First-principles molecular dynamics simulations were used to study a sample of liquid silica containing 3.84wt%H2O. It was found that, at 3000 and 3500K, water was almost exclusively dissolved as hydroxyl groups, the silica network was partially broken and the static and dynamic properties of the silica network changed considerably upon adding water. Water molecules, or free O–H groups, occurred only at the highest temperature; but were not stable and disintegrated rapidly. The structural properties of this system were compared with those of pure silica and sodium tetrasilicate melts at similar temperatures. These comparisons confirmed the concept of a partially broken tetrahedral network in the hydrous liquid, and suggested that the structure of the matrix was changed as much by the addition of water as it was by the addition of the same amount (in mol%) of sodium oxide. At larger length-scales, the correlations were qualitatively similar but seemed to be more pronounced in the hydrous silica liquid. A study was also made of the diffusion mechanisms of H atoms in the melt. It was found that HOSi2 triclusters and SiO dangling bonds played decisive roles as intermediate states for H diffusion.

First-Principles Molecular-Dynamics Simulations of a Hydrous Silica Melt - Structural Properties and Hydrogen Diffusion Mechanism. M.Pöhlmann, M.Benoit, W.Kob: Physical Review B, 2004, 70[18], 184209 (14pp)