Investigation of the short range order structures in sodium thioborosilicate mixed glass former glasses

Abstract

The very first detailed structural examination of glasses in the sodium thioborosilicate (NTBS) mixed glass former (MGF) system is reported. As in our previous studies of MGF systems, we have fixed the ratio of the modifier, Na2S, to the total glass former (GF), BS3/2 + SiS2, and varied the ratio of the two base GFs, the pure sodium thiosilicate (NTS) and sodium thioborate (NTB) end member binary glass systems. We have found that the 0.6Na2S + 0.4[xBS3/2 + (1-x)SiS2] series with its relatively high Na2S content is continuously glass forming for all x, 0 ≤ x ≤ 1, in large ~2 mm thick samples with only modest quenching was observed. Other higher and lower Na2S content glass series were investigated, but were found to be very poor glass formers and for this reason not further studied. Further, and advantageously, but not reported on here, this high Na2S content is also associated with glasses of the highest Na+ ion conductivities in either NTS or NTB binary glasses. Infrared, Raman, and 11B and 29Si magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopies were used to examine the short-range order (SRO) structures present in these glasses as a function of x. As expected from this high modifier content, the predominate finding is that these glasses are comprised mostly of depolymerized SRO units that possess one or at most two bridging sulfurs (BSs). Indeed, the main SRO structures observed in these glasses are the Si0, B0, and Si1 SRO units which contain no BSs and one BS, respectively. The Si1 and B0 SRO units are those expected from the composition of the binary series of glasses studied here, namely 3Na/Si for x = 0 and 3 Na/B for x = 1, respectively. However, the Raman spectra shows clear evidence of the disproportionate sharing of the Na+ towards the Si SRO units. This creates significant fractions of Si0 units for intermediate compositions. Likewise, a very close inspection and deconvolution of the NMR spectra and careful attention to charge balance shows that, as seen in other MGF systems, other SRO units with more BS structures, such as B1, B2, and B4 SRO units are formed from the transfer of the Na2S from the NTB subsystem to the NTS subsystem. The E1Si2 unit appears to be a common SRO unit in NTS (and other alkali) glasses and consists of two Si possessing two non-bridging sulfurs (NBSs) but linked together by two BSs in an edge-sharing motif. Due to the presence of the E1Si2 units (50 mol% Na2S) in a compositional series that is nominally 60 mol% Na2S, we find that the lack of sufficient Si0 SRO units to account for the extra Na2S necessarily produces a small amount of free, unreacted Na2S, <10 mol% of the total sodium sulfide in the system. Interestingly, the concentration of this “free” Na2S appears to increase with increasing NTB, x, content. Finally, as in nearly all non-oxide chalcogenide glasses, we do find a small amount of contaminant oxygen, always less than ~5 mol%, as a fraction of the total O + S, O/(O + S), in all of the glasses that increases in proportion to the amount of BS3/2 in the glass. This result is consistent with our experience that the starting material BS3/2 always contains more impurity O than the SiS2 starting material.