Low-temperature phases of the solid electrolyteRbAg4I5

Important structural features of the two low-temperature phases of ${\mathrm{RbAg}}_4$ ${\mathrm{I}}_5$, designated $\beta$,$\gamma$ in order of decreasing temperature, have been determined, even though the precise structure of the $\gamma$ phase is unattainable because of the large number of structural parameters and the limitations on available data. At 130 K, the $\beta$ phase is rhombohedral, space group $R32\left(D_3^7\right)$, with $a=11.17\mathrm{}\pm 0.01$ Å, $\alpha =90.1\mathrm{}°\pm 0.05°$. Most of the displacements of ions and equilibrium ${\mathrm{Ag}}^{+}$ ion sites, relative to the $\alpha$ phase, are small, the largest, 0.3 Å, being those for two ${\mathrm{Ag}}^{+}$ ion sites. The largest displacement of an ${\mathrm{I}}^{-}$ ion is 0.1 Å. The most significant difference between the $\alpha$ and $\beta$ phases is the preferential distribution of ${\mathrm{Ag}}^{+}$ ions, in the latter, over sets of sites formerly crystallographically equivalent. However, the distribution does not change very much over analogous sites: In $\alpha -\mathrm{Rb}{\mathrm{Ag}}_4{\mathrm{I}}_5$ the distribution is 0.9, 9.4, and 5.5 ${\mathrm{Ag}}^{+}$ ions in the $\mathrm{Ag}-c$, Ag-II, and Ag-III sites, respectively, while the analogous values in $\beta -\mathrm{Rb}{\mathrm{Ag}}_4{\mathrm{I}}_5$ are 1.3, 10.5, and 4.2. (The limits of error are probably about the same as in the $\alpha$ phase.) Neither $\alpha$- nor $\beta -\mathrm{Rb}{\mathrm{Ag}}_4{\mathrm{I}}_5$ can be ordered and therefore the 209-K transition is a "disorder-disorder" transition. The $\gamma -\mathrm{Rb}{\mathrm{Ag}}_4{\mathrm{I}}_5$ is trigonal, most probable space group $P321\mathrm{} \left(D_3^2\right)$, with $a=15.776\mathrm{}\pm 0.005$ Å and $c=19.320\mathrm{}\pm 0.005$ Å at 90 K. The unit cell has three times the volume of the rhombohedral cell and therefore contains 12 ${\mathrm{RbAg}}_4$