Deuterium is a special case for neutrons since it has comparable coherent and incoherent cross-sections. With D2 molecules, it is then possible to measure simultaneously Ds and Dt. Such an experiment was performed for different con­centrations of D2 adsorbed at 100 K in the NaX zeolite [19]. The self-diffusivities were checked from the broadenings measured at the same loadings for H2, after correcting from the mass difference between the two isotopes.

The values of Ds are plotted in Fig. 2.4. The slight increase of the self-diffusivity which is observed when the concentration increases was initially attributed to an interaction with the sodium cations [19]. This was later confirmed by atomistic computer simulations on the basis of the loading dependence of the partial molar configurational internal energy of the sorbate molecules, which indicated the existence of low-energy sites which are preferentially filled at low occupancy [20]. Molecules residing in these sites tend to exhibit lower translational mobility than molecules sorbed elsewhere in the intracrystalline space at higher occupancy.

The values of Dt, obtained at the same loadings, are also reported in Fig. 2.4. The width of the coherent scattering was found to show a minimum corresponding to the maximum of the structure factor. This line narrowing is characteristic of quasi­elastic coherent scattering and was first predicted by de Gennes [21].

It appears that at low D2 concentration, the self — and transport diffusivities are similar, but for higher loadings the transport diffusivity increases rapidly and exceeds the self-diffusivity, as expected from the contribution of the thermody­namic factor, Eq. (2.1). This means that the lattice gas model, which predicts that the transport diffusivity does not depend on the concentration, does not apply for this system. Only close to the saturation of the zeolite does the transport diffusivity

start to decrease, indicating that collective motions become affected by the packing density. The corrected diffusivity, D0, was obtained from Dt and from the ther­modynamic factor calculated by fitting a Langmuir isotherm to the adsorbed quantities. It is clear from Fig. 2.4 that for D2 in NaX the corrected diffusivity is not constant, this assumption being often made in the interpretation of macroscopic measurements.