Ionic metalKxC60: Cohesion and energy bands

Microscopic total-energy electronic-structure calculations for ${\mathrm{K}}_{\mathit{x}}$ ${\mathrm{C}}_{60}$ show that solid ${\mathrm{C}}_{60}$ weakly bonded via van der Waals forces is transformed upon potassium doping into a strongly condensed ionic metal in which both Madelung and kinetic energies contribute to its large cohesive energy and bulk modulus. We also find that K doping induces lattice contraction which results in nonrigid energy-band modification. The Fermi level for ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$ is found to be located close to a peak of the density of states.