Crystal structure and conductivity of the solid electrolyte $\alpha$-Rb${\mathrm{Cu}}_4$${\mathrm{Cl}}_3$${\mathrm{I}}_2$

Crystals of the solid electrolyte Rb${\mathrm{Cu}}_4$${\mathrm{Cl}}_3$${\mathrm{I}}_2$ are isostructural with Rb${\mathrm{Ag}}_4$${\mathrm{I}}_5$. The crystal of Rb${\mathrm{Cu}}_4$${\mathrm{Cl}}_3$${\mathrm{I}}_2$ on which the structure analysis was done belongs to space group $P{4}_{3}32 \left(O^6\right)$ with lattice constant 10.032±0.003 Å. As in Rb${\mathrm{Ag}}_4$${\mathrm{I}}_5$, there are three sets of tetrahedral sites for the charge carriers with a total of 56 sites. In Rb${\mathrm{Cu}}_4$${\mathrm{Cl}}_3$${\mathrm{I}}_2$, all the tetrahedra have both ${\mathrm{Cl}}^{-}$ and ${\mathrm{I}}^{-}$ ions at apices, but the coordination polyhedron about the ${\mathrm{Rb}}^{+}$ ions contain only ${\mathrm{Cl}}^{-}$ ions. The room-temperature distribution of charge carriers differs from that in Rb${\mathrm{Ag}}_4$${\mathrm{I}}_5$. Although the occupancies of the two sets of 24-fold sites are almost equal, that of the eightfold sites is still very low and thus overall the distribution is nonuniform. Room-temperature conductivities as high as 0.44 ${\mathrm{\Omega }}^{-1\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$ have been measured directly on a specimen having only 94% of theoretical density. Thus it is probable that the true electrolytic conductivity exceeds at least 0.47 ${\mathrm{\Omega }}^{-1\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. At the temperature of the transition of AgI to α-AgI, 419 K, the minimum value of the conductivity of Rb${\mathrm{Cu}}_4$${\mathrm{Cl}}_3$${\mathrm{I}}_2$ is 1.53 ${\mathrm{\Omega }}^{-1\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, about 17% higher than that of $\alpha$-AgI at the transition. Thus Rb${\mathrm{Cu}}_4$${\mathrm{Cl}}_3$${\mathrm{I}}_2$ has the highest conductivity of any solid electrolyte over a wide temperature range in the lower-temperature regions. Conductivity measurements between 130 and 383 K indicate that the $\alpha -\beta$ transition occurs at 221 K and a $\beta -\gamma$ transition at 170 K; the nature of the latter differs from that of the Rb${\mathrm{Ag}}_4$${\mathrm{I}}_5$ $\beta -\gamma$ transition. There is evidence that the average mobility of the ${\mathrm{Cu}}^{+}$ ion is higher than that of the ${\mathrm{Ag}}^{+}$ ion.