Structure of glasses in the pseudobinary system Ga(2)Se(3)-GeSe(2): violation of chemical order and 8-N coordination rule.

Structure of glasses in the pseudobinary system Ga2Se3-GeSe2 with Ga2Se3 content ranging from 6.3 to 30 mol % is investigated using a combination of Raman and multinuclear ((71)Ga, (77)Se) solid state nuclear magnetic resonance (NMR) spectroscopy. The results indicate that the structure of these glasses consists primarily of a corner sharing network of (Ge/Ga)Se4 tetrahedra with some fraction of edge-sharing GeSe4 tetrahedra and of ethane-like (Se3)Ge-Ge(Se3) units, in which the Ga, Ge, and Se atoms adopt coordination numbers of 4, 4, and 2, respectively. As expected, the concentration of metal-metal bonds increases with addition of Ga2Se3 as the glass structure becomes too deficient in Se to satisfy the tetrahedral coordination of both Ga and Ge by Se atoms alone. These metal-metal bonds are mostly limited to Ge-Ge homopolar bonds, indicating a violation of chemical order. At relatively high degrees of Se-deficiency, however, spectroscopic evidence suggests the formation of triply coordinated Se atoms as an alternate mechanism to accommodate the tetrahedral coordination of Ga and Ge atoms. This observation indicates a violation of the 8-N coordination rule and is reminiscent of oxygen triclusters in isoelectronic Al2O3-SiO2 glasses. Compositional variation of physical properties such as density, molar volume, optical band gap, glass transition temperature, and fragility are shown to be consistent with the proposed structural model.