A Unified View of Incommensurate-Commensurate Phase Transitions in A2BX4 Type Crystals

A unified treatment has been constructed which gives a perspective view of incommensurate(IC)-commensurate(C) phase transition schemes in A 2 BX 4 type crystals. We have shown that a microscopic treatment based on `Extended Antiferroelectric Interaction Ising' model can explain most of the observed features of IC-C phase transitions. The wide variety of the lock-in phases is attributed to the fact that there are three different ground states in the extended antiferroelectric interaction system, and that each substance passes through various transient lock-in phases in the process of reaching one of those ground states. Within this model, each substance is characterized by a point in a two-dimensional interaction parameter space. The unified view has been visualized by constructing a distribution map of the representative points of the hitherto observed materials in this interaction parameter space.