Non-Arrhenius behavior of the temperature dependence of superionic conductivity of a La$_{0.95}$Sr$_{0.05}$F$_{2.95}$ single crystal

The temperature dependence of the ionic electrical conductivity for a single crystal of the superionic conductor La$_{0.95}$Sr$_{0.05}$F$_{2.95}$ with the tysonite-type structure (sp. gr. $P\bar3c1$) was studied at 210–1073K. It was found that, in the temperature range studied, ionic conductivity exhibits non-Arrhenius behavior and satisfies the Vogel–Tammann–Fulcher equation: $\sigma_{dc}T^{1/2}=\sigma_0\exp(-\Delta H_{\mathrm{VTF}}/(T-T_0))$ with parameters $\sigma_0$ = 1.2 $\times$ 10$^2$ SK$^{1/2}$/cm, $\Delta H_{\mathrm{VTF}}$ = 0.18 eV, and $T_0$ = 85 K. This behavior of the $\sigma_{dc}(T)$ dependence is apparently due to the energy distribution of ion carrier (fluorine vacancies) hops due to the structural microheterogeneity of the solid solution. The application of the Vogel–Tammann–Fulcher mathematical formalism is of undoubted interest for the study of superionic fluorine-conducting solid solutions.