Journal article

Medium-range structure of vitreous SiO2 obtained through first-principles investigation of vibrational spectra

Using a density-functional framework, we investigate the vibrational spectra of vitreous SiO2 to determine to what extent these spectra provide information about the medium-range structure of the oxide network. We carry out a comparative study involving three model structures, which all feature a nondefective network of corner-sharing tetrahedra but differ through their Si-O-Si bond-angle distributions and ring statistics. We first address the results of typical diffraction probes. Fair agreement with experiment is achieved for the total neutron and total x-ray structure factors of all models, indicating limited sensitivity of these structure factors to the medium-range structure. The same consideration also applies to the Si-O and O-O partial structure factors. At variance, the Si-Si partial structure factor is found to be highly sensitive to the Si-O-Si bond-angle distribution. We then address typical vibrational spectra, such as the inelastic neutron spectrum, the infrared spectra, and the Raman spectra. For the inelastic neutron spectrum and the infrared spectra, the comparison with experiment is fair for all models, indicating poor sensitivity to the structural arrangement of tetrahedra. The only noticeable exception is the feature at similar to 800 cm(-1) which shifts to higher frequencies with decreasing Si-O-Si angles. At variance, the Raman spectra are shown to be very informative about the medium-range organization of the network through their sensitivity to the concentrations of three-membered and four-membered rings. Our study indicates that the considered experimental data are globally consistent with a medium-range structure characterized by an average Si-O-Si bond angle of 148 degrees and with small-ring concentrations as derived from the intensities of the experimental Raman defect lines. To describe the infrared and Raman couplings, our work also introduces parametric models which reproduce well the spectra calculated from first principles.


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