Sulfur speciation and network structural changes in silicate-phosphate glasses

Sulfur speciation and the network structure of glasses from the SiO2-P2O5-K2O-MgO-SO3 system were studied by Raman and MAS NMR techniques. Glasses were synthesized by melting powders containing reducing agents. Bulk composition and homogeneity of the glasses were confirmed by X-Ray Fluorescence Spectroscopy and Scanning Electron Microscopy, combined with analysis of microregions via Energy Dispersive X-Ray Spectroscopy. Raman spectra demonstrate that S-bearing glasses can contain more reduced forms of sulfur than SO42-, such as S2O42- , S2O32-, S2O52-, and polysulfides Sx(2-), depending on the reducing conditions. The P-31 and Si-29 MAS NMR spectra did not reveal peaks corresponding to P-S and Si-S linkages and indicate slight polymerization of the silico-oxygen and phospho-oxygen subnetworks.