Vibrational collapse of boroxol rings in compacted B2O3 glasses: a study of Raman scattering and low temperature specific heat

Low and high frequency Raman scattering of B2O3 glasses, compacted under GPa pressures, has been performed to investigate structural changes due to increasing atomic packing. Compacted glasses, annealed at ambient temperature and pressure, experience a time-dependent decrease of the density to a smaller constant value over a period of few months, displaying a permanent plastic deformation. Increasing densification determines a parallel and progressive decrease of the intensity of the Boson peak and the main band at 808 cm(-1), both these modes arising from localized vibrations involving planar boroxol rings (BB0O)(,) the glassy units formed from three basic BO3 triangles. The 808 cm(-1) mode preserves its frequency, while the BP evidences a well-defined frequency increase. The highfrequency multicomponent band between 1200 and 1600 cm(-1) also changes with increasing densification, disclosing a decreasing intensity of the 1260 cm(-1) mode due to oxygen vibrations of BO3 units bridging boroxol rings. This indicates the gradual vibrational collapse of groups formed from rings connected by more complex links than a single bridging oxygen. The observed behaviours suggest that glass compaction causes severe deformation of boroxol rings, determining a decrease of groups which preserve unaltered their vibrational activity. Growing glass densification stiffens the network and leads to a decrease of the excess heat capacity over the Debye prediction below 20 K, which is not accounted for by the hardening of the elastic continuum. By using the low-frequency Raman scattering to determine the temperature dependence of the heat capacity, it has been evaluated the density of low-frequency vibrational states which discloses a significant reduction of excess modes with increasing density.