Quasielastic neutron scattering study of hydrogen motion in C14- and C15-type ZrCr2Hx

In order to clarify the mechanisms of hydrogen diffusion in the cubic (C15) and hexagonal (C14) modifications of Laves phase ZrCr2, we have performed high-resolution quasielastic neutron scattering measurements on C15-ZrCr2H0.45 and C14-ZrCr2H0.5 over the temperature range 10-340 K. It is found that in both systems the diffusive motion of hydrogen can be described in terms of two jump processes: the fast localized H motion within the hexagons formed by interstitial (ZrCr2) sites and the slower hopping from one hexagon to the other. The experimental results are analysed to determine the hydrogen hopping rates and the tracer diffusion coefficients as functions of temperature. The motional parameters of hydrogen in the C15- and C14-type samples are found to be close to each other. Comparison of the motional parameters of hydrogen in ZrCr2Hx and in the related C15-type TaV2Hx shows that the localized H motion in ZrCr2 is slower, whereas the long-range H diffusion is much faster than in TaV2. These features are consistent with the difference between intersite distances in the hydrogen sublattices of ZrCr2Hx and TaV2Hx. (author)