ABSTRACT
We define ferroelastic crystals as those which undergo, or can be deemed to have undergone, at least one ferroelastic phase transition.
Ferroelastic phase transitions are a subset of ferroic phase transitions (cf. Fig. 5.2.2). Apart from the fact that they are nondisruptive phase transitions (NDPTs), they have the distinctive feature that they always involve a spontaneous shear distortion of the crystal lattice, such that there is a change of the shape (rather than the size) of the crystallographic unit cell. The case of the cubic-to-tetragonal phase transition in BaTiOa illustrates the point. It is a first-order phase transition, so that there is a change in the volume of the crystallographic unit cell. This fact is unimportant and irrelevant from the point of view of defining it as a ferroelastic phase transition.1 What is relevant is that there is a change of the shape of the unit cell: from a cube to a square prism, brought about by an elongation of the cell edge along one of the basis vectors, say as, and a reduction of the repeat distance along ai and a2. Since the elongation can occur along any of the basis vectors ai, a2, as, domain pairs can exist in the tetragonal phase, members of which differ in the direction chosen by the crystal for elongation. Such domain pairs are said to differ in relative spontaneous strain (cf. §11.1.3). If a phase transition results in the occurrence of at least one such domain pair, it is called a ferroelastic phase transition.
