C36, a new carbon solid

Under appropriate non-equilibrium growth conditions, carbon atoms form relatively stable hollow clusters of well-defined mass number1, collectively known as fullerenes. The mass production, purification and condensation of such clusters into a molecular solid is generally essential to full experimental characterization: the initial discovery2 of C60, for example, had to await a bulk synthesis method3 six years later before detailed characterization of the molecule was possible. Gas-phase experiments1,4,5 have indicated the existence of a wide range of fullerene clusters, but beyond C60 only a few pure fullerene solids have been obtained6, most notably C70. Low-mass fullerenes are of particular interest because their high curvature and increased strain energy owing to adjacent pentagonal rings could lead to solids with unusual intermolecular bonding and electronic properties. Here we report the synthesis of the solid form of C36 by the arc-discharge method3. We have developed purification methods that separate C36 from amorphous carbon and other fullerenes, to yield saturated solutions, thin films and polycrystalline powders of the pure solid form. Solid-state NMR measurements suggest that the molecule has D6h symmetry, and electron-diffraction patterns are consistent with a tightly bound molecular solid with an intermolecular spacing of 6.68 Å. We observe large increases in the electrical conductivity of the solid on doping with alkali metals.