Energetics and electronic structure of carbon doped aluminum clusters

The energetics and the electronic structure of AlnC clusters (n=3, 4, 5; 11 , 12, 13) have been studied by a global optimization of their geometry with out any symmetry constraint. The total energies of these clusters both in n eutral and charged states are calculated using an all-electron basis and th e generalized gradient approximation to the density functional theory. Whil e Al4C and Al12C clusters share some characteristic features of closed shel l structures, namely enhanced stability and low electron affinity compared to their neighboring sizes, their ionization potentials exhibit different b ehavior. These decrease steadily from Al3C to Al5C while that of Al12C is h igher than its neighbors. Carbon is found to form planar structures in smal l AlnC clusters (n=3, 4, 5) irrespective of their charge state although neu tral Al4C possesses a nearly degenerate tetrahedral isomer lying slightly h igher in energy from the planar configuration. The results agree well with experimental and previous theoretical data. In larger AlnC (n=11, 12, 13) c lusters, carbon occupies an interior site. In Al12C, carbon occupies the ce nter of an icosahedron while it is off-centered in Al11C and Al13C. As an e lectron is attached, the near degeneracies of the neutral Al4C is lifted wh ereas nondegenerate isomers of neutral Al12C yield nearly degenerate anions . Both these features produce complicated photoelectron spectra making iden tification of their adiabatic electron affinity a difficult problem. With t he exception of neutral Al12C, the bonding of carbon to aluminum atoms is g overned primarily by covalent interaction. The above calculations were also performed with a simplified basis by freezing the atomic cores of aluminum . In most cases, this simple basis yields results in good agreement with al l electron calculations. (C) 2001 American Institute of Physics.