Structure and physical properties of glasses in the system Ag2Se–Ga2Se3–GeSe2

Homogeneous glasses in the ternary system Ag ₂Se–Ga ₂Se ₃–GeSe ₂ are synthesized and their structure is characterized using Raman and one- and two-dimensional ⁷⁷Se, ⁷¹Ga and ⁶⁹Ga nuclear magnetic resonance (NMR) spectroscopy. The structure of these glasses consists predominantly of a charge-compensated network of corner-sharing (Ga/Ge)Se _4/2 tetrahedra where the negatively charged GaSe _4/2 tetrahedra are charge balanced by the Ag cations. The deficiency in Se required to satisfy the tetrahedral coordination of Ga in glasses with Ag ₂Se:Ga ₂Se ₃ < 1 is accommodated by the formation of homopolar Ge–Ge bonds. Progressive addition of Ag ₂Se to these glasses efficiently removes Ge–Ge bonding such that their concentration essentially goes to zero at the chemical threshold Ag ₂Se:Ga ₂Se ₃ = 1. Further addition of Ag ₂Se beyond this threshold (i.e., Ag ₂Se:Ga ₂Se ₃ > 1) results in the modification of the Ga,Ge–Se tetrahedral network via depolymerization and formation of non-bridging Se atoms. Moreover, all glasses irrespective of their Ag ₂Se:Ga ₂Se ₃ ratio contain a small fraction of Se–Se bonds, implying violation of chemical order. Therefore, the compositional evolution of the structure of these glasses displays the characteristics of both the isoelectronic alkali/alkaline-earth aluminosilicate glasses and the purely covalent, continuously alloyed chalcogenide networks. Such composition-dependent structural evolution is shown to be consistent with the corresponding variation in glass transition temperature and molar volume.