Large piezoelectric response in paraelectric region of PMN-32PT near morphotropic phase boundary

We report an effective piezoelectric coefficient of d_eff = 40 ± 6 pm/V in the paraelectric phase of (1-x)Pb(Mg_1/3Nb_2/3)O₃−(x)PbTiO₃ single crystals with x = 0.32 (PMN-32PT), which is near the morphotropic phase boundary (x = 0.30). This value for d_eff in the paraelectric phase is significantly larger than that for other ferroelectrics measured so far. Our detailed investigation of symmetry-disallowed piezoelectricity indicates bias effects due to alternating current (AC) electric fields (100-800 kHz) and AC field cooling. The application of moderate AC fields (100-350 V/cm) gives rise to a nonlinear response of the piezoelectric effect, leading to an irreversible increase in the piezoelectric coefficient in the paraelectric phase. Along with changes in the elastic properties, the observed poling due to AC electric fields is reminiscent of irreversible ferroelectric-ferroelastic domain wall motion in ferroelectric phases. Such behavior is absent in cubic SrTiO₃ where precursor effects are commonly observed at much lower temperatures.