Structure and Chemical Order in S–Se Binary Glasses

The compositional evolution of the structure and chemical order in binary S _xSe_{100- x} glasses (0 ≤ x ≤ 90) is investigated using a combination of high-resolution 2D ⁷⁷Se isotropic-anisotropic correlation NMR and Raman spectroscopy. The results indicate that the structure of S-Se glasses consists of two types of topological elements, namely polymeric [Se,S] _n chains and eight-membered Se _yS_{8 -y} rings (0 ≤ y ≤ 8). The relative concentration of Se atoms monotonically decreases in the chain elements and concomitantly increases in the ring elements with increasing S concentration. Moreover, the Se speciation results are consistent with an average heterocyclic ring composition of Se₁S₇ at low S content (≤40 at. % S), while the composition shifts to Se_1.5S_6.5 at higher S content (≥60 at. % S), indicating increasing incorporation of multiple Se atoms in each ring element. The Raman spectra suggest that -Se-Se- association is favored, when more than one Se atom is incorporated in chains and rings. As in their elemental forms, the S and Se atoms retain their preference of forming rings and chains in binary S _xSe_{100- x} glasses, which predicts a linear compositional variation in the relative fractions of these topological elements. This structural evolution is consistent with the corresponding variation in the T_g and molar volume, both of which exhibit a linear decrease with increasing S concentration.