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

Abstract

The coordination environments of Si and Se atoms and their connectivity in binary SiSe glasses with 0.05 ≤ x ≤ 0.33 are investigated using a combination of one- and two-dimensional Si andSe nuclear magnetic resonance (NMR) and Raman spectroscopy. The high-resolution correlated isotropic and anisotropic Si andSe 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 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.

NMR Spectroscopy in Glass Science: A Review of the Elements

“…Much like the case of oxygen described above, the main application of 77 Se NMR is to characterize the different bonding configurations between network polyhedra, especially homopolar and heteropolar bonding between various metal cations and Se, as demonstrated for binary Ge x Se 100−x glasses like that in Figure 12. Models for arsenic selenides, germanium selenides and other glasses have benefitted from accurate, quantitative assessment of selenium environments [84]. Interestingly, many of these selenide glasses have fairly low T g values, so variable temperature 77 Se NMR has been used to follow network dynamics, correlating different T-dependent line shapes with structural and viscous relaxation [85].…”

Structure and Connectivity in an Amorphous Silicon Oxycarbide Polymer-Derived Ceramic: Results from 2D 29Si NMR Spectroscopy

“…As noted above, the distinctive 29 Si and 13 C NMR isotropic chemical shifts δ iso have allowed the identification and quantitation of various Si and C environments in SiOC PDCs. However, besides the δ iso of a nuclide, which is characteristic of its coordination environment, the chemical shift anisotropy (CSA) carries important complementary information related to the symmetry of the local electronic environment of a nuclide and, in combination with δ iso , can often be useful in deciphering the intermediate-range order in a glass structure beyond the nearest-neighbor length scale. − Chemical shift is a a second-rank tensor with principal components δ xx , δ yy , and δ zz such that | δ zz – δ iso | ≥ | δ xx – δ iso | ≥ | δ yy – δ iso |. Therefore, the δ iso is defined as On the other hand, the CSA is characterized by the anisotropy Δ and the asymmetry parameter η (0 ≤ η ≤ 1), which are defined as The parameters Δ and η represent the deviation of the tensor from spherical and uniaxial symmetries, respectively.…”