Computer simulation of diffusion in silica liquid under temperature and pressure.

We have studied the diffusion mechanism in silica liquid following a new approach where the diffusion rate is estimated via the rate of SiO(x) → SiO(x±1) and the mean square displacement of Si particles per SiO(x) → SiO(x±1). Molecular dynamics simulation has been conducted for a model consisting of 1998 particles over a wide range of temperatures (3000-4500 K) and pressure (from 0 to 25.75 GPa). Our results show that the rate of SiO(x) → SiO(x±1) increases either with increasing the temperature or pressure. Further, we find that SiO(x) → SiO(x±1) is heterogeneously distributed through the network structure of the liquid. In particular, it is concentrated on a small section of Si particles in a low-temperature regime and at ambient pressure. The spatial localisation of SiO(x) → SiO(x±1) originates from the fact that the stable unit in low- and high-pressure regime is SiO4 and SiO6, respectively. The major change in the diffusion mechanism under pressure or temperature concerns the change in the distribution of SiO(x) → SiO(x±1) through the network structure. It is finally shown that the spatial localisation of SiO(x) → SiO(x±1) is responsible for the dynamics heterogeneity and the diffusion anomaly for silica liquid. This finding supports the concept that as the temperature approaches the glass transition point, SiO(x) → SiO(x±1) spatially localises such that the diffusivity drops and the dynamics are anomalously slow.