Molecular-dynamics investigation of hydrogen hopping in palladium

Diffusive hopping of hydrogen in ${\mathrm{PdH}}_{\mathrm{x}}$ has been reexamined with molecular-dynamics (MD) simulations in order to clarify earlier results of Gillan. We obtain the same values for the diffusion coefficient and width ${\Gamma }_q$ of the quasielastic neutron scattering function when Gillan’s potentials are used, but go beyond Gillan in performing a statistical analysis of the jumps. The analysis shows that over 95% of the jumps are uncorrelated and nearest neighbor, but the distribution of times between jumps is highly nonexponential. This latter feature accounts for the departure of ${\Gamma }_q$, noted by Gillan, from the Chudley-Elliott (CE) theory based on simple one-step nearest-neighbor hopping. A simple physical model of the hopping as a two-step process can explain the results and suggests they are highly sensitive to the H-Pd potential at short distances. A new MD simulation with a harder-core potential gives a ${\Gamma }_q$ in much better agreement with CE. We conclude that the non-CE behavior originally found by Gillan might be seen in real experiments on systems with suitably soft potentials.