Abstract
Measurements of the hydrogen diffusivity in thehydrogen-stabilized Laves phase compound C15-Hf Ti2Hx(3.9⩽x⩽4.2) have been performed by means of thepulsed-field-gradient nuclear magnetic resonance (PFG-NMR)between 220 and 500 K. The activation enthalpies for hydrogendiffusion obtained by fitting an Arrhenius expression to thediffusivities measured at a given concentration x areHa = 240 meV (x = 3.9), 210 meV (x = 4.0) and 235 meV (x = 4.2).The proton spin-lattice relaxation rates Γ1have been measured on the same samples over wide temperatureranges and at different resonance frequencies. Above about250 K, the Γ1 data are well represented by arelaxation model for jumps between tetrahedral [Hf Ti3] sites.A comparison with the PFG results shows that these jumpscorrespond to long-range diffusion. At lower temperatures, theΓ1 data indicate localized motion ofhydrogen, presumably within hexagons formed by six tetrahedral[Hf2Ti2] sites, which has no effect on the PFGmeasurements. Similar diffusion mechanisms have been observedpreviously in C15-ZrTi2Hx (x≈4).
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