Crystal structure of superconducting K₃Ba₃C₆₀: A combined synchrotron X-ray and neutron diffraction study

The crystal structure of the superconducting K₃Ba₃C₆₀ fulleride (T(c) = 5.4 K) has been studied by synchrotron X-ray and neutron powder diffraction between 10 and 295 K. It is body-centered cubic (bcc) at all temperatures with an essentially half-full t(1g)-derived band. Close contacts between Ba²⁺ and K⁺ ions and neighboring C₆₀ units imply a strong orbital hybridization, which leads to broadening of the conduction band. We find that the Ba²⁺ and K⁺ cations are disordered in the same distorted tetrahedral interstitial sites, (0, 1/2, 1/4 + δ), but they are displaced by a different distance (δ) from the center of the site. The resulting local distortions may be responsible for the observed complex relationship between T(c) and cubic lattice parameter in A₃Ba₃C₆₀ (A = Na, K, Rb, Cs) fullerides.