Hydrogen dynamics in Laves-phase hydride YFe₂H_2.6: Inelastic and quasielastic neutron scattering studies

Highlights: • H jump rates in YFe₂H_2.6 are determined from quasielastic neutron scattering. • Temperature dependence of the H jump rate is non-Arrhenius. • Fast two-site localized H motion within pairs of closely-spaced g sites. • Local lattice distortions resulting from hydrogenation of YFe₂. - Abstract: The vibrational spectrum of hydrogen and the parameters of H jump motion in the C15-type compound YFe₂H_2.6 have been studied by means of inelastic and quasielastic neutron scattering. It is found that hydrogen atoms occupying tetrahedral interstitial g (Y₂Fe₂) sites participate in the fast localized jump motion. The behavior of the elastic incoherent structure factor as a function of momentum transfer (measured up to Q_max ≈ 4 Å⁻¹) is consistent with the two-site motion of H atoms within pairs of closely spaced g sites. In the studied T range of 140–390 K, the temperature dependence of the jump rate of this localized motion is found to be non-Arrhenius; however, it can be described by two Arrhenius-like terms with the activation energies of 42 and 10 meV in the ranges 295–390 K and 140–240 K, respectively. Our results also indicate that hydrogen dynamics in YFe₂H_2.6 is affected by considerable local lattice distortions resulting from hydrogenation of YFe₂