Origin of ultrahigh piezoelectric activity of [001]-oriented ferroelectric single crystals at the morphotropic phase boundary

The physical origin of the ultrahigh piezoelectricity of [001]-oriented ferroelectric single crystals at the morphotropic phase boundary (MPB) is investigated via computer simulations using the phase field method. A wide composition range is explored and the existence of a monoclinic phase at the MPB is considered. The relative contributions from polarization rotation and domain wall motion are discriminated. The domain structures, hysteresis and hysteresis-free strain-electric field curves, and change of d33 as a function of composition for poled samples near the MPB predicted by the simulations are all in agreement with experimental observations. It is found that under small polarization anisotropy the ultrahigh piezoelectricity of [001]-oriented ferroelectric single crystals at the MPB originates mainly from polarization rotation. Although domain wall motion also takes place for poled samples with monoclinic nanodomain structures and contributes to the hysteresis, its contribution to the ultrahigh piezoelectricity is insignificant.