Origin of Conductivity Threshold in the Solid Electrolyte Glass System: ( Ag2 S)x( As2 S3)1−x

The electrical conductivity of (Ag₂S)_x(As₂S₃)_1-x glasses increases to display a step-like jump of nearly five orders of magnitude in the narrow composition range, 9% < x < 15% range. To elucidate the origin of this threshold behavior, we have now examined the molecular structure of these glasses in modulated-differential scanning calorimetry (MDSC) and Raman scattering experiments. Our MDSC results reveal bimodal glass transition temperatures (T_gs), a low-T_g and a high-T_g in the 7% < x < 40% range but unimodal ones outside this range. The low-T_g phase bears a similarity to that of the stoichiometric glass at x = 1/2, or AgAsS₂, and we identify it with a Ag-rich phase formed in these glasses once x > 7%. The Ag-rich phase is thought to percolate near x ~ 9%, and to contribute to the large jump in conductivity of the glasses. The high-Tg phase represents a semiconducting As₂S₃ glass phase alloyed with a few mole percent of Ag₂S, and it displays a reversibility window in the 8% < x < 13% range. The semiconducting phase becomes elastically flexible once x > 13%. Softening of the high-Tg phase lowers Ag⁺ ion migration energies and also contributes to the conductivity threshold.