NMR studies of hydrogen diffusion in the dihydrides of hafnium

Abstract The macroscopic diffusivity D and the spin-lattice relaxation rate Γ 1 of hydrogen in hafnium dihydrides, HfH x (1.71≤ x ≤1.94), were measured at temperatures up to 770 K. The activation enthalpy for hydrogen diffusion, obtained by fitting an Arrhenius expression to the diffusivities measured by PFG-NMR, increases slightly with increasing hydrogen concentration from H a =0.63 eV ( x =1.71) to H a =0.71 eV ( x =1.94). These H a values agree satisfactorily with those deduced from the dipolar spin-lattice relaxation Γ 1,dip . A combined analysis of both Γ 1 and PFG measurements shows that hydrogen atoms jump predominantly from occupied tetrahedral sites (T sites) to nearest-neighbour vacant T sites. The results for HfH x are compared with those for TiH x and ZrH x , the dihydrides of the other Group IVb transition metals.