Composition Dependence of the Glass-Transition Temperature and Molar Volume in Sodium Thiosilicophosphate Glasses: A...

The glass-transition temperature, T_g, and molar volume, V̅, are two physical properties known to exhibit the mixed glass former effect (MGFE), a nonlinear nonadditive increase or decrease from a linear ideal mixing behavior, in ternary glass systems, where the two glass forming species are varied, whereas the glass modifier content remains constant across the system. In the next of our continuing studies of the MGFE in ternary glasses, the T_g and molar volumes of two ternary glass forming series, 0.5Na₂S + 0.5[ xSiS₂ + (1 - x)PS_5/2], the 0.50 NSP series, and 0.67Na₂S + 0.33[ xSiS₂ + (1 - x)PS_5/2], the 0.67 NSP series, have been determined across the full glass forming range in both series, 0 ≤ x ≤ 1. The 0.50 NSP glasses were found to have a strongly negative MGFE in the T_g and a weaker MGFE in the molar volume. The 0.67 NSP series of glasses exhibited weak negative and strong positive MGFEs in the T_g and molar volumes, respectively. Using the short-range order (SRO) structure model for each glass series that was previously developed, the number of bridging sulfurs (BS) and nonbridging sulfurs (NBS) was determined and analyzed for each of these two series of glasses. A clear linear correlation was observed between the T_g and both the fraction of BSs, BS/(BS + NBS), and the number of BS per glass former, BS/GF in both series. The molar volumes of both series of glasses were analyzed using both ideal and real solution models of mixing. The molar volumes of the glasses were best fit to the molar volumes of all of the individual molar volumes of the various SRO units. In the ideal solution model, the molar volumes of the SRO units were only fit to molar volumes of the end member glasses, x = 0 and 1. In the real solution model, the molar volumes were best fit to the full composition dependence of the molar volume of all of the glasses. In both cases, the same molar volumes for the SRO units were used to fit both sets of molar volumes of both glass series. It was found that the best-fit molar volumes of both the P and Si SRO units were essentially the same at the same number of NBS/GF. In this study, therefore, it was observed that the MGFE in the T_g of the glass was linearly correlated with the number of BS per glass former, BS/GF, whereas the MGFE in the molar volumes of the glasses was correlated with the number of NBS per glass former, NBS/GF.