Effects of the degree of polymerization on the structure of sodium silicate and aluminosilicate glasses and melts: An 170 NMR study

摘要

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 (Na_2O/SiO2 ratio and Na_2O/A1_20_3 ratio), revealing in detail the extent of disorder (network con_nectivity and topological disorder) and variations of NMR parameters with the glass composition. In binary sodium silicate glasses [Na_2O-k(SiO_2)], the fraction of non-bridging oxygens (NBO_s, Na-О-Si) increases with the Na_2O/SiO_2 ratio (k), as predicted from the composition. The ~17O isotropic chemical shifts (17О ';so) for both bridging oxygen (BО) and NBO increase by about 10-15 ppm with the Si0_2 content (for k = 1-3). The quadrupolar coupling products of BO-s and NBO_s also increase with the Si0_2 content. These trends suggest that both NBO_s and BO_s strongly interact with Na; therefore, the Na distribu_tions around BO_s and NBO_s 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 ran_dom-network models. The peak width (in the isotropic dimension) and thus bond angle and length distributions of Si-O_Si and Na-О-Si increase with the Si0-2 content, indicating an increase in the topological disorder with the degree of polymer_ization. In the ternary aluminosilicate glasses [Na_2O]χ[Al_2O_3]1-_χSi0_2, the NBO fraction decreases while the AI-O-Si and Al_O-Al fractions apparently increase with increasing AI_2O_3 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 AI_2O_3 content at a constant mole fraction of Si0_2. Therefore, the activity of silica may decrease from depolymerized binary silicates to fully polymerized sodium alumino_silicate glasses at a constant mole fraction of Si0_2.