Thermodynamic and crystal-chemical conditions for the formation of rotationally ordered phases in rare earth perovskites

The conditions for the formation of a sequence of rotationally ordered phase states characteristic of the rare earth perovskites AB O 3 have been studied: Pm 3 m (C phase) → R 3 c (Rh phase) → Pnma (O phase). A necessary condition is the presence of crystal instabilities with respect to in-phase and antiphase rotations of the B O 6 octahedra, described by the order parameters ψ and φ, respectively. Both types of instability are caused by the tendency of the linear configuration of the B —O— B bonds to bend. Therefore, the degree of crystal instability with respect to the order parameters, φ and ψ, is not significantly different, and the order parameters themselves are competitors, where the occurrence of one prevents the occurrence of the other. Considering the results of phenomenological theory and analysis of phase transitions in rare earth aluminates, a scenario for the realization of this sequence has been developed and the factors responsible for this process have been identified. It is shown that the degree of deformation of the A —O and B —O bonds has a significant effect on the temperatures of the phase transitions. The important role of cation A in the realization of the Rh–O phase transition is noted. The process of formation of phase O during the Rh–O phase transition has also been studied.