Molecular Origin of Conductivity Threshold in the Solid Electrolyte (Ag2S)x(As2S3)1-xglass

Electrical conductivity of As2S3glass (10^-16Ohm cm^-1), is typical of a wide band-gap semiconductor. However, upon chemical alloying the solid electrolyte Ag2S, the electrical conductivity of (Ag2S)x(As2S3)1-xglasses increase to display a step-like jump of nearly 5 orders of magnitude in the narrow composition range, 0.09 < x <0. 15 range. To elucidate the origin of this threshold behavior, we have now examined the molecular structure of (Ag2S)x(As2S3)1-xglasses over a wide range of compsitin 0 < x < 0.50 in modulated-Differential Scanning Calorimetry (MDSC) and Raman scattering experiments. The thermal measurements reveal the existence of a reversibility window in the 0.09 < x < 0.13 range, a result that suggests glasses at x <0.08 to be stressed-rigid while those at x > 0.13 to be elastically floppy. The MDSC results also reveal unimodal Tgs at x < 0.08, but bimodal ones (low-Tg, high-Tg) at x > 0.12. The low-Tgphase first appears once x >0.09, and its Tgof 168 °C is quite similar to that of the stoichiometric glass at x = 0.50, or AgAsS2. We identify the low-Tgphase with a Ag-rich phase formed in these glasses once x >0.08. Furthermore, we identify the high-Tgphase with the semiconducting As2S3glass phase elastically modified by Ag2S additive. This particular phase becomes rigid but stress-free in the reversibility window as some Ag2S is alloyed. At higher x, the high-Tgphase becomes elastically floppy promoting Ag⁺ions to freely migrate.