Hydrogen jump motion in Laves-phase hydrides: Two frequency scales

Two types of hydrogen jump motion with different characteristic jump rates have been found to coexist in a number of Laves-phase hydrides. While the slower jump process is responsible for the long-range diffusion, the faster process corresponds to localized hydrogen motion over small groups of interstitial sites. In some of the Laves-phase compounds, the localized hydrogen motion is not 'frozen out' on the frequency scale of 107 to 108 s−1 down to 20 K. This paper reviews recent progress in investigations of the microscopic picture and systematics of H jump motion in Laves-phase hydrides. The relation between the hydrogen jump rates and the structure of hydrogen sublattice is emphasized.