About hopping mechanism for sodium diffusion in silicate liquids with low sodium concentrations: Molecular dynamics simulation

Molecular dynamics simulation is done for Na2O.4SiO2 melt at ambient pressure and 1873 K. The simulation shows that Na atoms are placed in O polyhedrons and not in Si ones. Moreover, 75.86% of total Na are placed in NBO and FO (non-bridging and free oxygen) polyhedrons, the volume of which is 27.36% of simulation box. We propose a new mechanism for sodium diffusion. Accordingly, Na moves collectively with network formers (O and Si) staying in the same polyhedron, and frequently displaces between different polyhedrons. The second motion type gives the essential contribution to sodium diffusivity, and leads to very fast sodium mobility. As NBO ↔ BO happens, Na atoms redistributed between O polyhedrons make the collective motion with network formers. Each BO (bridging oxygen), NBO polyhedron has 1 and 2 sites, respectively. The site energy for Na located in BO site is larger than NBO site, and the transition energy for Na moving from present site to neighboring BO site is larger than the one to NBO site. The hops are strongly correlated with each other at low temperature. Furthermore, the dynamical structure is found to be heterogeneous. During 150 ps, the system comprises two distinct regions occupied by SiO subnet and OO cluster. The region occupied by OO cluster resembles a preferential diffusion pathway for sodium, the volume of which is 20.5% of simulation box.