Soft modes in relaxor ferroelectrics

Abstract

The fact that the polarization-related electrostrictive coefficients Q in the relaxor type ferro-electrics are almost an order of magnitude smaller than in most normal ferroelectric perovskites can be explained under the assumption that the temperature dependence of the soft-mode TO phonon frequency is given as ω² _TO = A(T — T_c ), where the coefficient A in relaxor ferroelectrics is an order of magnitude smaller than the value in normal ferroelectric perovskites. The assumption also explains why the Curie-Weiss constants, C, in the relaxor crystals are several times larger than in the normal ferroelectrics. The soft-mode frequency in Pb(Mg_1/3Nb_2/3)O₃ crystals is predicted to be below 10 cm⁻¹ near the mean Curie temperature.