关键词: Morse stretch势场, 自扩散系数, SiO₂, 分子动力学

Abstract:

The diffusivity of elements in mineral controls the processes of ion exchange, isotope fractionation, and phase
transition. As one of the most important components of silicate melts, SiO2 plays a very important role in the earth mantle structure
and the dynamics process. This paper focuses on the ion diffusion mechanism in silica melt and the corresponding pressure when
the self-diffusion coefficient reaches its maximum value. We calculated the changing process of Si and O ions, self-diffusion
coefficient at temperatures of 3000 K by using the molecular dynamics (MD) simulation which contains 4500 ions in the silica
melt with Morse stretch potential. Calculations show that both Si and O self-diffusion coefficients increase at initial compression
and reach maximum values at 17.5 GPa. Self-diffusion coefficients for O are slightly greater than that for Si. The Si and O ions,
diffusion feature is caused by a defect-controlled transport mechanism and the five-fold coordination structure of Si ion is the main
reason for the rise of the diffusion coefficients in accord with the increasing pressure. The fact that the diffusion coefficient reaches
its maximum value means the change in the formation of five-fold Si in the liquid silica. This paper also calculated the relationship
between the average volume of [SiO2] unit and the pressure, which agrees well with the results of our experiment.

Key words: morse stretch potential； self-diffusion coefficient, SiO₂, molecular dynamics