13, 1377. Berglund, B. & Vaughan R.W. (1980) J. Chem. Phys., 73, 2037. Beshah, K., Zamir, D., Becla, P., Wolff, P.A. & Griffin, R.G. (1987) Phys. Rev. B, 36, 6420. Beshah, K., Becla, P., Griffin, R.G. & Zamir, D. (1993) Phys. Rev. B, 48, 2183. Blinc, R., Gregorovic, A., Zalar, B., Pirc, R., Laguta, V.V. & Glinchuk, M.D. (2000) Phys. Rev. B, 63, 024104. Borsa, F., Torgeson, D.R. & Shanks, H.R. (1977) Phys. Rev. B, 15, 4576. Bose, M. (1991) Phys. Rev. B, 44, 5343. Boussard-P16del, C., Le Floch, M.L., Fonteneau, G., Lucas, J., Sinbandhit, S., Shao, J., Angell, C.A., Emery, J. & Buzarr, J.Y. (1997) J. Non-Cryst. Solids, 209, 247. Braddock-Wilking, J., Lin, S.-H. & Feldman, B.J. (1999) Phys. Rev. B, 59, 9745. Braddock-Wilking, J., Lin, S.-H. & Feldman, B.J. (2001) Solid State Commun., 119, 19.


617

Braun, M. & Jana, C. (1995) Chem. Phys. Lett., 245, 19. Braun, M., Hartmann, P. & Jana, C. (1995) J. Mater. Sci.: Mater. Med., 6, 150. Bray, P.J. & Holubka, E.J. (1984) J. Non-Cryst. Solids, 67, 119. Brenn, U., Ernst, H., Freude, D., Herrmann, R., J~ihnig, R., Karge, H.G., K~irger, J., K6nig, T., M~idler, B., Pingel, U.-T., Prochnow, D. & Schwieger, W. (2000) Microporous Mesoporous Mater., 40, 43. Brieger, J., Merkle, R., Bertagnolli, H. & Muller K. (1998) Berichte Bunsen Gesells. Phys. Chem., 102, 1376. Bronnimann, C.E., Zeigler R.C. & Maciel, G.E. (1988) J. Amer. Chem. Soc., 110, 2023. Brow, R.K., Osborne, Z.A. & Kirkpatrick, R.J. (1992) J. Mater. Res., 7, 1892. Brunet, F. & Schaller, T. (1996) Amer. Mineralogist, 81, 385. Brunner, E. (1990) J. Chem. Soc. Faraday Trans., 86, 3957. Brunner, E. (1993) J. Chem. Soc. Faraday Trans., 89, 165. Brunner, E. & Sternberg, U. (1998) Prog. Nucl. Mag. Reson. Spectrosc., 32, 21. Bucher, J.P., Buttet, J., van der Klink, J.J. & Graetzel, M. (1989) Surface Sci., 214, 347. Bunker, B.C., Tallant, D.R., Balfe, C.A., Kirkpatrick, R.J., Turner, G.L. & Reidmeyer, M.R. (1987) J. Amer. Ceram. Soc., 70, 675. Bureau, B., Silly, G., Buzar6, J.Y., Emery J., Legein, C. & Jacobini, C. (1997) J. Phys.: Condensed Matter, 9, 6719. Bureau, B., Silly, G. & Buzar6, J.Y. (1999) Solid State Nucl. Mag. Reson., 15, 79. Bureau, B., Silly, G., Buzar6, J.Y. & Emery J. (1999a) Chem. Phys., 249, 89. Burum, D.P., Elleman, D.D. & Rhim, W.K. (1978) J. Chem. Phys., 68, 1164. Bussian, D.A. & Harbison, G.S. (2000) Solid State Commun., 115, 95. Cardile, C.M., Meinhold, R.H. & MacKenzie, K.J.D. (1987) J. Phys. Chem. Solids, 48, 881. Carduner, K.R., Rokosz, M.J., Tamor, M.A. & Vassell, W.C. (1991) Appl. Mag. Reson., 2, 647. Caullet, P., Delmotte, L., Faust, A.C. & Guth, J.L. (1995) Zeolites, 15, 139. Chan, J.C.C. & Eckert, H. (2001) J. Non-Cryst. Solids, 284, 16. Chang, W.K. Liao, M.Y. & Gleason, K.K. (1996) J. Phys. Chem., 100, 19653. Chaudhari, K., Das, T.K., Rajmohanan, P.R., Lazar, K., Sivasankar, S & Chandwadkar, A.J. (1999)J. Catalysis, 183, 281. Chen, Q., Springuel, M.A., Fraissard, J., Smith, M.L., Corbin, D.R. & Dybowski, C. (1992) J. Phys. Chem., 96, 10914. Clark, J.H., Goodman, E.M., Smith, D.K., Brown, S.J. & Miller, J.M. (1986) J. Chem. Soc., Chem. Commun., 675. Clayden, N.J. Dobson, C.M. & Fern, A. (1989) J. Chem. Soc., Dalton Trans., 843. Clayden, N.J. & Pugh, L. (1998) J. Mater. Sci. Lett., 17, 1563. Collins, M.J., Ratcliffe, C.I. & Ripmeester, J.A. (1986) J. Mag. Reson., 68, 172. Collins, M.J., Ripmeester, J.A. & Sawyer, J.F. (1987) J. Amer. Chem. Soc., 109, 4113. Corcoran, E.W. & Vaughan, D.E.W. (1989) Solid State Ionics, 32, 423. Cossement, C., Darville, J., Gilles, J.-M., Nagy, J.B., Fernandez, C. & Amoureux, J.-P. (1992) Mag. Reson. Chem., 30, 263. Crundwell, G.A., Cowans, B.A., Hopkins, J.A., Grutzner, J.B. & Kahr, B. (1999) J. Solid State Chem., 145, 327. Cruz, L.P., Savaraiult, J.-M., Rocha, J., Jumas, J.-C. & Pedrosa, de Jesus, J.D. (2001) J. Solid State Chem., 156, 349.

618


Dando, N.R. & Tadayyoni, M.A. (1990) J. Amer. Ceram. Soc., 73, 2242. Dann S.E. & Weller, M.T. (1996) J. Mater. Chem., 6, 1717. Delmotte, L., Soulard, M., Guth, F., Seive, A., Lopez, A. & Guth, J.L. (1990) Zeolites, 10, 778. Denecke, M.A., Gunber, W., Privalov, A.F. & Murin, I.V. (1992) Solid State lonics, 52, 327. Deng, F., Yue, Y. & Ye, C. (1998) Solid State Nucl. Mag. Reson., 10, 151. Deng, Z.Q., Lambert, J.F. & Fripiat, J.J. (1989) Chem. Mater., l, 375. Duddeck, H. (1995) Prog. Nucl. Mag. Reson. Spectrosc., 27, 1. Duke, C.V.A., Miller, J.M., Clark, J.H. & Kybett, A.P. (1990) J. Mol. Catalysis, 62, 233. Duke, C.V.A., Miller, J.M., Clark, J.H. & Kybett, A.P. (1990a) Spectrochim. Acta A, 46, 1381. Dupree, R., Warren, W.W. & DiSalvo, F.J. (1977) Phys. Rev. B, 16, 1001. Dupree, R., Lewis, M.H. & Smith, M.E. (1988) J. Appl. Crystallog., 21, 109. Eckert, H., Yesinowski, J.P., Silver, L.A. & Stolper, E.M. (1988) J. Phys. Chem., 92, 2055. Eldewik, A., Hook, J.M., Singh, N.K. & Howe, R.F. (1999) Mag. Reson. Chem., 37, $63. Engelhardt, G., Sieger, P. & Felsche, J. (1992) Ang. Chem. Int. Ed. Engl., 9, 1210. Fayon, F., Farnan, I., Bessada, C., Coutures, J., Massiot, D. & Coutures, J.P. (1997) J. Amer. Chem. Soc., 119, 6837. Fayon, F., Bessada, C., Massiot, D., Farnan, I. & Coutures, J.P. (1998) J. Non-Cryst. Solids, 232-234, 403. Fayon, F., Bessada, C., Douy, A. & Massiot, D. (1999) J. Mag. Reson., 137, 116. Fayon, F., Landron, C., Sakurai, K., Bessada, C. & Massiot, D. (1999a) J. Non-Cryst. Solids, 243, 39. F6ron, B., Guth, J.L. & Mimouni-Erddalane, N. (1994) Zeolites, 14, 177. Ferreira, A., Lin, Z., Rocha, J., Morais, C.M., Lopes, M. & Fernandez, C. (2001) Inorg. Chem., 40, 3330. Finlay, G.R., Hartman, J.S., Richardson, M.F., Williams, B.C. (1985) J. Chem. Soc., Chem. Commun., 159. Fischer, L., Harl6, V., Kasztelan, S. & d'Espinose de la Caillerie, J.-B. (2000) Solid State Nucl. Mag. Reson., 16, 85. Fletcher, J.P., Risbud, S.H. & Kirkpatrick, R.J. (1990) J. Mater. Res., 5, 835. Freitas, J.C.C., Emmerich, F.G., Cernicchiaro, G.R.C., Sampaio, L.C. & Bonagamba, T.J. (2001) Solid State Nucl. Mag. Reson., 20, 61. Freude, D., Hunger, M. & Pfeifer, H. (1987) Z. Phys. Chem. Neue Folge, 152, 171. Gabriel, A.O., Riedel, R., Dressier, W. Reichert, S., Gervais, C., Maquet, J. & Babonneau, F. (1999) Chem. Mater., 11, 412. Gabuda, S.P., Kozlova, S.G., Terskikh, V.V., Dybowski, C., Neue, G. & Perry, D.L. (1999) Chem. Phys. Lett., 305, 353. Gabuda, S.P., Kozlova, S.G., Terskikh, V.V., Dybowski, C., Neue, G. & Perry, D.L. (1999a) Solid State Nucl. Mag. Reson., 15, 103. Gavish, M., Vega, S. & Zamir, D. (1993) Phys. Rev. B, 48, 2191. Gervais, C., Babonneau, F., Maquet, J., Bonhomme, C., Massiot, D., Framery, E. & Vaultier, M. (1998) Mag. Reson. Chem., 36, 407. Gervais, C., Maquet, J., Babonneau, F., Duriez, C., Framery, E., Vaultier, M., Florian, P. & Massiot, D. (2001) Chem. Mater., 13, 1700.


619

Gleason, K.K., Petrich, M.A. & Reimer, J.A. (1987) Phys. Rev. B, 36, 3259. Golzan, M.M., Lukins, P.B., McKenzie, D.R., Vassallo, A.M. & Hanna, J.V. (1995) Chem. Phys., 193, 167. Goze Bac, G., Latil, S., Vaccarini, L., Bernier, P., Gaveau, P., Tahir, S., Micholet, V., Aznar, Rubio, A., Metenier, K. & Beguin, F. (2001) Phys. Rev. B, 63, 100302. Graetsch, H. & Topalovicdierdorf, I. (1996) Chem. der Erde, 56, 387. Grey, C.P., Dobson, C.M., Cheetham, A.K. & Jakeman, R.J.B. (1989) J. Amer. Chem. Soc., 111, 505. Grey, C.P. & Vega, A. (1995) J. Amer. Chem. Soc., 117, 8232. Grill, A., Meyerson, B.S., Patel, V.V., Reimer, J.A. & Petrich, M.A. (1987) J. Appl. Phys., 61, 2874. Groombridge, C.J., Coker, E.N. & Rees, L.V.C. (1993) Mag. Resort. Chem., 31, 1064. Gtinther, B. & Kanert, O. (1985) Phys. Rev. B, 31, 20. Haeberlen, U. (1976) High Resolution NMR in Solids, Supplement 1, Academic Press, New York. Hajji, L., Rachdi, F., Goze, C., Mehring, M. & Fischer, J.E. (1996) Solid State Commun., 100, 493. Han, O.C., Timken, H.K.C. & Oldfield, E. (1988) J. Chem. Phys., 89, 6046. Hannus, I., P~ilink6, I., L~iz~ir,K, Nagy, J.B. & Kiricsi, I. (1995) J. Mol. Struct., 349, 179. Harris, R.K. & Jackson, P.J. (1991) Chem. Rev., 91, 1427. Harris, R.K., Leach, M.J. & Thompson, D.P. (1989) Chem. Mater., 1, 336 Harris, R.K., Leach, M.J. & Thompson, D.P. (1990) Chem. Mater., 2, 320. Harris, R.K., Leach, M.J. & Thompson, D.P. (1992) Chem. Mater., 4, 260. Hartman, J.S., Narayanan, A. & Wang, Y. (1994) J. Amer. Chem. Soc., 116, 4019. Hartman, J.S., Richardson, M.F., Sherriff, B.L. & Winsborrow, B.G. (1987) J. Amer. Chem. Soc., 109, 6059. Hauck, D.S.B., Harris, R.K., Apperley, D.C. & Thompson, D.P. (1993) J. Mater. Chem., 3, 1005. Hayashi, S. & Hayamizu, K. (1990) Bull. Chem. Soc. Japan, 63, 913. Hayes, S., van Wtillen, L., Eckert, H., Even, W.R., Crocker, R.W. & Zhang, Z.-(1997) Chem. Mater., 9, 901. Herzog, K., Scholz, K. & Thomas, B.A. (1994) Solid State Nucl. Mag. Resort., 3, 1. Hill, G.J., Wu, J. & Hoch, M.J.R. (1999) Hyperfine Interact., 120/121, 81. Hinton, J.F., Metz, K.R. & Briggs, R.W. (1988) Prog. Nucl. Mag. Resort. Spectrosc., 20, 423. Hiroyama, Y. & Kume, K. (1988) Solid State Commun., 65, 617. Hirschinger, J. & Herv6, M. (1994) Solid State Nucl. Mag. Resort., 3, 121. Hoch, M.J.R. & Reynhardt, E.C. (1988) Phys. Rev. B, 37, 9222. Hoffner, F.M., Delmotte, L. & Kessler, H. (1993) Zeolites, 13, 60. Holl, M.M.B., Wolczanski, D.T., Proserpio, D., B ielecki, A. & Zax, D.B. (1996) Chem. Mater., 8, 2468. Hosono, H., Abe, Y. & Deguchi, J. (1992) J. Non-Cryst. Solids, 142, 103. Hou, X. & Kirkpatrick, R.J. (2000) Chem. Mater., 12, 1890. Hou, X., Kirkpatrick, R.J., Yu, P., Moore, D. & Kim, Y. (2000) Amer. Mineralogist, 85, 173. Hunger, M. (1996) Solid State Nucl. Mag. Resort., 6, 1.

