Anisotropic electrical conductivity and low-temperature phase transitions of the solid electrolyte ${\mathrm{Ag}}_{26}$${\mathrm{I}}_{18}$${\mathrm{W}}_4$${\mathrm{O}}_{16}$

Results of measurements of electrical conductivity of single crystals of ${\mathrm{Ag}}_{26}$ ${\mathrm{I}}_{18}$ ${\mathrm{W}}_4$ ${\mathrm{O}}_{16}$ in the temperature range 473-175 K show the existence of two first-order transitions at 246 and 197 K; the phases are labeled $\alpha$, $\beta$, $\gamma$ in order of decreasing temperature. Within experimental error, the principal conductivity axes of the $\alpha$ phase are $\overrightarrow{\mathrm{a}}$, $\overrightarrow{\mathrm{b}}$, and ${\overrightarrow{\mathrm{c}}}^{*}$ over its temperature range of existence. At 25°C, the principal conductivities are 0.120, 0.085, and 0.083 ${\mathrm{\Omega }}^{-1\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, respectively, and the average conductivity, 0.097 ${\mathrm{\Omega }}^{-1\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, is 1.6 times the best value obtained from conductivity measurements on polycrystalline material. It is shown that the ${log}_{10}\left(\sigma T\right)$ vs $T^{-1\mathrm{}}$ data for the $\alpha$ phase imply that the activation enthalpies of motion are temperature dependent. It is also likely that very few ${\mathrm{Ag}}^{+}$ ions are thermally excited from the immobile to the mobile category. The magnitudes of the activation enthalpies of motion of the $\beta$ and $\gamma$ phases appear to be constant, and suggest that there is greater (possibly complete) order in the $\gamma$ than in the $\beta$ phase and greater order in the $\beta$ than in the $\alpha$ phase.