Structure of B2O3 Glass at High Pressure: A 11B Solid-State NMR Study.

We report spectroscopic evidence for the pressure-induced structural changes in B2O3 glass quenched from melts at pressures up to 6 GPa using solid-state NMR. While all borons are tri-coordinated at 1 atm, the fraction of tetra-coordinated boron increases with pressure, being about 5% and 27% in the B2O3 glass quenched from melts at 2 and 6 GPa, respectively. The fraction of boroxol ring species increases with pressure up to 2 GPa and apparently decreases with further compression up to 6 GPa. Two densification mechanisms are proposed to explain the variation of boron species with pressure.