Displacement distributions in diffusion by atomic replacement: Ir atoms on Ir surfaces

From a measurement of two-dimensional displacement distributions in random-walk diffusion of Ir atoms on the Ir(001) surface, we find that the adatom jumps along the 〈001〉 direction instead of along the smoother closely packed 〈110〉 atomic-channel direction. The sites visited by one diffusing adatom therefore form a c(2×{}2) net of the substrate lattice. The displacement distributions measured at various temperatures agree with theoretical distributions for the discrete nearest-neighbor random walk on the c(2×{}2) lattice. This diffusion most probably occurs by an atomic-replacement mechanism of the adatom with a substrate atom. On the Ir(110) surface, an Ir adatom can displace along the [110] surface channel direction as well as across these channels. The latter displacements are known to occur by an atomic-replacement mechanism. The visited-site lattice agrees with the (1×{}1) net of the substrate. From the displacement distributions we conclude that {~}80% of the cross-channel jumps are in the 〈112〉 direction and {~}20% are in the 〈001〉 direction. The low-temperature displacement distributions agree best with Monte Carlo distributions for the discrete nearest-neighbor random walk, but at {~}260 K a significant number of the along-channel jumps extend beyond the nearest-neighbor distance. The energies needed for these atomic replacements on the (001) and (110) surfaces are found to be 0.84±{}0.05 and 0.71±{}0.02 eV, respectively. We also find a close correlation between atomic-relpacement self-diffusions and surface reconstructions.