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Hydrogen diffusion; Hafnium dihydrides; Nuclear magnetic resonance
Abstract:
The macroscopic diffusivity D and the spin-lattice relaxation rate Γ1 of hydrogen in hafnium dihydrides, HfHx (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 Ha=0.63 eV (x=1.71) to Ha=0.71 eV (x=1.94). These Ha values agree satisfactorily with those deduced from the dipolar spin-lattice relaxation Γ1,dip. A combined analysis of both Γ1 and PFG measurements shows that hydrogen atoms jump predominantly from occupied tetrahedral sites (T sites) to nearest-neighbour vacant T sites. The results for HfHx are compared with those for TiHx and ZrHx, the dihydrides of the other Group IVb transition metals.
