Nonlinear dielectric response of relaxor ferroelectrics

The calculations of nonlinear dielectric susceptibility on external de electric field is performed in the random field theory framework. The equations which describe the dependence of order parameter and dielectric susceptibility on temperature, de electric field, frequency, parameters of host lattice and random field sources are obtained. The numerical solutions of these equations for several random field sources concentrations and parameters have shown that nonlinear dielectric susceptibility at low frequencies smaller than linear one in dipole glass state, meanwhile in mixed ferroglass phase nonlinear part can be both negative and positive. No critical divergency of nonlinear susceptibility at freezing temperature was obtained in accordance with experiment. This proves that contrary to conventional spin glasses dipole glasses in relaxors are methastable states with long (up to infinity) relaxation times. The comparison of calculated and observed nonlinear susceptibility is carried out.