Spatial heterogeneous distribution of SiO(x) → SiO(x±1) reactions in silica liquid

We have numerically studied the diffusion mechanism in silica liquid following an approach where the diffusion rate is evaluated via the SiO(x) → SiO(x±1) reaction rate υ(react) and the mean square displacement of particles d(react) as a reaction happens. Five models at pressure up to 25 GPa and at a temperature of 3000 K have been constructed by molecular dynamic simulation. When applying pressure to the liquid, υ(react) increases monotonously because the Si-O bond becomes weaker with pressure. Meanwhile d(react) attains a maximum near the point of 10 GPa despite the particles move in a significantly smaller volume. Furthermore, the SiO(x) → SiO(x±1) reactions are spatially heterogeneously distributed in the liquid. Upon low pressure, most reactions happen with a small number of Si particles. This reaction localization causes the diffusion anomaly and dynamics heterogeneity in the liquid. With increasing pressure the diffusion mechanism changes from the heterogeneous spatial distribution of reactions to homogeneous one. The simulation also reveals two distinguished regions with quite different coordination environments where the reaction rate significantly differs from each other. These sets of Si particles migrate in space over time and form regions with so-called "fast" and "slow" Si particles. The result obtained here indicates the coexistence of low- and high-density regions, and supports the concept of polymorphism in silica liquid.