X-ray diffraction and phenomenological studies of the engineered monoclinic crystal domains in single crystal relaxor ferroelectrics

We present x-ray diffraction data, which demonstrate that after high field poling, solid solutions of lead zinc niobate with 8% lead titanate (PZN-PT) consist of a mixture of a dominant monoclinic phase and a remnant tetragon al phase. To understand the origin of this engineered monoclinic phase we s tudied a Devonshire phenomenological model that predicts the field induced phase transitions in PZN-PT. Although previous attempts to use this model f or these materials have matched the phase diagram and dielectric behavior, we show that the coefficients used in those models cannot be reconciled wit h recent data on electric field behavior. By modifying the coefficients we are able to explain the phase diagram, dielectric behavior, and high field phase transitions in compositions near the morphotropic phase boundary. Usi ng these coefficients, we find no stable monoclinic phase present in the mo del. This contradiction is explained by noting that within the engineered d omain state the tetragonal regions strain the rhombohedral regions, resulti ng in a metastable monoclinic phase. (C) 2000 American Institute of Physics . [S0021-8979(00)06811-0].