PNNL

Abstract

Orientation dependence of the longitudinal piezoelectric coefficients ( d*33 ) of a KNbO3 single crystal has been investigated as a function of temperature by using the Landau-Ginzburg-Devonshire thermodynamic phenomenological theory. It is shown that the maximum of d*33 is not always along the polarization direction of the ferroelectric phase. The enhancement of d*33 d along a nonpolar direction is attributed to the ferroelectric phase transition at which a polarization changes its direction. In the tetragonal phase, the maximum of d*33t for the tetragonal phase at high temperatures is along the tetragonal polar direction and then changes its direction toward the polar direction of the orthorhombic phase when close to the tetragonal-orthorhombic phase transition point. The maximum of d*33o of the orthorhombic phase depends on the competition of both high-temperature and low temperature ferroelectric phase transitions. In the rhombohedral phase, the maximum of d*33r is relatively insensitive to temperature due to the absence of any further phase transitions in the low temperature regime. These results can be generalized to the phase transitions induced by external electric-field, pressure, and composition variations.