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Acta Cryst. (1994). A50, 258-259

Electron diffraction techniques Vol. 2 (IUCr Monographs on Crystallography No. 4.)

Edited by John M. Cowley

Pp. vi + 423. Oxford: International Union of Crystallography/Oxford Univ. Press, 1993
Price £45.00. ISBN 0-19-855733-7

This is the second of two volumes, sponsored by the Commission on Electron Diffraction of the International Union of Crystallography, that are intended to be a comprehensive overview, written by acknowledged experts, of electron diffraction theory and its application to materials problems. A review of Volume 1 has already appeared Acta Cryst. (1993), A49, 677-678.

Over half of Volume 2 is the detailed Chapter 1 (222 pp.) on the analysis of defects by diffraction contrast and high-resolution electron microscopy, written by S. Amelinckx and D. Van Dyck of the University of Antwerp. This, in itself, could very well have been published as a separate monograph. These authors also have a second contribution to the volume, Chapter 4 (65 pp.), which is a very interesting treatment of electron diffraction from modulated structures. Other contributions include Chapter 2 (37 pp.) by J. K. Gjønnes of the University of Oslo, which discusses the analysis, in electron diffraction patterns, of continuous diffuse scattering caused by crystal disorder or thermal motion. Chapter 3 (49 pp.), by K. Yagi of the Tokyo Institute of Technology, is a description of reflection high-energy electron diffraction and reflection electron-microscopy techniques for the examination of crystal surfaces. Finally, the use of electron diffraction patterns and spectroscopic techniques (primarily EDS) to identify unknown materials is treated in Chapter 5 (45 pp.), by C. E. Lyman (Lehigh University) and M. J. Carr (Sandia National Laboratories), expanding the rather terse treatment of this topic in Volume 1.

For the investigator interested in inorganic materials, there is much to praise in this volume, as there was also in the preceding one. The theoretical treatments in Chapters 1 and 4, in particular, are quite thorough and will serve as a handy reference to researchers interested in the analysis of real crystal structures, including defects, disorder and deviations from simple stoichiometry. [It is a shame, however, that three important figures (1.4, 1.6 and 1.7) are missing.] Chapter 2 gives a balanced overview of how diffuse scattering problems have been analyzed and Chapter 3 is most fascinating in its treatment of surface dynamic processes. The difficulties of using crystallographic databases with analytical electron diffraction data are clearly portrayed by the authors of Chapter 5, who, nevertheless, give practical advice on how unknowns can be identified with such data. There is much in this volume that would be of use to the organic materials scientist also - especially since the advent of low-dose electron imaging techniques has made possible the analysis of defects in polymer crystals, for example, in terms of molecular (if not atomic) packing. Personally, I was most interested in the chapter on incommensurate structures because, after I read it, it became clear to me that this development could also be applied to the analysis of the phase separation of metastable paraffin solid solutions. Hence, as ever, the organic materials scientist follows the harbingers in inorganic materials research. Obviously the harbingers are not so much troubled by electron-beam-induced radiation damage as are the followers!

Excellent as the contributions to Volumes 1 and 2 of this series are, the reader should be cautioned that the predominant viewpoint of most authors therein is that no direct analysis of the experimental electron diffraction or microscopic data is possible and that only an indirect approach, starting froman assumed crystal structure model, can be used. This, of course, is a result of the complexity of the dynamical scattering theory. Even with an indirect interpretation of these data by image simulation, the viewpoint can also be quite negative. To quote a passage from Chapter 1: "Nevertheless, the technique is so tedious and so insensitive that, thus far, it has been only successful if the number of plausible structure models is very limited. In our view it is astonishing that such an expensive technique as HREM (high-resolution electron microscopy) is so dependent on the availability of prior information obtained from other techniques".

It is important, therefore, to point out that there are other, more optimistic, opinions. For example, a review of high-resolution electron microscopy in solid-state chemistry by L. Kihlborg of Stockholm University [Prog. Solid State Chem. (1990), 20, 101-133] demonstrates that a direct structure analysis is sometimes permitted, even with inorganic materials. This is exemplified by the numerous studies by S. Hovmöller and his collaborators e.g. Nature (London) (1984), 311, 238-241, and many more recent papers. The chapter on modulated structures was probably written before the publication of the ingenious analysis of ankangite by Xiang, Fan, Wu, Li & Pan [Acta Cryst. (1990), A46, 929-934], which describes the application of direct phasing methods to electron diffraction intensity data. Similar work has followed since then, such as that from the laboratory of Fan Hai-fu and Li Fang-hua. Thesestudies do not state, by any means, that the scattering theory is incorrect, but only that this theory can be exploited for collection of data sufficiently near the single-scattering approximation to permit a direct structure analysis to be carried out. Such analyses, moreover, can yield chemically reasonable results in agreement with (but not dependent upon) independent X-ray crystallographic studies. This has been recognized by organic electron crystallographers for years.

With these reservations in mind and with continued regrets that the organic field is not more fully represented (for instance by proteins or linear polymers, each with a long history of electron crystallographic structure determinations), there are no doubts in this reviewer's mind that these two volumes represent a significant addition to any electron diffractionist's bookshelf, with extensive material available for study in considering new research directions.

Douglas L. Dorset

Electron Diffraction Department
Medical Foundation of Buffalo, Inc.
High Street
Buffalo
NY 14203
USA


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