NMR studies of hydrogen diffusion in the hydrogen-stabilized Laves phase compound C15-Hf Ti₂H_x

Measurements of the hydrogen diffusivity in the hydrogen-stabilized Laves phase compound C15-Hf Ti₂H_x (3.9⩽x⩽4.2) have been performed by means of the pulsed-field-gradient nuclear magnetic resonance (PFG-NMR) between 220 and 500 K. The activation enthalpies for hydrogen diffusion obtained by fitting an Arrhenius expression to the diffusivities measured at a given concentration x are H_a = 240 meV (x = 3.9), 210 meV (x = 4.0) and 235 meV (x = 4.2). The proton spin-lattice relaxation rates Γ₁ have been measured on the same samples over wide temperature ranges and at different resonance frequencies. Above about 250 K, the Γ₁ data are well represented by a relaxation model for jumps between tetrahedral [Hf Ti₃] sites. A comparison with the PFG results shows that these jumps correspond to long-range diffusion. At lower temperatures, the Γ₁ data indicate localized motion of hydrogen, presumably within hexagons formed by six tetrahedral [Hf₂Ti₂] sites, which has no effect on the PFG measurements. Similar diffusion mechanisms have been observed previously in C15-ZrTi₂H_x (x≈4).