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

摘要

We use {\it ab initio} molecular dynamics simulations to study a sample ofliquid silica containing 3.84 wt.% H$_2$O.We find that, for temperatures of3000 K and 3500 K,water is almost exclusively dissolved as hydroxyl groups, thesilica network is partially broken and static and dynamical properties of thesilica network change considerably upon the addition of water.Water moleculesor free O-H groups occur only at the highest temperature but are not stable anddisintegrate rapidly.Structural properties of this system are compared to thoseof pure silica and sodium tetrasilicate melts at equivalent temperatures. Thesecomparisons confirm the picture of a partially broken tetrahedral network inthe hydrous liquid and suggest that the structure of the matrix is as muchchanged 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 arequalitatively similar but seem to be more pronounced in the hydrous silicaliquid. Finally, we study the diffusion mechanisms of the hydrogen atoms in themelt. It turns out that HOSi$_2$ triclusters and SiO dangling bonds play adecisive role as intermediate states for the hydrogen diffusion.