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 analogue Na6Mg are reported. The structure of Na6Pb has been optimized for a series of symmetry constraints (Oh, D3d, D3h, C5v, C3v and C2v). The resulting binding energies fall within a narrow range of less than 0.1 eV whereas a spread of more than 0.5 eV is calculated for Na6Mg. 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 Na6 moiety. Analysis of the bonding mechanism revealed two major contributions which increase the atomization energy of Na6Pb compared to Na6Mg: an enhanced charge transfer from the Na6 subsystem and a stronger polarization of the Pb atom. A significant contribution to the overall cluster stability comes from the interaction between the alkali atoms