Neutron Diffraction and Raman Studies of the Incorporation of Sulfate in Silicate Glasses

The oxidation state, coordination, and local environment of sulfur in alkali silicate (R₂O-SiO₂; R = Na, Li) and alkali/alkaline-earth silicate (Na₂O-MO-SiO₂; M = Ca, Ba) glasses have been investigated using neutron diffraction and Raman spectroscopy. With analyses of both the individual total neutron correlation functions and suitable doped-undoped differences, the S-O bonds and (O-O)_S correlations were clearly isolated from the other overlapping correlations due to Si-O and (O-O)_Si distances in the SiO₄ tetrahedra and the modifier-oxygen (R-O and M-O) distances. Clear evidence was obtained that the sulfur is present as SO₄ ^2- groups, confirmed by the observation in the Raman spectra of the symmetric S-O stretch mode of SO₄ ^2- groups. The modifier-oxygen bond length distributions were deconvoluted from the neutron correlation functions by fitting. The Na-O and Li-O bond length distributions were clearly asymmetric, whereas no evidence was obtained for asymmetry of the Ca-O and Ba-O distributions. A consideration of the bonding shows that the oxygen atoms in the SO₄ ^2- groups do not participate in the silicate network and as such constitute a third type of oxygen, "non-network oxygen", in addition to the bridging and non-bridging oxygens that are bonded to silicon atoms. Thus, each individual sulfate group is surrounded by a shell of modifier and is not connected directly to the silicate network. The addition of SO₃ to the glass leads to a conversion of oxygen atoms within the silicate network from non-bridging to bridging so that there is repolymerization of the silicate network. There is evidence that SO₃ doping leads to changes in the form of the distribution of Na-O bond lengths with a reduction in the fitted short-bond coordination number and an increase in the fitted long-bond coordination number, and this is consistent with repolymerization of the silicate network. In contrast, there is no evidence that SO₃ doping leads to a change in the distribution of Li-O bond lengths with a total Li-O coordination number consistently in excess of 4.