Combined atomistic simulation and quasielastic neutron scattering study of the low-temperature dynamics of hydrogen and deuterium confined in NaX zeolite

A direct comparison of quasielastic neutron scattering experimental measurements and molecular dynamics computer simulations is carried out for the first time for hydrogen and deuterium confined in the nanostructure of faujasite type zeolites at a temperature of 100 K, in order to investigate the dependence of sorption thermodynamics and sorbate dynamics on the sorbed phase concentration. The charged crystal framework of NaX is digitally reconstructed based on X-ray diffraction spectra, with the compensating Na+ positions determined by neutron diffraction data. For the quantum statistical mechanical description of the guest-guest and guest-zeolite interactions, the temperature-dependent effective potential of Feynman and Hibbs is employed, resulting from an approximation to the quantum mechanical path integral formulation of the motion. Computer simulation and neutron scattering are found in satisfactory agreement, both exhibiting a slight increase of the self-diffusivity and a maximum in the transport diffusivity as a function of sorbate loading. Our findings are further elaborated on the basis of the quasichemical mean field theory in conjunction with a model for surface transport due to Reed and Ehrlich, through which the computed and measured dynamical behavior can be related to the strength of interactions between the sorbate molecules.