Dynamics of hydrogen in Pr2Fe17H4 and Pr2Fe17H5 studied by quasielastic neutron scattering

Quasielastic neutron scattering (QENS) has been used to investigate the dynamics of hydrogen in Pr2Fe17H4, where isolated hexagons formed by the interstitial tetrahedral (t) sites of the metal lattice host mobile hydrogen atoms (on average, one H atom per hexagon). An activation energy of 0.10 eV has been measured for a localized jump process involving the hopping of these hydrogen atoms among adjacent vertices of each hexagon. This activation energy is somewhat lower compared to that for hydrogen hopping in Pr2Fe17H5, where each hexagon is occupied by two hydrogen atoms. The fraction of the hydrogen atoms immobilized on the t sites by defects in the host metal lattice is slightly higher in Pr2Fe17H4 compared to Pr2Fe17H5. These results are consistent with our earlier view of Pr2Fe17H5 as a system where the hopping of the t-site hydrogen atoms occurs through the correlated jumps of the two H atoms occupying each isolated hexagon.