Structure of haloform intercalated C60 and its influence on superconductive properties

CHCl3 and CHBr3 intercalated C-60 have attracted particular interest after a superconductivity transition temperature (T-c) of up to 117 K was discovered. We have determined the structure using synchrotron x-ray powder-diffraction and found that the expansion of the lattice mainly takes place in one dimension (triclinic b axis), leaving planes of C-60 molecules on an approximately hexagonal, slightly expanded lattice. We have performed tight-binding band structure calculations for the surface layer. In spite of the slight expansion of the layers, for the range of dopings where a large T-c has been observed, the density of states at the Fermi energy is smaller for C-60.2CHCl(3) and C-60.2CHBr(3) than for C-60. This suggests that the lattice expansion alone cannot explain the increase of T-c.