First-principles simulations were used to study a sample of liquid silica containing 3.84wt%H2O. It was found that, at 3000 to 3500K, the water was almost entirely 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 unstable, and disintegrated rapidly. The structural properties of this system were compared to those of pure silica and sodium tetrasilicate melts at equivalent temperatures. This confirmed the picture of a partially broken tetrahedral network in the hydrous liquid, and suggested that the structure of the matrix was as much changed 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 made of the diffusion of hydrogen atoms in the melt. It was found that HOSi2 triclusters and SiO dangling bonds played a decisive role, as intermediate states for hydrogen diffusion.

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