Structural and Topological Evolution in Si_xSe_1-x Glasses: Results from 1D and 2D ²⁹Si and ⁷⁷Se NMR Spectroscopy

The coordination environments of Si and Se atoms and their connectivity in binary Si_xSe_1-x glasses with 0.05 ≤ x ≤ 0.33 are investigated using a combination of one- and two-dimensional ²⁹Si and ⁷⁷Se nuclear magnetic resonance (NMR) and Raman spectroscopy. The high-resolution correlated isotropic and anisotropic ²⁹Si and ⁷⁷Se NMR spectra allow for the identification and quantitation of a variety of Si and Se environments. The results suggest that the structure of these glasses are characterized by a network with essentially perfect short-range chemical order, but with strong clustering at the intermediate range. Initial addition of Si to Se results in cross-linking of Se chain segments with nanoclusters of corner- and edge-shared SiSe_4/2 tetrahedra. These clusters percolate via coalescence near x ≥ 0.2 to finally form a low-dimensional network with high molar volume, at the stoichiometric composition (x = 0.33) that is composed of chains of edge-sharing tetrahedra cross-linked by corner-shared tetrahedra. This structural evolution is shown to be consistent with the compositional variation of the glass transition temperature and the molar volume of these glasses.