MODERN POWDER DIFFRACTION IN MATERIALS SCIENCE. Daniel Louër, Laboratoire de Cristallochimie, CSIM (URA CNRS 1495), Université de Rennes, 35042 Rennes cedex, France

The powder diffraction method offers a wide spectrum of applications to materials scientists. Most modern applications have benefited, in the recent years, from substantial advances in high resolution instrumentation and modelling of diffraction patterns, to overcome the line overlap problem, and from high-intensity X-ray synchrotron and neutron sources (see Langford and Louër, Rep. Prog. Phys. 1996, 59, 131). The development of new materials, e.g. high Tc ceramics, carbon-cage molecular compounds or in general zeolites, and the investigation of powder materials, in terms of structural and microstructural properties, in solid state chemistry or mineralogy, are widely based on the use of powder diffraction.

Modern powder diffraction applications include (i) analytical characterization, such as the improved identification of unknown materials from crystallographic databases using storage media, the introduction of fast search/match algorithms and quantitative determination of phase abundance using the Rietveld method; (ii) the extraction of 3-dimensional structural and microstructural information from 1-dimensional data and (iii) the use of dynamic and non-ambient diffraction to study the behaviour of materials subjected to external perturbation.

The most significant advance in the recent years has probably been the determination of crystal structures ab initio from powder data. More complex structures can be studied (e.g. up to 29 atoms in the asymmetric unit) by combining X-ray and neutron diffraction. New classes of compounds are being studied, such as coordination, organic or pharmaceutical compounds and new strategies for solving the phase problem have been introduced (e.g. entropy maximization, simulated annealing, atom-atom potential methods, direct methods,..).

Although the frontiers of powder diffraction have been extended by high-intensity radiation sources, most applications can be carried out with conventional sealed-tube X-ray sources, preferably by using monochromatic radiation.