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 Hf Ti2 Hx and ZrTi2 Hx with x 4, we have performed high-resolution quasielastic neutron scattering measurements on Hf Ti2 H3.5 , Hf Ti2 H4.0 and ZrTi2 H3.9 over the temperature range 10 - 327 K. It is found that the diffusive motion of hydrogen in these systems can be described in terms of at least two frequency scales of H hopping. A small fraction of H atoms participates in the fast localized motion within the hexagons formed by interstitial g (Hf2 Ti2 or Zr2 Ti2 ) sites; this fraction increases with increasing temperature. The slower jump process corresponds to H motion on the nearly completely filled sublattice of interstitial e (Hf Ti3 or ZrTi3 ) sites; transitions from one e site to another imply a number of jumps over intermediate g sites. The parameters of H motion in Hf Ti2 Hx and ZrTi2 Hx are found to be close to each other. The temperature dependence of the characteristic rate d -1 of the slower jump process in Hf Ti2 H4 is well described by the Arrhenius law with the activation energy of 0.17 ± 0.02 eV.