Dynamics of hydrogen in α-LaNi5hydride investigated by neutron scattering

Quasielastic and inelastic neutron-scattering experiments have been performed in order to determine the structural and dynamical properties of α-${\mathrm{LaNi}}_5$ hydride. A microscopic-jump model on the basis of the well-known ${\mathrm{CaCu}}_5$ structure of the compound (extended Chudley-Elliott model) has been used. It starts from the assumption of the hydogen atoms occupying two energetically different interstitial sites. This model has been applied to the long-range and localized diffusion of hydrogen in a single crystalline sample. To enhance the contrast for the incoherent scattering from the H nuclei, natural Ni was substituted with its ${}_{}{}^{60}{}_{}{}^{}\mathrm{Ni}$ isotope. An appreciable anisotropy of the long-range diffusion is not observable at 423 K. A fast and localized diffusive process was found which is confined to a hexagonal arrangement of six sites in the (001) lattice plane. The local hydrogen vibrational modes are resolved and assigned to specific vibrations taking into account the crystallographic directions in which they appear.