First-Principles Studies on the Structure and B–O Coordination of B₂O₃ Glass at High Pressure

The structures of amorphous materials at extreme pressures can be very different from those of their crystalline counterparts. Recent nonresonant inelastic scattering experiments on B2O3 glass performed up to ∼120 GPa have suggested that B–O coordination numbers higher than 4 were presented. In this study, B2O3 glass structures up to 350 GPa were generated from first-principles molecular dynamics (FPMD) calculations. X-ray and neutron diffraction patterns, B K-edge X-ray absorption spectra, and 11B NMR spectra were computed and compared to experiments. The nature of the B–O interactions was characterized by Bader's quantum theory of atoms in molecules (QTAIM) analysis and electron localization function (ELF). The theoretical results found that 5- and 6-coordinated B exists at high pressure. However, not all of the B and O contacts are covalently bonded. Instead, both BO5 and BO6 groups consisted of 4 short B–O covalent bonds with weaker B–O interactions with the other O atoms. Structure prediction calculations were performed at a similar pressure range to assess whether 6-coordinated B–O can exist in crystalline B2O3. No genuine 6-coordinated B was found. The high B–O coordination number in the glass is simply due to compaction at high pressure.