620


Huve, L., Saehr, D., Delmotte, L., Baron, J. & Le Dred, R. (1992) Comptes Rendue Acad. Sci. Paris Serie II, 315, 545. Huve, L., Delmotte, L., Martin, P., Le Dred, R., Baron, J. & Saehr, D. (1992a) Clays Clay Minerals, 40, 186. Iwasa, Y., Arima, T., Fleming, R.M., Siegrist, T., Zhou, O., Haddon, R.C., Rothberg, L.J., Lyons, K.B., Carter, H.L., Hebard, A.F., Tycko, R., Dabbagh, G., Krajewski, J.J., Thomas, G.A. & Yagi, T. (1994) Science, 264, 1570. J~iger, C., Gottwald, J., Spiess, H.W. & Newport, R.J. (1994) Phys. Rev. B, 50, 846. J~iger, C., Henning, T., Schl6gl, R. & Spillecke, O. (1999) J. Non-Cryst. Solids, 258, 161. Jana, C. & Braun, M. (1996) Biomater., 17, 2065. Jarman, R.H., Ray, G.J., Standley, R.W. & Zajac, G.W. (1986) Appl. Phys. Lett., 49, 1065. Jeschke, G., Hoffbauer, W. & Jansen, M. (1998) Solid State Nucl. Mag. Reson., 12, 1. Johnson, R.D., Meijer, G., Salem, J.R. & Bethune, D.S. (1991) J. Amer. Chem. Soc., 113, 3619. Johnson, R.D., Bethune, D.S. & Yannoni, C.S. (1992) Acc. Chem. Res., 25, 169. Jun, S., Chung, K.H. & Moon, C.H. (1996) Environ. Technol., 17, 655. Kagi, H., Parise, J.B., Cho, H., Rossman, G.R. & Loveday J.S. (2000) Phys. Chem. Minerals, 27, 225. Kao, H.M. & Grey, C.P. (1996) J. Phys. Chem., 100, 5105. Kaplan, S., Jansen, F. & Machonkin, M. (1985) Appl. Phys. Lett., 69, 2373. Kirkpatrick, R.J., Asalage, T., Phillips, B.L. & Montez, B. (1991) AlP Conf. Proc., 231, 261. Koch, W., Lutz, O. & Nolle, A. (1978) Z. Phys. A, 289, 17. Kohn, S.C. (1996) Amer. Mineralogist, 81, 1523. Kohn, S.C., Brooker, R.A. & Dupree, R. (1991) Geochim. Cosmochim. Acta, 55, 3879. Kohn, S.C., Dupree R. & Smith, M.E. (1989) Nature, 337, 539. Kohn, S.C., Dupree, R., Mortuza, M.G. & Henderson, C.M.B. (1991) Amer. Mineralogist, 76, 309. Koroglu, A., Apperley, D.C., Harris, R.K. & Thompson, D.P. (1996) J. Mater. Chem., 6, 1031. Kreinbrink, A.T., Sazavsky, C.D., Pyrz, J.W., Nelson, D.G.A. & Honkonen, R.S. (1990) J. Mag. Reson., 88, 267. Kruppa, D., Dupree R. & Lewis, M.H. (199 I) Mater. Lett., 11, 195. Kulshreshtha, S.K., Sasikala, R. & Sudarsan, V. (2001) J. Mater. Chem., 11,930. Kiimmerlen, J., Merwin, L.H., Sebald, A. & Keppler, H. (1992) J. Phys. Chem., 96, 6405. Kye, Y-S. & Harbison, G.S. (1998) Inorg. Chem., 37, 6030. Labouriau, A., Kim, Y.W., Chipera, S., Bish, D.L. & Earl, W.L. (1995) Clays Clay Minerals, 43, 697. Labouriau, A.L., Pietrass, T., Weber, W.A., Gates, B.C. & Earl, W.L. (1999) J. Phys. Chem. B, 103, 4323. L~i Manna, J., Braddock-Wilking, J., Lin, S.-H. & Feldman, B.J. (1999) Solid State Commun., 109, 573. Laperche, V., Lambert, J.F., Prost, R. & Fripiat, J.J. (1990) J. Phys. Chem., 94, 8821. Lathrop, D. & Eckert, H. (1990) J. Amer. Chem. Soc., 112, 9017. Lauterbur, P.C. & Burke, J.J. (1965) J. Chem. Phys., 42, 439.


621

Leach, M.J., Harris, R.K. & Thompson, D.P. (1990) Euroceramics, Eds de With, G., Terpstra, R.A. & Metselaar, R. Elsevier, The Hague, p. 140. Le Floch, M., Boussard-P16del, C., Fonteneau, Lucas, J., Sinbandhit, S., Emery, J. & Buzar6, J.Y. (1998) J. Phys.: Condensed Matter, 10, 5475. Lefmann, K., Buras, B., Pedersen, E.J., Shabanova, E.S., Thorsen, P.A., Rasmussen, F.B. & Sellschop, J.P.F. (1994) Phys. Rev. B, 50, 15623. Levy, D.H. & Gleason, K.K. (1992) J. Phys. Chem., 96, 8125. Levy, D.H. & Gleason, K.K. (1993) J. Vacuum Sci. Technol. A, 11, 195. Levy, D.H. & Gleason, K.K. (1993a) J. Electrochem. Soc., 140, 797. Lin, Z. Rocha, J. & Valente, A. (1999) J. Chem. Soc., Chem. Commun., 2489. Lin, Z., Rocha, J., Pedrosa de Jesus, J.D. & Ferreira, A. (2000) J. Mater. Chem., 10, 1353. Lohse, U., Jancke, K., Leffler, E. & Schaller, T. (1994) Cryst. Res. Technol., 29, 237. Luca, V., MacLachlan, D.J., Howe, R.F. & Bramley, R. (1995) J. Mater. Chem., 5, 557. Lukins, P.B., McKenzie, D.R., Vassallo, A.M. & Hanna, J.V. (1993) Carbon, 31, 569. MacFarlane, D.R., Browne, J.O., Bastow, T.J. & West, G.W. (1989) J. Non-Cryst. Solids, 108, 289. MacKenzie, K.J.D., Meinhold, R.H., McGavin, D.G., Ripmeester, J.A. & Moudrakovski, I. (1995) Solid State Nucl. Mag. Resort., 4, 193. Maekawa, H., Saito, T. & Yokohama, T. (1998) J. Phys. Chem. B, 102, 7523. Mal, N.K., Ramaswamy, V., Ganapathy, S. & Ramaswamy, A.V. (1995)Appl. Catalysis A, 125, 233. Maniwa, Y., Sato, M., Kume, K., Kozlov, M.E. & Tokumoto, M. (1996) Carbon, 34, 1291. Marian, C. & Gastreich (2001) Solid State Nucl. Mag. Reson., 19, 24. Maricq, M.M. & Waugh, J.S. (1979) J. Chem. Phys., 70, 3300. Martinez, A., Rojo, J.M., Iglesias, J.E., Sanz, J. & Rojas, R.M. (1994) Chem. Mater., 6, 1790. Martinez-Juarez, A., Rojo, J.M., Iglesias, J.E. & Sanz, J.E. (1995) Chem. Mater., 7, 1857. Mastikhin, V.M., Mudrakovskii, I.L. & Zamaraev, K.I. (1987) React. Kinetics Catalysis Lett., 34, 161. Mastikihn, V.M., Tersikh, V.V., Lapina, O.B., Filimonova, S.V., Seidl, M. & Kn6zinger, H. (1995) Solid State Nucl. Mag. Reson., 4, 369. Maxwell, R., Lathrop, D., Franke, D. & Eckert, H. (1990) Ang. Chem. Int. Ed., 29, 882. McNamara, K.M. & Gleason, K.K. (1994) Chem. Mater., 6, 39. McNamara, K.M., Levy, D.H., Gleason, K.K. & Robinson, C.J. (1992) Appl. Phys. Lett., 60, 580. Meinhold, R.H. (1987) J. Phys. Chem. Solids, 48, 927. Mennitt, P.G., Shatlock, M.P., Bartuska, V.J. & Maciel, G.E. (1981) J. Phys. Chem., 85, 2087. Mercier, C., Montagne, L., Sfihi, H., Palavit, G., Boivin, J.C. & Legrand, A.P. (1998) J. Non-Cryst. Solids, 224, 163. Miller, J.M. (1996) Prog. Nucl. Mag. Resort. Spectrosc., 28, 255. Mitra, S. & Gleason, K.K. (1993) Diamond Rel. Mater., 2, 126. Moran, L.B., Berkowitz, J.K. & Yesinowski, J.P. (1992) Phys. Rev. B, 45, 5347. Moskvich, Y.N., Rozanov, O.V., Sukhovsky, A.A. & Aleksandrova, I.P. (1985) Ferroelectrics, 63, 83. Mtiller, U., Hoffbauer, W. & Jansen, M. (2000) Chem. Mater., 12, 2341.

622


Mtiller-Warmuth, W., Schulz, G.W., Neuroth N., Meyer, F. & Deeg, E. (1965) Z. Naturforsch., 20a, 902. Mundus, C., Taillades, G., Pradel, A. & Ribes, M. (1996) Solid State Nucl. Mag. Reson., 7, 141. Murthy, V.S.R., Lewis, M.H., Smith, M.E. & Dupree, R. (1989) Mater. Lett., 8, 263. Navage, D.M., Interrante, L.V., Marchetti, P.S. & Maciel, G.E. (1991) Chem. Mater., 3, 721. Neue, G., Dybowski, C., Smith, M.L. & Barich, D.H. (1994) Solid State Nucl. Mag. Reson., 3,115. Neue, G., Dybowski, C., Smith, M.L., Hepp, M.A. & Perry, D.L. (1996) Solid State Nucl. Mag. Reson., 6, 241. Ngokoli-Kekele, P., Springuel-Huet, M.A., Man, P.P., Thoret, J., Fraissard, J. & Corbin, D.R. (1998) Microporous Mesoporous Mater., 25, 25. Niessen, H.G., Van Buskirk, M., Dybowski, C., Corbin, D.R., Reimer, J.A. & Bell, A. (2001) J. Phys. Chem. B, 105, 2945. Nolle, A. (1977) Z. Natulforsch., 32a, 964. Noma, H., Tateyama, H., Nishimura, S. & Inoue, K. (1997) Chem. Lett., 199. Nordin, J.P., Sullivan, D.J., Phillips, B.L. & Casey, W.H. (1999) Geochim. Cosmochim. Acta, 63, 3513. Orion, I., Rocha, J., Jobic, S., Abadie, V., Brec, R., Fernandez, C. & Amoureux, J.-P. (1997) J. Chem. Soc., Dalton Trans., 3741. Papenguth, H.W., Kirkpatrick, R.J., Montez, B. & Sandberg, P.A. (1989) Amer. Mineralogist, 74, 1152. Pan, Y. (1995) Solid State Nucl. Mag. Reson., 5, 263. Panich, A.M. & Doert, T. (2000) Solid State Commun., 114, 371. Panich, A.M. & Gasanly, N.M. (2001) Phys. Rev. B, 63, 195201. Panich, A.M., Goren, S.D., Frenkel Ben-Yakar, L., Eder, M. & Gritzner, G. (2001) Physica C, 356, 129. Petrich, M.A., Gleason K.K. & Reimer J.A. (1987) Phys. Rev. B, 36, 9722. Pfeifer, H., Freude, D. & Hunger, M. (1985) Zeolites, 5, 274. Pfeifer, H. (1988) J. Chem. Soc., Faraday Trans. I, 84, 3777. Pfeifer, H., Freude, D. & K~irger, J. (1991) Stud. Surface Sci. Catalysis, 65, 89. Phillips, B.L., Burnley, P.C., Worminghaus, K. & Navrotsky, A. (1997) Phys. Chem. Minerals, 24, 179. Pietrass, T. & Taulelle, F. (1997) Mag. Reson. Chem., 35, 363. Pietrass, T., Taulelle, F., Lavela, P., Olivier-Fourcade, J., Jumas, J.C. & Steuernagel, S. (1997) J. Phys. Chem. B, 101, 6715. Pietrass, T., Kneller, J.M., Assink, R.A. & Anderson, M.T. (1999) J. Phys. Chem. B, 103, 8837. Rasmussen, J.T., Hohwy M., Jakobsen, H.J. & Nielsen, N.C. (1999) Chem. Phys. Lett., 314,239. Ratcliffe, C.I., Ripmeester, J.A. & Tse, J.S. (1985) Chem. Phys. Lett., 120, 427 Raupach, M., Barron, P.F. & Thompson, J.G. (1987) Clays Clay Minerals, 35, 208. Regnier, P., Lasaga, A.C., Berner, R.A., Han, O.H. & Zilm, Z.W. (1994) Amer. Mineralogist, 79, 8O9. Reimer, T., Schmidt, B., Behrens, H. & Dupree, R. (2000) Solid State Nucl. Mag. Resort., 15,201.


623

Resing, H.A., Hodgeman, D.K.C., Baker, J.A. & Poziomek, E.J. (1985) Carbon, 23, 395. Reynhardt, E.C. & High, G.L. (2001) Prog. Nucl. Mag. Reson. Spectrosc., 38, 37. Richardson, M.F., Hartman, J.S., Guo, D. & Winsborrow, B.G. (1992) Chem. Mater., 4, 318. Robert, E., Whittington, A., Fayon, F., Pichavant, M. & Massiot, D. (2001) Chem. Geol., 174, 291. Sakida, S., Hayakawa, S. & Yoko, T. (1999) J. Non-Cryst. Solids, 243, 1. Sakida, S., Hayakawa, S. & Yoko, T. (1999a) J. Non-Cryst. Solids, 243, 13. Sakida, S., Hayakawa, S. & Yoko, T. (1999b) J. Ceram. Soc. Japan, 107, 395. Sakida, S. & Kawamoto, Y. (2000) Chem. Lett., 1046. Sakida, S., Hayakawa, S. & Yoko, T. (2000) J. Phys.: Condensed Matter, 12, 2579. Sakida, S. & Kawamoto, Y. (2001) J. Non-Cryst. Solids, 290, 15. Sakida, S., Hayakawa, S. & Yoko, T. (2001) J. Amer. Ceram. Soc., 84, 836. Sakida, S., Shojiya, M. & Kawamoto, Y. (2001 a) J. Non-Cryst. Solids, 284, 22. Sangill, R., Rastrup-Andersen, N., Bildsoe, H., Jakobsen, H.J. & Nielsen, N.C. (1994) J. Mag. Reson. A, 107, 67. Santos, R.A., Wind, R.A. & Bronnimann, C.E. (1994) J. Mag. Reson. B, 105, 183. Schaller, T., Dingwell, D.B., Keppler, H., Kn611er, W., Merwin, L. & Sebald, A. Geochim. Cosmochim. Acta, 56, 701. Schaller, T. & Sebald, A. (1995) Solid State Nucl. Mag. Reson., 5, 89. Schmidt, W.R., Interrante, L.V., Doremus, R.H., Trout, T.K., Marchetti, P.S. & Maciel, G.E. (1991) Chem. Mater., 3,257. Schonwandt, B.V. & Jakobsen, H.J. (1999) J. Solid State Chem., 145, 10. Schotte-Darie, C., Delmotte, L., Kessler, H. & Benazzi, E. (1994) Solid State Nucl. Mag. Reson., 3, 43. Schotte-Darie, C., Kessler, H. & Benazzi, E. (1994a) Stud. Surface Sci., 83, 3. Schweizer, S., Spaeth, J.M. & Bastow, T.J. (1998)J. Phys. : Condensed Matter, 10, 9111. Schwerk, U., Engelke, F., Kleber, R. & Michel, D. (1993) Thin Solid Films, 230, 102. Scotti, N., Kockelmann, W., Senker, J., Tra[3el, S.T. & Jacobs, H. (1999) Z. Anorg. Allg. Chem., 625, 1435. Sebald, A., Merwin, L.H., Dollase, W.A. & Seifert, F. (1990) Phys. Chem. Minerals, 17, 9. Sebald, A. (1994) NMR Basic Principles and Progress, Eds Blumich, B. & Kosfeld, R., Springer-Verlag, Berlin, Vol. 31, p.91. Senegas, J., Bobe, J.M. & Reau, J.M. (1994) Solid State Commun., 89, 983. Shelimov, B., Lambert, J-F., Che, M. & Didillon, B. (1999) J. Amer. Chem. Soc., 121,545. Sheng, T.C., Kirszensztejn, P., Bell, T.N. & Gay, I.D. (1994) Catalysis Lett., 23, 119. Shi, J., Wessels, M. & Ross, J.H. (1993) Phys. Rev. B, 48, 8742. Sjoberg, J., Harris R.K. & Apperley, D.C. (1992) J. Mater. Chem., 2, 433. Smith, K.A. & Burum, D.P. (1989) J. Mag. Reson., 84, 85. Spearing, D.R., Stebbins, J.F. & Farnan, I. (1994) Phys. Chem. Minerals, 21, 373. Stebbins, J.F. & Zeng, Q. (2000) J. Non-Cryst. Solids, 262, 1. Storek, W., Peuker, C. & Geissler, H. (2000) Glastechnische Berichte, 73, 373. Tang, X.P., Kleinhammes, A., Shimoda, H., Fleming, L., Bennoune, K.Y., Sinha, S., Bower, C., Zhou, O. & Wu, Y. (2000) Science, 288, 492. Thayer, A.M., Steigerwald, M.L., Duncan, T.M. & Douglas, D.C. (1988) Phys. Rev. Lett., 60, 2673.

