DILUTION AND CLUSTERING IN MODEL CLUSTERS OF SMALL BIMETALLIC RUGE AND RUSN AGGREGATES - A DENSITY-FUNCTIONAL STUDY

Density Functional Calculations (DFT) are reported for 13 atom model c lusters of RuSn and RuGe bimetallic particles with cubooctahedral stru cture. The purpose of this study is to analyze the major factors which govern the dilution effect for Sn or Ge, which is one of the two aspe cts of their topological segregation on the Ru surface, the other one being the site preference. Site preference controls the location of th e atoms of the second element at low- or high-coordination sites, wher eas the dilution effect is governed by their capacity to make preferen tially homonuclear or heteronuclear bonds. This study has shown that i sland formation is controlled by a preparation step which defines the most stable facet. In the case of Ge-3 and Sn-3 clusters, a (111) face t is less favorable than a (100) facet and the energetics preference f or the (100) geometry is far more pronounced for Ge than for Sn. Combi ned with preparation, stabilizing Ru-M bonds depends on the electronic interactions between both metal elements and on the morphologies of t he island and of the particle. For the cubooctahedral model we have ch osen, the Ru-M interactions favor the (111) island for Ge and the (100 ) island for Sn. Finally, island formation with respect to a diluted t opology must also be discussed in terms of the binding energy between M atoms. The stabilization induced by M-M bonding is shown to be the m ost favorable for Ge-3 (100).