In the principle of optical crystallography and crystal symmetry, it is found that at room temperature, the 0.67 PMN-0.33 PT crystals near the morphotropic phase boundary composition present not only a rhombohedral $3m$ phase but also a monoclinic m phase with various angles of polarization rotation, the third ferroelectric phase in the PMN-PT solid solution system. The polarization rotation readily driven by composition suggests that it can be easily induced by electric field as well, thereby providing the structure base of the ultrahigh piezoelectric performance of 0.67 PMN-0.33 PT single crystals.

• Received 11 December 2000

DOI:https://doi.org/10.1103/PhysRevB.64.020102