624


Tinet, D., Faugere, A.M. & Prost, R. (199 l) J. Phys. Chem., 95, 8804. Tong, Y.Y., van der Klink, J.J., Clugnet, G., Renouprez, A.J., Laub, D. & Buffat, P.A. (1993) Surface Sci., 292, 276. Tong, Y.Y. & van der Klink, J.J. (1994) J. Phys. Chem., 98, 11011. Tong, Y.Y. & van der Klink, J.J. (1994a) J. Phys. : Condensed Matter, 6, L533. Topi, B., von Kienlin, A., G61zh~iuser, A. & Haeberlen, U. (1988) Phys. Rev. B, 38, 8625. Tunstall, D.P., Patou, S., Liu, R.S. & Kao, Y.H. (1999) Mater. Res. Bull., 34, 1513. Turner, G.L., Kirkpatrick, R.J., Risbud, S.H. & Oldfield, E. (1987) Amer. Ceram. Soc. Bull., 70, 675. Tycko, R. (1994) Solid State Nucl. Mag. Reson., 3, 303. Tycko, R., Dabbagh, G., Fleming, R.M., Haddon, R.C., Makhija, A.V. & Zahurak, S.M. (1991) Phys. Rev. Lett., 67, 1886. Tycko, R., Dabbagh, G., Vaughan, G.B.M., Heiney, P.A., Strongin, R.M., Cichy, M.A. & Smith, A.B. (1993) J. Chem. Phys., 99, 7554. Tycko, R., Haddon, R.C., Dabbagh, G., Glarum, S.H., Douglass, D.C. & Mujsce, A.M. ( 1991) J. Phys. Chem., 95, 518. Ueda, S. & Shimizu, T. (1979) Physica Status Solidi (B), 95, 279. van Wtillen, L. & Jansen, M. (2001) J. Mate:'. Chem., 11,223. Velu, S., Suzuki, K., Okazaki, M., Osaki, T., Tomura, S. & Ohashi, F. (1999) Chem. Mate:'., 11, 2163. Vinogradova, I.S., Sukhovskii, A.A. & Khizbullin, F.F. (1989) J. Solid State Chem., 78, 209. Vogt, F.G., Gibson, J.M., Aurentz, D.J., Mueller, K.T. & Benesi, A.J. (2000) J. Mag. Reson., 143, 153. Wagner, G.W., Na, B.-K. & Vannice, M.A. (1989) J. Phys. Chem., 93, 5061. Wang, F. & Grey, C.P. (1995) J. Amer. Chem. Soc., 117, 6637. Wang, F. & Grey, C.P. (1997) Chem. Mater., 9, 1068. Wang, F. & Grey, C.P. (1998) Chem. Mater., 10, 3081. Wark, M., Schwenn, H.J., Warnken, M., Jaeger, N.I. & Boddenberg, B. (1995) Stud. Sulface Sci. Catalysis, 97, 205. White, D.J., Cox, E.R., Westrick, N., Pan, Y., Gwynn, A. & Arends J. (1995) J. Dental Res., 74, 401. Willig, A., Sapoval, B., Liebler, K. & V6ri6, C. (1976) J. Phys. C, 9, 1981. Winzek, N., Hentsch, F., Mehring, M, Mattausch, H., Kremer, R. & Simon, A. (1990) Phys. C, 168, 327. Withers, A.C., Kohn, S.C., Brooker, R.A. & Wood, B.J. Geochim. Cosmochim. Acta, 64, 1051. Wrackmeyer, B., Horchler, K., Sebald, A., Merwin, L.H. & Ross, C. (1990) Ang. Chem. Int. Ed., 29, 807. Wrackmeyer, B. (1999) Ann. Rep. Nucl. Mag. Reson. Spectrosc., 38, 203. Wu, X. & Zilm, K.W. (1993) J. Mag. Reson. A, 102, 205. Yang, J.-H., Clark, L.A., Ray, G.J., Kim, Y.J., Du, H. & Snurr, R.Q. (2001) J. Phys. Chem. B, 105, 4698. Yang, K.H. & Woo, A.J. (1996) Bull. Korean Chem. Soc., 17, 696. Yannoni, C.S., Bemier, P., Bethune, D.S., Meijer, G. & Salem, J.R. (1991 a) J. Amer. Chem. Soc., 113, 3190.


625

Yannoni, C.S., Johnson, R.D., Meijer, G., Bethune, D.S. & Salem, J.R. (1991) J. Phys. Chem., 95, 9. Yano, T., Mizuno, J., Shibata, S., Yamane, M., Inoue, S. & Onoda, Y. (1997) J. Non-Cryst. Solids, 213-214, 345. Yesinowski, J.P. & Eckert, H. (1987) J. Amer. Chem. Soc., 109, 6274. Yesinowski, J.P., Eckert, H. & Rossman G.R. (1988) J. Amer. Chem. Soc., 110, 1367. Yesinowski, J.P. & Mobley, M.J. (1983) J. Amer. Chem. Soc., 105, 6191. Yoko, T., Kiyoharu, K., Miyaji, F. & Sakka, S. (1992) J. Non-Cryst. Solids, 150, 192. Zax, D.B., Vega, S., Yellin, N. & Zamir, D. (1987) Chem. Phys. Lett., 138, 105. Zax, D.B., Zamir, D. & Vega, S. (1993) Phys. Rev. B, 47, 6304. Zeng, Q., Nekvasil, H. & Grey, C.P. (1999) J. Phys. Chem. B, 103, 7406. Zeng, Q. & Stebbins, J.F. (2000) Amer. Mineralogist, 85, 863. Zeng, Q., Stebbins, J.F., Heaney, A.D. & Erdogan, T. (1999) J. Non-Cryst. Solids, 258, 78. Zhao, P., Prasad, S., Huang, J., Fitzgerald, J.J. & Shore, J.S. (1999) J. Phys. Chem. B, 103, 10617.


Chapter 10

NMR of Other Quadrupolar Nuclei 10.1.

10.2. 10.3.

10.4.

6Li and 7Li NMR 10.1.1 General Considerations 10.1.2 6'7LiNMR of Crystalline Solids 10.1.3 Relation between 6Li Chemical Shifts and Structure 10.1.4 6'7LiNMR of Fast Lithium Ion Conductors 10.1.5 6'7LiNMR of Glasses 9Be NMR 51V NMR 10.3.1 General Considerations 10.3.2 5~V NMR of Vanadium Oxides and the Vanadates 10.3.3 51V NMR of Zeolites and Catalysts 63Cu and 65CuNMR 10.4.1 63CuNMR of Superconductors and Superfast Ionic Conductors

10.5. 69Ga and 71Ga NMR 10.5.1 General Considerations 10.5.2 69'71GaNMR of Crystalline Compounds 10.5.3 69'71GaNMR of Other Compounds 10.6. 87RbNMR 10.6.1 General Considerations 10.6.2 87RbNMR of Crystalline Compounds 10.6.3 87RbNMR of Rubidium Fullerides 10.7. 93Nb NMR 10.8. 133CsNMR 10.8.1 General Considerations 10.8.2 133CsNMR of Crystalline Caesium Compounds 10.8.3 133CsNMR of Minerals and Zeolites 10.8.4 ~33CsNMR of Fullerides, Superionic Conductors and Semiconductors 10.9. 139LaNMR References

629 629 630 634 636 638 639 642 642 642 646 649 650 653 653 655 657 658 658 658 661 662 665 665 666 669 673 674 678


Chapter 10

NMR of Other Quadrupolar Nuclei The quadrupolar nuclei of greatest importance to materials science (27A1 and 170) have been dealt with in Chapters 5 and 6 respectively. Two other important quadrupolar nuclei (23Na and liB) have also been treated separately in Chapter 7. The present chapter deals with a number of the other quadrupolar nuclei encountered in solid state NMR studies of inorganic materials.

10.1. 6Li AND 7Li NMR

10.1.1 Generalconsiderations Both the lithium nuclides are suitable for NMR spectroscopy. The spin = 3/2 nucleus VLi is commonly used since it has a high natural abundance (92.5%) and favourable receptivity, but the quadrupole moment ( - 4 . 0 • 10 -3~ e m 2) can give rise to relatively broad lines from Li in non-symmetrical sites. The Larmor frequency of the spin = 1 nucleus 6Li is about 2.6 times smaller than 7Li and it has a much lower natural abundance (7.5%) and hence a less favourable receptivity, but its quadrupole moment is also significantly lower than 7Li and its homonuclear dipole-dipole interactions are much weaker, giving narrower resonance lines under MAS conditions. Thus, although 6Li has in the past been less widely used in solid-state NMR studies, its use can be preferable in circumstances requiring the resolution of two close resonances. Because of its smaller quadrupolar interaction, the 6Li shift may also more closely approximate the isotropic chemical shift and provide a better measure of the Li bonding environment, but these benefits may be offset by the relaxation times which are often very long, and require longer data acquisition times. Comparing 2 isotopes with very different interactions can also be used to understand the source of relaxation and hence the motion responsible. Figure 10.1A shows the 7Li MAS NMR spectrum of Li-containing beryl, one of the few materials in which the 7Li-VLi homonuclear dipole-dipole interactions are sufficiently small to be removed by magic angle spinning, revealing the second-order quadrupolar lineshape. In most other materials, dipolar-dipolar broadening renders the 7Li lineshape broad and featureless even under MAS conditions. By contrast, the 6Li MAS NMR spectrum of the same sample (Figure 10.1B) shows a sharp resonance very close to the isotropic chemical shift, since the second-order quadrupolar shift is negligible. 6'7Li NMR chemical shifts are commonly measured with respect to aqueous LiC1 solution.

629

630


A

B

7Li

6Li

ated ~observed '

2.4

'

'

'

0.8

~

-0.8

...... _ ~ ....

I ....

, .........

6

I .....

9........

3

I ..............

0

l ..........

-3

Li shift ( p p m ) w.r.t. L i C I Figure 10.1. A. 7Li MAS NMR observed and simulated spectra of Li-substituted beryl, spinning speed 10 kHz. Simulation parameters XQ = 0.66 MHz, r I = 0.2. B. 6Li MAS NMR spectrum of the same sample, spinning speed 6 kHz. From Xu and Stebbins (1995), by permission of the copyright owner.

10.1.2 6'7Li NMR of crystalline solids A s e l e c t i o n o f 6Li and 7Li N M R shifts for l i t h i u m c o m p o u n d s are s h o w n in T a b l e 10.1.

Table 10.1. 6'7Li NMR parameters for lithium compounds. Compound

~iso (ppm)*

LiI LiBr LiA1Si4Olo (petalite) LiA1Si206 (spodumene) LiA1SiO4 (eucriptite) Nao.3(Mg,Li)3SiaOlo(F,OH)2 (hectorite) K(Li,A1)3(Si,A1)40~o(F,OH)2 (lepidolite) BeAlzSi6018 (beryl) LiA1Si206.H20 (bikitaite) LiAla(Si3A1)Olo(OH)8 (cookeite) Laponite LizSiO3

- 2.312 - 2.152, - 0.359 0.1 - 1.0 0.2 - 0.8 - 0.8 to - 1.0 0 . 9 , - 1 . 0 , - 1.5 0.1 - 0.6 - 0.735 0.200, 0.44

Li2Si205 Li4SiO4 NaLiSiO4 NaLi3SiO4 LiSiON LiSieN3

0.2 1.5, 0.8, 0.2, - 0.7 - 0.69 0.76 0.27I" 1.315"

Reference

6Li Bond et al. (1991) Bond et al. (1991) Xu & Stebbins (1995) Xu & Stebbins (1995) Xu & Stebbins (1995) Xu & Stebbins (1995) Xu & Stebbins (1995) Xu & Stebbins (1995) Xu & Stebbins (1995) Xu & Stebbins (1995) Bond et al. (1991) Bond et al. ( 1991), George et al. (1998) Xu & Stebbins (1995) Xu & Stebbins (1995) Gee et al. (1997) Gee et al. (1997) Kempgens et al. (1999) Kempgens et al. (1999)

631

NMR of Other Quadrupolar Nuclei

Table 10.1. (Continued) Compound

~iso ( p p m ) *

NaLiSO4 NaLi3SiO4 LisB7S13 LIPS3 Li4P2S6 Li3PS4 LivPS6 LiSiON LiSizN3

- 0.69 0.47 -2 1.76"* 1.45"* 2.80** 2.08** 0.17t 1.27t

Reference

7Li

Gee et al. (1997) Gee et al. (1997) Griine et al. (1995) Eckert et al. (1990) Eckert et al. (1990) Eckert et al. (19900 Eckert et al. (1990) Kempgens et al. (1999) Kempgens et al. (1999)

* Chemical shifts quoted with respect to LiC1 solution. ** With respect to solid LiC1 ~ determined at 7.05 T

Lithium orthosilicate, Li4SiO4, has an interesting structure containing LiO4, LiO5 and LiO6 units. One of the LiO4 sites has one very long Li-O bond, making it effectively LiO3. These polyhedra are connected by edge-sharing to form the three-dimensional structure. The 6Li MAS NMR spectrum of an enriched sample (Figure 10.2A) shows 3 well-resolved peaks at 1.5 ppm (LiO3), 0.8 ppm (LiO4), - 0.7 ppm (LiO6) and a shoulder at about 0.2 ppm attributed to LiO5 units (Xu and Stebbins 1995). As the temperature of the sample is raised, these resolved spectral peaks merge into a single broad line due to hopping of the Li + between all the sites (Xu and Stebbins 1995a). Two-dimensional 6Li NMR exchange spectra (Figure 10.2 B) have been used to provide a detailed picture of the hopping rates of the Li + among the various sites. The two-dimensional spectra are determined at various mixing times. If the mixing time of the experiment is faster than the rate of exchange between the sites, the normal one-dimensional spectrum appears on the diagonal and there are no other peaks. As the mixing time becomes longer, crosspeaks appear at the coordinates of the 2 peaks involved in Li + exchange, allowing exchange rates and activation energies to be determined for the various sites (Xu and Stebbins 1995a). The static 7Li NMR spectra of LizSiO3 recorded as a function of temperature from ambient to 1150~ show a well-defined quadrupolar lineshape at all temperatures, indicating that the Li in this compound is not sufficiently mobile to display an averaged isotropic environment even at temperatures 50~ below the melting point (George et al. 1998). The room-temperature second-order quadrupolar lineshape can be simulated by assuming XQ = 0.15 MHz and xl = 0.65, but above 600~ an additional narrower quadrupolar lineshape appears, with XQ = 0.03 MHz and xl = 0. These results have been tentatively explained in terms of a partial dynamical averaging of the spectra caused by Li + exchange from one position to another. Complete averaging, evidenced

