A NEUTRON-SPECTROSCOPY STUDY OF 2-DIMENSIONAL HYDROGEN DIFFUSION IN THE HYDRIDE HALIDE YBRH0.78

The self-diffusion of H in the substoichiometric rare-earth hydride ha lide YBrH0.78 was investigated by incoherent quasielastic neutron scat tering. YBrH0.78 exhibits a layered structure in which the H atoms occ upy tetrahedral interstitial sites located in Y bilayers, which are sa ndwiched by Br bilayers. As this arrangement suppresses H transfer bet ween different Y bilayers, the present study investigates an essential ly two-dimensional H diffusion within a given Y bilayer. The self-diff usion coefficient of the H was determined to be between 750 and 900 K. At 900 K, its value is D = (1.2 +/- 0.4) X 10(-6) cm2 S-1. The temper ature dependence can be described by an Arrhenius relation with an act ivation energy E = (0.48 +/- 0.12) eV. The momentum-transfer dependenc e of the measured spectra suggests the presence of noticeable correlat ion effects in the H diffusion.