Theory of a Structural Phase Transition in Perovskite-Type Crystals. II. Interaction with Elastic Strain

Abstract

The interaction between soft-optical-mode coordinates and the elastic strain at structural phase transitions that have as soft modes linear combinations of the triply degenerate ${\ensuremath{\Gamma}}_{25}$ optical mode at the $R$ corner of the Brillouin zone are discussed with the help of a model Hamiltonian. The effect of the interaction on the soft-mode frequencies, the angle of rotation, and the strain distortions are calculated. The theory is compared with experimental results for SrTi${\mathrm{O}}_{3}$ and LaAl${\mathrm{O}}_{3}$, and good agreement is obtained with the available experimental data.