New solid electrolyte: CsCu4Cl3I2

The solid electrolyte $\alpha$-Cs${\mathrm{Cu}}_4$ ${\mathrm{Cl}}_3$ ${\mathrm{I}}_2$ is thermodynamically stable above 431 K and to at least 505 K. It is isostructural with Rb${\mathrm{Cu}}_4$ ${\mathrm{Cl}}_3$ ${\mathrm{I}}_2$. At 431 K it has a conductivity of $0.76{\Omega }^{-1\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, at 419 K the conductivity is $0.72{\Omega }^{-1\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$ and extrapolation to 298 K gives $0.29{\Omega }^{-1\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. When corrected for deviation of bulk from x-ray density, the expected values are 0.84, 0.80, and $0.35{\Omega }^{-1\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, respectively. Its activation enthalpy of motion is 0.112 eV. There are two first-order phase transitions: $\beta -\alpha$ at 431 K and $\gamma -\beta$ at 418 K; both are very sluggish. The $\alpha$ phase does not reconvert to the $\beta$ phase; it returns to the $\gamma$ phase below ∼418 K. At 298 K the conductivity of the $\gamma$ phase is $5.3\times {10}^{-6\mathrm{}}$ ${\mathrm{\Omega }}^{-1\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, at 425.5 K, the conductivity of the $\beta$ phase is $0.11{\Omega }^{-1\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. The enthalpy of activation of motion of the $\beta$ phase is 0.77 eV. The $\gamma$ phase is orthorhombic with $a=14.242\mathrm{}$, $b=24.984\mathrm{}$, $c=11.712\mathrm{}$ Å, probable space groups $\mathrm{Cmca}$ or $C2\mathrm{ca}$, and 16 Cs${\mathrm{Cu}}_4$ ${\mathrm{Cl}}_3$ ${\mathrm{I}}_2$ in the unit cell.