In order to clarify the mechanisms of H diffusion in the cubic (C15) and hexagonal (C14) modifications of this Laves phase, high-resolution quasi-elastic neutron-scattering measurements were performed on C15-type Cr2ZrH0.45 and C14-type Cr2ZrH0.5 at temperatures ranging from 10 to 340K. It was found that, in both systems, the diffusive motion of H could be described in terms of 2 jump processes. These were a fast localized H motion within the hexagons that were formed by interstitial (Zr2Cr2) sites, and a slower hopping from one hexagon to another. The experimental results were analyzed in order to determine the H hopping rates and the tracer diffusion coefficients as a function of temperature (figure 14). The H motional parameters in the C15- and C14-type samples were found to be close to each other. A comparison of the motional parameters of H in Cr2ZrHx and in C15-type V2TaHx showed that the localized H motion was slower in the former, whereas long-range H diffusion was much faster than in the latter. These features were consistent with the difference in intersite distances of the H sub-lattices of Cr2ZrHx and V2TaHx.

A.V.Skripov, M.Pionke, O.Randl, R.Hempelmann: Journal of Physics - Condensed Matter, 1999, 11[6], 1489-502