Strain coupling in perovskite structural transitions: A first principles approach

Effects of strain coupling on ferroelectric transitions in the perovskite oxides are investigated through analysis of first-principles effective Hamiltonians and the related “eight-site” model. When strain coupling is set to zero in these models, classical Monte Carlo simulations show a single second-order cubic-rhombohedral transition. Mean field analysis of the full models, including strain coupling, leads to a second-order transition directly to a phase with the same symmetry as the ground state. Only in classical Monte Carlo simulations of the full models are intermediate tetragonal and orthorhombic phases and first-order transitions observed. This suggests that nonanalytic strain-related fluctuation corrections are essential for producing the observed transition behavior in ferroelectric perovskites.