Spatial distribution of cations through Voronoi polyhedrons and their exchange between polyhedrons in sodium silicate liquids

We study the spatial distribution of cations (Si and Na) through Voronoi polyhedrons around O and their exchange between polyhedrons in molecular dynamics models of sodium-silicate liquids. The result shows that most Na atoms reside in polyhedrons with specific configurations of Si and Na numbers. The fraction of polyhedrons with the same Si and Na index varies strongly with SiO2 content of the liquid. The average volume for polyhedrons with Si = 0, 1 and Na = 1, 2, 3 is found to vary from 31.78 to 35.09 Å3, and is smaller than 27.27 Å3 for the ones with Na = 0. The observed very fast diffusivity of sodium is caused by high rate of Na exchange between polyhedrons. The Na exchange is realized by two ways: 1) hopping between nearest neighbor polyhedrons and 2) collective displacement of a group of Na along a chain of polyhedrons. Within the time scale of the simulation the structure is strongly heterogeneous with a separate diffusion pathway where the majority of Na move, and a sodium-free silica domain. By analyzing the flow of Na and density of Na, Si, O in a fixed cubic lattice we find the exhibition of dynamics and chemical heterogeneity in the liquid.