Hydrogen motion in C15-type ZrMo₂H_x: Quasielastic neutron scattering and nuclear-magnetic-resonance studies

In order to determine the parameters of H motion in the cubic (C15-type) Laves phase ZrMo₂, we have performed quasielastic neutron scattering measurements and nuclear-magnetic-resonance measurements of the proton spin-lattice relaxation rate on ZrMo₂H_x (0.3 < x < 1.0) in the temperature range 10–430 K. The diffusive motion of hydrogen in this system can be described in terms of two jump processes: the fast localized H motion within the hexagons formed by interstitial g (Zr₂Mo₂) sites and the slower hopping from one hexagon to the other. The hopping rate τ^-1_l of the faster process is found to be of the order of 10¹⁰s^-1 at 300 K. The characteristic frequency τ^-1_d of the slower process at the same temperature is about 6 × 10⁷s^-1. Comparison of the motional parameters of hydrogen in ZrMo₂H_x with those in the related C15-type systems TaV₂H_x and ZrCr₂H_x shows that the value of τ_d/τ_l for ZrMo₂H_x lies between the corresponding values for TaV₂H_x and ZrCr₂H_x. This is consistent with the changes in the ratio of g-g distances r₁ (within the hexagons) and r₂ (between different hexagons).