In order to clarify the mechanisms of hydrogen diffusion in the cubic (C15)
and hexagonal (C14) modifications of Laves phase ZrCr2, we have performed
high-resolution quasielastic neutron scattering measurements on C15-ZrCr2H0
.45 and C14-ZrCr2H0.5 over the temperature range 10-340 K. It is found that
in both systems the diffusive motion of hydrogen can be described in terms
of two jump processes: the fast localized H motion within the hexagons for
med by interstitial (Zr2Cr2) sites and the slower hopping from one hexagon
to the other. The experimental results are analysed to determine the hydrog
en hopping rates and the tracer diffusion coefficients as functions of temp
erature. The motional parameters of hydrogen in the C15- and C14-type sampl
es are found to be close to each other. Comparison of the motional paramete
rs of hydrogen in ZrCr2Hx and in the related C15-type TaV2Hx shows that the
localized H motion in ZrCr2 is slower, whereas the long-range H diffusion
is much faster than in TaV2. These features are consistent with the differe
nce between intersite distances in the hydrogen sublattices of ZrCr2Hx and
TaV2Hx.