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 (O_h, D_3d, D_3h, C_5v, C_3v and C_2v). 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.