Abstract
The proton spin-lattice relaxation rate Γ 1 in ZrClH 0.5 shows one maximum and two shoulders, which indicates three distinct modes of hydrogen motion. The frequency dependence of the spin-lattice relaxation rate in the rotating frame, Γ 1,ρ, shows clearly that hydrogen diffusion is confined to two dimensions. Direct measurements of the long-range diffusivity D xy were performed by pulsed field gradient (PFG) spin-echo NMR. These PFG experiments confirmed the two-dimensional character of hydrogen diffusion. The temperature dependence of D xy yields an activation enthalpy, H a=0.49 eV, which is in good agreement with the value H a=0.47 eV deduced from the frequency-dependent Γ 1,ρ data. The different processes of hydrogen motion are discussed on an atomistic scale. The Γ 1 and Γ 1,ρ curves measured in ZrClH 1.0 are consistent with a single mechanism of hydrogen diffusion. Both the relaxation data and the PFG data reveal a strongly reduced hydrogen mobility in ZrClH 1.0 compared to ZrClH 0.5. From the analysis of the echo amplitude in the PFG experiments it is concluded that the hydrogen diffusion is restricted to two dimensions as well. The D xy values show Arrhenius behavior with an activation enthalpy of H a=0.58 eV.
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