Structural phase transitions I. Landau theory

Structural phase transitions can be studied by a variety of techniques; macroscopic fields, X-ray, light and neutron diffraction, and resonance methods. The information which can be obtained by these different techniques about the order parameter and susceptibility at structural phase transitions is described and compared. Landau theory coupled with the ‘soft mode’ concept provides a simple picture of many structural phase transitions in terms of relatively few phenomenological constants. A review is given of the application of this type of theory to the wide variety of different phenomena which can occur at structural phase transitions. In particular, emphasis is placed on the nature of the fluctuations within Landau-Ginzburg theory and on the role of secondary order parameters. The nature of incommensurate phases and of lock-in phase transitions are discussed using both the conventional lattice dynamical approach and the soft soliton theories. The parameters of the Landau theory can, in principle, be obtained from a microscopic model. We critically discuss the attempts that have been made to develop models for anharmonic crystals, order-disorder systems and various electronic instabilities.