HYDSIL

Abstract

A crucial step for the understanding of geological processes (e.g. volcanic eruptions) as well as the materials properties of technological glasses is the understanding of microscopic mass transport in silica (SiO₂) and mixtures of silica with other oxides. Of particular interest are silicates containing water. Even small amounts of water reduce the viscosity of a silicate melt by orders of magnitude. Despite its importance for glass technology and geosciences, many aspects of the structural arrangement and the diffusive transport of water in silicates are largely unknown. In this project, mixtures of SiO₂ and water (hydrous silica) will be studied by ab-initio simulations. Born-Oppenheimer molecular dynamics (BO-MD) simulations based on density functional theory (DFT) will be performed with the Quickstep code which employs a hybrid basis set of Gaussian functions and plane waves. This scheme is particularly suitable for the simulation of large systems and thus we will be able to study systems of about 500 particles on a time scale of about 50 ps. Melts and glasses with different water content ranging from 1 wt% to 10 wt% H₂O will be considered. The first aim is to analyze how the water molecules are built into the Si-O network. Here, the accuracy of the simulation can be directly tested by comparing the static structure factor to recent neutron scattering data on hydrous silica. The second aim is to elucidate the mechanism how OH or other water-based units diffuse through the Si-O network. Also in this case, our simulation results can be compared to recent quasi-elastic neutron scattering experiments on the proton dynamics in hydrous silica. On the other hand, the detailed information provided by the simulation helps to better understand the results from the neutron scattering experiment.