Observation of Stabilized Monoclinic Phase as a “Bridge” at the Morphotropic Phase Boundary between Tetragonal Perovskite PbVO3 and Rhombohedral BiFeO3

A morphotropic phase boundary (MPB) with coexisting tetragonal and monoclinic phases was observed in perovskite-type (ABO3) (1 – x)­PbVO3–xBiFeO3 solid solutions that were synthesized with a high-pressure and high-temperature method. At x = 0.95, the single monoclinic phase as an intermediate between tetragonal (PbVO3) and rhombohedral (BiFeO3) phases was identified by Rietveld refinement of synchrotron X-ray powder diffraction data at room temperature, and the monoclinic phase persisted in a wide temperature range. This phase also showed a polarization rotation as a function of temperature. In the tetragonal phase with 0 ≤ x ≤ 0.8, a large tetragonal distortion existed in a wide temperature range from room temperature to ∼900 K, and it showed little dependence on temperature, indicating that the thermal stability of the perovskite structure was enhanced compared with pristine PbVO3, which decomposes at 570 K. Correspondingly, the spontaneous polarization (PS), which originates from the hybridization between A/B site cations and oxygen, maintained a high value (>100 μC/cm2) in the tetragonal phase. The present study not only provides direct evidence of a monoclinic phase acting as a “bridge” between morphotropic phases but also suggests the potential of polar perovskites with strong polarity, i.e., PbVO3, as high-performance piezoelectric materials.