Statistical theory of orbital ordering in perovskites based on a three-minimum two-sublattice model

Abstract

The existence of tetragonally distorted octahedra found in a high-temperature cubic phase of some perovskites (LaMnO3, KCrF3, etc.) and the estimations performed make it possible to consider the observed orbital orderings as order–disorder phase transitions. The conditions of formation of ferro- and antiferrodistortive phase states described by the order parameters corresponding to reciprocal lattice vectors k = 0 and k 1/2(b 1 + b 2 + b 3), have been determined using a three-minimum two-sublattice model. The model is shown to be applicable for description of phase transitions in some ion–molecular crystals.