We report the first in situ boron $K$-edge inelastic x-ray scattering (IXS) spectra for alkali borate glasses (${\mathrm{Li}}_2{\mathrm{B}}_4{\mathrm{O}}_7$) at high pressure up to 30 GPa where pressure-induced coordination transformation from three-coordinated to four-coordinated boron was directly probed. Coordination transformation (reversible upon decompression) begins around 5 GPa and the fraction of four-coordinated boron increases with pressure from about 50% (at 1 atm) to more than 95% (at 30 GPa) with multiple densification mechanisms, evidenced by three distinct pressure ranges for $(d{}^{[4]}\mathrm{B}/dP{)}_T$. The lithium $K$-edge IXS spectrum for Li-borate glasses at 5 GPa shows IXS features similar to that at 1 atm, suggesting that the Li environment does not change much with pressure up to 5 GPa. These results provide improved understanding of the structure of low-$z$ glass at high pressure.

• Received 25 September 2006

DOI:https://doi.org/10.1103/PhysRevLett.98.105502