Abstract
The application of modern light scattering techniques to the study of structural phase transitions is reviewed with emphasis upon observed departures from predictions of the simple soft mode theory for defect free systems. Particular attention is given to the so-called central peak phenomena evident in several light scattering spectra which arise from (a) very low frequency collective excitations and/or (b) defects. Such dynamic processes as entropy fluctuations and phonon density fluctuations are shown to contribute, via non linear coupling, to inelastic singular central peaks. In addition, static processes - associated with defects - are assigned responsibility for singular elastic central peaks accompanying several transitions. Materials to be discussed include KDP, Pb5Ge3O11, SrTiO3, and K(HxD1-x)3(SeO3)2.
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