The Landau model of Slonczewski and Thomas (1970) for the improper ferroelastic ${\mathit{O}}_{\mathit{h}}^1$-${\mathit{D}}_{4\mathit{h}}^{18}$ phase transition in perovskite-structure compounds has been extended by including spatial gradient terms of the three-component primary order parameter (OP) and applied to calculate the OP profile and the strain distribution for antiphase and for twin boundaries in the tetragonal phase. In order to obtain quasi-one-dimensional kink-type solitary-wave solutions for which the OP and the strain depend only on the coordinate normal to the interface plane, lateral surface forces are required which allow for the shape change associated with ferroelastic interfaces, but which prevent expansion or contraction within the boundary plane. Numerical application to ${\mathrm{SrTiO}}_3$, including calculation of the thickness and energy of both types of interphase boundaries versus temperature, is also presented.

• Received 2 October 1989

DOI:https://doi.org/10.1103/PhysRevB.41.4334