Tracer and chemical diffusion of hydrogen in BCC metals

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Abstract
Quantities relevant to the chemical and tracer diffusion coefficients, namely the tracer correlation factor, the mobility correlation factor and the site-availability factor are calculated for an interacting lattice gas model using a Monte Carlo method. The origin and nature of the mobility correlation factor is discussed. Calculations are performed for diffusion by nearest-neighbour hopping between the tetrahedral sites in a BCC host, for each of four different site-blocking models (no double occupancy, multiple blocking to first, second or third neighbour). Comparisons are made with previous experimental work on NbHx and TaHx where it is found that for x<0.25 the measured correlations are greater than those calculated for any model, which is consistent with the presence of additional long range attractive interactions known to be important in this region; for 0.25<x0.6 the atoms are considerably more mobile than predicted by the third-neighbour blocking model. This is to be expected because at these concentrations the assumption of complete site exclusion is unrealistic.