Glass formation and structural analysis of Na4P2S7-xOx, 0 ≤ x ≤ 7 sodium oxy-thiophosphate glasses

Abstract

The short-range order (SRO) structures of glasses in the Na4P2S7-xOx, 0 ≤ x ≤ 7 system were investigated on samples prepared by planetary ball milling (PBM). Like other glasses prepared by PBM, the glassy nature of the prepared materials was confirmed by x-ray diffraction (XRD) showing that they were structurally amorphous and by differential scanning calorimetry (DSC) showing that each composition exhibited a reproducible glass transition temperature (Tg). The SRO structure of these glasses were determined using a combination of Raman, Fourier Transform Infrared (FT-IR), and 31P Magic Angle Spinning NMR (MAS NMR) spectroscopies to investigate how oxygen and sulfur are bonded in the various SRO structural units that were found in these glasses. The three spectroscopic techniques gave further evidence that the resulting glasses were completely reacted from their starting polycrystalline materials Na2S, P2S5 and P2O5. It was found that significant disproportionation reactions occur in these compositions in which the original as-batched and expected pyro-phosphate SRO units, P1 = 2 Na/P, for x = 0 undergo a transformation to form charge compensated amounts of ortho-phosphate (P0, 3 Na/P) and meta-phosphate (P2, 1 Na/P) SRO structures with increasing x such that there is a conservation of Na+ charge, on average 2Na/P. In this nomenclature, the superscript is the total number of bridging sulfurs (BSs) and bridging oxygens (BOs), three minus the superscript is the number of Na per P, and four minus the superscript is the total number of non-bridging sulfurs (NBSs) and non-bridging oxygens (NBOs) on each of the SRO units. As seen in earlier work on other mixed oxy-sulfide (MOS) glass forming systems, the stronger Lewis base O=, compared to S=, is found to preferentially form BOs in the form of P2 units by bonding to the stronger Lewis acid P+5, compared to Na+. This leaves the weaker Lewis base S= to bond to the weaker Lewis acid, Na+, in the form of NBSs on the charge compensation required P0 groups. Using both charge balance and quantitative 31P MAS NMR measurements, a complete composition map of all of the SRO structures present in these glasses has been determined.