CROSSOVER IN A NON-ANALYTICAL BEHAVIOR OF DIELECTRIC NONLINEARITY IN PBMG1 3NB2/3O3 RELAXOR FERROELECTRIC/

The dielectric response of the relaxer ferroelectric PbMg1/3Nb2/3O3 (P MN) is found to be a non-analytical function of the ac field, with the absolute value of the non-linear component of the polarization given by \P-nl\ alpha E-m(gamma(omega,T)). The dependence on the temperature , T, and frequency, omega, of the exponent gamma manifests itself in t he form of a crossover in gamma(T) from approximately 2 to 3 upon cool ing, whose position depends upon the frequency of the applied ac held. From the comparison of y(w, T) with the linear complex dielectric per mittivity, epsilon(l) (omega, T), of PMN, the following can be correl ated: (i) gamma approximate to 2 in the regime of the quasi-static res ponse, and y starts deviating from 2 simultaneously with the onset of the frequency dispersion; and (ii) gamma approximate to 3 when the spe ctrum of relaxation times in PMN becomes flat. These findings can be u nderstood in terms of the change in the type of the motion of the inte rphase boundaries of the microscopic polar regions existing in PMN, na mely in the change of the scale on which the interphase boundaries can move.