Quasielastic neutron scattering study of hydrogen motion in C15-type TaV2Hx

In order to clarify the mechanism of hydrogen diffusion in cubic Laves phase TaV2, we have performed high-resolution quasielastic neutron scattering measurements on TaV2Hx (x = 0.6 and 1.1) in the temperature range 10-320 K. It is found that the diffusive motion of hydrogen in this system can be described in terms of Mo jump processes: the fast localized H motion within hexagons formed by interstitial g (Ta2V2) sites and the slower hopping from one hexagon to the other. This model is also supported by neutron diffraction measurements showing that the sublattice of g sites in TaV2Dx is Split into hexagons well separated from each other. The behaviour of the elastic incoherent structure factor for TaV2Hx suggests that only a fraction of the H atoms participate in the fast localized motion, and this fraction increases with temperature. Comparison of the properties of TaV2Hx and other Laves phase hydrides with g-site occupation shows a clear correlation between the parameters of the two jump processes and the g-g distances r(1) (within the hexagons) and r(2) (between different hexagons).