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Abstract

We have investigated the electronic structure of Ba-doped C₆₀ films with Ba concentrations of up to x≈12 (Ba_xC₆₀) by applying valence-band photoemission and x-ray-absorption spectroscopy. A crystal orbital (CO) formalism based on a semiempirical Hamiltonian of the intermediate-neglect-of-differential-overlap type has been employed to derive solid-state results for the Ba-doped C₆₀ fullerides. Using x-ray diffraction, we show three distinct phases for the bulk Ba_xC₆₀ system with Ba concentrations of up to x=6. In all cases, the experimental observations strongly indicate that fulleride formation leads to the occupation of hybrid bands on both sides of the Fermi level. The theoretical data indicate that the alkaline-earth atoms are essentially monovalent and hybridize strongly with the π-type functions of the C₆₀ network. The Ba atoms in the Ba_xC₆₀ fullerides deviate from the limit of complete charge transfer as a consequence of the competition between covalent Ba-C₆₀ bonding and ionic contributions. Furthermore, it is shown that the calculated density-of-state profiles reproduce the photoemission data in the extreme outer valence-band region.