ORBITAL DISPROPORTIONATION OF CONDUCTION-ELECTRON DENSITY IN CUBIC LATTICES WITH THREEFOLD DEGENERATE SITE ORBITALS

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