STABILITY OF SMALL IRIDIUM CLUSTERS NEAR THE IR(111) SURFACE

Iridium atoms deposited on the Ir(111) surface associate to form diffe rent cluster configurations (isolated adatoms, dimers, trimers...). In order to understand the various arrangements observed at the surface, we have computed the effective interactions of small clusters adsorbe d on Ir(111). Our model is an extension of the DCA (direct configurati onal averaging) methods. We work in a tight binding framework using th e recursion method and the ''orbital peeling'' scheme to compute direc tly the energies from the electronic structure. Both ''bulk'' sites (i .e. which continue the fee lattice) and ''surface'' sites (i.e. which sit in an hcp arrangement and create a fault plane) are considered. Fo llowing our results, the effective interaction energy of first neighbo ur pairs is the most attractive, favouring compact clusters formation at the surface. Concerning the trimers we show that the compact triang le (2D) is more stable than the linear chain (1D) and than the triangl e formed by second neighbor impurity pairs. Our conclusions are in goo d agreement with the available experimental data.