Structure and properties of off-stoichiometric Ba-Ga-Ge selenide glasses

Off-stoichiometric glasses in the Ba-Ga-Ge-Se system with Se excess or deficiency (BGGS ± Se) are synthesized and their structure is characterized using Raman and multinuclear (77Se, 71Ga) magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The results reveal that the structure of these off-stoichiometric BGGS ± Se glasses consists of a charge-compensated, partially covalent network of corner-sharing (Ga/Ge)Se4 tetrahedra where [GaSe4]− tetrahedra charge balance the Ba2 + cations. Addition of excess Se to a stoichiometric BaSe-Ga2Se3-GeSe2 glass with BaSe:Ga2Se3 > 1 results in the formation of Se-Se linkages, while Se deficiency is accommodated via the formation of homopolar Ge-Ge bonds, in agreement with the continuously-alloyed structural scenario of purely covalent chalcogenide glasses. Such composition-dependent structural evolution is shown to be consistent with the corresponding variation in density and glass transition temperature.