Synthesis and structural characterization of stoichiometric Li-Ga-Ge Sulfo-selenide glasses

Homogeneous glasses in the mixed-chalcogen pseudo-ternary system Li2S-Ga2Se3-GeSe2 are synthesized and their structure is characterized using Raman and one- and two-dimensional 6Li, 77Se, and 71Ga nuclear magnetic resonance (NMR) spectroscopy. The structure of these glasses can be described as a charge-compensated network predominantly consisting of corner sharing (Ga/Ge) (Se,S)4/2 tetrahedra. The compositional evolution of the atomic structure is heavily influenced by the Li2S:Ga2Se3 ratio R where charge compensation is accommodated by the formation of different structural units and preferential chemical ordering for S atoms. Glasses with R < 1 are deficient in chalcogens required to satisfy the tetrahedral coordination of Ga and consequently form ethane-like X3Ge-GeX3 (X = S, Se) units and S atoms preferentially participate in these structural units. On the other hand, the structure of chalcogen-excess glasses with R > 1 are characterized by the formation of non-bridging Se (NBSe) and S (NBS) sites. The Se atoms show a preference over S for these non-bridging sites and form Ge-NBSe linkages, while the S atoms preferentially bond to Ga, resulting in the formation of GaS4/2 tetrahedra. This structural scenario is shown to be consistent with the corresponding changes in the glass transition temperature.