Solid state electronic structure of Ba3C60 - A model approach

The band structure of Ba3C60 has been modeled by a crystal orbital (GO) formalism of the intermediate neglect of differential overlap (INDO) type. The fulleride crystallizes in a novel A15 structure with three crystallographically non-equivalent alkaline-earth atoms in the unit cell. The symmetry of the space group is <Pm(3)over bar n>. To define the underlying unit cell for the CO calculations, two formula units are necessary. A finite band gap is predicted for the fulleride system, a theoretical result which is in line with the resistivity maximum observed in alkaline-earth fullerides with three donor atoms per C-60 soccerball. The alkaline-earth atoms are essentially monovalent and hybridize strongly with the pi-type functions of the C-60 network. The Ba-to-C-60 charge transfer (CT) is responsible for an almost perfect alignment between formal CC ''double'' and ''single'' bonds. The nature of the Ba-C interaction is discussed on the basis of covalent and ionic two-center energies.