Origin of excess low-energy vibrations in densified B2O3 glasses

Low-temperature experiments of Raman scattering and heat capacity have been performed in a B₂O₃ glass, pressure quenched from 1200 °C in order to obtain the density as largest as possible (ρ = 2373 kg/m³). When compared to those of compacted B₂O₃ glasses having smaller density, the Raman spectrum of this glass exhibits a strong decrease of the intensities of the Boson peak and the band at 808 cm^?1, both the features being determined by the decrease of the boroxol ring population. Moreover, the Boson peak exhibits a large shift to 68 cm^?1 (from 26 cm^?1 observed in normal vitreous B₂O₃). The high atomic packing of the glassy network also leads to a marked decrease of the excess heat capacity over the Debye T³-behaviour characterizing the crystal. The density g(ν) of low-frequency vibrational states has been assessed by using the low-frequency Raman intensity to determine the temperature dependence of the low-temperature heat capacity. The observations performed over a wide range of glass densities are compared to the predictions of theoretical models and computer simulations explaining the nature of the Boson peak. Consistency with the results of a simulation study concerning the vibrations of jammed particles leads to evaluate a nanometre length scale which suggests the existence of poorly packed domains formed from several connected boroxols. These soft regions are believed to be the main source of low-frequency optic-like vibrations giving rise to the Boson peak.