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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.

The temperature dependence of the third-order nonlinear dielectric constant and the time dependence of the linear dielectric constant under a biasing field in relaxor ferroelectrics, (1-x)Pb(Mg1/2W1/2)O3–xPbTiO3 with x=0.5, were measured near the temperature where the dielectric constant shows a maximum value. It was found that the nonlinear dielectric constant shows a broad peak without any indication of divergence. It was also found that the dielectric constant under the biasing field changes slowly with time. The nonlinear response of relaxor ferroelectrics was discussed.