High-resolution 77 Se MAS NMR spectroscopy has been conducted at 11.7 T to investigate the short-and intermediate-range structure and chemical order in binary Ge x Se 100-x glasses with 5 e x e 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 GeSe 4 tetrahedra in either corner-sharing or edge-sharing configuration. Assignments of corner and edge-shared tetrahedra were made based on the 77 Se MAS NMR spectrum of crystalline β-GeSe 2 . Analysis of the compositional variation of the relative concentrations of these Se sites indicates that the structure of Ge x Se 100-x glasses in this composition range can be described as a randomly interconnected network of GeSe 4 tetrahedra and chains of Se atoms. The implications of this structural model are discussed in relation to the composition dependence of the glassforming ability and kinetic fragility of the corresponding parent liquids.

Clustering of rare-earth dopants in GeAs sulfide glasses was studied by fluorescence spectroscopy of Pr-doped glasses and by EPR measurements of Gd-doped samples. The linewidth of the g ∼ 2 resonance of Gd 3+ , as well as the relative intensity of emission from the 1 D 2 level of Pr 3+ , was used as a relative measure of rare-earth clustering. Rare earths were found to have low solubility in uncodoped GeAs sulfide glasses, which also displayed poor fluorescence efficiency due to severe clustering. Codoping such glasses with Ga greatly enhanced rare-earth solubility and dispersal, particularly for Ga:rare earth ratios ≥ 10:1, as evidenced by the narrower EPR resonances and more intense luminescence of Gd-and Pr-doped glasses, respectively. In, P and Sn were also observed to 'decluster' rare earths, although less efficiently than Ga, whereas codoping with I was found to have no effect on clustering. These phenomena are explained by a structural model in which (1) rare-earth dopants and codopants are spatially associated and (2) rare-earth dispersal is accomplished by a statistical distribution of codopants in tetrahedral network sites.

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