Thermal expansion measurements on unpoled, multidomain, single crystals of PMN-PT in the MPB region reveal that one of the ⟨001⟩ directions is different than the other two, contrary to the crystallographic equivalency that is expected for threefold rotations about ⟨111⟩ derived from the prototypic-cubic phase. This unexpected behavior may be related to the method of seeded crystal growth. On heating into the tetragonal phase, the unique axis persists, consistent with the preferential alignment of $c$-axis domains. Poling along the unique direction further enhances the anisotropy in thermal expansion coefficients, leading to an increase in the magnitude of the strain induced on heating through the subsequent rhombohedral-to-tetragonal and tetragonal-to-cubic phase transformations. Room-temperature poling in the rhombohedral phase in a direction normal to the unique direction reorients the preferred domain alignment in the higher temperature tetragonal phase. This realignment disappears upon further heating into the high-temperature cubic phase, i.e., thermal depoling reestablishes the original unique direction. When the room-temperature poled crystals are heated under open-circuit conditions, the temperature at which the tetragonal-to-cubic phase transformation is completed is shifted to higher temperatures relative to unpoled crystals, indicating an increase in the temperature range of stability for the tetragonal phase as a result of incomplete randomization of domain orientations at the rhombohedral-to-tetragonal phase transformation. Under short-circuit conditions, the tetragonal-to-cubic phase transformation temperature is not shifted, indicating a complete randomization of polarization directions on heating above the rhombohedral-to-tetragonal phase transformation temperature. The thermal expansion characteristics for the phase transformation behavior are related to poling conditions and dielectric properties.

• Received 19 July 2005

DOI:https://doi.org/10.1103/PhysRevB.72.214110