Structure of ferroelastic domain and thermoelastic effect in lead phosphate single crystal

A ferroelastic crystal is defined as a crystal which has one or more orientation states in the absence of mechanical stress and can be switched from one to another of these states by a mechanical stress. Pb ₃(PO ₄) ₂ is a well-known representative ferroelastic crystal. The crystal has two types of domain walls, the W _m wall and the W _b, wall, and both the walls are represented as crystallographically prominent planes of fixed indices. Two contiguous domains of the W _m wall and W _b, wall can be deduced one from the other by mirror symmetry and by a rotation of 180°. A ferroelastic material has different temperature dependence of the strain from a paraelastic one. It was also reported that an intermediate phase exists between the para- and ferroelastic phases. In this study, the domain-switching process of lead phosphate under a sinusoidal mechanical stress has been investigated by using modified thermomechanical analysis. Distinct synchronization phenomena between the periodic stress and the sample temperature oscillation were observed. This characteristic feature is understood as a thermoelastic effect enhanced by the thermoelastic domain switching due to the external stress, in which the thermoelastic effect depends on the sign and change of magnitude of the thermal expansion coefficient of the sample.