Quasielastic neutron scattering study of hydrogen motion in C15-type HfMo2H0.26

In order to evaluate the parameters of H motion in the cubic (C15-type) Lav es phase HfMo2, we have performed quasielastic neutron scattering measureme nts on HfMo2H0.26 in the temperature range 15-390 K. The results are consis tent with the fast localized I-I motion within the hexagons formed by inter stitial g(Hf2Mo2) sites. The slower jump process corresponding to H hopping from one hexagon to another has been probed by recent proton NMR measureme nts [A. V. Skripov et al., J. Phys.: Condens. Matter 11, 10 393 (1999)]. Co mparison of the NMR and the quasielastic neutron scattering results shows t hat the ratio of the characteristic hopping rates for the two jump processe s, tau (d)/taul, is 2.4X 10(3) at 300 K; this ratio increases with decreasi ng temperature. For the series of CIS-type AB(2) compounds TaV2-HfMo2-ZrMo2 -ZrCr2, it is found that the increase in the ratio of the metallic radii of the elements A and B, R-A/R-B, leads to the decrease in tau (d)/tau (l).