Theoretical analysis of the temperature-field phase diagrams of perovskite-type ferroelectrics

The temperature-field phase diagrams are discussed within the Landau theory for perovskite-type ferroelectrics, which undergo successive transitions of first order. The sequences of the cubic-tetragonal-orthorhombic-rhombohedral phases and the cubic-rhombohedral-orthorhombic-tetragonal phases are considered, and the field is applied along the [100], [110], or [111] direction. The topology of the phase diagrams obtained are found to depend much on the degree of the anisotropy of the Landau potential functions in the order parameter space. When the free energy function is nearly isotropic, the critical end points associated with all the possible ferroelectric phases, stabilized successively with decreasing temperature, appear at a certain field and temperature. Only the one associated with the highest temperature ferroelectric phase does that at a fairly high field. It is pointed out that the critical end points experimentally observed in the relaxor ferroelectrics like Pb(Mg1/3Nb2/3)O-3 suggests that the transition in the relaxer, if any, must be of first order.