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 25 GPa and at temperature of 3000 K. 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 Si-xi -> Si-xi +/- 1 and O-zeta -> O-zeta +/- 1 which is significantly different between low- and high-pressure samples. For low-pressure sample the transitions Si-4 -> Si-5,Si- Si-5 -> Si-4, O-2 -> O(3)and O-3 -> O-2 are dominant, meanwhile for high-pressure sample there are transitions Si-4 -> Si-xi +/- 1, with xi = 4, 5,6 and O-zeta -> O-zeta +/- 1 with zeta=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. (C) 2012 Elsevier B.V. All rights reserved.