ABSENCE OF A METALLIC PHASE AT HIGH-PRESSURES IN C60

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 silicon 3, or it may result in a covalent solid such as diamond 4. 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 volume 5 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.