Origin of conductivity threshold in the solid electrolyte, glass system:: (Ag2S)x(As2S3)1-x

The electrical conductivity of (Ag2S),(x)(AS(2)S(3))(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(g)s), a low-T-g and a high-T-g in the 7% < x < 40% range but unimodal ones outside this range. The low-Tg 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 similar to 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-T-g phase lowers Ag+ ion migration energies and also contributes to the conductivity threshold.