Solids with high ionic conductivity in group 1 halide systems

Abstract

Phase diagrams have been determined for the systems MI+AgI (M = Na, K, Rb, Cs, Tl or NH₄), MBr+AgBr (M = K, Rb or Cs) and MI+CuI (M = K or Rb). A group of isomorphous compounds MAg₄I₅(M = K, Rb, NH₄) with exceptionally high ionic conductivity has been identified. At 20°C the conductivity of polycrystalline RbAg₄I₅ is 0.124 ± 0.006 ohm^–1 cm^–1, i.e., higher than the ionic conductivity of any other known solid. At this temperature RbAg₄I₅ is stable, but KAg₄I₅ and NH₄Ag₄I₅ slowly disproportionate to AgI and K₂AgI₃ or (NH₄)₂AgI₃. The ternary system KI+RbI+AgI contains a complete range of solid solutions (K,Rb) Ag₄I₅. Analogous bromides have not been found, but a compound KCu₄I₅ is stable between 257 and 332°.