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

Abstract

Homogeneous glasses in the ternary system Ag2Se–Ga2Se3–GeSe2 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)Se4/2 tetrahedra where the negatively charged GaSe4/2 tetrahedra are charge balanced by the Ag cations. The deficiency in Se required to satisfy the tetrahedral coordination of Ga in glasses with Ag2Se:Ga2Se3<1 is accommodated by the formation of homopolar Ge–Ge bonds. Progressive addition of Ag2Se to these glasses efficiently removes Ge–Ge bonding such that their concentration essentially goes to zero at the chemical threshold Ag2Se:Ga2Se3=1. Further addition of Ag2Se beyond this threshold (i.e., Ag2Se:Ga2Se3>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 Ag2Se:Ga2Se3 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.