Silver diffusion and isotope effect in silver rubidium iodide

The diffusion coefficient of silver in silver rubidium iodide (${\mathrm{Ag}}_4$Rb${\mathrm{I}}_5$) has been measured in both superionic phases using radiotracer ${}_{}{}^{110m}{}_{}{}^{}\mathrm{Ag}$ and serial sectioning with a low-temperature sectioning apparatus. The activation energies for diffusion in $\alpha$-Rb${\mathrm{Ag}}_4$ ${\mathrm{I}}_5$ and $\beta$-Rb${\mathrm{Ag}}_4$ ${\mathrm{I}}_5$, respectively, are 0.11±0.01 and 0.20±0.04 eV. These values are the same, within experimental error, as the observed activation energies for conduction. An isotope effect for diffusion has also been measured in both superionic phases. ${}_{}{}^{105}{}_{}{}^{}\mathrm{Ag}$ and ${}_{}{}^{110m}{}_{}{}^{}\mathrm{Ag}$ radioisotopes were used with gamma spectroscopy and energy discrimination. The effect is small, with no significant temperature variation, with the value at 333 K being 0.12±0.02. The second-order phase transition at 208 K has a small effect, if any, on the magnitude of the effect. The data suggest that a highly cooperative transport mechanism is responsible for the unusually high values of both the conductivity and diffusion coefficient. Although it is not possible to deduce the particular mechanism involved, theories involving ionic polarons, or cooperative motion, such as crowdions or solitons, seem consistent with the observed results.