Density-driven structural transformations inB2O3glass

Abstract

The method of in situ high-pressure neutron diffraction is used to investigate the structure of ${\mathrm{B}}_{2}{\mathrm{O}}_{3}$ glass on compression in the range from ambient to 17.5(5) GPa. The experimental results are supplemented by molecular dynamics simulations made using a newly developed aspherical ion model. The results tie together those obtained from other experimental techniques to reveal three densification regimes. In the first, ${\mathrm{BO}}_{3}$ triangles are the predominant structural motifs as the pressure is increased from ambient to 6.3(5) GPa, but there is an alteration to the intermediate range order which is associated with the dissolution of boroxol rings. In the second, ${\mathrm{BO}}_{4}$ motifs replace ${\mathrm{BO}}_{3}$ triangles at pressures beyond 6.3 GPa and the dissolution of boroxol rings continues until it is completed at 11--14 GPa. In the third, the B-O coordination number continues to increase with pressure to give a predominantly tetrahedral glass, a process that is completed at a pressure in excess of 22.5 GPa. On recovery of the glass to ambient from a pressure of 8.2 GPa, triangular ${\mathrm{BO}}_{3}$ motifs are recovered but, relative to the uncompressed material, there is a change to the intermediate range order. The comparison between experiment and simulation shows that the aspherical ion model is able to provide results of unprecedented accuracy at pressures up to at least 10 GPa.