Mechanisms of electrostriction and giant piezoelectric effect in relaxor ferroelectrics

Two mechanisms of electrostriction in relaxor ferroelectrics of lead magnesium niobate (PMN) type are proposed. The first is based on the interaction of the soft-lattice polarization and lattice distortions with the charge transfer fluctuations and with the active ion displacements for the localized Nb–O–Nb and O–Nb–O triads in the disordered relaxor state. The second mechanism is due to the effect of simultaneous interaction of quasilocal vibrations with the soft-lattice polarization and lattice distortions. In the latter case, the quasilocal Ti-O stretching mode in PMN and lead zinc niobate (PZN) doped with lead titanate (PT) is considered. While the first mechanism explains the magnitude of electrostriction coefficients in undoped PMN and PZN, the second one explains the drastic increase of electrostriction upon PT doping. It is also shown that a giant piezoelectric effect appears in both cases under the action of an applied electric field, and the field dependence of the piezoelectric coefficients is calculated.