Simulations of H diffusion in Nb were performed for a system consisting of 432 Nb atoms and 8 H atoms at 450 and 580K. A description proposed by Finnis, Sinclair and Gillan was used for the interatomic interactions. The results were compared with quasi-elastic neutron-scattering data, and the model reproduced quite well both the distinct deviation from simple jump-diffusion behavior and the 'anomalous' Debye-Waller factor. To reveal the details of the H motion, the residence-time distribution at the stable sites (T sites) as well as the correlation character among consecutive jumps were evaluated. It was found that the residence-time distribution was composed of two distinct contributions; one narrow component with a short residence time of the order 35fs, and one broad component with a roughly exponential decay. The narrow component corresponded to rapid H atom movement among two or more sites belonging to a so-called 4T configuration. The typical decay time of the broad component was found to be of the order of 160 and 300fs in the time intervals 60 < t < 300fs and 300 < t <600fs, respectively. This was compared with the mean residence time, of 324fs, derived from the diffusion constant. Substantial contributions of second-nearest neighboring jumps were also found, but the division between nearest- and second-nearest neighboring jumps was ambiguous. The diffusive and the vibrational motion of the H atom could not be clearly separated and the time spent and the spatial excursion performed in the jump phase were not negligible.

Molecular-Dynamics Simulation of Hydrogen Diffusion in Niobium. Li, Y., Wahnström, G.:  Physical Review B, 1995, 51[18], 12233-45