Bridging, non-bridging and free (O2-) oxygen in Na2O-SiO2 glasses: An X-ray Photoelectron Spectroscopic (XPS) and Nuclear Magnetic Resonance (NMR) study

High resolution Na 1s, O 1s and Si 2p core level XPS spectra of six Na2O-SiO2 glasses ranging in composition from 100 to 45mol % SiO2 have been collected using the Kratos Ultra Axis instrument with its unique charge compensation system. The O 1s spectra for the glasses are well resolved so that bridging oxygen (BO, Si-O-Si) and non-bridging oxygen (NBO, Na-O-Si) signals can be accurately fitted and quantified without resorting to constraints or assumptions. The same samples were analysed by 29Si MAS NMR to obtain Q-species abundances from which BO and NBO proportions were calculated. Similar BO:NBO ratios were obtained by both methods over the entire compositional range studied. They are also consistent with most previous XPS and NMR results for glasses containing more than ~65mol % SiO2. Our XPS and NMR experimental results, however, differ somewhat from previously published XPS and NMR results for glasses containing less than about 65mol % SiO2. Na is mobile in the X-ray beam and mobility causes BO:NBO ratios to increase with time of exposure. Na mobility here has been circumvented to yield reliable BO:NBO ratios of the glasses. The ratios are lower than previously reported in XPS studies and are similar to ratios obtained from our 29Si MAS NMR results on the same glasses. The XPS and 29Si MAS NMR results also indicate the presence of a third oxygen species in sodic glasses. As has been proposed for CaSiO3 glass and for sodic and potassic glasses containing La, we suggest that O2a degree is present in sodic glasses at small concentrations. The O2a degree content correlates with increased soda content and may be associated with, and instrumental in development of, three dimensional percolation channels in the glasses. The XPS O 1s line width of the BO peak is broader than the NBO peak, indicating more than one contribution to the BO peak. As observed in crystalline Na metasilicate and Na disilicate, BO of Na-silicate glasses may be of two types, one arising from BO bridging two Si atoms, and the second BO signal arising from BO bonded not only to two Si atoms but also to Na.