Structure of Haloform Intercalated C 60 and Its Influence on Superconductive Properties

Abstract

CHCl 3 and CHBr 3 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 .