Lattice-gas model for alkali-metal fullerides: body-centered-cubic structure

A Coulomb lattice-gas model with a host lattice screening mechanism is adapted to describe the ordering phenomena in alkali-metal fullerides of body-centered-cubic structure. It is assumed that the electric charge of an alkali ion residing at a tetrahedral interstitial site is completely screened by its first-neighbor C$_{60}$ molecules. The electronic energy of the C$_{60}^{x-}$ ion is also taken into consideration as a charged spherical shell. By means of these assumptions an effective (short range) pair interaction is obtained between two alkali ions. The resultant lattice-gas model is analyzed by using two- and six-sublattice mean-field approximations. The thermodynamic properties are summarized in phase diagrams for different shell radii.