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 f ield gradient nuclear magnetic resonance technique. The diffusivity of hydrogen in LaNi4.8Sn0.2H5.8 is characterized by a higher mobility an d a lower activation enthalpy than observed in LaNi5.0H6.0. The diffus ivities 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, respective ly. 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 re laxation rates Gamma(1) and Gamma(1 rho) were measured on the same sam ples in the temperature range between 100 and 350 K. Tl-le D values an d the relaxation data are found to be consistent with respect to the a ctivation enthalpies. For long-range diffusion in both alloys. The hop ping rates extracted from the Gamma(1 rho) maxima correspond to long-r ange diffusion, and both sets of measurements indicate faster motion f or the Sn-containing hydride.