Molecular origin of the giant conductivity enhancement in (Ag$_{2}$S)$_{x}$(As$_{2}$S$_{3})_{1-x}$ glasses

The solid electrolyte additive Ag$_{2}$S is found to homogeneously alloy with base As$_{2}$S$_{3}$ glass at low concentrations ( x $<$ 6 {\%}, single: T$_{g}$ = T$_{g}^{high} \quad \sim $ 210C$_{ })$, but it rapidly segregates as a Ag-rich glass phase at medium concentrations ( 6{\%} $<$ x $<$ 20{\%}, bimodal : T$_{g}^{high}$ and T$_{g} \quad ^{low} \quad \sim $ 170C ), and becomes the principal glass phase populated at higher x $>$ 35 {\%} (single: T$_{g}^{low})$ as revealed by modulated calorimetric measurements. The stoichiometry of the Ag-rich (T$_{g}^{low}$ phase) is suggested to be near AgAs$_{3}$S$_{7}$ at x $\sim $ 25{\%} but becomes closer to that of Smithite (AgAsS$_{2})$ at x $>$ 40{\%}, as revealed by Raman scattering. In the 9{\%} $<$ x $<$ 14{\%} composition range, one observes, in calorimetric experiments, the opening of a reversibility window, and a pronounced increase in the fractional population, R(x) of the Ag-rich glass phase, both of which correlate well with the 5-orders of magnitude increase in electrical conductivity$^{1,2}$ across this compositional interval. In the same interval molar volumes on our samples show a local plateau. These observations suggest a new interpretation of the giant electrical conductivity enhancement observed at x $>$ 15{\%} in the present electrolyte glass system. $^{1}$ E.A. Kazakova and Z.U.Borisova, Fiz. Khim.Stekla \textbf{6}, 424(1980).