title: Concentration of small ring structures in vitreous silica from a first-principles analysis of the Raman spectrum.
abstract: Using a first-principles approach, we calculate Raman spectra for a model structure of vitreous silica. We develop a perturbational method for calculating the dielectric tensor in an ultrasoft pseudopotential scheme and obtain Raman coupling tensors by finite differences with respect to atomic displacements. For frequencies below 1000 cm(-1), the parallel-polarized Raman spectrum of vitreous silica is dominated by oxygen bending motions, showing a strong sensitivity to the intermediate range structure. By modeling the Raman coupling, we derive estimates for the concentrations of three- and four-membered rings from the experimental intensities of the Raman defect lines.
authors.affiliation: Institut de Théorie des Phénomènes Physiques (ITP), Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
authors.forename: P
authors.initials: P
authors.lastname: Umari
authors.forename: Xavier
authors.initials: X
authors.lastname: Gonze
authors.forename: Alfredo
authors.initials: A
authors.lastname: Pasquarello