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 HfCr2H chi(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 tau(-1)(l) and a slower process with the rate tau(-1)(d) associated with H jumps leading to long-range diffusion. In the interval 175-400 K, the temperature dependences of both tau(-1)(l) and tau(-1)(d) are reasonably described by an Arrhenius law. For HfCr2H0.74 the corresponding activation energies derived from the QENS data are 122 +/- 7 meV for tau(-1)(l) and 148 +/- 7 meV for tau(-1)(d); the ratio tau(d)/tau(l) at room temperature is close to 16. The long-range hydrogen mobility is found to decrease with increasing H content.