Nonlinear dielectric constant in relaxor ferroelectrics

We have measured the linear and the third-order nonlinear dielectric constants in (1 – x)Pb (Mg1/2W1/2)O3–xPbTiO3 with concentrations x from 0·075 to 0·6. The third-order nonlinear constant shows a broad maximum in the temperature dependence and the maximum value steeply increases as x is increased. The third-order dielectric constant is positive for all the concentrations measured. The experimental results are discussed on the basis of a few models proposed to explain the linear dielectric properties.