Cohesive energy of potassium doped C60 solids

We report cohesive energy calculations for alkali metal doped C60 solids. Model calculations are presented for K1C60, K2C60, K3C60, K4C60 and K6C60. In this work, the C60 molecule is modelled as a uniform spherical shell with appropriate surface density of carbon atoms, while the ionised alkali atoms, forming the cations are taken to be point charges. Part of the electrons freed by those ionised K atoms are distributed on the C60 molecule making it an anion, while the rest (say x) are assumed to form a delocalised electron gas. This electron gas screens the Coulomb interaction between the various anion and cations. We also account for on-shell Coulomb repulsion between the electrons on the C60 shell. With these assumptions the total cohesive energy is calculated taking into consideration van der Waals and screened Coulomb interaction between different ions. On minimising the energy thus calculated with respect to x, the fraction of electrons forming electron gas, we find that x is zero i.e. total charge transfer from cation to anion is favoured. Thus ionic character of K doped C60 solids is established on the basis of the model. Comparison of the total energy thus obtained has been made with other calculations. We also show the phase instability of K2C60 system.