ATOMIC REPLACEMENT AND ADATOM DIFFUSION - RE ON IR SURFACES

The behavior of vapor-deposited Re adatoms on Ir surfaces has been studied with the field ion microscope (FIM). On the Ir{113} surface, diffusion of Re adatoms occurs by atomic hopping along the surface channels above approximately 250 K. On the Ir{001} and {110} surfaces, atomic replacement of Re adatoms with substrate atoms will occur around 300 and 250 K, respectively; thus, diffusion of Re adatoms will not occur. Instead, the atomic replacement will induce self-diffusion of these Ir surfaces. On the Ir{110} surface, an atomic replacement takes one step, whereas on the Ir{001} surface, it takes two steps: first, an Ir atom is displaced out of the substrate to form a Re-Ir dimer-vacancy complex near 240 K. Around this temperature, the dimer can also change its orientation by 90-degrees without moving to a new site. Above 280 K, the complex can dissociate. Upon dissociation, the Re atom will incorporate into the substrate lattice and diffusion of the Ir adatom will occur. These elementary atomic steps can be seen directly in the FIM and the activation energies of these steps have also been measured. For heterosystems, this low-temperature atomic replacement is a mechanism of single atomic site alloying of the top surface layer. It will also induce self-diffusion of the substrate. It can already occur at or below room temperature for refractory metals.