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 C-60 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 + delta), but they are displaced by a different distance (delta) 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 A(3)Ba(3)C(60) (A = Nat K, Rb, Cs) fullerides.