Connected structural and ferroelectric phase transitions in perovskites in terms of the deformation order parameters

Based on Landau's phenomenological theory, the patterns of coupled structural and ferroelectric phase transitions in alloys prone to tetragonal distortions have been investigated. Through numerical experiments, it has been demonstrated that the presence of a weak coupling between the ferroelectric and structural subsystems, provided that the critical temperature of the ferroelectric transition is above the critical temperature of the structural phase transition, will induce weak tetragonal lattice distortions in the interval between the critical temperatures. As the level of coupling increases, the critical temperatures of both transitions converge and may ultimately coincide in the limit.