By measuring 2-dimensional displacement distributions during the random-walk diffusion of Ir atoms on the (001) surface, it was found that the adatom jumped along - [001]-type directions instead of along the smoother close-packed [110]-type atomic channel directions. The sites which were visited by a diffusing adatom therefore formed a c(2 x 2) net on the substrate lattice. The displacement distributions which were measured at various temperatures agreed with theoretical distributions for discrete nearest-neighbor random walks on the c(2 x 2) lattice. It was suggested that this diffusion probably occurred via atomic replacement of the adatom by a substrate atom. On the (110) surface, an Ir adatom could move along the [110] surface channel direction as well as across these channels. The latter displacements were known to involve an atomic replacement mechanism. The visited-site lattice agreed with the 1 x 1 net of the substrate. On the basis of the displacement distributions, it was concluded that about 80% of the cross-channel jumps were in [112]-type directions and about 20% were in [001]-type directions. The low-temperature displacement distributions agreed best with Monte Carlo distributions for a discrete nearest-neighbor random walk. However, at about 260K a significant number of the along-channel jumps extended beyond the nearest-neighbor distance. The energies which were required for these atomic replacements on the (001) and (110) surfaces were 0.84 and 0.71eV, respectively. There was also a close correlation between atomic replacement self-diffusion and surface reconstruction.

T.T.Tsong, C.Chen: Physical Review B, 1991, 43[3], 2007-17