Does C₆₀ have a liquid phase?

Abstract

ABOVE a substance's liquid–vapour critical point (T_c), the distinction between the liquid and vapour phases disappears. Below the triple point (T_t), meanwhile (at which solid, liquid and vapour coexist), only the solid and vapour are stable. The liquid range, T_c/T_t, depends on the nature of the intermolecular forces: for argon, T_c/T_t = 1.8, whereas for sodium the ratio is 7.5. But might there be molecular substances that have no liquid phase at all? Here we present results which suggest that C₆₀ is such a substance. We map out the phase diagram using computer simulations in which the C₆₀ molecules are represented by spheres interacting via Lennard-Jones potentials summed over all 60 carbon atoms. We find that the sublimation line passes above the metastable liquid-vapour coexistence curve. By drawing an analogy with the aggregation of colloidal particles, we expect that solid C₆₀ formed by nucleation from the vapour phase will be amorphous rather than crystalline.