Features of the formation of the rotationally ordered phases in perovskites

Based on the phenomenological theory, the conditions for the formation of a sequences of the rotationally ordered phase states observed in orthoaluminates, orthoferrites and several other ABX3 compounds with a perovskite structure have been studied. It was shown that the specificity of the phase state formation is due to the competition between two critical order parameters characterizing the in-phase and antiphase rotations of the octahedra. The condensation of one order parameter blocks the appearance of the other. The blocking can be overcome by the interaction of the competing order parameters with the displacements of the cation A. In this case, the degree of stretching of the A–X bonds is of great importance. The consequence of the removal of the blocking is the formation of the phase states described by two competing order parameters. The conditions under which a change in the critical order parameter occurs have discussed.