632

Multinuclear Solid-State NMR of Inorganic Materials B

4

6

LiO 4 3

uo~/i 4

~4-6 3-6 3

I

-1

6Li shift (ppm) w.r.t. LiC! Figure 10.2. A. 6Li MAS NMR spectra of 6Li-enriched Li4SiO4, from Xu and Stebbins (1995), with permission of the copyright owner. B. Two-dimensional pure-absorption exchange 6Li NMR spectra of Li4SiO4 at two different mixing times. Note at the slower mixing time of 47 ms (top spectrum) only the peaks corresponding to the primary units are seen. At the faster mixing time of 188 ms (lower spectrum) peaks corresponding to Li exchange between the various sites appear. The horizontal axis is to2. Adapted from Xu and Stebbins (1995a). by the collapse of the spectrum to a single narrow peak is not observed, suggesting that the Li motion does not sample a wide enough set of site geometries and orientations for their time average to achieve spherical symmetry (George et al. 1998). The 6Li MAS NMR spectrum of LizSiO3 consists of a single narrow peak with a typically tetrahedral shift (0.44 ppm) but the relaxation time (hundreds of seconds) is too long for most practical purposes (George et al. 1998). Lithium aluminate, LiA1508 with the inverse spinel structure, is a material with possible applications in ceramic blankets for thermal control of fusion reactors. 6Li and 7Li NMR has been used to measure the spin-lattice relaxation of lithium in this compound (Stewart et al. 1995). The results indicate that 6Li relaxes most significantly through interactions with paramagnetic impurities, whereas 7Li relaxes much more strongly through dipole-dipole interactions. Small unexpected differences in the crystal structure of LiKSO4 have been revealed by a single-crystal 7Li NMR study which has indicated XQ and xl values of 0.025 MHz and 0.15 respectively for this compound (Lim et al. 1996, Lim and Jeong 2001). These results, taken together with 39K parameters determined for the same sample, indicate a non-axially symmetric EFG tensor, suggesting that the LiO4 unit is slightly distorted from its expected symmetry, possibly resulting from an artifact of the crystal growing conditions. The temperature dependence of the 7Li XQ and xl values of single-crystal LiKSO4 indicates the occurrence at 190 K of a first-order transition to ferroelastic domains characterised by lowering of the Li site symmetry (Lim et al. 1997). The


633

7Li and 39K NMR spectra of this material, measured at 180 K, have allowed the structure of the ferroelastic phase to be directly inferred from the domain pattern of the low-temperature NMR spectra (Lira and Jeong 2000). The temperature dependence of the 7Li NMR spectrum of single-crystal LiRbSO4 has been determined in the temperature range 140-400 K. The room-temperature value of XQ (20.4 kHz) decreases with increasing temperature and has been interpreted in terms of the torsional frequency of Li-O (Lim et al. 1997a). The 7Li NMR spectrum of single-crystal LiCsSO4 has also been determined, and shows 3 sets of signals explained in terms of 3 types of growth twin-domains rotated with respect to each other by 60 ~ around the c-axis (Lira and Jeong 1999). The temperature dependence of these spectra indicates a second-order phase transition to ferroelastic domains with lowered Li site symmetry below 200 K (Lira and Jeong 1998). Measurements of the temperature dependence of the 7Li NMR spectrum of singlecrystal LiNHaSO4 demonstrate the occurrence of a first-order phase transition at 285 K (Lim et al. 2000a). The room temperature values of • and Xl (25 kHz and 0.22 respectively) increase with decreasing temperature, explained in terms of a change in the Li-O torsional frequency. Differences in the temperature dependence of the 7Li XQ values for the series of single crystals LiXSO4 (where X = K, Rb, Cs and NH4) have been explained in terms of differences in the torsional motion of the LiO4 tetrahedra about the x-axis of the EFG tensor, which, in turn, can be related to differences in the atomic weight of the X ion (Lim et al. 2000). Laponite, [Mg,Li]6SisOzo(OH)4Nao.sv.nH20, a synthetic form of hectorite, is a trioctahedral layer silicate with the structure of talc in which some octahedral Mg is substituted by Li, the charge balance provided by interlayer Na + or Ca 2+. These substitutions give the material useful cation exchange properties and also influence its thermal decomposition behaviour. The thermal decomposition of laponite has been studied by a combination of VLi, 298i and 25Mg MAS NMR (MacKenzie and Meinhold 1994). Loss of interlayer water at about 200~ produces very little change in the 7Li spectra, but just prior to dehydroxylation at 650~ discontinuities in the downfield trend of the 7Li shift with increasing temperature suggest the movement of interlayer Na closer to the tetrahedral sheets, influencing the Li in the octahedral sites (MacKenzie and Meinhold 1994). A similar downfield shift with increasing temperature reported in the 6Li spectra of heated laponite has been ascribed to the movement of Li from trioctahedral sites to edge sites (Bond et al. 1991). By contrast with the spinel LiTi204, which contains only tetrahedral Li sites, the superconducting spinel phase Li~.33Ti1.6704 contains both tetrahedral and octahedral Li sites, but the chemical shift difference between them is too small to be resolved even with very high MAS spinning speeds. The static 7Li spectrum is considerably broader than that of LiTi204 due to dipolar broadening between 7Li sites. This observation has led to an interesting experiment in which the Li-Li separation was increased by

634


isotopically diluting the sample with enriched 6Li (Dalton et al. 1994). The resulting 7Li MAS NMR spectrum showed structure resolvable into 2 Gaussian lineshapes with shifts of 0.2 and - 0.09 ppm. These resonances were identified as the tetrahedral and octahedral resonances respectively, by comparison with the known Li shifts in other compounds (8 for octahedral Li = - 0.1 to - 0.6 ppm, 8 for tetrahedral Li = 2.4 ppm) (Dalton et al. 1994). The lithium silicon nitride phases LiSiON and LiSi2N3 have been studied by 6Li and 7Li NMR at 2 magnetic field strengths (Kempgens et al. 1999). The spectra of both compounds consist of an intense central resonance with associated spinning side band manifolds. Although the difference between the isotropic chemical shift is small (Table 10.1), the 2 phases can readily be distinguished by their 7Li XQ and xI values (130 kHz and 0.55 for LiSiON and 100 kHz and 0.9 for LiSizN3). The quadrupolar interaction in the 7Li NMR spectra is much larger than the chemical shift interaction, making it difficult to accurately determine the small CSA and the relative orientation of the 2 interactions by 7Li NMR. These parameters can, however, be determined much more accurately from the 6Li NMR spectra (Kempgens et al. 1999).

10.1.3 Relation between 6Li chemical shifts and structure The 6Li NMR spectra of silicates are dominated by chemical shift effects, and this, together with their superior resolution makes them potentially useful for providing structural information. The 6Li chemical shifts (peak positions) of a series of silicate and aluminosilicate minerals derived from MAS NMR measurements have been shown to correlate well with the Li coordination numbers derived from single-crystal X-ray measurements (Figure 10.3A) (Xu and Stebbins 1995). This systematic decrease in the 6Li chemical shift with increasing oxygen coordination number (CN) resulting from increased shielding can be described by the linear relationship: 6 (rLi) = -0.608(CN) + 2.91

(~o.1)

This trend, which is in the same direction as for 27A1, 29Si, 23Na and 25Mg, appears to be related to the increase in Li-O bond length and Li + ionicity with increasing coordination number. A similar trend in the 6Li isotropic shift with ionicity has also been reported in sulphide-based glasses (Eckert et al. 1990). This simple relationship between the 6Li shifts and Li coordination number has been found not to hold for crystalline lithium phosphates, indicating that other factors must be taken into account (Alam et al. 1999). By analogy with 23Na NMR (Chapter 7), Alam et al. (1999) sought a relationship between the 6Li shift and the average degree of polymerisation in the phosphate tetrahedra as reflected by the number of non-bridging

635


oxygen atoms per phosphate tetrahedron. The scatter in this relationship indicates that it does not satisfactorily describe the situation in the lithium phosphates. However, the chemical shift parameter (A) derived for 23Na shifts by Koller et al. (1994) in terms of the bond valences of the neighbouring oxygens was found by Alam et al. (1999) to give a good linear relationship with 6Li shifts (Figure 10.3B), described by

6Li + (ppm) -- 4.30A - 5.85

(10.2)

Similarly, a linear relationship was found for the Li sites in crystalline Li4SiO4 (Figure 10.3B), described by

(10.3)

6Li ~ (ppm) -- 7.50A - 8.52

By contrast with the results for 23Na (Chapter 7), both these lines have a positive slope. This unexplained descrepancy suggests that the formalism for defining the parameter A requires refinement, although the treatment is potentially useful within limited groups of similar compounds. A

B 2

/ / p / / /

~, ]

/

~o

/

o 0

_~

9

I ....

i ....

-1

....

i ....

4

i ....

6

i ....

l .+++:+ _

8

Li coordination number

-2 I

I 1.1

v

P 1.3

:

4 1.5

Chemical shift parameter A

Figure 10.3. A. Relationship between the 7Li chemical shift of lithium silicates and aluminosilicates and the Li coordination number (CN). The open circles indicate the probable peak assignments of Li4SiO4, crosses indicate the hypothesized LiO6 and LiOs sites in Li-substituted beryl, with all other samples indicated by +. From Xu and Stebbins (1995), with permission of the copyright owners. B. Relationship between the 6Li chemical shift and the chemical shift parameter A (defined by equation 7.7, Chapter 7) for crystalline lithium phosphates (solid circles) and crystalline Li4SiO4 (open circles). From Alam et al. (1999), by permission of Elsevier Science.

636


10.1.4 6'7Li N M R o f f a s t lithium ion conductors

One of the most important practical applications of lithium compounds is as fast ion conductors with potential electronic applications such as solid electrolytes for lithium batteries. Li20 is a fast ion conductor in which the Li ions occupy a simple cubic sublattice with the antifluorite structure. Both MAS and static 7Li NMR spectra of Li20 have been reported, the former recorded as a function of temperature up to 1000 K (Xie et al. 1995). The effect of introducing vacancies on the Li sites by doping with LiF has been studied by high-temperature static 7Li NMR, which reveals the interaction of the Li defects > 600 K and the appearance of 2 distinct quadrupolar interactions at about 900 K. Measurements of the relative intensities of the satellite peaks as a function of temperature have provided evidence of thermal dissociation of an impurity-vacancy complex (Xie et al. 1995). The mechanism of Li motion in the novel thioborate LisBvS13 has been investigated by measuring the relaxation rates of 7Li as a function of temperature up to 650 K (Grtine et al. 1995). This compound displays pronounced Li + mobility but with rather complex relaxation behaviour indicating the operation of 3 different processes by which Li ions move within the crystal. Below room temperature, the lithium ions move in the extended channels in the structure, unhindered by the presence of S atoms such as those located in the channels of related thioborate compounds. A second process becoming significant at about 200 K involves jumping of the Li + between the holes of the porous anionic network, while the third process, above about 300 K, results from the movement of Li + between more isolated sites via pathways which become increasingly accessible because of thermal activation (Grtine et al. 1995). Lithium intercalation of compounds such as SnS2 are of technical interest for photochromic display materials and lithium electrodes which reversibly take up and release Li +. 6'7Li and ll9Sn NMR has been used to investigate the location of the Li insertion sites in this material (Pietrass et al. 1997). The 7Li spectra show a central transition which can be decomposed into 2 components with different XQ values corresponding to Li in octahedral and tetrahedral interlayer sites. As the Li concentration increases, the additional ions enter tetrahedral intralayer sites surrounded by 3 tin and 4 sulphur atoms and characterised by a broad VLi NMR spectral component. Further insertion of Li results in the material becoming amorphous by rupture of the layers (Pietrass et al. 1997). LiCoO2, an important electrode material for secondary lithium batteries, occurs in 2 polytypes, both of which have been investigated by 6'7Li and 59Co NMR at 3 magnetic fields (Siegel et al. 2001). Both polytypes show only 1 Li resonance corresponding to lithium in octahedral coordination with oxygen, with similar 7Li XQ values (25-36 kHz for the 02 polytype and 31-39 kHz for the 03 polytype). The superionic compound Li3Scz(PO4)3, studied by 7Li NMR up to 575~ (Vashman et al. 1992) has revealed the operation of three types of Li ion motion and allowed

N M R of Other Quadrupolar Nuclei

637

their activation energies to be determined. Both the 7Li quadrupole parameter and spinlattice relaxation rate change abruptly in the vicinity of a phase transformation at about 530 K. The 6Li NMR spectra of a 6Li-enriched sample were also measured as a function of temperature, and it was possible to derive a precise value of the quadrupole moment for 7Li of (2.56 + 0.05) • 10 -2 barn from observation of the 6Li and 7Li quadrupole spectra in the same compound (Vashman et al. 1992). The 7Li NMR spectra of the superionic conducting compound Li3Inz(PO4)3 have been obtained at temperatures up to 520 K, and, together with the corresponding 31p NMR spectra, provide evidence of phase transitions in this material at about 380 K and 420 K, at which the lithium ions are re-distributed between the different crystallographic sites (Pronin et al. 1990). Measurements of the 7Li relaxation rates indicate the presence of Li+-Li + contact pairs in the crystal lattice, leading to a suggested model for Li ion transport involving the movement of an interstitial configuration of metastable Li + pairs (Pronin et al. 1990). Lithium-doped BPO4, another candidate ceramic electrolyte material for lithium batteries has been studied by 7Li NMR relaxation and linewidth measurements of samples with Li doping levels up to 20 mol % (Dodd et al. 2000). Comparison of the NMR data with values of the second moment calculated for both random and homogeneous models of Li distribution indicate the existence of Li clusters with an internuclear distance of --~ 3A, possibly consisting of 1 Li ion fixed at a boron vacancy with additional 2 Li ions in the conduction channels surrounding the vacancy. The atomic jump time, determined from measurements of the 7Li motional narrowing behaviour, indicate a maximum in the Li ionic mobility at the 10 mol % doping level (Dodd et al. 2000). 7Li NMR has been used to study the processes by which Li ions move through the structures of the ionic conducting ceramic materials lithium lanthanum titanate and lithium aluminium titanium phosphate (Nairn et al. 1996). The 7Li static NMR spectra of Lio.33Lao.svTiO3show the quadrupolar powder pattern associated with significant Li ionic mobility, with a room-temperature XQ value of 900 Hz. A second Li site which becomes apparent in these spectra at higher temperatures has been attributed to the presence of less mobile defects. The 7Li NMR spectrum of Lil.3Alo.3Til.7(PO4)3 shows a powder pattern with a large room-temperature • (about 45 kHz) increasing smoothly to about 54 kHz at 400 K probably due to a temperature-induced lattice distortion (Nairn et al. 1996). Lithium vanadate bronzes are intercalated compounds with potential applications for lithium battery technology, since Li can be reversibly inserted into these structures by electrochemical reaction. 7Li NMR has been used to study the structure of "y-Lio.95V205 (Cocciantelli et al. 1992) and a series of related bronzes LixV205 (Cocciantelli et al. 1992a). The 7Li NMR spectrum of the ~/-phase indicates the presence of a single Li site, but as the Li content is increased beyond x = 1, new lines can be resolved, corresponding

638


to the Li sites in the 8-phase ( - 12 ppm) and g-phase ( - 10 ppm) which co-exist with the y-phase (5-15 ppm) in these compositions (Cocciantelli et al. 1992, 1992a).

