Molecular-dynamics simulation of hydrogen diffusion in niobium

Molecular-dynamics simulations of H diffusion in Nb are performed for a system consisting of 432 Nb atoms and 8 H atoms at two different temperatures: T=450 and 580 K. For the interatomic interactions we use a description proposed by Finnis, Sinclair, and Gillan. We compare our results with quasielastic-neutron-scattering data and our model reproduces 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’’ are evaluated. We find that the residence-time distribution is composed of two distinct contributions; one narrow component with a short residence time of the order 35 fs, and one broad component with roughly exponential decay. The narrow component corresponds to that the H atom moves rapidly among two or more sites belonging to what has been called a 4T configuration. The typical decay time of the broad component is found to be of the order of 160 fs and 300 fs in the time intervals 60<t<300 fs and 300<t<600 fs, respectively, which should be compared with the mean residence time derived from the diffusion constant, ${\mathrm{\tau }}_{\mathrm{res}}$=${\mathit{a}}_0^2$/48${\mathit{D}}_{\mathit{s}}$=324 fs. We also find substantial contributions of second-nearest-neighboring jumps, but the division between nearest- and second-nearest-neighboring jumps is ambiguous. The diffusive and the vibrational motion of the H atom cannot be clearly separated and the time spent and the spatial excursion performed in the ‘‘jump phase’’ are not negligible.