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

Abstract

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 SO42– groups, confirmed by the observation in the Raman spectra of the symmetric S–O stretch mode of SO42– 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 SO42– 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.