Hydrogen motion in C14-type HfCr2Hx: quasielastic neutron scattering and NMR studies

In order to study the mechanism and parameters of hydrogen diffusion in the hexagonal (C14-type) Laves phase HfCr2, we have performed quasielastic neutron scattering (QENS) measurements in HfCr2H0.74 and nuclear magnetic resonance (NMR) measurements of the proton spin-lattice relaxation rate in HfCr2Hx (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 ?l?1 and a slower process with the rate ?d?1 associated with H jumps leading to long-range diffusion. In the interval 175?400?K, the temperature dependences of both ?l?1 and ?d?1 are reasonably described by an Arrhenius law. For HfCr2H0.74 the corresponding activation energies derived from the QENS data are 122 ? 7?meV for ?l?1 and 148 ? 7?meV for ?d?1; the ratio ?d/?l at room temperature is close to 16. The long-range hydrogen mobility is found to decrease with increasing H content.