Topology of ferroelectric phase diagrams with morphotropic phase boundary and their piezoelectric properties.

Ferroelectric materials at morphotropic phase boundary (MPB) exhibit large piezoelectric coefficients and thus are widely used as piezoelectric actuators and sensors. Three different phase diagram topologies have been reported in various ferroelectric MPB systems: one with a cubic(C)-tetragonal(T)-orthorhombic(O)-rhombohedral(R) quadruple point; another with a C-T-R triple point and the other with a C-T-R along with a T-O-R triple point. However, it is unclear which phase diagram topology would give the best piezoelectric property. Here in this work, we obtain the above three different MPB phase diagram topologies by Landau free energy. It was found that among the three phase diagram topologies, the one with a T-O-R triple point exhibits superior piezoelectric property at the T-O-R triple point because of the small polarization anisotropy required by the simultaneous thermodynamic equilibrium of three ferroelectric phases. This work could guide the design of piezoelectric materials. [ABSTRACT FROM AUTHOR]