C₃₆, a new carbon solid

Abstract

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 D_6h 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.