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Abstract
The longitudinal strains ξ 3 of soft relaxor ferroelectric ceramics PbMg 1/3 Nb 2/3 O 3 -PbTiO 3 (PMN-PT) were measured as a function of the electric field strength E 3 . The electric field dependences of longitudinal differential piezoelectric coefficients d 33 (E 3 ) and electrostriction coefficients M 33 (E) were measured from the virgin ξ 3 (E 3 ) curves and hysteresis loop branches. It was shown that taking into account the polarization nonlinearity (that is, the E 3 dependence of dielectric susceptibility) leads to nonmonotonic electric field dependences d 33 (E 3 ) and M 33 (E 3 ). The large magnitude of the dielectric susceptibility of relaxor ferroelectric ceramics is responsible for the giant electrostriction being positive in low electric fields and negative in strong ones.