Boroxol ring modification driven by plastic deformations of compacted B 2 O 3 glasses

Structural changes of glassy B2O3, compacted under GPa pressures at temperatures below the glass transition, have been monitored by Raman spectroscopy, disclosing a progressively decreasing intensity of both the Boson peak and the main band at 808 cm−1 with increasing densification. Both these components are determined by vibrational activity of boroxol rings (B3O6), the glassy super-structural units formed by connected BO3 planar triangles. It is suggested that growing density by glass compaction (up to 13%) causes severe deformation of boroxol rings, determining a decrease of structural units which preserve unaltered their vibrational activity. The density of compacted glasses, annealed at ambient temperature and pressure, is time-dependent and decreases to a lower constant value, but still higher than that of normal vitreous B2O3, evidencing a permanent plastic deformation. The final structural configuration is achieved over a period of few months during which the local strain is partially relaxed allowing some boroxol groups to recover their original shape and vibrational modes. Growing glass densification gives rise to a substantial hardening of the network with a well-defined linear increase of both the bulk and rigidity moduli, while leaving nearly constant the Poisson's ratio. Also the frequency νBP of the Boson peak shifts from 26 cm−1 in v-B2O3 up to 37 cm−1 in the glass having the highest density (2069 kg/m3), this increase being stronger than that expected from the simple hardening of elastic continuum. The observed effects are consistent with the gradual vibrational collapse of boroxol rings driven by densification and emphasize the basic role of these glassy units in determining the softening degree and the low-energy vibrational dynamics of v-B2O3.