10.1.5 6, 7Li N M R o f glasses

A series of binary lithium silicate glasses have been studied by 6Li MAS NMR, showing a linear relationship between the average isotropic chemical shift and the glass composition (Figure 10.4A) (Gee et al. 1997). This relationship, which is similar to that found for 23Na isotropic shifts in binary sodium silicate glasses, indicates that for both nuclei the isotropic chemical shift becomes more positive (i.e. the bonding becomes more covalent) as the concentration of non-bridging oxygen species in the glass increases (Gee et al. 1997). Both 6Li and 7Li NMR spectra have been reported for binary lithium silicate glasses and their crystallisation products (Dupree et al. 1990). On thermal recrystallisation of the glasses, the widths of the 6Li NMR spectra decrease, indicating a significant contribution by chemical shift dispersion to the linewidth of the glass. The static 7Li NMR spectra of the crystallised glasses exhibit splitting due to dipolar coupling of isolated pairs of 7Li nuclei with a separation of -~ 2.1/k (Dupree et al. 1990). The "mixed alkali" effect in silicate glasses refers to the observation that systems containing more than 1 alkali cation show ionic conductivity and dielectric behaviour which does not follow a simple linear combination of the properties of the pure components, but can often show a marked minimum in these properties at about the equiatomic composition. 7Li, 23Na and 29Si MAS NMR has been used to investigate this effect in (Li,Na) disilicate glasses (Ali et al. 1995). The 7Li and 23Na linewidths and shifts were found to change continuously as a function of composition, suggesting that the alkali ions are uniformly mixed rather than segregated into Li and Na-rich domains. This conclusion, which contradicts previous glass structure models, has been confirmed by 23Na-{ 7Li} Spin Echo Double resonance (SEDOR) studies (Gee and Eckert 1996), and by 29Si{VLi} and 29Si{23Na} Rotational Echo Double Resonance (REDOR) NMR results (Gee et al. 1997). The REDOR experiments were used to selectively enhance those silicon sites most strongly coupled to either Li or Na ions, allowing a comparison of their spectroscopic parameters. The isotropic chemical shifts of both the 7Li and 6Li MAS NMR spectra of (Na,Li) disilicate glasses have been found to become more positive with increasing Na content of the glasses (Figure 10.4B), following a similar trend found for the 23Na shifts (although the chemical shift range in 6'7Li is smaller, making the relationship with composition more subtle). These monotonic compositional dependencies of the alkali chemical shifts provide further evidence against glass structural models involving cation clustering (Gee et al. 1997). Both 6Li and 7Li MAS NMR has been used to investigate the local Li coordination environment in a series of binary lithium phosphate glasses (Alam et al. 1999). The 6Li chemical shifts, which approximate closely to the isotropic chemical shifts, increase

639


A

B

0.4

~

0.3

w _t_

0.2 G 13.5

* chemicalshiftsrelativeto Ga(H20)63+

Xl

Reference

0.85 Massiot et al. (1995), (1999) 0.08, 0.01 Massiot et al. (1995), (1999) ND Bradley et al. (1993) ND Bradley et al. (1993) ND Miyaji et al. (1992) 0.05 Massiot et al. (1999) 0.03 Massiot et al. (1999) ND Massiot et al. (1999) ND Massiot et al. (1999) 0.7 Massiot et al. (1999) ND Bradley et al. (1993) 0.51 Massiot et al. (1999) 0.45 Massiot et al. (1999) ND Timken & Oldfield (1987) ND Bayense et al. (1989), Chen et al. ( 1991), Kentgens et al. (1991) ND Bradley et al. (1993) ND Timken & Oldfield (1987) ND Timken & Oldfield (1987) ND Timken & Oldfield (1987) ND Thomas et al. (1983) ND Massiot et al. (1999) ND Massiot et al. (1999)

655


9 r

3ooo

-~

150 "

~

-

~

CN=6

.p...~

~ -150 -50

|

0

l

50

,

100

27A| shift (ppm) w.r.t. AI(H20)63§ Figure 10.10. Plot of the 71Ga chemical shifts of gallium compounds with only oxygen in the first

coordination sphere vs. the 27A1 chemical shifts of the analogous aluminium compounds. The scatter is attributed to the fact that not all the shift values may be the isotropic shifts. From Bradley et al. (1993), by permission of John Wiley and Sons Ltd.

The 71Ga NMR parameters of the well-defined 5-coordinated Ga site in the compound LaGaGe207 have been determined from its static spectrum (Massiot et al. 1999). The giso value (75.8 ppm) falls between the typical shift range for Ga (~v~ and Ga (vI~, with a XQ value of 15 MHz and an estimated CSA of --~100 ppm (Massiot et al. 1999).

10.5.2 69,71GaN M R o f crystalline compounds Both the 69Ga and 71Ga MAS NMR spectra of ~-Ga203 have been reported (Massiot et al. 1995). One-half of the Ga atoms in this oxide are in tetrahedral sites and the other half are in octahedral sites. The spectral widths extend over at least 200 kHz and are thus too broad to be usefully narrowed by MAS. However, the static spectra from both 69Ga and 71Ga show similar features (Figure 10.11) and can be simulated with 2 second-order quadrupolar lineshapes corresponding to the tetrahedral and octahedral sites, although the site population ratio derived from the simulations show a systematic underestimation of the wider contribution. The resulting XQ values for the 2 sites (Table 10.4) indicate that the tetrahedral site is more distorted than the octahedral, consistent with the known crystal structure of this compound (Massiot et al. 1995). GaPO4 is similar to A1PO4 in being able to crystallise in structural forms related to the silica polymorphs quartz and cristobalite. The 71Ga NMR spectra of both GaPO4 structures have been recorded, the NMR parameters (Table 10.4) being consistent with the more symmetrical tetrahedral Ga environment in the cristobalite form (Massiot et al. 1999). Very fast spinning MAS speeds (up to 35 kHz) have been used to distinguish the tetrahedral and octahedral Ga sites in the 71Ga NMR spectra of a series of Ga-rich fluoro-amphiboles NaCazMg4Ga3Si6Oz2F2, since more conventional spinning

656


A

69Ga

B

~

10000

'

~-------ksimulated oct

oct

F--~] '

71Ga

~. o

'-iooo0'

tet

~ _ c

~~J

2000

tet 0

-2000

Ga shift (ppm) w.r.t. Ga(H20)63+ Figure 10.11. A. Observed and simulated 7 T static 69Ga NMR spectrum of [3-Ga203 showing the octahedral and tetrahedral contributions to the simulation. B. The corresponding 7 T static 71GaNMR spectrum and simulation. From Massiot et al. (1995), by permission of the

copyright owner. speeds (14 kHz) resulted in an overlap of the spinning sidebands of the tetrahedral and octahedral resonances (Sherriff et al. 1999). The resulting 7~Ga MAS NMR spectra (Figure 10.12A) showed a single tetrahedral peak at 230 ppm and 2 low-intensity peaks attributed to an octahedral quadrupolar doublet at about 40 ppm. The crystal structure of this mineral indicates that the occupation of the octahedral sites should be about onethird of the tetrahedral sites; the NMR measurement therefore significantly underestimates the intensity of the octahedral Ga site, possibly due to large quadrupolar effects associated with this distorted site (Sherriff et al. 1999). The formation of germanate phases with the mullite structure (Ga6Ge2013, (Ga,A1)6Ge20~3) from sol-gel precursors has been studied by multinuclear solid state NMR, including 71Ga MAS NMR (Meinhold and MacKenzie 2000). Many of the 71Ga spectra were dominated by the quadrupolar lineshape of 13-Ga203 from the structural units of the intermediate phase a-Ga4GeO8 or from unreacted starting material. The 71Ga NMR spectrum of crystalline Ga6GeeO13 was found to be broad and featureless, as were the 69Ga MAS NMR spectra of these samples. Static 69Ga NMR spectra obtained for quenched and annealed samples of the clinopyroxene LiGaSi206 have been interpreted as indicating the presence of 2 different electronic states of the octahedral Ga ions (Ohashi et al. 1995) A series of crystalline gallosilicate molecular sieves with the zeolite [3 structure synthesised by a rapid method from alkali-free hydrogels have been studied by NMR methods, including 7~Ga MAS NMR (Occelli et al. 1999). The 71Ga spectra (Figure 10.12B) show that most samples contain only tetrahedral Ga in framework

657


A

BB

Oct

et ~ O c t

I

L

!

400

f

r

200

t

F

0

-

i

I

p

I

P

300

100

-100

71Ga shift (ppm) w.r.t. Ga(H20)63+ Figure 10.12. A. 71Ga MAS NMR spectra of 2 gallium-fluoro amphiboles spun at 28 kHz. The most Ga-rich sample (upper) shows a small octahedral Ga feature which may arise from a gallium sapphirine impurity. From Sherriff et al. (1999), by permission of the Mineralogical Society of America. B. 71GaMAS NMR spectra of 2 galliosilicate molecular sieves with the ~ zeolite structure. The tetrahedral Ga spectrum (upper) is typical of gallium in framework sites. The additional octahedral Ga resonance (lower spectrum) arises from extra framework Ga generated during thermal treatment of the sample. From Occelli et al. (1999), by permission of Elsevier Science. sites, but one sample clearly showed the presence of extra-framework octahedral Ga, due in part to the thermal treatment of the sample after synthesis. Gallium has also been incorporated into the structure of a microporous titanosilicate ETS-10 which unlike other zeolite-type materials contains framework atoms in octahedral coordination. 71Ga MAS NMR of the gallium-substituted sample ETGS-10 shows the presence of only tetrahedral Ga, indicating that its isomorphous substitution occurs only on the silicon sites to avoid the neighbouring titanium (Rocha et al. 1995). In this respect the behaviour of gallium is similar to that of aluminium in the analogous titanoaluminosilicate ETAS-10. A 69Ga and 71Ga MAS NMR study of a series of gallium analogues of the zeolite ZSM-5 at 2 applied magnetic fields has allowed the value of XQ (1.9-2.2 MHz) to be determined from the difference of the peak position of the 69Ga and 71Ga resonances (Kentgens et al. 1991). Changes in the linewidth as a function of the magnetic field revealed the presence of both second-order quadrupolar broadening, and broadening due to a chemical shift distribution.

10.5.3 69"71Ga N M R o f other compounds

A series of caesium gallate glasses x C s 2 0 . ( 1 - x)Ga203 where x = 0.3 to 0.7 have been studied by 69'71Ga NMR (Zhong and Bray 1987). Both static and MAS conditions were

658


used to establish the presence of tetrahedral and octahedral coordination in these glasses. The results indicate that in glasses with Ga:Cs ratios of less than 3:7, the Ga is in solely tetrahedral network-forming sites, but as the gallium content increases, the excess Ga enters octahedral sites. The 71Ga NMR spectra of the semiconductors GaAs and InGaAs show a single resonance in GaAs but the spectrum of InGaAs consists of a sharp intense peak as in GaAs, but with an underlying broad, weak resonance. Under MAS conditions, the InGaAs spectrum shows an extensive and complex sideband structure, by comparison with the simpler MAS spectrum of GaAs (Kushibiki and Tsukamoto 1986).

10.6. 87RbNMR

10.6.1 General considerations Rubidium compounds are important in a number of areas of materials science, as catalysts for ammonia synthesis and oxidation of methane, as a component of some glasses and as a dopant metal in buckminsterfullerene (C6o) causing it to become superconducting at 28 K. There are 2 NMR-active rubidium isotopes, 85Rb (I = 5/2, natural abundance 72.8%) and 87Rb (I = 3/2, natural abundance 27.2%). The sensitivity of 87Rb is greater than that of 85Rb, but its residual homonuclear dipolar broadening is larger and its relaxation time tends to be longer (100-300 ms for simple Rb salts) which is an advantage for acquiring DAS spectra. Most of the published solid state rubidium NMR uses 87Rb as the nucleus of choice, although the larger quadrupole moment of 85Rb can be useful in providing an indication of the number of chemically different Rb sites present in a salt (Cheng et al. 1990). The simple rubidium salts such as the halides and nitrate have small • values giving rise to narrow resonances, whereas the chromate, acetate, sulphate and hydroxide have larger XQ values giving wider central transition lineshapes (Cheng et al. 1990). The NMR interaction parameters of a number of rubidium compounds are collected in Table 10.5.

10.6.2 S7Rb NMR of crystalline compounds Dynamic Angle Spinning (DAS) has been used to obtain the 87Rb NMR spectra of several rubidium salts (Baltisberger et al. 1992), including RbNO3 which contains 3 inequivalent Rb sites and could not be resolved in the static NMR spectrum (Cheng et al. 1990). DAS NMR was found to narrow the 87Rb spectral lines significantly more than MAS or VAS (variable angle spinning), except in the case of RbC1 (Figure 10.13A), in which the nucleus is in a cubic environment with no second-order quadrupolar

659


Table 10.5. 87RbNMR interaction parameters for rubidium compounds. Compound RbF RbCI RbBr RbI Rb2SO4 site 1 site 2 RbzCO3 site 1 site 2 RbzCrO4 site 1 site 2 RbC104

Reference

0 0, 0

0 0, 0

0 0 2.6 3.2 5.0 3.2 5.2 11.5

0 0 0.89 0.13 0.75 1.0 0.48 0.75

3.2, 3.2 4.3 1.83 2.07 1.85 2.8 6.9

0.16, 0.10 0.77 0.12 1.00 0.48 0.3 0.47

Cheng et al. (1990) Cheng et al. (1990), Baltisberger et al. (1992) Cheng et al. (1990) Cheng et al. (1990) Cheng et al. (1990), Baltisberger et al. (1992) Cheng et al. (1990) Cheng et al. (1990) Cheng et al. (1990), Baltisberger et al. (1992) Cheng et al. (1990) Cheng et al. (1990), Baltisberger et al. (1992) Cheng et al. (1990) Baltisberger et al. (1992) Baltisberger et al. (1992) Baltisberger et al. (1992) Cheng et al. (1990) Cheng et al. (1990)

XQ

(MHz)

50 128, 127 155 183 46.4, 42 3.0, 16 18.9 - 7.0 - 47.4, - 11 52.8 3.8, 16.2 30.5 - 26.2 - 26.8 - 30.9 0 7.6 -

RbOH.H20 RbNO3 site 1 site 2 site 3 RbOOCH.H20 Rb acetate.H20

"q

~iso

(ppm)*

* chemical shifts referred to RbNO3 solution

broadening. MAS is able to average the homonuclear dipolar interaction present in this compound but DAS will not, resulting in a broader DAS spectrum. DAS allowed the MAS powder patterns of RbNO3 to be separated (Figure 10.13B) and the quadrupolar parameters for each site to be determined by single-site simulation (Baltisberger et al. 1992). The isotropic shifts derived by DAS for a number of rubidium salts other than RbC1 are considerably different from those from the static 87Rb spectra (Table 10.5); the DAS parameters are claimed to be more reliable since they do not depend on simulations requiting a number of adjustable parameters (Baltisberger et al. 1992). A triplequantum MAS NMR approach has also been used to separate the 3 Rb sites in RbNO3, demonstrating the usefulness of this technique for such closely overlapping sites (Fernandez and Amoureux 1996). By simultaneously selecting the 2 mirror coherence transfer pathways (0)(_+ 3 ) ( - 1) the pure-phase 2D 87Rb spectrum of RbNO3 was obtained (Figure 10.14) with a significant gain in sensitivity. The resulting isotropic chemical shifts and quadrupolar products are in agreement with those determined by DAS measurements (Baltisberger et al. 1992). 87Rb NMR has been used to measure the temperature dependence of the secondorder shifts of the central transition of RbSCN, a compound which undergoes an


660 A RbCI

MAS -10 -

I

I

!

observed

A

DAS I

,,|

145 ~

-30

\

~

JAM __

L

-25

, ,,

_L_ _L_

t

105

125

RbNO3 /

t

simulated

11

-20 -50

' S 3 -45 ' -33 -45 -20

S7Rb shift (ppm) w.r.t. RbCI soln.

I

-30

-35 -40 S7Rb shift ( p p m ) w.r.t RbC! soln. Figure 10.13. A. 87Rb MAS and DAS NMR spectra of RbC1 (upper spectra) and RbNO3 (lower spectra) acquired at 11.7 T. B. 87Rb powder pattern cross-sections through the F2 dimension of a pure-phase MASdetected DAS spectrum of RbNO3 acquired at 11.7 T with simulations of the lineshapes from the 3 sites. From Baltisberger et al. (1992) by permission of the American Chemical Society.

~geetlon 3

s~ection 2 section 1

30 25

~,

20

-20 30

-40

v2 (ppm) Figure 10.14. 87Rbtwo-dimensional triple-quantum MAS NMR spectrum of RbNO3 showing the three Rb sites and their corresponding anisotropic sections. From Fernandez and Amoureux (1996), by permission of the copyright owner.


