Theory of a structural phase transition in perovskite-type crystals. II. Interaction with elastic strain

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 Γ25 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 SrTiO3 and LaAlO3, and good agreement is obtained with the available experimental data. © 1970 The American Physical Society.