Mechanisms of structural accommodation of Se deficiency in binary Ga2Se3–GeSe2 glasses: Results from 77Se MATPASS/CPMG NMR spectroscopy

• Violation of the 8 − N rule is observed in the binary Ga2Se3–GeSe2 glasses.
• Quantitative 2D 77Se MATPASS/CPMG NMR spectra are reported.
• Direct spectroscopic evidence of three-coordinated Se atoms is provided.
• Three-coordinated Se atoms resemble oxygen triclusters in peraluminous oxide glasses.
• Self-consistent structural model is proposed based on Se speciation from NMR results.

The nature of the selenium coordination environments in the structure of binary Ga2Se3–GeSe2 chalcogenide glasses is investigated using two-dimensional 77Se magic angle turning-phase adjusted spinning sidebands/Carr–Purcell–Meiboom–Gill (MATPASS/CPMG) nuclear magnetic resonance (NMR) spectroscopy. The structural network of these glasses consists of corner and edge-sharing (Ga/Ge)Se4 tetrahedra in which the coordination numbers of Ga, Ge, and Se atoms are primarily 4, 4, and 2, respectively. Consequently, the addition of Ga2Se3 to GeSe2 results in the formation of Ge–Ge homopolar bonds to satisfy the coordination constraints of the constituent atoms. These Ge–Ge bonds are randomly distributed throughout the network such that any clustering of (Se3/2)Ge–Ge(Se3/2) ethane-like units is avoided to the maximum extent. However, for glasses containing ≥ 30 mol% Ga2Se3, the appearance of three-coordinated Se atoms provides an alternate mechanism to accommodate for the Se deficiency in the limit where the Ge–Ge bonds are nearly saturated. These three-coordinated Se atoms are formed by corner-sharing between three GaSe4 tetrahedra that is reminiscent of oxygen triclusters shared by three AlO4 tetrahedra reported in peraluminous oxide glasses and liquids.