Polyamorphic transitions in vitreous B₂O₃ under pressure

We have studied the nature of structural transitions in B₂O ₃ glass under pressure using molecular dynamics simulations, based on a newly developed coordination-dependent charge transfer potential, to complement the results from our earlier Brillouin and Raman scattering experiments and to interpret these findings. This interaction model allows for charges to re-distribute between atoms upon the formation and rupture of chemical bonds, and accommodates multiple coordination states for a given species in the course of the simulation. The macroscopic observables of the simulated vitreous B₂O₃, such as the variation of density and elastic modulus with pressure, agree well with those seen in experiments. The compaction of simulated structures is based on a polyamorphic transition that involves transitory four-coordinated boron atoms at high pressures. While the coordination of boron completely reverts to trigonal upon pressure release, without this transitory coordination increase permanent densification would not be manifest in the recovered glass. The response of vitreous B₂O ₃ to pressure is virtually independent of the concentration of boroxol rings in the structure. In simulated glass, boroxol rings dissolve when subject to pressure, which explains the disappearance of the breathing mode in the Raman spectrum of compressed B₂O₃ glass.