Antiphase boundaries in improper ferroelastic crystals: orientation and transformation of boundary structure

Thick antiphase boundaries (APB) in improper ferroelastic crystals are studied in the framework of the Landau expansion. Making use of the compatibility restrictions, two qualitatively different types of boundaries are predicted. The stress-free ones have preferred temperature-dependent orientations, for which an explicit formula is calculated. The strained boundaries, in contrast, possess non-mechanical stress along the boundary plane also in the crystallographic lock-in orientation with minimum energy. The transition between linear and rotational APB structures can appear as a minimum in the preferred APB orientation versus temperature curve. Consequently, the linear and orientational structures can be distinguished as well as determination of the APB transition temperature. A comparison with experimental observations is given.