Book and Article reviews

Introduction to X-ray Powder Diffractometry

Authors: Ron Jenkins and Robert L. Snyder

As might be expected, Jenkins and Snyder have written a really useful book. It covers basic crystallography, production of X-rays, instrumentation, sample preparation, data collection and reduction, phase identification and quantitativ p hase analysis. The book concentrates on Bragg-Brentano geometry, being the most common for routine work, and is very practical describing how to get started once you are in the lab with your sample.

Many students use powder diffraction as one of several different techniques to c haracterise materials. For them the chapters on sample preparation, data collection and redu ction, phase identification, databases and quantitative phase analysis give a clear and comprehensive guide of how to find out what crystalline phases are in the sample. There are diagrams and flow charts to illustrate the steps in data reduction and phase identification respec tively as well as tables which show the effect of errors. Descriptions of what lies behind commercial software for finding peak positions and identifying crystalline phases are helpful, particularly for the occasions when the black box (on default settings) might not give the correct answer.

I believe that most of the information on dif fractometer alignment has not appeared before in a textbook. With the recent round-robin tests showing that many diffractometers are not optimally aligned this is very timely, particularly as the participants of such tests are likely to be more conscientious than the many other non-participants. The disappointment in this chapter is that the descriptions all relate to a family of Philips diffractometers and it would have been good to cover a wider range of instruments. The discussion of systematic aberrations and choice of slits is instructive with more clear diagrams. Routine instrumental checks and the use of standards also rarely appear in textbooks. They tend to be considered by laboratories that concentrate on X-ray work or are seeking some sort of accreditation and are unfortunately forgotten or never considered by others. For anyone trying to get the best out of a diffractometer and wanting to have an understanding of possible errors, these chapters are most helpful.

As well a s including new material, the book draws on recent and old literature. There are some historical descriptions which add interest without obscuring the main aim of hel ping people to do powder diffraction. The book is titled $Introduction$ and thus there is little or no mention of profile analysis, texture measurements, glancing angle work, residual strain, indexing, structure determination and the use of synchrotron or neutron sources. A companion volume is planned in which one expects that such top ics will be addressed.

If you have a Bragg-Brentano diffractometer which is being used for powder diffraction, I recommend that you obtain a copy of this book.

Mary Vickers January 1997

Article reviewed in issue 60 March 97

1997 will mark 30 years since the development of the Rietveld method whic h, according to Langford and Louër, was one of the main factors responsible for the renaissance of powder diffraction methods. The breadth and scope of this review article clearly reflect the power and applicability of such techniques today, in the characterisation of modern materials at the atomic level. It is also apparent that this renaissance is far from complete; future developments are likely to increase further the versatility and scope of powder techniques.

For the industrial c rystallographer in particular, there was a previous renaissance 30 years before Rietveld when the work of Hanawalt et al. conceived the forerunner of the PDF. The topic of qualitative phase identification is only one of many covered by Langford and Louër in their review of developments in powder diffraction in recent times.

The review is accessible to all users and researchers in powder diffraction as the basic principles, aspects of instrumentation and experimental considerations are well described, yet the authors lead us through to recent state-of-the-art developments and applications. Of course, many of the key advances in the technique in recent years are in those areas where the authors themselves have played leading roles; namely in total pattern fitting, line profile analysis, indexing and ab initio structure determination. These are fully reviewed, starting with fundamental principles and clearly describing how the contributions from various workers have evolved into tod ay's interpretative techniques.

Although brief mention is made of synchrotron and neutron sources, the theme of the review is the powder technique itself, irrespective of the source used; this inevitably leads to a bias towards laboratory X-ray examples, reflecting the volume of work in this area.

Each section is illustrated by examples taken from key papers. For example, the use of Rietveld refinement in structural studies on High Tc superconductors. The discussion of line profil e analysis is, not surprisingly, thorough and includes the classic example of direction dependant effects in ZnO. The treatment of ab initio structural studies is fully up-to-date with mention of maximum entropy, simulated annealing, lattice energy minimisation and anomalous dispersion approaches, among others. The topics of non-ambient diffraction, dynamic diffraction and, in particular, diffraction at high pressures make a welcome appearance and include examples using conventional sources and neut rons as well as synchrotron X-rays.

A concise section on quantitative phase analysis includes conventional methods as well as Rietveld-based approaches. This account is well referenced and has a short account of applications. Indeed one of the main purposes of a review such as this is to allow ready access to the key publications in a particular area of work; the comprehensive set of references appended to this article will satisfy anyone with this purpose in mind. The concluding section o f the review summarises well the past successes of powder diffraction and what were the key developments. Some indication is also given of the future. This includes the increasing ease of experimentation on individual grains of a polycrystalline sample leading to a convergence of powder and single crystal methods. The future of the technique in the characterisation of real materials does indeed seem secure.
Craig Adam
Staffordshire University

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