Orbital disproportionation of conduction-electron density in cubic lattices with threefold degenerate site orbitals

The instability of a high-symmetry distribution of conduction electrons in t-type orbitals in cubic lattices is investigated within the mean-field approach. All essential bielectronic and one-site vibronic interactions involving the threefold degenerate orbitals are taken into account. It is shown that the degeneracy of the site orbitals may give rise to a special kind of electronic instability, described as orbital disproportionation of the conduction-electron density at each site. The criterion for instability of the cubic band structure with respect to a homogeneous disproportionation, including both spin- and charge-density redistributions, is established. Low-symmetry nuclear distortions may reinforce the tendency for disproportionation of charge density between t orbitals. A model calculation suggests that electronic charge disproportionation may be responsible for the unusual band structure of allcali-metal-doped fullerides.