Lattice parameters of alkali-metal-doped C₆₀ fullerides

Lattice constants of alkali-metal-doped A₃C₆₀ fullerides with face-centered cubic crystal structure are reported for a wide range of lattice parameters, using Li, Na, K, Rb, Cs and their binary mixtures. It is shown that the size of the alkali-metal cations in the tetrahedral interstitial sites controls the lattice parameters. The observed lattice parameters are discussed in comparison with other body-centered cubic A₆C₆₀ and rock salt type A₁C₆₀ phases. The field repulsive interactions among the C₆₀ molecules other than the electrostatic Madelung energy must be taken into account to understand the lattice parameters of A₃C₆₀ in the small lattice parameter region, different from standard alkali-halide ionic crystals.