In-situ observation of the structural change in MgO-B2O3-SiO2 glass at high pressure and the permanent structural change

A 41.6MgO-27.1B2O3-31.2SiO2 glass was synthesized by container-less levitation method. B K-edge X-ray absorption near edge structure (XANES) spectroscopy was used to quantify the coordination of boron in the as-made glass, resulting in ~72% of boron in 3-fold coordination. Raman spectroscopy indicated that the glass structure includes depolymerized borate units, which are pyroborate (B2O54−) and ring and/or chain-type metaborate (B3O63−, (BO2O−)∞) groups. The structural changes in the glass under hydrostatic condition were investigated by high-pressure Raman spectroscopy within a diamond anvil cell at pressures up to 8.5 GPa. On the borate structure, metaborate groups decreased with pressure. In contrast, triborate groups, which include tetrahedrally coordinated boron and have a fully polymerized borate network, increased; suggesting that the coordination transformation of B and polymerization were induced via compression. However, the modification of the borate structure units seems to be reversible at least up to 8.5 GPa. As for the silicate network, Q0 and Q1 structures were responsible to compression. Specifically, Q0 could be replaced by Q1 with elevating pressure, which means that the silicate network becomes polymerized with pressure. Whereas, structure of glass samples recovered from the pressure higher than 5 GPa displayed depolymerized silicate network, as dissociation of Q2 to Q1. The observed structural changes in the silicate network implied a partial change of the coordination number of Si4+ ions, which can be enhanced by the presence of a modifier cation with high field strength such as Mg2+.