Measurements were made of the temperature and azimuthal dependence of the diffusion rates of H and D on Ni (111) at 65 to 240K. The diffusion rates varied by 8 orders of magnitude. The diffusion of H crossed over from a type of activated behavior, at temperatures above 110K, to another type of activated behavior below 110K; with a lower activation energy. After ruling out the possibility of a step effect, it was suggested that the cross-over occurred from classical over-barrier hopping to quantum-mechanical under-barrier tunnelling.

Diffusion of Hydrogen on Ni(111) over a Wide Range of Temperature: Exploring Quantum Diffusion on Metals. G.X.Cao, E.Nabighian, X.D.Zhu: Physical Review Letters, 1997, 79[19], 3696-9

Figure 17

Diffusivity of H in Ni

(Cathodic current density. Circles: 10mA/cm2,

filled squares: 30mA/cm2, open squares: 50mA/cm2)