  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.
