Effects of the degree of polymerization on the structure of sodium silicate and aluminosilicate glasses and melts: An O NMR study

Revealing the atomic structure and disorder in oxide glasses, including sodium silicates and aluminosilicates, with varying degrees of polymerization, is a challenging problem in high-temperature geochemistry as well as glass science. Here, we report ¹⁷O MAS and 3QMAS NMR spectra for binary sodium silicate and ternary sodium aluminosilicate glasses with varying degrees of polymerization (Na₂O/SiO₂ ratio and Na₂O/Al₂O₃ ratio), revealing in detail the extent of disorder (network connectivity and topological disorder) and variations of NMR parameters with the glass composition. In binary sodium silicate glasses Na₂O-k(SiO₂)], the fraction of non-bridging oxygens (NBOs, Na-O-Si) increases with the Na₂O/SiO₂ ratio (k), as predicted from the composition. The ¹⁷O isotropic chemical shifts (¹⁷O δ_iso) for both bridging oxygen (BO) and NBO increase by about 10-15 ppm with the SiO₂ content (for k = 1-3). The quadrupolar coupling products of BOs and NBOs also increase with the SiO₂ content. These trends suggest that both NBOs and BOs strongly interact with Na; therefore, the Na distributions around BOs and NBOs are likely to be relatively homogenous for the glass compositions studied here, placing some qualitative limits on the extent of segregation of alkali channels from silica-enriched regions as suggested by modified random-network models. The peak width (in the isotropic dimension) and thus bond angle and length distributions of Si-O-Si and Na-O-Si increase with the SiO₂ content, indicating an increase in the topological disorder with the degree of polymerization. In the ternary aluminosilicate glasses Na₂O]_xAl₂O₃]_1−xSiO₂, the NBO fraction decreases while the Al-O-Si and Al-O-Al fractions apparently increase with increasing Al₂O₃ content. The variation of oxygen cluster populations suggests that deviation from “Al avoidance” is more apparent near the charge-balanced join (Na/Al = 1). The Si-O-Si fraction, which is closely related to the activity coefficient of silica, would decrease with increasing Al₂O₃ content at a constant mole fraction of SiO₂. Therefore, the activity of silica may decrease from depolymerized binary silicates to fully polymerized sodium aluminosilicate glasses at a constant mole fraction of SiO₂.