661

antiferroelectric phase transition at 435 K from a high-temperature paraelectric tetragonal phase to a low-temperature antiferroelectric orthorhombic phase (Blinc et al. 1995). The 87Rb NMR results provide a physical picture of the structural transformation in terms of a disordering process connected with the formation of dynamic clusters or microdomains which are embedded in a long-range-ordered matrix below the transition temperature and which become random above the transition temperature (Blinc et al. 1995). 8VRb NMR has also been used to study the low-temperature phase of RbzZnC14, a one-dimensionally modulated incommensurate crystal which exhibits successive phase changes through 4 phases. Single-crystal measurements of the change in the 87RbNMR lineshape as a function of temperature in the vicinity of the low temperature commensurate phase change at 74.6 K have confirmed the crystal structure of this phase, while an anomaly in the temperature dependence of the spin-lattice relaxation time was interpreted in terms of the condensation of the soft mode inducing this transformation (Apih et al. 1992). Alkali metals such as rubidium are added to the surface of alumina catalysts to improve their efficiency in facilitating the partial oxidation of ethylene to ethylene oxide. 87Rb NMR has been used to gain an understanding of the interactions between Rb salts and the reactive sites of ~/-alumina (Cheng and Ellis 1989). When the alumina was impregnated with solutions of RbI, RbC1, RbNO3 and RbzSO4 at concentrations giving a submonolayer coverage, 4 rubidium species were identified after oven drying. Two of these species, described as surface salts, have 87Rbchemical shifts which depend on the nature of the impregnating anion, while the other species (described as surface species) are only weakly bonded to their oxo-anion and have chemical shifts which are essentially independent of the anion. The 87Rbrelaxation times suggest that both the surface salts and surface species can exist in a disordered form containing interstitial vacancies which provide a mechanism for migration of Rb + from site to site (Cheng and Ellis 1989). Alkalides and electrides are stoichiometric salts containing alkali metal cations complexed by crown ethers. Charge balance is provided by the alkali metal anions (alkalides) or trapped electrons (electrides). 87Rb and 85Rb NMR has been used to study a number of rubidium alkalides, electrides and related compounds (Kim et al. 1996). Spin-echo NMR measurements were used to obtain reliable values of giso, Xo and qq for these compounds.

10.6.3 87Rb NMR of rubidium fuUerides. Solid C6o (buckminsterfullerene) can be intercalated by alkali metal atoms to form MxC6ocompounds where x = 1--~6. The discovery that M3C6odisplays superconductivity, with transition temperatures as high as 33 K, has promoted considerable research interest, including a number of solid state NMR studies of both the structure and superconductivity mechanisms in these compounds.

662


Above 450 K, the 87Rb NMR spectrum of Rb3C6o contains 2 sharp resonances arising from rubidium in non-equivalent octahedral and tetrahedral sites (Walstedt et al. 1993). The octahedral peak appears at about 52 ppm and the tetrahedral peak is at about 195 ppm with an octahedral:tetrahedral intensity ratio of 1:2, consistent with the known crystal structure. As the temperature is lowered these resonances broaden and shift slightly and a third tetrahedral resonance appears; at 200 K the 3 resonances occur at 40 ppm (octahedral), 165 ppm (tetrahedral) and 270 ppm (new tetrahedral). The formation of the second tetrahedral site has been explained in terms of alkali-metal vacancies which occur only in the tetrahedral positions (Apostol et al. 1996). Measurements of the 87Rb and 85Rb relaxation rates indicate a quadrupole relaxation mechanism involving phonons, and no change in either the NMR spectrum or the relaxation rates was found in the vicinity of Tc for this compound (Corti 1993). High-temperature superconductivity has also been demonstrated in mixed alkali metal fullerides, including the ternary alkali compound KRbCsC6o which has a Tc of 28-29 K (Maniwa et al. 1993). The 87Rb NMR spectra of this and the related binary alkali compound K2RbC6o (Figure 10.15A) show that whereas the rubidium ions occupy predominantly octahedral sites ( - 1 5 0 ppm) in the latter, they are located primarily in tetrahedral sites (about 0 ppm) in KRbCsC6o. These results, together with 133Cs NMR, provide evidence that the alkali metal atoms are site-selectively intercalated into the face-centred-cubic C6o lattice according to their size, with the largest ion (Cs) preferentially occupying the octahedral sites and the smaller K and Rb ions occupying mainly tetrahedral sites (Maniwa et al. 1993). 87Rb NMR has been used to study the electronic properties and phase transitions in another rubidium fulleride, RbC6o, in which the rubidium is located in octahedral sites of the NaC1 structure (Tycko et al. 1993). The 87Rb NMR spectra (Figure 10.15B) indicate a phase transition in this compound at about 300 K, the low-temperature phase containing a broad octahedral Rb resonance at - 120 ppm, being replaced above the transition temperature by a narrower tetrahedral line with a chemical shift decreasing strongly from 615 ppm at 353 K to 410 ppm at 473 K. The NMR data indicate that the high-temperature phase is a paramagnet in which the electronic dynamics are dominated by electron-electron effects. The electron-spin susceptibility is greatly reduced in the low-temperature phase in which most of the unpaired electron spins have become paired (Tycko et al. 1993).

10.7. 93Nb

NMR

93Nb is a nucleus with spin = 9/2, a relatively large quadrupole moment (Table 1.2, Chapter 1) and 100% natural abundance. 93Nb NMR is difficult since, in the solid state, electric field gradients arising from the electronic cloud at the nucleus can interact

663


A

B

RbC6o

T(K)

Oct

/ ~

/

K2RbC6 o

Tet

313 KRbCsC6~

323

I

200

I

0

I

~.

| ....

-200

87Rb shift (ppm) w.r.t. RbCl soln.

800

400

0

-400

a7Rb shift (ppm) w.r.t. RbCI soln.

Figure 10.15. A. 87RbNMR spectra of the rubidium fullerides K2RbC6o(upper) and KRbCsC6o (lower), from Maniwa et al. (1993). B. 87RbNMR spectra of the rubidium fulleride RbC6oat various temperatures. Note the broad octahedral Rb resonance in the low-temperature phase progressively replaced by the narrower tetrahedral resonance above the phase transition temperature. From Tycko et al. (1993). Both diagrams used by permission of the copyright owners.

with the nuclear electric quadrupole, giving rise to considerable spectral broadening. However, the technical importance of a number of niobates as piezoelectric and optoelectric ceramic materials has provided the stimulus for several recent 93Nb NMR studies using techniques such as high-speed MAS, DAS and MQMAS to overcome broadening problems. Chemical shifts have been quoted in the literature with respect to solid Nb205 or a saturated solution of NbC15 in wet acetonitrile, the latter being the more commonly used reference substance. The 93Nb NMR spectra of a number of alkali and lead niobates have been acquired using MAS, DAS, MQMAS and two-dimensional nutation NMR to improve the resolution and determine values of the quadrupolar products PQ for these materials (Prasad et al. 2001). The 9.4 T 93Nb MAS NMR spectrum of LiNbO3 spun at 25 kHz (Figure 10.16A) shows a lineshape dominated by the second-order quadrupolar interaction, with only marginal improvement in resolution at a field of 14.1 T. For most of the metal niobates, the second-order interaction is not removed by MAS alone, even at high magnetic fields and fast spinning speeds. The expected improvement in 93Nb resolution provided by DAS spectroscopy is offset by significant homonuclear

664


MAS

B

DAS

C

MQMAS

D

2D nutation

14.1 T

.1 00 ~ -750 -1000 -1250

lu

93Nb shift ( p p m )

-1000" 'r

,,I

.........

i .........

-1000

, .........

w .

.

.

.

~

-1400 ~

Frequency (ppm)

.

.

.

.

.

.

.

i,

11,ti, i! o

I

~

100

~

I ......... t ........ ~ ......... r ........

300

Frequency(ppm)

,

, ,

1

3

......... ......................... ,........ ,.....

5

Frequency (vm~)

w.r.t. NbCIs in acetonitrile Figure 10.16. 93NbNMR spectra of LiNbO3. A. MAS NMR spectra acquired at 14.1 T (spinning speed 18 kHz) and 9.4 T (spinning speed 25 kHz). B. DAS NMR spectrum also showing the 1D projection of the isotropic dimension. C. Triple-quantum MAS NMR spectrum also showing the 1D projection of the isotropic dimension. D. Pure-phase 2D nutation spectrum also showing the 1D projection of the nutation dimension. From Prasad et al. (2001) by permission of the copyrightowner.

Nb-Nb dipolar interactions which dominate the centreband in the isotropic dimension of the DAS NMR spectrum of LiNbO3 (Figure 10.16B). Although the use of MQMAS NMR to improve the resolution is hampered by the large quadrupole interactions of 93Nb, making multiple-quantum excitation and conversion less efficient, the triplequantum 93Nb spectrum of LiNbO3 (Figure 10.16C) shows an improvement in resolution of approximately an order of magnitude over the DAS spectrum (Prasad et al. 2001). The numerous spinning sidebands in the isotropic dimension of the MQMAS NMR spectrum arise typically from rotor modulation of the anisotropic chemical shift and quadrupolar interactions in the conversion period being different from that of the excitation period. Since the 93Nb quadrupolar products PQ of LiNbO3 and the related alkali niobates determined by MQMAS NMR are large (22.1-22.7 MHz), twodimensional nutation spectroscopy has been used to provide complementary information. The 2D nutation spectrum of LiNbO3 (Figure 10.16D) shows a single Nb site with its centre of gravity at 4VRF corresponding to a XQ value of 20 MHz (Prasad et al. 2001). Lead magnesium niobate, Pb(Mgo.33Nbo.66)O3, a relaxor ferroelectric material with a high dielectric constant and useful electrorestrictive properties, occurs in both perovskite and pyrochlore structures. The perovskite contains Nb(V) in the multiple B-sites of the structure, with a 93Nb MAS NMR spectrum showing 2 distinct Nb environments for which the quadrupolar parameters were determined from the triplequantum MAS NMR spectrum (Cruz et al. 1999). The broader of these 2 resonances, centred at - 980 to - 1000 ppm with respect to NbC15 in acetonitrile, has been assigned to a range of axial or rhombic Nb(ONb)6_•215 B-site configurations occurring in the Nb-rich regions (Fitzgerald et al. 2000). The shift of the other sharper

665


Table 10.6. 93Nb NMR interaction parameters for niobium compounds. Compound

giso(ppm)*

XQ (MHz)

xI

Reference

LiNbO3

0.82

KNbO3

- 1050

22.1t, 22.1 22.7 t, 19.7 23.1

0

NaNbO3

- 1004, - 1009 - 1073

PbNb206

- 1113, - 1090

16.8 t, 19

0.5

1003 978 995 1013 975 999 951 995

13.C 17.0 t 13.7* 16.6 t 17.9 t 18.9 t 20.6 ~ 13.7 t

-

Prasad et al. (2001), Kind et al. (1968) Prasad et al. (2001), Kind et al. (1968) Fitzgerald et al. (2000), Kind et al. (1968) Prasad et al. (2001), Prasad et al. (1999) Prasad et al. (2001) Prasad et al. (2001) Prasad et al. (2001) Prasad et al. (2001) Prasad et al. (2001) Prasad et al. (2001) Prasad et al. (2001) Prasad et al. (2001)

- 1014

26.8 t

-

Prasad et al. (2001)

Pb2Nb207 " Pb3Nb4013 PbsNb4015 " Pb3Nb208 " Pbl.g3Nb1.vlMgo.2906.39 (perovskite) Pb1.83Nbl.71Mg0.2906.39 (pyrochlore)

-

0.80

* chemical shiftsrelativeto NbC15in acetonitrile t quadrupolarproductvaluesPQ

r e s o n a n c e at - 902 p p m is u n u s u a l for B-site N b ( O N b ) 6 configurations, and has b e e n tentatively e x p l a i n e d in terms of 1" 1 M g / N b ordering in M g - r i c h regions w h e r e the local s y m m e t r y is nearly cubic (Fitzgerald et al. 2000). T h e 93Nb M A S N M R and M Q M A S N M R spectrum of the pyrochlore form contains a single resonance suggesting a d i s t r i b u t i o n of isotropic c h e m i c a l shifts a n d q u a d r u p o l e c o u p l i n g s a t t r i b u t e d to disorder in the N b e n v i r o n m e n t (Cruz et al. 1999). Structural details of b o t h the perovskite f o r m of lead titanium niobate and a series of lead niobate p y r o c h l o r e s h a v e also b e e n studied by 93Nb D A S N M R (Prasad et al. 2001) and by 2D nutation s p e c t r o s c o p y (Prasad et al. 1999, Fitzgerald et al. 2000, Prasad et al. 2001).

10.8.

133CsN M R

10.8.1

Generalconsiderations

133Cs is a spin I - 7/2 nucleus of 100% natural a b u n d a n c e and a very small quadrupole m o m e n t of - 3.4 x 10 -31 m 2. 133Cs in inorganic c a e s i u m c o m p o u n d s (Table 10.7) shows a m o d e r a t e c h e m i c a l shift range (about 300 ppm). T h e c h e m i c a l shifts are normally referenced to aqueous CsC1 solution.

666


Table 10.7.

133CsNMR interaction parameters of caesium compounds.

Compound

~iso (ppm)*

XQ (kHz)

xl

Reference

CsOH.H20 CsBr

ND 258.2, 260.3 223.2, 228.1 275 0

98.2 0

"~0 0

0

0

ND 135, 126 115 168 95 140

ND 0.09, 0.08 0 0.45 0 ND

ND, 38.0 ND ND 370 270 272 134 134 ND ND ND ND 148 181 153 212 207 105 ND

ND

Amm & Segel (1986) Mooibroek et al. (1986), Haase et al. (1977) Mooibroek et al. (1986), Haase et al. (1977) Haase et al. (1977) Mooibroek et al. (1986), Tarasov et al. ( 1991) Tarasov et al. (1990a) Tarasov et al. (1990) Mooibroek et al. (1986) Mooibroek et al. (1986), Haase et al. (1977) Mooibroek et al. (1986), Haase et al. (1977) Haase et al. (1977) Haase et al. (1977) Tarasov et al. (1992) Tarasov et al. (1992) Mooibroek et al. (1986) Mooibroek et al. (1986) Mooibroek et al. (1986) Hartman et al. (1998) Kohn et al. (1994) Kohn et al. (1994) Kohn et al. (1994) Kroeker et al. (1997) Lim et al. (1999) Lim et al. (1997b) Lim et al. (1997b) Lim & Jeong (1999a) Dupree et al. (1982) Dupree et al. (1980)

CsC1 CsI CsC104 CsBrO4 CsIO4 CsCN CsNO3 CszSO 4

Cs2CO3 Cs2CrO4 CsTcO4 site 1 site 2 CsAuC13 CsSD ( - 81~ CsSeD ( - 61 ~ CsA1TiO4 Cs2CdSi5012 Cs2ZnSi5012 CszMgSisOl2 CsCd(SCN)3 LiCsSO4 CsMnC13 site 1 site 2 CsPbC13 Cs3Sb CsAu

ND ND 135.3 - 9.2, - 14.9 100, 91.7 152.7 - 61.3 - 83** - 115"* 128 257 276 66.0 77.1, 25.7 64.4, 8.0 62.9, - 4.3 94.4 620 375

ND ND --~0 --~0 0 0 0 ND ND ND ND 0.98 0.92 0 0 0.38 ND ND

* chemical shiftsquotedwithrespectto aqueousCsC1solution ** chemical shifts quotedwithrespect to aqueousCsBrsolution

10.8.2

133C$N M R

of crystalline

caesium compounds

A comprehensive single-crystal 133Cs N M R study of Cs2CrO4 has yielded the magnitude and orientations of the

133Cs chemical

shielding and quadrupolar tensors for the 2

crystallographically distinct Cs sites in this compound, indicating that the chemical shielding and quadrupolar interactions are not coincident for the 2 distinct caesium positions (Power et al. 1994). The temperature d e p e n d e n c e of the 133Cs N M R interaction parameters of solid CsC104, CsBrO4 and CsIO4 have been determined as a function of temperature. In

667


CsC104 the temperature dependence of XQfor both 133Cs and 35C1 is linear, with a negative temperature coefficient, but the temperature dependence of the 133Cs -q-value shows an anomaly at 342 K (Figure 10.17A) which is not related to any abrupt structural change but to a change in the relative values of the tensor components of the electric field gradient (Tarasov et al. 1991). 133Cs and 79'81Br NMR of CsBrO4 show lineshapes dominated by quadrupolar effects. An increase in 133Cs • and a decrease in ~q with increasing temperature (Figure 10.17B) is thought to be associated with anisotropic behaviour of the lattice a-parameter (Tarasov et al. 1990a). The behaviour of CsiO4 is more complex, being shown by 133Cs and 127INMR to involve 2 phase transitions, one at 243-300 K and the other at 420M40 K. The first phase transition shows temperature hysteresis effects, and the 2 phases can coexist over a finite temperature range. The 127I NMR spectra also suggest the samples are showing reverse piezomagnetism which is unusual in non-magnetic crystals (Tarasov et al. 1990). 133Cs NMR measurements have been used to study the nature of phase transitions occurring in several caesium compounds. CsSCN undergoes a first-order structural phase transition at 470 K from a low-temperature orthorhombic antiferroelectric phase to a high-temperature cubic paraelectric phase. Measurements of the ~33Cs spin-lattice relaxation time of this compound suggest that the high-temperature cubic phase is a rotationally-disordered plastic phase (Furukawa et al. 1991). A single crystal study of the relaxation rates of this compound in the vicinity of the phase transition indicates the onset of large amplitude reorientations of the thiocyanate groups, with no evidence of orthorhombic microdomains above the transition temperature (Blinc et al. 1995a), by

A

CsCIO 4 I

t

B

I

'

I

CsBrO4

t20

200

ZQ " ~ " " "'--"-'~"

N

11o N

100 rJ~

- 0.4

0

0.4

~, 100

q

e~

-. , 100

I 200

.

