Chemical Bonding State of Sulfur in Na<sub>2</sub>S-SiO<sub>2</sub> Glasses

The structure and electronic states of Na2S-SiO2 oxysulfide glasses were investigated and the role of sulfur in the glasses is discussed. Na2S formed stable glass with SiO2 at an alkali concentration range similar to that of Na2O-SiO2 systems by the conventional melt-quench method. The obtained glasses were reddish brown in color and a remaining of sulfur was confirmed. X-ray photoelectron spectroscopy (XPS) was used to study the chemical bonding state of bridging and nonbridging anionic species in Na2S-SiO2 glass. The S2p binding energy of the glass was observed to be approximately 161eV. This value is lower than that of orthorhombic crystalline sulfur and almost the same as the value of Na2S. From the results, the electron density around sulfur was near to that of S2-. Furthermore, by 29Si magic angle spinning (MAS) and nuclear magnetic resonance (NMR) measurement, three clear-cut peaks were detected between 0 and -120ppm. It was assumed that a sulfur atom was joined to silicon via substitution for an oxygen atom, and was present as nonbridging sulfur in Na2S-SiO2 glass with low alkali ion concentration. On the other hand, it was revealed that a portion of the sulfur ions existed in an isolated state from the glass-network structure at high alkali content. The glass composition changed to a chemical composition that was more silica rich than predicted. The sulfur ions made sulfide clusters with the sodium ions isolated from the glass-network.