Quasielastic neutron scattering study of H motion in the hydrogen-stabilized C15-type phases HfTi2Hx and ZrTi2Hx

In order to clarify the mechanism of H diffusion in the hydrogen-stabilized phases HfTi2Hx and ZrTi2Hx with x approximate to 4, we have performed high -resolution quasielastic neutron scattering measurements on HfTi2H3.5, HfTi 2H4.0 and ZrTi2H3.9 over the temperature range 10-327 K. It is found that t he diffusive motion of hydrogen in these systems can be described in terms of at least two frequency scales of ii hopping. A small fraction of H atoms participates in the fast localized motion within the hexagons formed by in terstitial g (Hf2Ti2 or Zr2Ti2) sites; this fraction increases with increas ing temperature. The slower jump process corresponds to H motion on the nea rly completely filled sublattice of interstitial e (HfTi3 or ZrTi3) sites; transitions from one e site to another imply a number of jumps over interme diate g sites. The parameters of H motion in HfTi2Hx and ZrTi2Hx are found to be close to each other. The temperature dependence of the characteristic rate tau(d)(-1) of the slower jump process in HfTi2H4 is well described by the Arrhenius characteristic rate td law with the activation energy of 0.1 7 +/- 0.02 eV.