Diffuse scattering and structural phase transitions

On the basis of numerous experiments using the mono-Laue method on the diffuse scattering of X-rays and electrons we have developed the Comès-Lambert-Guinier concept of the existence of co-operative thermal vibrations in perfect crystals in which one-dimensional or two-dimensional atomic objects, i.e. chains or planes of atoms, are moving in a double-well potential with conservation of coherence. For an approximate description of such a motion we have utilized the pseudospin Ising model approach which is widely used in the theory of structural phase transitions. These ideas proved to be successful for the description of diffuse scattering patterns, and their temperature evolution in close connection with structural phase transitions. The developed theory gives the possibility through the observation of diffuse scattering at high temperatures and its analysis to predict the structural phase transition (and even the subsequent cascades of transitions) and to give the correct explanation for physical anomalies in the crystals, either with distortive (weak) or reconstructive (strong) transitions. A wide variety of crystallochemical situations and transitions in crystals of the perovskite family and in metals with a bcc lattice are analysed in detail. A number of new experimental diffuse scattering patterns from crystals of various types are obtained.