Piezoelectric softening in ferroelectrics: ferroelectric versus antiferroelectric PbZr_1-xTi_xO_3

The traditional derivation of the elastic anomalies associated with ferroelectric (FE) phase transitions in the framework of the Landau theory is combined with the piezoelectric constitutive relations instead of being explicitly carried out with a definite expression of the FE part of the free energy. In this manner it is shown that the softening within the FE phase is of electrostrictive and hence piezoelectric origin. Such a piezoelectric softening may be canceled by the better known piezoelectric stiffening, when the piezoelectric charges formed during the vibration are accompanied by the depolarization field, as for example in Brillouin scattering experiments. It is therefore possible to evaluate the average piezoelectric coupling from the usual elastic measurements of unpoled ceramics, where the piezoelectric stiffening does not occur. As experimental validation, we present new measurements on Zr-rich lead zirconate titanate (PZT), where the FE phase transforms into antiferroelectric on cooling or doping with La, and a comparison of existing measurements made on FE PZT with low frequency and Brillouin scattering experiments