Universal phase diagram for high-piezoelectric perovskite systems

Strong piezoelectricity in perovskite-type PbZn1-xTixO3 (PZT) and Pb(Zn1/3N b2/3)O-3-PbTiO3 (PZN-PT) systems is generally associated with the existence of a morphotropic phase boundary (MPB) separating regions with rhombohedra l and tetragonal symmetry. An x-ray study of PZN-9% PT has revealed the pre sence of an orthorhombic phase at the MPB, and a near-vertical boundary bet ween the rhombohedral and orthorhombic phases, similar to that found for PZ T 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 hi gh anharmonicity in these oxide systems. (C) 2001 American Institute of Phy sics.