A mechanism for fluorine diffusion in sodium fluoroberyllate liquid and glass was deduced using a molecular dynamics study. It was shown that the separation of F from the neighborhood of Be occurred only when the Be had a coordination number of five or more. The diffusion mechanism involved a F ion entering the coordination sphere of a fourfold Be ion to make it fivefold. Following this, a different F ion left the neighborhood to make the Be fourfold again. The diffusion mechanism explained qualitatively the concentration dependence of the viscosity in alkali fluoroberyllate fluids. At temperatures below the computer glass transition, where no macroscopic F diffusion occurred during a run, some F could still separate from fivefold Be by distances greater than 0.5Å. In the glass, about 40% of the Be ions were fivefold-coordinated by F. These motions required an activation energy of 0.1eV (compared to 0.7eV for macroscopic diffusion).

Defects and Fluorine Diffusion in Sodium Fluoroberyllate Glass: a Molecular Dynamics Study. Brawer, S.A.: The Journal of Chemical Physics, 1981, 75[7], 3516-21