Mixed cation Ag₂S-Tl₂S-GeS₂ glasses: macroscopic properties and Raman scattering studies

Mixed cation Ag₂S-Tl₂S-GeS₂ glasses containing 40 and 50 mol% of metal sulfide have been synthesized and analyzed for the first time using thermal analysis, conductivity measurements and Raman spectroscopy. The composition dependences of the ionic conductivity and glass transition temperature T _g showed the presence of classical mixed cation effect with the minimum of both glass transition temperature and room temperature conductivity and the activation energy maximum centered at r = Ag/(Tl + Ag) ≈ 0.2. Raman spectroscopy measurements reveal that the structure of the mixed glasses is not a simple mixture of the end-members, pure thallium (r = 0) and silver (r = 1) thiogermanates. The edge-sharing ES-Ge₂S₆ dimers and corner-sharing CS-Ge _n S_3n+1 (n = 2 or 3) oligomers disappear more quickly than is predicted by the stoichiometry relations. The systematic evolution of the vibrational properties as a function of the silver fraction r also suggests that the both cations are not separated spatially in the glass network but rather located in close proximity to each other.