High-Conductivity Solid Electrolytes: MAg$_{4}$I$_{5}

The solid electrolytes MAg$_{4}$I$_{5}$, where M may be potassium, rubidium, or ammonium, have an ionic conductivity of 0.2 (ohm cm)$^{-1}$ at 20 °C. Although pure CsAg$_{4}$I$_{5}$ does not form, partial substitution of cesium into the M position was obtained. The high-conducting phases of KAg$_{4}$I$_{5}$ and RbAg$_{4}$I$_{5}$ may be quenched to low temperatures, where they transform to resistive phases at -136 ° and -155 °C, respectively.