STRUCTURAL STABILITY OF ADATOM ISLANDS ON FCC(111) TRANSITION-METAL SURFACES

The energetics of monomer; dimer, and triangular trimer adatom islands on the (111) surface of fee transition metals of the same chemical sp ecies is calculated in the tight-binding scheme as a function of the d band filling. Both the adatom and their nearest neighbor substrate at oms are allowed to relax. We investigate all possible atomic configura tions of the system arising from the existence of (a) two types of ads orption sites: normal (fee) and fault (hcp) sites, and (b) both types of borders that may exist for triangles of adatoms: A, having (001), ( 010), and (100) facets and B, having (11(1) over bar), (1(1) over bar1 $), and (<(1)over bar 11>) facets. It is found that there is an invers ion of relative stability from fault to normal sites when the d band f illing is larger than 8.2d electrons per atom for monomers, 7.85 for d imers, 7.5 for trimers of type A, and 7.6 for trimers of type B. There is also an inversion of stability for trimers from type B to type A w hen the d band filling is larger than 7.95. All these results an in ve ry good agreement with experiments on Ir for which the d band filling is similar or equal to 7.5--7.6: monomers and dimers prefer to stick a t fault sites while trimers settle at both sites. Furthermore, triangl es of type B are energetically more favored than triangles of type A. The case of other transition metals is also discussed.