SYNTHESIS AND CHARACTERIZATION OF ALKALI-METAL FULLERIDES - AXC60

Alkali metal fullerides (A(x)C60) are a subject of considerable current interest because of the occurrence of superconductivity for A3C60 at temperatures surpassed only by the high T(c) copper oxides. The preparation and characterization of A(x)C60 (A = alkali metal, x = 2,3,4,6) by powder X-ray diffraction, NMR (C-13, Na-23 and Rb-87), and d.c. magnetization are reported. The structures are described as intercalation compounds of the FCC structure of pristine C60 or of hypothetical BCC or BCT structures. The structures and phase diagrams can be rationalized on the basis of ion size and electrostatic considerations. Only the A3C60 compounds are metallic (and superconducting). The superconducting T(c) increases nearly linearly with unit cell size. EHT (Extended Huckel Theory) calculations and C-13 NMR relaxation measurements indicate higher densities of states for the higher T(c) compositions.