First principles calculations of Raman spectra: relationships between structure and vibrations
Résumé
Raman spectroscopy is one of the key experimental methods for the structural
analysis of glasses, thanks to its simple application and its sensitivity to variations of
composition or atomic-scale order. However, the inherent disorder of the glassy
structure results to broad and overlapping peaks in the spectra, rendering their
quantitative analysis a challenging task and resulting in a mostly phenomenological
interpretation. This drawback can be addressed by the use of modern and accurate
theoretical tools, such as ab-initio vibrational spectroscopies. In this work we have
studied three simple sodosilicate glasses, prepared using a combined classical/ab-
initio approach. For these structures, we have used density functional theory (DFT) to
calculate their vibrational density of states, as well as their Raman and IR spectra.
The main advantage of the DFT calculations is that the resulting spectra can be
directly correlated with the structural models and, most importantly, the vibrational
spectrum can be decomposed into exact contributions arising from individual
structural units. In the case of the sodosilicate glasses, we have identified
correlations between spectral characteristics and stoichiometry, the effect of the
presence of bridging and non-bridging oxygens, the contributions of smaller or larger
Si-O-Si angles, as well as the ones arising from the presence of the so-called Q-species. These results open the way for the study of even more complex glasses and
aid in the interpretation of experimental spectra.