Insight into sodium silicate glass structural organization by multinuclear NMR combined with first-principles calculations

Short and medium range order of silica and sodium silicate glasses have been investigated from a quantitative analysis of Si-29 MAS NMR and Na-23, O-17 MQMAS NMR spectra. The method described enables the extraction of the underlying O-17 NMR parameter distributions of bridging oxygens (BOs) and non-bridging oxygens (NBOs), and yields site populations which are confirmed by Si-29 NMR data. The extracted NMR parameter distributions and their variations with respect to the glass chemical composition can then be analyzed in terms of local structural features (bond angles and bond lengths, coordination numbers) with the help of molecular dynamics simulations combined with first-principles calculations of NMR parameters. Correlations of relevant structural parameters with Na-23, Si-29 and O-17 NMR interactions (isotropic chemical shift delta(iso), quadrupolar coupling constant C-Q and quadrupolar asymmetry parameter eta(Q)) are re-examined and their applicability is discussed. These data offer better insights into the structural organization of the glass network, including both chemical and topological disorder. Adding sodium to pure silica significantly diminishes the Si-O-Si bond angles and leads to a longer mean Si-O bond length with a slight decrease of the mean Na-O bond length. Moreover, the present data are in favor of a homogeneous distribution of Na around both oxygen species in the silicate network. Finally, our approach was found to be sensitive enough to investigate the effect of addition of a small quantity of molybdenum oxide (about 1 mol%) on the O-17 MAS spectrum, opening new possibilities for investigating the Mo environment in silicate glasses. (C) 2011 Elsevier Ltd. All rights reserved.