Displacive phase transitions
Among the structural phase transitions, displacive phase transitions comprise those that only require small collective displacements of individual atoms. A small displacement of atoms in this context amounts to fractions of the nearest neighbour interatomic distances, i.e. generally at most a few tenths of an ångstrom. Displacive transitions occur spontaneously and reversibly at specific pressure and temperature conditions. Because of this, their direct observation is inextricably linked to the use of in situ methods, usually requiring a non-trivial sample environment, e.g. high-pressure cells, furnaces or cryostats. This definition puts displacive phase transitions in contrast to those structural phase transitions that involve significant diffusion of atoms, e.g. cation ordering transitions or entirely reconstructive phase transitions.
As this introductory text should serve as a guide to the analysis of experimental data, it will be predominantly concerned with the theory of displacive phase transitions and not with the experimental techniques employed to obtain the necessary data. Alarge number of in-depth review articles and textbooks devoted to the subject has already appeared in the recent past. It is therefore not the aim of this text to introduce every imaginable aspect of displacive phase transitions. Many of the details that are necessarily being omitted can be found elsewhere (e.g. Binder, 1987; Salje, 1992a, 1992c, 1993; Dove, 1997; Carpenter et al., 1998a; Carpenter & Salje, 1998). What this text is trying to achieve is to transport a general picture of the theory and its application to experimental data, accompanied by an explanation of technical terms where they might appear.