Study of sodium diffusion in silicate glasses. Molecular dynamics simulation

Molecular dynamics simulation is carried out to study diffusion in sodium silicate glasses (NS1, NS2, NS3, NS4) at temperatures of 973, 1173 and 1373 K. The result shows that the structure consists of network region where more than 83% of total Si and O are present, and Na-polyhedron region in which most Na-polyhedrons possess several non-bridging oxygens. The Na-polyhedron region changes slightly with temperature, and significantly with SiO2 concentration. During 150 ps the Si and O atoms vibrate around fixed points, while Na atoms move from one Na-polyhedron to another. The network region is static, while the Na-polyhedron region is seen dynamically. The glasses exhibit the dynamics heterogeneity. The simulation shows that Na atoms reside in a small part of Na-polyhedron region and move frequently through pathways consisting of polyhedrons with high local sodium density. Moreover, they move between polyhedrons often by small displacements and rarely by large jumps. We establish the expression for diffusion constant D Na via average resident time in polyhedron t RP and mean square displacement of Na per polyhedron delta. The dependence of D Na on delta and lnD Na on t RP is found to be linear.