  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)O3 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 (on the order of 1000 pC/N) is
predicted in PbTiO3 under uniaxial compression.
