Nuclear magnetic resonance studies of hydrogen diffusion in LaNi5.0H6.0 and LaNi4.8Sn0.2H5.8

Diffusion measurements of hydrogen in LaNi5.0H6.0 and LaNi4.8Sn0.2H5.8 were carried out between 270 and 365 K using the alternating pulsed field gradient nuclear magnetic resonance technique. The diffusivity of hydrogen in LaNi4.8Sn0.2H5.8 is characterized by a higher mobility and a lower activation enthalpy than observed in LaNi5.0H6.0. The diffusivities at room-temperature, D(300K), are 9.2x10-(12) m(2) s(-1) and 4.6x10(-12) m(2) s(-1) for LaNi4.8Sn0.2H5.8 and LaNi5.0H6.0, respectively. A fit of an Arrhenius expression, D =D(0)exp(-H(a)lk(B)T), to the diffusivities yields an activation enthalpy of H-a = 0.29 eV for LaNi5.0H6.0 and H-a = 0.22 eV for LaNi4.8Sn0.2H5.8. The NMR spin-lattice relaxation rates Gamma(1) and Gamma(1 rho) were measured on the same samples in the temperature range between 100 and 350 K. Tl-le D values and the relaxation data are found to be consistent with respect to the activation enthalpies. For long-range diffusion in both alloys. The hopping rates extracted from the Gamma(1 rho) maxima correspond to long-range diffusion, and both sets of measurements indicate faster motion for the Sn-containing hydride.