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 SiO_x. 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 SiO_x, x = 3, 4, … 7 or OSi_y, y = 1, 2, 3, 4. The diffusivity strongly depends on the rate of transitions Si^ξ → Si^ξ ± 1 and O^ζ → O^ζ ± 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 Si^ξ → Si^ξ ± 1 with ξ = 4, 5, 6 and O^ζ → O^ζ ± 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.