||Langford JI, UNIV BIRMINGHAM,SCH PHYS & SPACE RES,BIRMINGHAM B15 2TT,W MIDLANDS,ENGLAND|
UNIV RENNES 1,CHIM SOLIDE & INORGAN MOLEC LAB,GRP CRISTALLOCHIM,CNRS,URA 1495,F-35042 RENNES,FRANCE
||The powder diffraction method, by using conventional X-ray sources,was devised independently in 1916 by Debye and Scherrer in Germany and in 1917 by Hull in the United States. The technique developed steadily and, half a century later, the 'traditional' applications, such as phase identification, the determination of accurate unit-cell dimensions and the analysis of structural imperfections, were well established. There was then a dramatic increase of interest in powder methods during the 1970s, following the introduction by Rietveld in 1967 of his powerful method for refining crystal structures from powder data. This has since been used extensively, initially by using neutron data and later with X-rays, and it was an important step towards extracting 3-dimensional structural information from 1-dimensional powder diffraction patterns, in order to study the structure of crystalline materials. Similarly, techniques which do not involve structural data have been introduced for modelling powder diffraction patterns, to extract various parameters (position, breadth, shape, etc.) which define the individual reflections. These are used in most applications of powder diffraction and are the basis of new procedures for characterizing the microstructural properties of materials. Many subsequent advances have been based on this concept and powder diffraction is now one of the most widely used techniques available to materials scientists for studying the structure and microstructure of crystalline solids. It is thus timely to review progress during the past twenty years or so. Powder data have been used for the identification of unknown materials or mixtures of phases since the late 1930s. This is achieved by comparison of experimental data with standard data in crystallographic databases. The technique has benefited substantially from the revolution in the development of storage media during the last decade and from the introduction of fast search/match algorithms. Phase identification sometimes precedes a quantitative analysis of compounds present in a sample and powder diffraction is frequently the only approach available to the analyst for this purpose. A new development in quantitative analysis is the use of the Rietveld method with multi-phase refinement.
A major advance in recent years has occurred in the determination of crystal structures ab initio from powder diffraction data, in cases where suitable single crystals are not available. This is a consequence of progress made in the successive stages involved in structure solution, e.g. the development of computer-based methods for determining the crystal system, cell dimensions and symmetry (indexing) and for extracting the intensities of Bragg reflections, the introduction of high resolution instruments and the treatment of line-profile overlap by means of the Rietveld method. However, the intensities obtained, and hence the moduli of the observed structure factors, are affected by the overlap problem, which can seriously frustrate the determination of an unknown crystal structure. Although numerous structures have been solved from powder data by using direct or Patterson methods, the systematic or accidental total overlap of reflections continues to focus the attention of a number of crystallographers. New approaches for the treatment of powder data have been devised, based on maximum entropy methods and 'simulated annealing', for example, to generate structural models. Additionally, resonant diffraction (anomalous scattering) is used as an aid to structure solution.
There has been spectacular progress in characterizing the microstructural properties which arise from various types of structural imperfection. The principal advance has been the 3-dimensional reconstruction of 'anisotropic' (direction- or hkl-dependent) features or properties of polycrystalline materials. These include the shape of diffracting domains and the distribution of the size, structural 'mistakes' induced during the formation or subsequent treatment of a sample and dislocations or other forms of lattice distortion. The main innovation here has been a comparison of experimental data with those derived from a physical model based on data from other techniques or from prior knowledge of the behaviour of the material.
Most aspects of powder diffraction are brought together in analysing data from experiments carried out under non-ambient conditions, a field that continues to expand as more intense sources of radiation become available. Such experiments can be carried out over a wide range of temperature and at ever increasing pressures. Chemical or solid-state reactions and other processes, such as phase transformations, can be followed in situ by means of time-resolved diffraction.
For the benefit of the reader who is unfamiliar with powder diffraction, a resume of the basic principles underlying the various techniques and applications is included. Sources of radiation, modern instrumentation and detectors are also considered, since these have played a major role in the progress of powder diffraction during the past two decades. Numerous examples are discussed throughout the review, in order to illustrate the main applications and procedures. Powder diffraction is interdisciplinary and these are inevitably drawn from various branches of science. However, it should be remembered that, in the main, the use of powder diffraction is frequently a 'means to an end', albeit an important stage in a study of polycrystalline materials.