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 AB(2)H(x) are consistent with the coexistence of two types of H motion: the fast localized jumps within the hexagons formed by interstitial g (A(2)B(2)) 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 R(A) and R(B) of the elements A and B forming the AB(2) compound. We conclude that the behavior of the two frequency scales of H motion is determined mainly by R(A)/R(B). A new type of localized H motion is predicted for compounds with R(A)/R(B) > 1.35.