Long jumps in diffusion of iridium on W(110)

Diffusion of single iridium atoms has been examined on W(110) using the field ion microscope to measure mean-square displacements as well as the distribution of displacements. The Arrhenius plot of the diffusivity, obtained from the mean-square displacements, against the reciprocal temperature gives a straight line without any indication of special behavior. From the distribution of displacements at temperatures of 331 K or lower, we find that for these conditions nearest-neighbor single jumps dominate; longer transitions are negligible. As the temperature is increased, however, the contribution of long jumps becomes more important, and at 371 K is preeminent. Both the activation energies and prefactors of the long jumps are found to be significantly higher than for nearest-neighbor transitions. The energetics of vertical and horizontal jumps are the same, unlike earlier results for the diffusion of tungsten on W(110). At elevated temperatures, the rate of nearest-neighbor jumps deviates significantly from the Arrhenius plot, suggesting that for iridium all the jump processes originate from nearest-neighbor transitions. At temperatures higher than studied here, at T similar to 400 K, single jumps are expected to disappear, to be replaced by longer transitions.