SITE SEGREGATION IN MODEL CLUSTERS OF SMALL BIMETALLIC RUGE AND RUSN AGGREGATES

The results of density functional calculations are reported for RuGe a nd RuSn model clusters, with Sn and Ge atoms located either at a high coordination site (central location) or at a low coordination site (co rner location). The model clusters have been first studied at a fixed bulk geometry. Then, in order to simulate small bimetallic particles, their structures have been allowed to relax. The relative stabilities of the model clusters support the conclusions that the most stable agg regates are those where Sn and Ge are located at low coordination site s. However, when relaxation occurs, the clusters distort in order to a ccommodate Sn or Ge at the center of the first layer. This distorsion induces a reduction of the coordination of this central atom. The rela xation effects stabilize the RuGe particle, with Ge at both sites, lea ding to only a small preference for the corner site. At the opposite, the distorsion necessary to accommodate a central Sn atom costs too la rge amount of energy, which leads to a large preference for Sn to segr egate at a corner site. Quantum chemical calculations are shown to be very helpful in order to quantify the cluster distorsions and the site preference effects when Ru is replaced by Sn or Ge.