Signature of small rings in the Raman spectra of normal and compressed amorphous silica: A combined classical and ab initio study

TL;DR

This study combines classical and ab initio simulations to analyze how small rings in amorphous silica influence Raman spectra, revealing that peak intensities are not directly proportional to small ring concentration.

Contribution

It provides a detailed computational analysis linking small ring structures to specific Raman spectral features in amorphous silica, clarifying their behavior under compression.

Findings

Modes of fourfold rings persist under compression but have weak Raman intensity.

Modes of threefold rings dominate the D2 peak and change with compression.

The area of D1 and D2 peaks does not directly reflect small ring concentration.

Abstract

We calculate the parallel (VV) and perpendicular (VH) polarized Raman spectra of amorphous silica. Model SiO2 glasses, uncompressed and compressed, were generated by a combination of classical and ab initio molecular-dynamics simulations and their dynamical matrices were computed within the framework of the density functional theory. The Raman scattering intensities were determined using the bond-polarizability model and a good agreement with experimental spectra was found. We confirm that the modes associated to the fourfold and threefold rings produce most of the Raman intensity of the D1 and D2 peaks, respectively, in the VV Raman spectra. Modifications of the Raman spectra upon compression are found to be in agreement with experimental data. We show that the modes associated to the fourfold rings still exist upon compression but do not produce a strong Raman intensity, whereas the ones associated to the threefold rings do. This result strongly suggests that the area under the D1 and D2 peaks is not directly proportional to the concentration of small rings in amorphous SiO2.