Transport of rare gases and molecular water in fused silica by molecular dynamics simulation

The dynamics and the diffusivity of rare gases and molecular water in fused silica are investigated by molecular dynamics simulation. It is shown that the values of the diffusivity of the rare gases in simulated liquid silica do not differ greatly between each other, in contrast to the situation observed at lower temperatures in the glassy state. Furthermore, the calculated activation energies associated with the diffusion process exhibit much higher values in liquid silica than those measured in silica melts below the glass transition temperature. It is found also that molecular water behaves mostly like noble gas atoms except near room temperature, where the kinetic energy is not sufficiently large to prevent the formation of strong hydrogen bonds between water and the oxygen atoms of the silica network. The vibrational motions and the hopping motions of gas molecules trapped in the silica matrix also are investigated in detail.