Low-frequency dielectric spectroscopy of the relaxor ferroelectric Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃

The complex dielectric permittivity (ε = ε′ - iε″) of relaxor ferroelectric 0.75Pb(Mg_1/3Nb_2/3)O₃-0.25PbTiO₃ ceramics with perovskite-type structure is studied in the frequency range of 10^-2-10⁵ Hz as a function of temperature. The dielectric response in the temperature range of the diffused ε(T) peak is determined by contributions from two different polarization mechanisms: "universal relaxor polarization" and "conventional relaxor polarization." The overlapping dielectric responses related to these two polarization mechanisms are separated from each other and studied individually. The conventional relaxor polarization shows Kohlrausch-Williams-Watts relaxation behavior with a temperature-independent spectral shape and a non-Arrhenius (linear) temperature dependence of the logarithm of the most probable relaxation frequency. The universal response is characterized by the susceptibility Χ_u continuously growing (without any loss peak) with decreasing frequency according to the universal relaxation law Χ_U∝f^n-1. The real part of the universal relaxor susceptibility shows a critical behavior Χ′_U = C_χ(T-T₀)^-2 at T>T0, starting from a temperature several degrees above T₀. The parameters C_χ and T₀ slightly depend on frequency according to fractional power relations. We show that it is the universal relaxor dispersion, not the conventional relaxor dispersion, that is related to the freezing of dipole dynamics which is characteristic of relaxor ferroelectrics.