Experimental evidence for Na coordination to bridging oxygen in Na-silicate glasses: Implications for spectroscopic studies and for the modified random network model

X-ray Photoelectron Spectroscopy (XPS) O 1s spectra provide experimental verification of at least two types of bridging oxygen in Na-silicate glasses. Oxygen of the Si-O-Si moiety (i.e., BO) is two-fold coordinate whereas O of the second type is at least 3-fold coordinate, with Na being the third atom to which BO is coordinated (i.e., a BO-Na moiety). The conclusion is supported by many molecular dynamics (MD) simulations on Na and Na-Ca silicate glasses, ab-initio MO calculations, and by X-ray diffraction studies (XRD) of crystalline Nasilicates. The existence of at least two types of BO has implications for numerous types of spectroscopy. The fits to the O 1s XPS spectra indicate that the BO signals span a binding energy range of about 0.5 eV or 50 kJ. This energy range is likely to affect nuclear, electronic and perhaps some vibrational spectroscopies in that each BO and BO-Na moiety probably will yield a separate signal (perhaps overlapping) in many types of spectroscopy including O 1s XPS, O-17 NMR and Raman spectroscopies. According to the modified random network (MRN) model of silicate glasses, the locations of NBO, BO and BO-Na are related closely to the structure of the glasi, with NBO located at the interface between the ionic channels and the covalent network and BO located within the network Some BO atoms of the network are, however, located near channels and in sufficiently dose proximity to Na atoms for the BO to be coordinated to Na, thus giving rise to the BO-Na moiety. There are then, three distinct zones of oxygen atoms, a NBO zone at the interface between the channels and network, a BO-Na zone located within the covalent network but close to the channels, and a BO zone located deep within the covalent network. The X-ray beam damages, to some extent, Na-silicate glasses. The effect generally is to increase slightly the total BO content of the glass at the expense of NBO, thus increasing the polymerization of the glass. Polymerization may cause channels to become segmented into unconnected pockets where ionic species are concentrated. This damage has not been observed for crystalline silicates although additional studies are required. The implication is that the structures of glasses are much more fragile than crystalline structures and are readily altered. All glasses so far studied have shown beam damage with changes to BO:NBO ratios. (C) 2014 Elsevier B.V. All rights reserved.