The behavior of vapor-deposited Re adatoms upon Ir surfaces was studied by means of field-ion microscopy. On {113} surfaces, the diffusion of Re adatoms occurred via atomic hopping along the surface channels at temperatures above about 250K. On {001} and {110} surfaces, atomic replacement of Re adatoms by substrate atoms occurred at about 300 and 250K, respectively. Thus, the diffusion of Re adatoms would not occur. Instead, atomic replacement would induce self-diffusion on these surfaces. On {110} surfaces, atomic replacement required one step whereas, on {001} surfaces, it required 2 steps. Firstly, an Ir atom was displaced out of the substrate so as to form a Re-Ir dimer-vacancy complex near to 240K. At around this temperature, the dimer could also change its orientation by 90° without moving to a new site. At temperatures above 280K, the complex could dissociate. Upon dissociation, the Re atom was incorporated into the substrate lattice, and diffusion of the Ir adatom could occur. These elementary atomic steps could be seen directly in the field-ion microscope, and the activation energies of the steps were also measured. In heterosystems, this low-temperature atomic replacement involved a mechanism of single atomic site alloying of the top surface layer. It could also induce self-diffusion of the substrate. It could occur at, or below, room temperature in the case of refractory metals.

C.Chen, T.T.Tsong, L.Zhang, Z.Yu: Physical Review B, 1992, 46[12], 7803-7