One hundred years of diffuse scattering

Over the 100 years since the discovery of the diffraction of X-rays by crystals, structure determination based on the analysis of Bragg peaks has grown into a very precise, widely applicable, and definitive tool. This conventional crystallography is based on the assumption that a crystal consists of a three-dimensional array of identical units. Real materials, however, only approximate this ideal and their diffraction patterns contain, in addition to sharp Bragg peaks, a weak continuous background known as diffuse scattering. Diffuse scattering occurs when there are departures of any kind from the ideal lattice. The properties of many important materials are dependent not simply on the average crystal structure yielded by the Bragg analysis but are often crucially dependent on the departures from ideality (disorder) that can only be revealed by analysis of the diffuse scattering. Diffuse scattering has been known and studied since the very earliest days of crystallography but because of the generally very low intensities and the diversity of effects that can give rise to it, the field has largely remained the realm of a relatively few specialist research groups. However, in recent years with the advent of synchrotron sources, latest high-resolution and high-dynamic-range X-ray pixel detectors and powerful computers for analysis and modelling, the problems that limited development of diffuse scattering methods have now largely been solved. Current methods are now capable of tackling virtually any disorder problem to yield details of structure and dynamics that goes far beyond the confines of the average unit cell description of structure. In this paper, we outline how diffuse scattering methods developed over the course of a century since the birth of X-ray crystallography and review the wide range of different areas and materials to which the methods have been applied.