NMR studies of hydrogen diffusion in the hydrogen-stabilized Laves phase compound C15-HfTi2Hx

Measurements of the hydrogen diffusivity in the hydrogen-stabilized Laves phase compound C15-HfTi2Hx (3.9 less than or equal to x less than or equal to 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 Gamma(1) have been measured on the same samples over wide temperature ranges and at different resonance frequencies. Above about 250 K, the Gamma(1) data are well represented by a relaxation model for jumps between tetrahedral [HfTi3] sites. A comparison with the PFG results shows that these jumps correspond to long-range diffusion. At lower temperatures, the Gamma(1) 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 approximate to 4).