Hydrogen motion in C15-type ZrMo2Hx: Quasielastic neutron scattering and nuclear-magnetic-resonance studies

In order to determine the parameters of H motion in the cubic (C15-type) La ves phase ZrMo2, we have performed quasielastic neutron scattering measurem ents and nuclear-magnetic-resonance measurements of the proton spin-lattice relaxation rate on ZrMo2Hx(0.3 less than or equal to x less than or equal to 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 l ocalized H motion within the hexagons formed by interstitial g (Zr2Mo2) sit es and the slower hopping from one hexagon to the other. The hopping rate t au(l)(-1) of the faster process is found to be of the order of 10(10) s(-1) at 300 K. The characteristic frequency tau(d)(-1) Of the slower process at the same temperature is about 6 x 10(7) s(-1). Comparison of the motional parameters of hydrogen in ZrMo2Hx with those in the related C15-type system s TaV2Hx and ZrCr2Hx shows that the value of tau(d)/tau(l) for ZrMo2Hx lies between the corresponding values for TaV2Hx and ZrCr2Hx. This is consisten t with the changes in the ratio of g-g distances r(1) (within the hexagons) and r(2) (between different hexagons). [S0163-1829(99)00433-6].