.

. . 300

Temperature

.

.

0 400

K

~.2 q

"~

0.2

90 80

|

100

,

,

I

,

,

I

160 220 Temperature

,

,

J

280

K

Figure 10.17. A. Temperature dependence of the 133CsNMR parameters XQand ~1for polycrystalline CsC104. From Tarasov et al. (1991). B. Temperature dependence of the 133CsNMR parameters Re and "q for polycrystalline CsBrO4. Adapted from Tarasov et al. (1990a).

668


contrast with KSCN and RbSCN. 133Cs NMR has also been used to study the phase transitions occurring in CsSnC13 and CsPbBr3 (Sharma et al. 1991). Single-crystal 133Cs NMR studies have been reported of CsCd(SCN)3 allowing the relative orientations of the EFG tensors to be determined (Kroeker et al. 1997), and of ferroelastic CsPbC13 in which the NMR measurements revealed the presence of a twinned crystal structure (Lim and Jeong 1999a). The temperature dependences of the 133Cs quadrupolar parameters have also been determined for single crystal LiCsSO4 (Lim et al. 1999) and single crystal CsMnC13 (Lim et al. 1997b). A 133Cs NMR study of CsTcO4 has revealed the presence of 2 crystallographically inequivalent caesium sites of which the relative populations vary with temperature. The results provide information about changes in the crystal field potential in the vicinity of the cations accompanying a first-order phase transition from an orthorhombic to a tetragonal form at 389 K (Tarasov et al. 1992). An incommensurate phase existing in single-crystal Cs2HgBr4 over a narrow temperature range has been studied by 133Cs NMR. Below the onset temperature of the incommensurate phase the compound occurs as a paraelectric phase, changing to an antiferroelectric phase at higher temperatures. Changes in the 133Cs resonance intensity and lineshape at 243 K provide evidence of the existence of the incommensurate phase, and unequivocally indicate a soliton lattice in this phase (Boguslavskii et al. 1990). 133Cs NMR has been used to investigate an unusual phase transition which occurs in the compound CsCuC13 at 4.2 K under the influence of a magnetic field applied parallel to the c-axis. At a critical value of the magnetic field (11.19 T) the 133Cs resonance abruptly disappears, with no hysteresis in the sweep direction of the magnetic field. Analysis of the spectral lineshapes above and below this critical field value suggests that the anomalous change in the magnetisation occurs as a result of the quantum spin fluctuations of a one-dimensional ferromagnet with s - 1/2 (Chiba et al. 1993). The compound CsOH.H20 exists as a hexagonal [3-phase at room temperature, undergoing a slight modification to a hexagonal a-phase above 340 K. Below 232 K the Cs and O atoms of the hexagonal [3-phase undergo a further small change resulting in the structure becoming monoclinic. A ~33Cs NMR study of "pseudosingle crystal" CsOH.H20 shows at room temperature all 7 expected energy level transitions, from which the XQ value are derived (Amm and Segel 1986). The 133Cs XQvalues show an almost linear decrease with increasing temperature throughout the hexagonal [3 to hexagonal c~ transition (Figure 10.18), but a discontinuity of about 13 kHz ocurs at the hexagonal [3 to monoclinic transition at 236 K, indicating that the monoclinic phase has a longer longitudinal relaxation time than the hexagonal [3-phase. The 133Cs NMR results, complemented by ~H NMR, indicate that the hexagonal [3-phase is affected by an additional relaxation mechanism such as atomic diffusion, but the

669


160 ~o N

120

rj~

80

~176 1

monoclinic

hexagonal

hexagonal

(a)

40

!

I

I

I

200

300

400

500

Temperature K

Figure 10.18. Change in the 133Csquadrupole coupling constant XQwith temperature for pseudosingle crystal CsOH.H20. Note the 13 kHz discontinuity at the monoclinic to hexagonal 13transition at 236 K. Adapted from Amm and Segel (1986). change in ~33Cs XQ, which continues down to 180~ must involve a different mechanism (Amm and Segel 1986).

10.8.3 133CsNMR of minerals and zeolites 133Cs and 295i MAS NMR has been used to study 3 caesium compounds with the structure of leucite (Kohn et al. 1994). The 133Cs NMR spectra of Cs2CdSisO12, Cs2ZnSisO12 and Cs2MgSisO12 all show 2 narrow resonances of approximately equal area, consistent with the expected occurrence of 2 alkali sites in leucite structures with 6 tetrahedral T-sites. This result for Cs2ZnSisO12 is not consistent with the proposed structure which predicts 3 Cs sites with relative occupancies of 2:1:1, suggesting a need to reassess the structural space group in the light of the NMR data. The ~33Cs shifts are influenced by the framework cation, becoming more negative from Cd to Zn to Mg (Table 10.7) (Kohn et al. 1994). Barium hollandite, Bal.4(A1,Ti)2.28Ti6016, is an important component of Synroc, a synthetic material developed for the immobilisation of high-level waste from nuclear reactor fuel. The hollandite component of Synroc takes up alkali metal ions such as radioactive Cs + by substitution for Ba 2+ in the structural channels. This uptake has been studied by 133Cs MAS NMR which shows a single resonance at 211 ppm from Cs in the channel sites in the absence of paramagnetic ions (Hartman et al. 1998). Replacement of A13+ by Ti 3+ in the channel walls causes the 133Cs NMR peak to broaden and shift to 640 ppm, and also provides a sensitive means of monitoring the formation of water-soluble CsA1TiO4 which, if present, would compromise the aqueous durability of Synroc.

670


Cation adsorption onto phyllosilicate minerals is an important process with practical consequences for soil/water systems, sediments, natural hydrothermal processes, metamorphic environments and waste disposal sites. The adsorption of Cs + by a number of clay minerals has been studied by 133Cs NMR which provides information about the number and nature of the adsorption sites, and the hydration state of the cation. 133Cs MAS NMR at various temperatures shows that adsorption of Cs on the clay mineral hectorite, (Mg,Li,A1)3Si4Olo(OH)2.Cs+o.33, occurs in several distinctly different chemical sites between which motional averaging occurs at about - 40~ if interlayer water is present (Weiss et al. 1990). Below about - 60~ motional averaging of the adsorbed Cs is sufficiently slow for 2 Cs resonances to be resolved, 1, at about - 30 ppm, arising from Cs relatively tightly bound to the basal oxygens, the other, at about - 8 to + 30 ppm, arising from Cs in a region of compositional gradient (called the Gouy diffuse layer). After dehydration of the hectorite at 500~ the NMR spectra indicate that the adsorbed Cs remains in the interlayer in 2 new sites giving rise to 133Csresonances at about 30 and - 120 ppm. The latter peak corresponds to more highly coordinated Cs (CN --~ 12) located in the hexagonal holes formed by oxygen atoms on both sides of the interlayer, and the former peak corresponds to less highly coordinated Cs (CN ---9) associated with the hexagonal hole on only one side of the interlayer and interacting with fewer oxygen atoms on the opposite side (Weiss et al. 1990). ~33Cs NMR has been used to study the hydration state of the cation in Cs-exchanged vermiculite, a swelling mica mineral with a typical formula (Mg,Ti,Fe,A1)3 (Si,A1)4Olo(OH)2X2+o.45 (Laperche et al. 1990). The results indicate that the 133Cs isotropic chemical shift is directly related to the hydration state of the mineral and depends on the configuration of the oxygens from the lattice and water ligands and the back-donation from the ligand to the cation. The large • value found for 133Cs in the vermiculite interlayer (about 6.7 MHz) results from an appreciable degree of stacking disorder due to the large radius of Cs + which prevents its engagement with the pseudohexagonal lattice oxygen network. The corresponding value of ~1 is close to unity (Laperche et al. 1990). 133Cs MAS NMR has been used to examine the structural sites occupied by Cs adsorbed on a variety of phyllosilicate minerals with a view to determining possible relationships between the 133Cschemical shift and the chemical and structural parameters of the clays (Weiss et al. 1990a). Significant differences are found between the 133Cs NMR spectra of samples in the form of an aqueous slurry and those fully dehydrated by heating at 450~ consistent with an increase in the direct bonding of the exchanged Cs + to the basal oxygen atoms with increasing dehydration. For the hydrated slurry samples, a reasonable correlation was found between the ~33Cs chemical shift and the ratio of tetrahedral A1 to total tetrahedral atoms of the clay mineral, with the data for dioctahedral and trioctahedral minerals falling on different lines (Figure 10.19A). The 133Cspeak becomes less shielded as the content of tetrahedral A1

671


increases, since although motional averaging occurs in these fully hydrated samples, the position of the Cs resonance is the weighted average of the peaks from the various hydrated sites. Only very poor correlations were found with the degree of tetrahedral distortion and with the total layer charge of the hydrated clay minerals. The 133Cs NMR spectra of the fully dehydrated samples contain 2 resonances, each of which shows a reasonable correlation with the degree of tetrahedral A1 substitution (Figure 10.19B), but with no separate trends apparent for dioctahedral and trioctahedral minerals. Somewhat similar correlations were also found with the total layer charge of the fully dehydrated minerals (Weiss et al. 1990a). Calcium silicate hydrates are nanocrystalline porous materials of variable composition and poor crystallinity analogous to the compounds occurring in hydrated cements. The behaviour of these materials is of practical interest in determining the possible performance and long-term durability of storage facilities for nuclear waste and other hazardous substances. In a study aimed at improving the understanding of the surface chemistry of calcium silicate hydrate compounds, their interaction with CsC1 and NaC1 has been studied by 133Cs and 23Na NMR (Viallis et al. 1999). The NMR results indicate that both Cs and Na have an affinity for the calcium silicate hydrate surface, on which they are located in a diffuse ion swarm. Freeze-drying changes the environment of the adsorbed cations, reflected in the 133Cs chemical shift (200-250 ppm) A

B

20 4

[]

10

dioctahedral

~

~~ t

o

~

dioctahedral + trioctahedraltrioctahedral

0 ~-~ .40

r~ r~

-10

"~

trioctahedral

-20 0

0.06

0.12

AI(IV)/(AI(W)+ Si)

{ 0.18

J ~ -120~ -

- -- ~

dioctahedral + trioctahedral

0.12

0.24

- 1 i

0

Al~

[

,

- t

0v) + Si)

Figure 11).19. A. Relationship between the 8.45 T 133CsMAS NMR room temperature chemical shifts of fully hydrated Cs-exchanged clay minerals and their degree of tetrahedral A1 substitution. Open squares denote the dioctahedral minerals, open circles denote the trioctahedral minerals. Note that due to motional averaging in these samples, only one caesium resonance is observed. B. The same relationship for samples fully dehydrated at 450~ The 2 lines correspond to the 2 133Cs resonances observed in these samples. Note the similar behaviour of the dioctahedral and trioctahedral minerals when dehydrated. From Weiss et al. (1990a) by permission of the Mineralogical Society of America.

672


arising from inner-sphere surface complexes formed by interaction of the dehydrated Cs cations with the oxygen atoms of the bridging Si units. The Cs involved in these inner-sphere complexes occurs in 2 distinct environments, with and without chloride in the coordination sphere (Viallis et al. 1999). ~33Cs NMR has been used to monitor the dehydration of the Cs-exchanged zeolite mordenite (Chu et al. 1987). Both the static and MAS 133Cs NMR spectra of the fully hydrated material show a single resonance at - 64 ppm arising from motional averaging of the fully hydrated Cs + (Figure 10.20). Progressive dehydration results in the migration of the Cs into zeolite lattice sites characterised by 133Cs resonances resolved by MAS as peaks at - 157 and - 2 4 ppm (Figure 10.20B). The more intense and broader resonance can be fitted by 2 peaks, at - 157 and - 186 ppm with XQ = 3.1 MHz and ~1 = 0.6. The quadrupolar fitting parameters of the - 24 ppm resonance are very similar. The 2 Cs sites with similar chemical shifts have been identified as being near the centre of an 8-membered oxygen ring in the mordenite structure, with the other site located off-centre of a 6-membered oxygen ring (Chu et al. 1987). Binary caesium-lanthanum oxides supported on the mesoporous molecular sieve MCM-41 have potential catalytic applications for base-catalysed reactions. A 133Cs MAS NMR study of this system revealed shorter Cs-O bond lengths in the A

Static

B MAS---~Lab 59 PAS---)Rotor 1--~ Rotor 2---~Lab 76 Transitions central 55 satellite 58 symmetric 78 Transmitters 116 Transverse relaxation (T2) 99 TRAPDOR

Subject Index

1H-27A1,),-alumina 1H-23Na hydrous glasses 548 introduction 172, 182 31p-zvA1 T-alumina 293,450 31p-zvA1 glasses 446, 447 31p-Z3Na glasses 446, 447 Trapezoidal multiplication 129 Triclusters 27A1 276, 285,312 170 375 Tungsten, see 183W Two-dimensional experiments introduction 12, 90 from solution NMR 157

U Ultrasonic narrowing

78

V 51V CSA 643 oxides 642 phase transitions 646 shift ranges 643 solid solutions 646 surface layers 647 zeolites 649 Variable angle spinning 75 Vegard's law 2~ 609

W 183W

CP 474 tungstates 473 Water aluminosilicate glasses, 23Na 414, 417 aluminosilicate glasses 170 387 IH 536 WHH-4 79 Window functions 128 Windowed observation 79 Work hardening 687

X

XANES 3 129Xe gas 601 surface probe

602

719

X-rays 3 X-sialons 27A1 321 29Si 250 89y 464 XY CP 17 7

Y 89y atomic distributions 465 CSA 464 hydrides 468 nitrides 464 oxides 462 phosphors 467 rare earth doping 464, 465 relaxation times 462 shift ranges 463 sialons 464 sintering aids 464 superconductors 467

Z Zeeman interactions classical 24 quantum 25 Z-filter 162 Zeolite, see Mineral Index Zero-order coherence 33 Zero-order phase corrections Zirconates 23Na 407 170 355 91Zr 516 Zirconia in silica 242 67Zn correlations 513 CSA 512 oxides 511 shift range 512 sulphides 511 91Zr metal alloys 516 oxides 515 zircon 515 zirconates 516

