Morphotropic phase boundary in the Pb(Zn1/3Nb2/3O3)-BaTiO3-PbTiO3 system

The morphotropic phase boundary (MPB) in the relaxor ferroelectric system P b(Zn1/3Nb2/3O3)-BaTiO3-PbTiO3 (PZN-BT-PT) with 15 mol% BT was investigated through dielectric permittivity and high-temperature X-ray diffraction meas urements. It was revealed that MPB is a broad composition region extending from 12 to 18 mol% PT, within which the temperatures of the permittivity ma ximum are close to the ending temperatures for the phase transformation fro m coexisting rhombohedral and tetragonal phases to cubic phase on heating. When the specimen is cooled, the starting temperatures for the rhombohedral -to-tetragonal phase transition increase with increasing PT content. The la rge thermal hysteresis observed by X-ray diffraction is caused by the phase transformation between rhombohedral and tetragonal phases. On cooling, the MPB curves toward the PT-rich side, so that ceramics within this compositi on range undergoe successive phase transitions from cubic to rhombohedral a nd from rhombohedral to tetragonal phase. The diffuseness of the paraelectr ic-to-ferroelectric phase transition is remarkably decreased by the additio n of PT. The enhanced dielectric permittivity peak values for the MPB compo sitions are correlated with the reduced lattice distortion and phase coexis tence.