Abstract
In order to search for Pb-free relaxor substances, the conditions to derive morphotropic phase boundary (MPB) are studied in perovskite-oxides. Before a microscopic theory for displacive type phase transition, the phenomenological approach is extended by combining with a first-principles computer simulation in several perovskite compounds. The MPB region in mixed perovskite-oxides is described by an anisotropy of the fourth-order term of polarizations, S, as 0 ≤ S ≤ 1.7, which is the region corresponding to individual perovskite-oxides with successive phase transitions. Basing on the present theoretical results, the definition of the MPB is discussed with explaining the pressure effect and the electric-field dependence in relaxor ferroelectrics.
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