Monte Carlo simulations of hydrogen diffusion in hydrogen loaded cubic Laves-phases (AB2Hx) were performed in order to express the tracer diffusivity DT and the dipolar contribution Γ1ip to the proton spin-lattice relaxation rate in terms of the parameters of different microscopic diffusion models. The discussed diffusion models were based on the assumption that hydrogen exclusively occupies so-called g sites (A2B2 tetrahedral interstices). The following hopping processes were included: 1) Jumps from a g site to its three nearest-neighbor g sites. Two of these jumps leading through equivalent saddle points were described by a hopping rate vgg2, the third by a hopping rate vgg1. 2) Two equivalent jumps to second nearest-neighbor g sites via an intermediate nearest neighbor e site (AB3 tetrahedral interstice), described by a hopping rate vgeg. Mainly discussed were two special cases of this model in which either vgg1 or vgeg was set equal to zero. Setting both, vgg1 and vgeg equal to zero results in local motion on closed paths and vanishing long-range diffusion. In order to account for correlations between subsequent hydrogen jumps a geometrical correlation factor for a particle diffusing on an empty lattice of g sites was introduced and determined as a function of the ratios between hopping rates. Additional correlation effects at finite hydrogen concentration due to blocking of sites by adjacent hydrogen atoms were described by a further multiplicative correlation factor which was found to be essentially independent of the ratios between the hopping rates. The results of the simulations may be used to obtain relations between DT and Γ1ip . In particular DT(Tmax) where Tmax was the temperature at which Γ1ip had its maximum, was calculated for different diffusion models and compared with experimental results obtained on ZrCr2H0.2. According to this preliminary analysis both special cases of the model (vgeg = 0 or vgg1 = 0) were compatible with experiment, if the possibility that hydrogen blocked its nearest-neighbour g sites was taken account of.

Monte Carlo Simulation of Hydrogen Diffusion in Cubic Laves-Phases: Correlation Effects and Nuclear Spin-Lattice Relaxation Rate of Proton Spins. A.Herrmann, L.Schimmele, G.Majer, A.Seeger: Defect and Diffusion Forum, 1997, 143-147, 963-70