Nonlinear dielectric response of relaxor ferroelectrics

The behavior of the third-order nonlinear dynamic dielectric susceptibility chi(3)(omega) in relaxor ferroelectrics is investigated in the framework of the spherical random-bond-random-field model. It is shown that there are two distinct contributions to chi(3)(omega): The first one chi(3)(omega)(I), which has been studied earlier, is due to the intrinsic nonlinearity of the rigid spherical model; the second contribution chi(3)(omega)(II) arises from the field modulation of the average coupling between polar clusters. The frequency and temperature dependence of chi(3)(omega)(II) is calculated and compared with chi(3)(omega)(I). In the static limit, the results for chi(3)(omega)(II) are in qualitative agreement with the measured field-cooled static nonlinear dielectric permittivity in Pb1-xLax(ZryTi1-y)(1-x/4)O-3 ceramics.