Self-adsorption sites on a close-packed surface: Ir on Ir(111)

The binding sites for adsorption of a single iridium atom on a close-packed iridium (111) plane have been mapped out in the field ion microscope by observing the location of one iridium atom after diffusion over the surface at T{~}100 K. Both surface (hcp) and bulk (fcc) sites are found occupied, but the former are favored, with the free energy for adatoms {~}370 cal/mol lower on surface sites. This free-energy difference is small compared to the barrier of 6.2 kcal/mol which separates equivalent sites in surface diffusion.