The Vogel-Fulcher law as a criterion for identifying a mixed ferroelectric-glass phase in potassium tantalate doped with lithium

The dynamic dielectric response and the nonlinear dielectric susceptibility of K_1?xLi_xTaO₃ (x=0.010, 0.016, 0.030) compounds are measured in a dc electric field in the temperature range 4≤T≤150 K. It is found that the permittivity ?′ of K_1?xLi_xTaO₃ samples with two lower concentrations of lithium impurities decreases in an electric field E. For samples with a lithium concentration x=0.030, the permittivity ?′ decreases in electric fields E>1 kV/cm and increases in fields E<0.5 kV/cm. The observed dependences of the maximum of the permittivity on the temperature and the frequency of the measuring field obey the Arrhenius law for samples with lower concentrations of lithium impurities (x=0.010, 0.016) and the Vogel-Fulcher law for samples with a higher lithium concentration (x=0.030). The results of the theoretical treatment performed in the framework of the random-field theory are consistent with the experimental data. It is established that the Arrhenius law is valid for dipole glass phases, whereas the Vogel-Fulcher law holds true for a mixed ferroelectric-glass phase in which the short-range and long-range polar orders coexist. The inference is made that the results of measurements of the dielectric response can be used to identify a mixed ferroelectric-glass phase in any disordered ferroelectric material.