130


Mineral Index akermanite 298i of 212 25Mg of 479, 481 albite 298i ordering of 210 23Na of 145,402, 410 23Na temperature dependence of 405 298i of 213 27A1-298i CP of 229 23Na-298i CP of 230 1H-29Si CP and HETCOR of 549 albite glass 23Na of 147 1H-29Si CP of 228 170 of 337, 380 170 MQ MAS of 379 1H in 546 19F-298i CP of fluorine-doped 559 13C of CO2-treated 573 alexandrite 9Be of 640, 641 alite 27A1 of guest ions in 315 allophane thermal decomposition of 216, 313 amphibole 298i ordering in 210 analcime 298i of 213 23Na of thermal dehydration 412 analcite 1Hof 539, 540 anatase 170 of 349, 351 conversion to rutile 351 49Ti of 506, 507 195pt of supported catalyst 603 andalusite 27A1 of 144, 277,282 298i of 212 anorthite 29Si of 213 antigorite 25Mg of 481 apatite

43Ca of 503 IH of biomineralisation 550 apophyllite 298i of 213 aragonite 43Ca of 503,504 13C of 573 armenite 298i of 213 augelite 27A1 of 282 bauxite dissolution in NaOH 299 bayerite thermal decomposition of 291 27A1 of 292 17Oof 373 beidellite 298i of 213 belite 27A1 of guest ions in 315 benitoite 298i of 213 beryl 298i of 213 7Li of lithium-containing 629, 630 9Be of 639, 640,641 bikitaite 6Li of 630 boehmite set-up compound for 1H CP 177 ground 284, 294 27A1 of 292 thermal decomposition of 291 in anodised films 293 1H-170 CP of 345 170 SATRAS of 340 170 of 273 boracite 11B of 421 boralite 11B of 431,432 borax 23Na of 401 11B of 421 721

722


brookite lVO of 349 49Ti of 506, 507 brucite 25Mg of 480 thermal decomposition of 484 brushite IH of 540 buddingtonite ~H of 541 calcite 43Ca of 503,504 13Cof 573 carnegieite 29Si of 213 cancrinite 298i of 213 celsian from gels 242, 298 137Ba of 523,524 chabazite 29Si of 213 chiolite 27A1 of 309 19F of 554, 555 chlorite 298i of 213 chondrite 298i of 212 170 of 362, 386 chrysotile thermal decomposition of 217,485 25Mg of heated 482 clinoenstatite 29Si of 212 170 DAS andDOR of 341 170 of 362 clinohumite ~70 of 362, 386 clinopyroxene 1Hin 543 cloverite 19F of 559 coesite 29Si of 213 ab initio calculations 224 170 DAS of 347 lVO of 359 relation between ~70 CQ and structure 346 relation between 170 h and structure 360 colemanite 11B of 421

cookeite 6Liof 630 cordierite 29Si ordering in 210 298i of 213 from gels 242, 298 corundum 27A1 of 291 ground 294 cristobalite 298i of 213 ab initio calculations 224 17Oof 359 relation between 170 h and structure structural relation to glass 367 cryolite 27A1of 309 19F of 554 dakeite 19F-29Si CP of fluorine-doped 559 danburite 29Si of 213 11B of 421 datolite 298i of 212 JiB of 421 1H of 540 dawsonite 23Na of 400, 413 diamond JH in 545 13C of 564, 566 diaspore ~H of 537 diopside 29Si of 212 170 DAS and DOR of 341 ~70 of 361,363 25Mg of 479, 481 dolomite 25Mgof 480, 481 thermal decomposition of 484 elbaite ~H of 540 emerald 29Si of 213 endellite 29Si of 213 enstatite from chrysotile 216 25Mg of 481 1Hin 543

360

M i n e r a l Index

~-eucryptite 298i ordering in 210 298i of 213 170 of glass 380 eucryptite 6Li of 630 faujasite 27A1 of 287, 289 SAPO with similar structure 306 170 MQ of 343,344 170 ab initio calculations 348 17Oof 363 23Na of 418 feldspars relation between 298i ~ and structure 221 in carbothermal sialon formation 248 23Na of 414 1H of 542 ferrierite ab initio calculations 224, 348 170MQof 343 17Oof 363 fluoroapatite 1H of biomineralisation 550 19F of 555 31p-19F REDOR of 556 13C of carbonate-substituted 573 fluorohydroxyapatite 19F of 550, 551,555 fluorophlogopite 298i of 213 fluoroscandium paragsite 19F of 556 forsterite from chrysotile 216, 485 170 DAS and DOR of 341 170 ab initio calculations 348 170 of 361,362 25Mg of 481 25Mg exchange in 483 1H in 543 fresnoite 298i of 213 17Oof 365 gallium fluoroamphibole 71Ga of 657 garnet 1H in 542 gehlinite 298i of 212 gibbsite ground 284, 294

723

27A1 of 292 thermal decomposition of 291 gmelilite 298i of 213 grandidierite 298i of 212 27A1 of 282 liB of 421 25Mg of 479,480, 481 graphite 13C of 564, 566 gypsum 43Ca of 503 1H of 540 halloysite thermal decomposition of 216 harkerite 27A1 of 277 hectorite 298i of 213 thermal decomposition of 217,485,633 25Mg of 481 113Cd of adsorbed cadmium 589 6Li of 630 133Cs of adsorbed caesium 670 heulandite 298i of 213 hollandite sol-gel preparation 313 133Cs of barium-containing Synroc 669 holtedahlite 32p of 438 1H of 540 hyalite 1H of 540 hydrocalumite 35C1 of interlayer ions 495 hydromagnesite 25Mg of 480, 481 hydrotalcite thermal decomposition of 313,484, 486 25Mg of 480, 481 35C1 of interlayer ions 495 15N of adorbed nitrate 582, 583 778e of interlayer Se species 586 119Sn of tin-containing 593 hydroxyapatite formation from Bioglass 257 31p of in bioglasses 451 1H of 537, 540,541 ilerite 1H of 540


724 illite

29Si ordering in 29Si of 213

298i ordering in

210 thermal decomposition of ilmenite 49Ti of 506 imogolite thermal decomposition of inderite liB of 421 inyoite liB of 421 jadeite

298i of

313

212

lepidolite 29Si of 213 6Li of 630 leucite

relation between 29Si ~ and structure 220 relation between 27A1 8 and structure 279 39K of 501

133Cs of caesium-substituted 216, 313

27A1 of 301 19F-27A1CP ofF-doped glass 559 kalsilite 298i of 213 39K of 501 kanemite 23Na of 411 kaolinite 27A1 of 146 set-up compound for ~H CP 174, 227 29Si of 213 conversion to sialon 248,252, 319 thermal decomposition of 215,222, 310 23Na of NaOH-leached 413 1H-29Si CP of 228 778e of intercalated dimethylselenoxide 587 113Cd of adsorbed cadmium 589 kenyaite 23Na of 411 kernite set-up compound for ~H CP 177 11B of 421 kyanite 27A1 of 177, 277 29Si of 212 laponite 6Li of 630 thermal decomposition of 633 larnite in portland cement hydration 258 170 DAS and DOR of 341 17Oof 362 lawsonite

29Si of 212 43K of 503

210

lorenzenite 29Si of 213 magadiite 23Na of 411 magnesite 25Mg of 480, 481 thermal decomposition of margarite 29Si ordering in 210

298i of

669

484

213

makatite 23Na of 408, 411 1H of 540 metakaolinite 298i of 215 27A1 of 284, 310 conversion to geopolymer 304 mica IH in 542 microcline 298i of 213 vapour-phase formation 319 23Na of 410 milarite 298i of 213 monetite 1H of 540 montmorillonite 298i of 213 thermal decomposition of 216, 312 1H-29Si CPof 228 25Mg of 481 39K of exchanged ions 500 19F of fluoride-exchanged 558 l l3Cd of adsorbed cadmium 589, 590 l l9Sn of interlayer tin 593 monticellite 29Si of 212 mordenite 27A1 of 289 ~33Cs of exchanged caesium 672 mullite 298i of 212 27A1 of 276 from gels 240, 294 ground and reheated 285

Mineral Index

as a sialon oxidation product 248,253,320, 321 mechanochemical preparation 243 relation to X-sialon 251,321 muscovite 29Si of 202, 213 29Si ordering in 210 thermal decomposition of 216, 313 39K of exchanged ions 500 N-melilite 27A1 and 29Si of Sm compound 323 89y of 465 15N of 577 narsarsukite 29Si of 213 natrolite 295i of 204, 213 nepheline 29Si of 213 170 of glass 380 23Na of 412 1Hin 542 19F-29Si CP of F-doped 559 13C of CO2-treated glass 573 octadecasil 19F-29Si CP of 229 octosilicate 23Na of 411 1H of 540 oligoclase 295i ordering in 210 29Si of 213 olivine 29Si of 212 omphacite 29Si of 212 orthoclase 39K of 500 orthoenstatite 29Si of 212 25Mg of, from talc decomposition 485 palygorskite 29Si of 213 25Mg of 481 paragonite 295i of 213 pargasite 1H of 540 pectolite 295i of 212 1Hof 540 43K of 503

725

penkvilksite 29Si of 213 periclase 25Mg of 480 petalite 6Li of 630 phenacite 29Si of 212 phlogopite 29Si ordering in 210 29Si of 213 25Mg of 481 phonolite 1H of 549 phosphoellenbergerite 31p of 438 1H of 540, 541 piemontite 295i of 212 pseudoboehmite thermal decomposition of 221 27Alof 292 in anodised films 293 pyrope 29Si ordering in 210 25Mg of 481 pyrophyllite 295i of 213 thermal decomposition of 216, 312 ground 312 1H of 540, 541 quartz 29Si of 213 ab initio calculations 224 170 of 359 relation between 170 h and structure 360 structural relation to glass 367 1Hin 542 rankinite 29Si of 212 reedmergnite 11B of 421 ruby 170 of 372 rutile 170 of 349, 351 49Ti of 506, 507 formation from gel 508 sanidine 170 of 387 170 of glass 388 vapour phase formation of 319

726


sapphire 27A1 CSA of 272 saponite 298i of 213 scolecite 298i of 204, 213 senegalite 27A1 of 282, 284 sepiolite 298i of 213 25Mg of 481 1H of 542 51V of vanadium-impregnated catalysts 648 serpentine 298i of 213 sillimanite 298i of 212 29Si ordering in 210 27Alof 277 sodalite 29Si of 213 relation between 298i ~ and structure 220 35C1 of encapsulated AgC1 495,496 9Be of beryllium-containing 640 relation between 9Be ~ and structure 641 sorensenite ~H of 540 sphene 298i of 212 spinel degree of disorder 273 spodumene 298i of 213 6Li of 630 stilbite 298i of 213 170 of 376 170 MQ MAS of 378 stishovite 298i of 225,226 170 of 359 talc 298i of 213 thermal decomposition of 217,485 1H-170 CP of 345 25Mg of 481 1H of 540 thaumasite 29Si of 226 thompsonite 298i of 204, 213 thorveitite

298i of 212 tielite sol-gel preparation 313 titanite 298i of 213 topaz 298i of 212 thermal decomposition of 216, 313 1H of 540 19F of 555 tourmaline 298i of 213 11B of 421 tremolite 298i of 213 1H of 539, 540,541 19F of 556 tridymite 29Siof 210, 211,213 tugtupite 9Be of 639, 641 tveitite 19F of 554 ulexite I~B of 421 ultramarine 29Si ordering in 209 vermiculite 298i orderingin 210 298i of 213 39K of exchanged ions 500 113Cd of exchanged cadmium 590 133Cs of exchanged caesium 670 vesuvianite 27A1 of 283 vinogradovite 298i of 213 wadeite 298i of 226 170 of 362 whitlockite 3~p of in bioglasses 451 wollastonite 170 of 76, 362 170 DAS and DOR of 341 29Si of 213 xonotlite in portland cement hydration 259 zeolite 27A1 of 149 170 DAS of 157, 158 170 DOR and MQ of 342, 376, 377

Mineral Index

170 enrichment of

336

298i of 206, 209 relations between 298i ~ and structure 219, 220, 221 relation between 170 8 and structure 342, 348 ab initio calculations 224, 348 27A1 of 287 1H-ZVA1TRAPDOR of 290 surface catalytic sites of 290 23Na of 418,419 liB of 431 l~ of catalyst supported by 472 1H of 538 19F of fluorinated 558 19F of adsorbed CF4 558 19F-29Cp of F-containing 562 113Cd of exchanged cadmium 590 119Sn of tin-substituted 594

129Xe of adsorbed xenon 602 195pt of supported catalyst 604 2~ of thallium-containing 606 9Be of beryllium-substituted 641 51V of vanadium-containing catalysts 71Ga of gallium-containing 656 139La of lanthanum-substituted 677 zircon 298i of 212 91Zr of 514, 515 zirconia 91Zr of 515 zoisite 27A1 of 144 zorite 298i of 213 zunyite 27A1 of 278

727

649

Multinuclear Solid-State NMR of Inorganic Materials

NMR imaging of materials

Synthesis of inorganic materials

Multinuclear Solid-State Nuclear Magnetic Resonance of Inorganic Materials (Pergamon Materials Series)

Molecular Materials (Inorganic Materials Series)

Principles of Inorganic Materials Design

Handbook of Inorganic Electrochromic Materials

Solution Processing of Inorganic Materials

Principles of Inorganic Materials Design

Principles of Inorganic Materials Design

Inorganic Materials Synthesis and Fabrication

The NMR Probe of High-Tc Materials

Inorganic Materials Synthesis and Fabrication

Nanohybridization of Organic-Inorganic Materials (Advances in Materials Research)

Inorganic Materials Chemistry Desk Reference, Second Edition

Surface Engineering Measurement Standards for Inorganic Materials

Inorganic Materials Chemistry Desk Reference, Second Edition

NMR Imaging of Materials (Monographs on the Physics and Chemistry of Materials, 57)

NMR Spectroscopy of Glycoconjugates

NMR Spectroscopy of Annulenes

NMR of Polymers

NMR Spectroscopy of Glycoconjugates

Handbook of Inorganic Chemicals

Mechanisms of Inorganic Reactions

Handbook of Inorganic Chemicals

Handbook of Inorganic Chemicals

Handbook of Inorganic Chemicals

Handbook of Inorganic Chemicals

Handbook of Inorganic Chemicals

NMR Studies of Translational Motion

Inorganic Chemistry

Multinuclear Solid-State NMR of Inorganic Materials

NMR imaging of materials

Synthesis of inorganic materials

Multinuclear Solid-State Nuclear Magnetic Resonance of Inorganic Materials (Pergamon Materials Series)

Molecular Materials (Inorganic Materials Series)

Principles of Inorganic Materials Design

Handbook of Inorganic Electrochromic Materials

Solution Processing of Inorganic Materials

Principles of Inorganic Materials Design

Principles of Inorganic Materials Design

Inorganic Materials Synthesis and Fabrication

The NMR Probe of High-Tc Materials

Inorganic Materials Synthesis and Fabrication

Nanohybridization of Organic-Inorganic Materials (Advances in Materials Research)

Inorganic Materials Chemistry Desk Reference, Second Edition

Surface Engineering Measurement Standards for Inorganic Materials

Inorganic Materials Chemistry Desk Reference, Second Edition

NMR Imaging of Materials (Monographs on the Physics and Chemistry of Materials, 57)

NMR Spectroscopy of Glycoconjugates

NMR Spectroscopy of Annulenes

NMR of Polymers

NMR Spectroscopy of Glycoconjugates

Handbook of Inorganic Chemicals

Mechanisms of Inorganic Reactions

Handbook of Inorganic Chemicals

Handbook of Inorganic Chemicals

Handbook of Inorganic Chemicals

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