Structure, Connectivity, and Configurational Entropy of GexSe100-x Glasses: Results from 77Se MAS NMR Spectroscopy

High-resolution Se-77 MAS NMR spectroscopy has been conducted at 11.7 T to investigate the short-and intermediate- range structure and chemical order in binary GexSe100-x glasses with 5 <= x <= 33.33. Four distinct Se environments are observed for the first time, corresponding to Se-Se-Se and Ge-Se-Se linkages as well as Ge-Se-Ge sites where the Se atom is shared by two GeSe4 tetrahedra in either corner-sharing or edge-sharing configuration. Assignments of corner and edge-shared tetrahedra were made based on the Se-77 MAS NMR spectrum of crystalline beta-GeSe2. Analysis of the compositional variation of the relative concentrations of these Se sites indicates that the structure of GexSe100-x. glasses in this composition range can be described as a randomly interconnected network of GeSe4 tetrahedra and chains of Se atoms. The implications of this structural model are discussed in relation to the composition dependence of the glass-forming ability and kinetic fragility of the corresponding parent liquids.