Absence of a metallic phase at high pressures in C₆₀

Abstract

THE bonding between molecules in bulk solid C₆₀ is extremely weak, making it a narrow-band semiconductor with an energy gap of 1.5 eV (ref. 1) for the face-centred cubic phase. Doping with alkali metals produces a metallic state which can be superconducting at temperatures as high as 33 K (ref. 2). The intermolecular coupling should have an important influence on the conductivity of the pure, semiconducting state: stronger coupling might induce a transition to a metallic or possibly even superconducting state, as is the case for silicon³, or it may result in a covalent solid such as diamond⁴. We have explored these possibilities by measuring the electrical resistivity of solid granular C₆₀ up to pressures of 25 GPa. Our results show that the magnitude of the gap and the resistivity decrease with increasing pressures as the sample volume⁵ decreases. But eventual gap closure to give a metallic state is not observed; instead, there is a sudden transition at ^15–20 GPa to a more insulating phase, possibly with covalent intermolecular bonding.