NMR studies of hydrogen diffusion in the hydrogen-stabilized Laves phase compound C15-Hf Ti2Hx

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Abstract
Measurements of the hydrogen diffusivity in the hydrogen-stabilized Laves phase compound C15-Hf Ti2Hx (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 Ha = 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-ZrTi2Hx (x≈4).