Phenomenological Theory of Morphotropic Phase Boundary with Monoclinic Phase in Solid-Solution Systems of Perovskite-Type Oxide Ferroelectrics

The phase diagram with monoclinic phase near the morphotropic phase boundary (MPB) and the dielectric constant in a solid-solution system of perovskite-type ferroelectrics are discussed on the basis of the Landau–Devonshire-type free-energy function expressed in terms of polarization components. It is confirmed that the origin of the large dielectric response near the MPB can be attributed to the instability perpendicular to spontaneous polarization, called the “transversal instability”.