Cubic-tetragonal elastic phase transformations in solids

The cubic-tetragonal transformation driven by the elastic instability $(C_{11}-C_{12})\to 0$ may be followed by further rearrangements in which $\frac{c}{a}>1\mathrm{}$ changes to $\frac{c}{a}<1\mathrm{}$ 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 terminated in a bicritical point. Examples of both types of behavior are discussed briefly.