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

Аннотация

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(b₁ + b₂ + b₃), 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.