The diffusion of H in sodium aluminium sodalite (NaAlSi-SOD) was modelled by using classical molecular dynamics, allowing for full flexibility of the host framework, at 800 to 1200K. From these simulations, the self-diffusion coefficient was determined to be a function of temperature and the H uptake at low equilibrium H concentration was estimated at 573K. The influence of the cation distribution over the framework on H self-diffusion was investigated by comparing results using a low-energy fully ordered cation distribution with those obtained by using a less ordered distribution. The cation distribution was found to have a surprisingly large influence upon the diffusion, which appeared to be due to the difference in framework flexibility for different cation distributions, the occurrence of correlated hopping in case of the ordered distribution, and the differing nature of the diffusion processes in both systems. Compared with previously reported calculations of all-silica sodalite (all-Si-SOD), the H diffusion coefficient of sodium aluminium sodalite was higher in the case of the ordered distribution and lower in case of the disordered distribution. The H uptake rates of all-Si-SOD and NaSiAl-SOD were comparable at about 1000K, and lower for all-Si-SOD at about 400K.

Effect of Cation Distribution on Self-Diffusion of Molecular Hydrogen in Na3Al3Si3O12 Sodalite - a Molecular Dynamics Study. A.W.van den Berg, S.T.Bromley, E.Flikkema, J.C.Jansen: Journal of Chemical Physics, 2004, 121[20], 10209-16