Structural Characterization of the Short-Range Order in High Alkali Content Sodium Thiosilicophosphate Glasses.

The composition dependence of the short-range order (SRO) structure in highly modified mixed glass former sodium thiosilicophosphate glasses, yNa2S + (1 - y)[xSiS2 + (1 - x)PS5/2], were investigated using infrared (IR), Raman, and 29Si and 31P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopies. Both the y = 0.5 and 0.67 glasses undergo disproportionation reactions among the Si and P SRO structures, which lead to various and complex SRO structural units for the Si and P, as shown via the spectra used to characterize the glasses. In the y = 0.5 series, the compositionally expected and experimentally observed SRO units are the P1 and Si2 units in the two binary end-member glasses, where the superscript is the number of bridging sulfur atoms on the P or Si units. However, in the ternary mixed glasses, 0 < x < 1, these units were found to react to form P0 (more highly modified, y = 0.60) and Si3 (less highly modified, y = 0.33) units, indicating preferential association of Na+ ions with the P SRO structures. The Raman spectra were used to resolve the heretofore incompletely studied Si3 SRO unit, which was otherwise difficult to elucidate using 29Si MAS NMR alone. In the y = 0.67 series glasses, the expected P0 and Si0 SRO units were observed for the end-member binary glasses. Like in the y = 0.5 series, the 29Si MAS NMR showed that edge-sharing Si2 (ESi2, y = 0.5) structures were also present in these highly modified glasses, which meant Na2S was not completely incorporated in the network. Evidence of this was shown in the Raman spectra in the form of polysulfide structures Na2Sx (x = 2, 4).