Structural changes and elastic characteristics of permanently densified vitreous B2O3

Inelastic light-scattering spectra of normal and permanently densified B2O3 glasses were investigated over the frequency range between 6 and 1600 cm-1. Densification from 1826 to 2320 kg/m3 was obtained by loading B2O3 glasses in a multianvil apparatus for synthesis at 2 and 4 GPa; they were fused at temperatures between 1400 and 1500 K and then were quenched at those pressures. It is shown that increasing densification gives rise to (i) a growing decrease of the intensity of the strong band at 808 cm-1, ascribed to the breathing vibration of boroxol rings, implying a reduction in the number of rings in the network and (ii) the appearance of a band at 775 cm-1 in the glass compacted at 4 GPa, which is assigned to the vibrations of structural units containing tetrahedral BO4 groups. The low-frequency Raman scattering includes the boson peak (BP), which dominates the spectra between 10 and 100 cm-1. Densification significantly decreases the intensity of the BP and shifts its position from about 26 cm-1, through 39 cm-1 (2-GPa glass), up to 68 cm-1 (4-GPa glass). These increases are stronger than those expected from the substantial hardening of the elastic continuum: The elastic moduli increase up to about a factor of 5 compared to those of normal glass. These observations imply that densification drives the system toward a structure having a more efficient packing of molecular units, causing substantial variations of the short- and medium-range orders with the formation of boron atoms in the four-fold-coordinated state at a quenching pressure of 4 GPa.