The embedded-atom method was used to study the structures, adsorption energies, migration paths, and energy barriers to Ir adatom and small-cluster diffusion on the face-centered cubic (100), (110) and (111) surfaces of Ir. It was found that the barrier to single-adatom diffusion was lowest for the (111) surface, higher for the (110) surface, and highest for the (100) surface. Exchange mechanisms of adatom diffusion were energetically favored on (100) and (110) surfaces. On all 3 surfaces, Ir2 dimers with the nearest-neighbor spacing were the most stable. On the (110) surface, the Ir2 dimers diffused collectively along the <110> channel, while motion perpendicular to the channel walls involved successive 1-atom and correlated jumps. On the (111) surface, the Ir2 dimer diffused in a zig-zag manner on hexagonal close-packed and face-centered cubic sites without breaking up into 2 single atoms. On the (100) surface, diffusion of the Ir2 dimer occurred via successive 1-atom exchanges with substrate atoms, together with a 90 rotation of the Ir2 dimer. This mechanism was found to require the surprisingly low activation energy of 0.65eV. This was 0.14eV lower than the energy for single adatom exchange on the (100) surface. Trimers were found to have a 1-dimensional structure on (100) and (110) surfaces, and a 2-dimensional structure on the (111) surface. An experimentally observed sudden decrease in the barrier to tetramer diffusion on the (111) surface was confirmed.

C.M.Chang, C.M.Wei, S.P.Chen: Physical Review B, 1996, 54[23], 17083-96