Ferroelectric properties of monoclinic (formula presented) crystals

A monoclinic phase was recently discovered near the morphotropic phase boundary in several high-performance piezoelectric perovskite solid solutions, but its properties have not been reported. In this paper the dielectric, piezoelectric, and ferroelectric properties of the monoclinic Pm phase in the (formula presented) perovskite system are studied. In a (001)-oriented crystal of composition (formula presented) ferroelectric hysteresis loops with remanent polarization of 23 (formula presented) are displayed. In poled monoclinic crystals, under a unipolar drive up to 10 kV/cm, the domain walls remain unchanged and the polarization and longitudinal strain change almost linearly, but the piezoelectric response (formula presented) is much weaker than in the rhombohedral phase of close composition. The relative dielectric permittivity of the Pm phase is also smaller (with a small-signal value of -2500), but the piezoelectric constant (formula presented) and the electromechanical coupling factor (formula presented) are practically the same as in the rhombohedral phase. The properties of the various phases in the range of the morphotropic phase boundary are related to the different rotation paths of the polarization vector induced by the external drive.