Freezing of dipole dynamics in relaxor ferroelectric Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ as evidenced by dielectric spectroscopy

Complex dielectric susceptibility (χ′ + jχ″) has been studied at f = 10^-2-10⁵ Hz in the relaxor ferroelectric 0.75Pb(Mg_1/3Nb_2/3)O₃-0.25PbTiO₃. In addition to the well known relaxor dispersion observed at temperatures around and below the temperature of permittivity maximum T_m, a new and comparatively weak dispersion has been disclosed at T > T_m. It is described by the 'universal relaxation law', χ″ = cot(nπ/2)χ′ ∞ f^n-1. The temperature dependence of the exponent n follows the Vogel-Fulcher relation, indicating a process of freezing into the nonergodic state.