Structural changes ofB2O3through the liquid-glass transition range: A Raman-scattering study

Abstract

Raman scattering of ${\mathrm{B}}_2$ ${\mathrm{O}}_3$ has been performed from room temperature to 1273 K to study structural changes as the glass transforms into the melt via the supercooled regime. It is found that a structural model containing threefold six-member planar boroxol rings and chains of ${\mathrm{BO}}_3$ triangles can explain the experimental spectra. From the behavior of the internal vibrations of the boroxol rings and the ${\mathrm{BO}}_3$ triangles, we conclude that these molecular units do not change significantly as the temperature increases whereas the connectivity of the units is strongly affected. Still, network connectivity is observed in this ‘‘strong’’ liquid far above ${\mathit{T}}_{\mathit{m}}$, where the presence of transverse acoustic modes have also been reported. This supports the idea of a relation between structural properties and the dynamics of the liquid-glass transition as suggested in the ‘‘strong-fragile’’ classification scheme. The structural changes are demonstrated by the intensity profile of the spectra. It is shown that the strongest vibrational mode at 808 ${\mathrm{cm}}^{\mathrm{-}1}$, attributed to the breathing mode of boroxol rings, decreases rapidly in intensity as the temperature is raised above the glass transition temperature ${\mathit{T}}_{\mathit{g}}$. The high-frequency multicomponent band at 1200–1600 ${\mathrm{cm}}^{\mathrm{-}1}$ also displays anomalous temperature behavior above ${\mathit{T}}_{\mathit{g}}$. A significant redistribution of the intensity from the two narrow lines at ∼1210 and ∼1260 ${\mathrm{cm}}^{\mathrm{-}1}$ into the broad band at ∼1325 ${\mathrm{cm}}^{\mathrm{-}1}$ is found. The observed effects are consistent with a gradual breakup of boroxol rings, which change into chains of ${\mathrm{BO}}_3$ triangles as the temperature increases above ${\mathit{T}}_{\mathit{g}}$. From a detailed analysis of the temperature dependence of the spectra, the structure is estimated to consist of about half of the number of atoms in boroxol rings at ${\mathit{T}}_{\mathit{g}}$. Heating the glass to the melting temperature leads to breaking of about 1/3 of the boroxol rings.