Determination of the quasistatic linear and non-linear dielectric response in relaxor materials

Temperature dependence of the first ε1 and third harmonic dielectric response ε3 in relaxor systems was studied both as a function of the dc electric field by utilising the charge accumulation technique and as a function of the ac electric field. Additional non-linear contribution was found in the ac field dependence of ε1 in lead magnesium niobate single crystals and in lanthanum-modified lead zirconate titanate ceramics. This dominating contribution in the field dependence of ε1(ω) has an origin in the fact that the increasing field shifts the relaxor state from the glassy to a particular crossover state in the glassy-to-ferroelectric crossover region. This was confirmed by quasistatic measurements of the total third-order non-linear dielectric response ε3t as a function of the dc electric field throughout the glassy-to-ferroelectric crossover region. It is shown that the total third-order non-linear dielectric response can be understood within the extended spherical random bond-random field model of relaxor ferroelectrics.