Hydrogen dynamics in Ce2Fe17H5: inelastic and quasielastic neutron scattering studies.

The vibrational spectrum of hydrogen and the parameters of H jump motion in the rhombohedral Th2Zn17-type compound Ce2Fe17H5 have been studied by means of inelastic and quasielastic neutron scattering. It is found that hydrogen atoms occupying interstitial Ce2Fe2 sites participate in the fast localized jump motion over the hexagons formed by these tetrahedral sites. The H jump rate ??1 of this localized motion is found to change from 3.9???109?s?1 at T?=?140?K to 4.9???1011?s?1 at T?=?350?K, and the temperature dependence of ??1 in the range 140?350?K is well described by the Arrhenius law with the activation energy of 103?3?meV. Our results suggest that the hydrogen jump rate in Th2Zn17-type compounds strongly increases with decreasing nearest-neighbor distance between the tetrahedral sites within the hexagons. Since each such hexagon in Ce2Fe17H5 is populated by two hydrogen atoms, the jump motions of H atoms on the same hexagon should be correlated.