Piezoelectric response and free-energy instability in the perovskite crystals BaTiO3, PbTiO3, and Pb(Zr,Ti)O3

The question of the origin of the piezoelectric properties enhancement in perovskite ferroelectrics is approached by analyzing the Gibbs free energy of tetragonal BaTiO3, PbTiO3, and Pb(Zr,Ti)O-3 in the framework of the Landau-Ginzburg-Devonshire theory. The flattening of the Gibbs free-energy profile appears as a fundamental thermodynamic process behind the piezoelectric enhancement. The generality of the approach is demonstrated by examining the free-energy flattening and piezoelectric enhancement as a function of composition, temperature, electric field, and mechanical stress. It is shown that the anisotropy of the free-energy flattening is the origin of the anisotropic enhancement of the piezoelectric response, which can occur either by polarization rotation or by polarization contraction. Giant enhancement of the longitudinal piezoelectric response (d(33)proportional to 10(3) pC/N) is predicted in PbTiO3 under uniaxial compression.