About diffusion mechanism in silica liquid under pressure

We perform a molecular dynamic simulation to study the diffusion mechanism in silica liquid under pressure up to 25GPa and at temperature of 3000K. We find that total O―Si―O angle distribution can be expressed by a simple relation between partial O―Si―O angle distribution and fractions of units SiOx. Specifically, we demonstrate that these liquids consist of identical units SiO4, SiO5 and SiO6 and have common partial O―Si―O angle distribution. We also show that each particle undergoes a series of stages where the particle locates in unchanged unit SiOx, x=3, 4, … 7 or OSiy, y=1, 2, 3, 4. The diffusivity strongly depends on the rate of transitions Siξ→Siξ±1 and Oζ→Oζ±1 which is significantly different between low- and high-pressure samples. For low-pressure sample the transitions Si4→Si5, Si5→Si4, O2→O3 and O3→O2 are dominant, meanwhile for high-pressure sample there are transitions Siξ→Siξ±1 with ξ=4, 5, 6 and Oζ→Oζ±1 with ζ=2, 3, 4. This finding may be common for diffusion in all network-forming liquids. The simulation also reveals the spatially heterogeneous dynamics in low-pressure liquid where a large cluster of immobile particle exists for the time that a number of particles move over several inter-particle distances. ► We examine the relation between total bond O―Si―O angle distribution and fraction of network units for silica liquids under pressure ► The anomalous behavior of diffusion is observed for both Si and O ► The diffusivity strongly depends on the rate of transitions Siξ→Siξ±1 and Oζ→Oζ±1 ► The heterogeneous dynamics is observed in low-pressure liquid ► A large cluster of immobile particles is found in low-pressure sample.