General Characteristics of Structural Phase Transitions in Crystals

The great majority of crystals can exist in several crystalline phases. As a rule, such phases appear to be stable in a certain range of temperatures and pressures and the transition from one phase to another is accompanied by significant discontinuous changes in the volume and entropy of the crystal. This involves the displacement of various structural elements to distances of the order of the unit cell dimensions and no limitations are imposed on a change in the crystal symmetry. The temperature hysteresis of a phase transition, i.e., the difference between the transition temperatures upon cooling and heating of the crystal, may reach hundreds of degrees and is an indication of the possible formation of relatively stable metastable states: one of the crystalline phases can exist in the region of temperatures and pressures in which another phase is more stable.