Giant electrostriction and stretched exponential electromechanical relaxation in 0.65Pb(Mg _1/3Nb _2/3)O ₃-0. 35PbTiO ₃ crystals

Unipolar electric field-induced strain was studied in the [001] _cub-oriented 0.65Pb(Mg _1/3Nb _2/3)O ₃-0.35PbTiO ₃ crystals with the composition close to the morphotropic phase boundary, which are known to show enhanced piezoelectric properties. Polarized-light microscopy was used to study the domain structure. In the samples containing the mixture of rhombohedral and tetragonal ferroelectric phases, a giant electrostrictive effect was found to exist after poling, in addition to the piezoelectric effect. The a domains of the tetragonal phase with polarization perpendicular to the poling field were observed. Their existence in the poled crystals is due to the elastic interactions with the rhombohedral domains. The electromechanical response of the tetragonal a domains, which should be purely electrostrictive due to the crystal symmetry, is suggested to provide the intrinsic contribution to the observed electrostriction. The extrinsic contribution to electrostriction is explained by the 90° reorientation of the a domains into the c domain state. Upon application of the driving field, the onset of extrinsic electromechanical response was found to follow the stretched exponential relaxation pattern.