Dynamics and diffusion mechanism in network forming liquid under high pressure: A new approach

The static and dynamic properties of liquid silica (SiO2) are investigated by molecular dynamics simulation at temperature of 3200 K and in the 0–25 GPa pressure range. To clarify diffusion mechanism and the anomalous diffusivity under compression, we have traced the time evolution of breakage and formation of the Si–O bond in the basic structural units SiOx (x = 4, 5, 6). The investigation reveals that atomic diffusion is realized through the transition Si[n] ↔ Si[n+1] (Si[n] means that Si atom has n coordinated oxygens) and the instability of units SiO5 is the cause of the anomalous diffusivity. Moreover, the transitions Si[n] ↔ Si[n+1] are not uniformly distributed but strongly localized in the space. The distribution of coordination units SiOx in network structure is not uniform but tends to form clusters of SiO4, SiO5 and SiO6 and this is the origin of localization of transitions resulted in the spatially heterogeneous dynamics in liquid SiO2.