Universal phase diagram for high-piezoelectric perovskite systems

Abstract
Strong piezoelectricity in perovskite-type PbZn1−xTixO3 (PZT) and Pb(Zn1/3Nb2/3)O3–PbTiO3 (PZN–PT) systems is generally associated with the existence of a morphotropic phase boundary (MPB) separating regions with rhombohedral and tetragonal symmetry. An x-ray study of PZN–9% PT has revealed the presence of an orthorhombic phase at the MPB, and a near-vertical boundary between the rhombohedral and orthorhombic phases, similar to that found for PZT between the rhombohedral and monoclinic phases. We discuss the results in the light of a recent theoretical paper by Vanderbilt and Cohen [Phys. Rev. B 63, 94108 (2001)], which attributes these low-symmetry phases to the high anharmonicity in these oxide systems.