M. Stoll et al., NUCLEAR-MAGNETIC-RESONANCE STUDIES OF HYDROGEN DIFFUSION IN THE LAYER-STRUCTURED SYSTEM ZRCLHX, Journal of alloys and compounds, 253, 1997, pp. 435-440
The proton spin-lattice relaxation rate Gamma(1) in ZrClH0.5 shows one
maximum and two shoulders, which indicates three distinct modes of hy
drogen motion. The frequency dependence of the spin-lattice relaxation
rate in the rotating frame, Gamma(1,rho), shows clearly that hydrogen
diffusion is confined to two dimensions. Direct measurements of the l
ong-range diffusivity D-xy were performed by pulsed field gradient (PF
G) spin-echo NMR. These PFG experiments confirmed the two-dimensional
character of hydrogen diffusion. The temperature dependence of D-xy yi
elds an activation enthalpy, H-a=0.49 eV, which is in good agreement w
ith the value H-a=0.47 eV deduced from the frequency-dependent Gamma(1
,rho) data. The different processes of hydrogen motion are discussed o
n an atomistic scale. The Gamma(1) and Gamma(1,rho) curves measured in
ZrClH1.0 are consistent with a single mechanism of hydrogen diffusion
. Both the relaxation data and the PFG data reveal a strongly reduced
hydrogen mobility in ZrClH1.0 compared to ZrClH0.5. From the analysis
of the echo amplitude in the PFC experiments it is concluded that the
hydrogen diffusion is restricted to two dimensions as well. The D-xy v
alues show Arrhenius behavior with an activation enthalpy of H-a=0.58
eV.