First principles study of the surface of silica and sodium silicate glasses

TL;DR

This study uses ab initio molecular dynamics to explore the structural, vibrational, and electronic properties of silica and sodium silicate glass surfaces, revealing defect concentrations, vibrational signatures, and bonding characteristics influenced by surface features and sodium content.

Contribution

It provides detailed atomic-level insights into the surface structure and properties of silica and sodium silicate glasses using first-principles simulations, highlighting the effects of sodium on defect reduction and vibrational behavior.

Findings

Surfaces have more dangling bonds and 2M rings than bulk.

Na2O reduces structural defects on the surface.

2M rings exhibit a vibrational signature at ~850 cm$^{-1}$.

Abstract

We use \textit{ab initio} molecular dynamics simulations to investigate the properties of the dry surface of pure silica and sodium silicate glasses. The surface layers are defined based on the atomic distributions along the direction ($z-$direction) perpendicular to the surfaces. We show that these surfaces have a higher concentration of dangling bonds as well as two-membered (2M) rings than the bulk samples. Increasing concentration of Na$_2$O reduces the proportion of structural defects. From the vibrational density of states, one concludes that 2M rings have a unique vibrational signature at a frequency $\approx850$~cm$^{-1}$, compatible with experimental findings. We also find that, due to the presence of surfaces, the atomic vibration in the $z-$direction is softer than for the two other directions. The electronic density of states shows clear the differences between the surface and interior and we can attribute these to specific structural units. Finally, the analysis of the electron localization function allows to get insight on the influence of local structure and the presence of Na on the nature of chemical bonding in the glasses.