Comparative study of alkali metal-doped C60 solids

A simple model has been proposed to calculate the cohesive energy and other related bulk properties of alkali-doped C60 solids. In our earlier work we have successfully explained the bulk properties of K-, Rb- and Cs-doped C60 solids. The system is treated as an ionic solid, where van der Waals interactions have also been taken into account. The nature of interactions does not change when doped with sodium or other alkali atoms. The present work specifically deals with bulk properties of sodium and mixed alkali-doped M3C60 solids and in general alkali-doped C60 solids. Highly doped sodium systems are of extreme importance due to clustering of sodium atoms at octahedral sites. Our results for lattice constant are in good agreement with other experiments and calculations. Our method also allows us to estimate the distribution of excess charge among the clusters. We find that the Na atoms at tetrahedral sites are fully ionized, i.e. exist as Na+ while those at octahedral sites, which exist as clusters whenever n>3 in NanC60, are partially ionized such that the total charge on C60 molecule is six-electron units.