Quasielastic neutron scattering studies of H motion in Laves-phase compounds

The results of our quasielastic neutron scattering measurements for a number of cubic Laves-phase hydrides AB2Hx are consistent with the coexistence of two types of H motion: the fast localized jumps within the hexagons formed by interstitial g (A2B2) sites and the slower hopping from one hexagon to another. The analysis of these results has revealed the relation between the hydrogen-hopping rates, the g-g distances, and the ratio of the metallic radii RA and RB of the elements A and B forming the AB2 compound. We conclude that the behavior of the two frequency scales of H motion is determined mainly by RA/RB. A new type of localized H motion is predicted for compounds with RA/RB>1.35. (orig.)