Hydrogen dynamics in the hexagonal Ho2Fe17H4 and Y2Fe17H4.2: Inelastic and quasielastic neutron scattering studies

The vibrational spectra of hydrogen and the parameters of H jump motion in the hexagonal Th2Ni17-type compounds Ho2Fe17H4 and Y2Fe17H4.2 have been studied by means of inelastic and quasielastic neutron scattering. It is found that hydrogen atoms occupying interstitial Ho(Y)2Fe2 sites in both compounds participate in the fast localized jump motion over the hexagons formed by these tetrahedral sites. The temperature dependence of the H jump rate is well described by the Arrhenius law over wide T ranges (100-340 K for Ho2Fe17H4 and 140-360 K for Y2Fe17H4.2) with the activation energies of 54 (4) meV and 84 (7) meV, respectively. For Ho2Fe17H4, the localized hydrogen jump motion is found to be the fastest among all R2Fe17 hydrides studied so far. At room temperature, the H jump rate in Ho2Fe17H4 derived from our quasielastic neutron scattering data reaches 6.4 × 1011 s−1.