Raman studies of vitreous SiO2versus fictive temperature

The first-order Raman spectra of vitreous Si${\mathrm{O}}_2$ have been studied as a function of sample fictive temperature $T_F$ for $900°\mathrm{C}\le T_F\le 1550°\mathrm{C}$. Three observations were made: (1) The broad features assigned to vibrations associated with the continuous random network of $v-\mathrm{Si}{\mathrm{O}}_2$ shift in frequency in a manner consistent with densification by reduction of the average Si-O-Si angle $\theta$; (2) the sharp "defect" lines at 492 and 606 ${\mathrm{cm}}^{-1\mathrm{}}$ show little or no shift in frequency; (3) both defect line intensities exhibit Arrhenius behavior, $\exp \left(\frac{-\Delta E}{k_B{T}_F}\right)$, with well-defined activation energies $\Delta E$ of 0.14 and 0.40 eV, respectively. These observations are interpreted using a central-force network dynamics model and changes in the number of planar threefold and regular fourfold ring defects.