Analysis of Ferroelectricity and Enhanced Piezoelectricity near the Morphotropic Phase Boundary

Abstract

In this Chapter, we discuss the phase diagram, dielectric constants, elastic constants, piezoelectricity and polarization reversal in the vicinity of a morphotropic phase boundary (MPB) in perovskite-type ferroelectrics and rare-earth–Fe₂ compounds from a theoretical viewpoint based on a Landau-type free-energy function. It has been clarified that the instability of the order parameter perpendicular to the radial direction in the order-parameter space near the MPB is induced by the isotropy or small anisotropy of the free-energy function; this is called the transverse instability. It should be noticed that the origins of the enhancement of the responses near the MPB both in the perovskite-type ferroelectrics and the rare-earth–Fe₂ compounds are the same. At the end of this Chapter, we point out that the transverse instability is a common phenomenon, appearing not only in the perovskite-type ferroelectric oxides, but also in magnetostrictive alloys consisting of rare-earth–Fe₂ compound, in the low-temperature phase of hexagonal BaTiO₃ and in shape memory alloys.