Polarization rotation and monoclinic phase in relaxor ferroelectric PMN-PT crystal

A monoclinic phase is evidenced between rhombohedral and cubic phases in a <111>-cut single crystal PMN-33%PT from polarizing microscope observation of domain structures. Some rules for interpreting these observations for various cuts are reviewed, to illustrate how particular cuts are useful for distinguishing among particular types of phases and domains. Near 360 K the structure begins to distort front the rhombohedral toward the tetragonal phase through monoclinic domains (probably M-A-type but perhaps M-B-type). However, the present <111>-cut crystal seems to disfavor the tetragonal phase and persists in the monoclinic phase up to Tsimilar to420 K, where the cubic phase begins to develop. Temperature-dependent orientations of optical indicatrices of domains indicate polarization rotations within the monoclinic planes. In addition, a previous electric-field-cooled process enhances long-range order in domain patterns observed upon heating. At room temperature, with increasing E-field along [111], the crystal undergoes successive phase transformations via polarization rotations (perhaps M-A-type), i.e. rhombohedral --> tetragonal (associated with 90degrees-domain walls) --> [111] rhombohedral. As E-field decreases, very different domain patterns appear (as compared with those with increasing field) which indicate a strong thermal hysteresis relation between E-field and strain.