Origin of Conductivity Threshold in the Solid Electrolyte Glass System: $( \hbox{Ag}_{2} \hbox{S})_{x}( \hbox{As}_{2} \hbox{S}_{3})_{1-x}$

The electrical conductivity of (Ag
₂
S)
_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-T
_g
phase bears a similarity to that of the stoichiometric glass at x = 1/2, or AgAsS
₂
, 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 As
₂
S
₃
glass phase alloyed with a few mole percent of Ag
₂
S, 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.