First-principles molecular-dynamics simulations of a hydrous silica melt: Structural properties and hydrogen diffusion mechanism

TL;DR

This study uses ab initio molecular dynamics to explore the structural changes and hydrogen diffusion mechanisms in hydrous silica melts at high temperatures, revealing the predominance of hydroxyl groups and the role of specific clusters in hydrogen mobility.

Contribution

It provides detailed insights into the structural properties and hydrogen diffusion mechanisms in hydrous silica melts using first-principles simulations, highlighting the impact of water addition.

Findings

Water exists mainly as hydroxyl groups at high temperatures.

The silica network is partially broken by water addition.

Hydrogen diffusion involves triclusters and dangling bonds.

Abstract

We use {\it ab initio} molecular dynamics simulations to study a sample of liquid silica containing 3.84 wt.% H$_2$O.We find that, for temperatures of 3000 K and 3500 K,water is almost exclusively dissolved as hydroxyl groups, the silica network is partially broken and static and dynamical properties of the silica network change considerably upon the addition of water.Water molecules or free O-H groups occur only at the highest temperature but are not stable and disintegrate rapidly.Structural properties of this system are compared to those of pure silica and sodium tetrasilicate melts at equivalent temperatures. These comparisons confirm the picture of a partially broken tetrahedral network in the hydrous liquid and suggest that the structure of the matrix is as much changed by the addition of water than it is by the addition of the same amount (in mole %) of sodium oxide. On larger length scales, correlations are qualitatively similar but seem to be more pronounced in the hydrous silica liquid. Finally, we study the diffusion mechanisms of the hydrogen atoms in the melt. It turns out that HOSi$_2$ triclusters and SiO dangling bonds play a decisive role as intermediate states for the hydrogen diffusion.