The study of diffusion in network-forming liquids under pressure and temperature

In this paper, the molecular dynamics simulation is applied to investigate the diffusion in silica liquids under different temperature and pressure. We show that the diffusion is controlled by the rate of effective SiO_x→SiO_x±1 and OSi_y→OSi_y±1 reaction. With increasing the pressure, the rate of reaction increases and the Si–O bond is weaker. Moreover, the reactions are not uniformly distributed in the space, but instead they happen frequently or rarely in separate regions. We also reveal two motion types: free and correlation motion. The correlation motion concerns the moving of a group of atoms which is similar to that of the diffusion of a super-molecule in the liquid. A detailed analysis of the movement of atoms from specified set shows the clustering of them which indicates structure and dynamics heterogeneity. Further, we find that the correlation motion is very important for the diffusion in network-forming liquid. The observed phenomena such as diffusion anomaly, dynamics heterogeneity and dynamical slowdown are originated from the correlation motion of atom.