Mechanism of electrotransport in superionic conductor Sr$_{0.7}$La$_{0.15}$Lu$_{0.15}$F$_{2.3}$ with the fluorite-type structure

The ionic conductivity of the superionic conductor Sr$_{0.7}$La$_{0.15}$Lu$_{0.15}$F$_{2.3}$ with the fluorite-type structure (type CaF$_2$, sp. gr. $Fm\bar3m$) had measured by the impedance spectroscopy in the temperature range 385–794 K. Bulk crystals of a three-component solid solution (lattice parameter $a$ = 5.7726(1)$\mathring{\mathrm{A}}$) had obtained from the melt by the directed crystallization technique. Temperature dependence ionic conductivity $\sigma_{dc}(T)$ satisfies the Arrhenius–Frenkel equation with the activation enthalpy of the electrical conductivity $\Delta H_\sigma$ = 0.706 $\pm$ 0.05 eV. The value of $\sigma_{dc}$ is 1.5 $\cdot$ 10$^{-5}$ S/cm at 500 K. Based on the analysis of electrophysical and structural studies of fluorite-type solid solutions in SrF$_2$–LaF$_3$ and SrF$_2$–LuF$_3$ systems, the microscopic model of ion transport in the Sr$_{0.7}$La$_{0.15}$Lu$_{0.15}$F$_{2.3}$ crystal was proposed. The concentration and mobility of charge carriers were calculated: $n_{mob}$ = 7.8 $\cdot$ 10$^{20}$ cm$^{-3}$ и $\mu_{mob}$ = 1.2 $\cdot$ 10$^{-7}$ cm$^2$/Vs at 500 K.