A LATTICE-GAS MODEL FOR ALKALI-METAL FULLERIDES - BODY-CENTERED-CUBICSTRUCTURE

A Coulomb lattice-gas model with a host-lattice screening mechanism is adapted to describe the ordering phenomena in alkali-metal fullerides of body-centred-cubic structure. It is assumed that the electric char ge of an alkali ion residing at a tetrahedral interstitial site is com pletely screened by its first-neighbour C-60 molecules. The electronic energy of the C-60(x-) ion is also taken into consideration as a char ged spherical shell. By means of these assumptions an effective (short -range) pair interaction between two alkali ions is obtained. The resu ltant lattice-gas model is analysed by using two-and six-sublattice me an-held approximations. The thermodynamic properties are summarized in phase diagrams for different shell radii.