ON THE ELECTRONIC AND GEOMETRIC STRUCTURE OF BIMETALLIC CLUSTERS - A COMPARISON OF THE NOVEL CLUSTER NA6PB TO NA6MG

Density functional studies of the abundant cluster Na6Pb and of its an alogue Na6Mg are reported. The structure of Na6Pb has been optimized f or a series of symmetry constraints (O-h, D-3d, D-3h, C-5v, C-3v and C -2v). The resulting binding energies fall within a narrow range of les s than 0.1 eV whereas a spread of more than 0.5 eV is calculated for N a6Mg. These findings indicate a high structural flexibility of Na6Pb. The Pb atom exhibits a propensity to occupy a highly coordinated site in contrast to Mg which, in the most stable structures, is attached to the 'outside' of a Na-6 moiety. Analysis of the bonding mechanism rev ealed two major contributions which increase the atomization energy of Na6Pb compared to Na6Mg: an enhanced charge transfer from the Na-6 su bsystem and a stronger polarization of the Pb atom. A significant cont ribution to the overall cluster stability comes from the interaction b etween the alkali atoms.