Cubic-tetragonal elastic phase transformations in solids

The cubic-tetragonal transformation driven by the elastic instability (C11-C12)→0 may be followed by further rearrangements in which c/a>1 changes to c/a<1 or vice versa. These transformations are investigated by Landau theory. Unlike the cubic-tetragonal transformation, which must be first order, the subsequent reordering may be either a single first-order transformation or involve two second-order transformations through an intervening orthorhombic phase. The three-phase boundaries terminate in a bicritical point. Examples of both types of behavior are discussed briefly