Quasielastic neutron scattering study of hydrogen motion in C15-type TaV₂H_x

In order to clarify the mechanism of hydrogen diffusion in cubic Laves phase TaV₂, we have performed high-resolution quasielastic neutron scattering measurements on TaV₂H_x (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 two 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 TaV₂D_x is split into hexagons well separated from each other. The behaviour of the elastic incoherent structure factor for TaV₂H_x 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 TaV₂H_x 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 r₁ (within the hexagons) and r₂ (between different hexagons).