All-halide superionic glasses

To date all the highest-conducting Ag⁺ and alkali cation glasses have involved admixture of a halide, usually the iodide, with some oxyanion salt of the same cation. In this paper we report cases where the oxyanion can be omitted. Instead, a second cation (Cs⁺ or Rb⁺) is introduced to provide, together with a mixture of anions, the inmobile quasi-lattice through which the mobile cations, Ag+ or Cu+, can migrate. The materials are evidently vitreous state analogs of the well-known superionic crystals CsAg₄I₅ and RbCu₄I₅. In these new systems, conductivities at 25°C can reach record values. Glass transition temperatures, however, are lower than for oxyanion- containing glasses of the same conductivity. Since they contain only heavy monovalent ions, the far IR transparency of the new glasses is unprecedented. The characteristics of stress relaxation for both electrical and mechanical stresses are studied and are compared with those of the oxyanion + halide glasses. The all-halide glasses show less exponential relaxation (broader distributions of relaxation times) despite equivalence of all Ag⁺. They nevertheless show smaller deviations from the Arrhenius law in their temperature dependences.