In order to study the mechanism and parameters of hydrogen diffusion in the hexagonal (C14-type) Laves phase HfCr 2 , we have performed quasielastic neutron scattering (QENS) measurements in HfCr 2 H 0.74 and nuclear magnetic resonance (NMR) measurements of the proton spin-lattice relaxation rate in HfCr 2 H x (0.30 x 1.62) over the temperature range 11-424 K. It is found that the diffusive motion of hydrogen in this system can be described in terms of at least two jump processes: a fast localized H motion with the jump rate τ −1 l and a slower process with the rate τ −1 d associated with H jumps leading to longrange diffusion. In the interval 175-400 K, the temperature dependences of both τ −1 l and τ −1 d are reasonably described by an Arrhenius law. For HfCr 2 H 0.74 the corresponding activation energies derived from the QENS data are 122±7 meV for τ −1 l and 148 ± 7 meV for τ −1 d ; the ratio τ d /τ l at room temperature is close to 16. The long-range hydrogen mobility is found to decrease with increasing H content.

In order to study the mechanism and parameters of hydrogen diffusion in A15-type intermetallic Nb 3 Al, we have performed high-resolution quasielastic neutron scattering measurements in Nb 3 AlH x ͑x = 0.13 and 1.77͒ over the temperature range 10-407 K. The experimental results are consistent with a coexistence of two hydrogen jump processes: the fast H motion along the chains formed by interstitial d ͑Nb 4 ͒ sites and the slower H jumps from one chain to another. It is found that the modified Chudley-Elliott model taking into account the structure of the d-site sublattice gives a qualitative description of the experimental data for Nb 3 AlH 0.13 . The faster jump process in Nb 3 AlH 0.13 corresponding to the long-range H diffusion along the d-site chains is characterized by the activation energy of 194 meV. In Nb 3 AlH 1.77 the d-site chains are more than half filled, and the faster jump process is well described in terms of a model of local H jumps within pairs of nearestneighbor d sites on the chains.