Abstract
As preliminaries of a microscopic theory for displacive phase transition, the first-order transition is investigated within a phenomenological study that provides a good explanation of experimental results on the electric field dependence and pressure effect in relaxor ferroelectrics. The morphotropic phase boundary (MPB) region in mixed perovskite oxides is described in the same phase diagram corresponding to individual perovskite oxides with successive phase transitions. On the basis of the calculated field-dependent phase transitions, the origin of the MPB is examined in comparison with that in the case of BaTiO3 with the first-order displacive transition.