Mixed cation effect in Ag2S-Tl2S-GeS-GeS2 glasses: Conductivity and tracer diffusion studies

Mixed cation effect has been studied for thallium-silver thiogermanate glasses in the (Ag2S)x(Tl2S)50-x(GeS)25(GeS2)25 system, where Ag2S fraction r = Ag2S / (Ag2S + Tl2S) = 0, 0.25, 0.5, 0.75, and 1.0. The glass transition temperature changes from 160 °C for r = 0 to 231 °C for r = 1.0 and exhibits a negative deviation from additivity, with a slight minimum Tg ≈ 157 °C observed for compositions in the range 0.1 ≤ r ≤ 0.3. The room temperature conductivity σ298 increases by 5 orders of magnitude with increasing silver concentration, ranging between 10− 8 S cm− 1 for the Tl-Ge-S system and 10− 3 S cm− 1 for the Ag-Ge-S glass with a minimum at σ298 ≈ 10− 9 S cm− 1. The activation energy decreases from 0.66 to 0.33 eV, respectively, with the maximum at Eσ ≈ 0.69 eV. The minimum of both Tg and σ298 and the Eσ maximum are observed in the same composition range, 0.1 ≤ r ≤ 0.3. The 204Tl tracer diffusion coefficient at 170 °C decreases with silver concentration from 10− 10 cm2 s− 1 for (Tl2S)50(GeS)25(GeS2)25 glass to 10− 12 cm2 s− 1 for (Ag2S)50(GeS)25(GeS2)25 glass. The obtained results indicate a classical mixed cation effect.