Nucleation mechanism for the direct graphite-to-diamond phase transition

Graphite remains stable at pressures higher than those of its equilibrium coexistence with diamond. This has proved hard to explain, owing to the difficulty in simulating the transition with accuracy. Ab initio calculations using a trained neural-network potential now show that the stability of graphite and the direct transformation of graphite to diamond can be accounted for by a nucleation mechanism. Graphite and diamond have comparable free energies, yet forming diamond from graphite in the absence of a catalyst requires pressures that are significantly higher than those at equilibrium coexistence1,2,3,4,5,6,7. At lower temperatures, the formation of the metastable hexagonal polymorph of diamond is favoured instead of the more stable cubic diamond2,5,6,7. These phenomena cannot be explained by the concerted mechanism suggested in previous theoretical studies8,9,10,11,12. Using an ab initio quality neural-network potential13, we carried out a large-scale study of the graphite-to-diamond transition assuming that it occurs through nucleation. The nucleation mechanism accounts for the observed phenomenology and reveals its microscopic origins. We demonstrate that the large lattice distortions that accompany the formation of diamond nuclei inhibit the phase transition at low pressure, and direct it towards the hexagonal diamond phase at higher pressure. The proposed nucleation mechanism should improve our understanding of structural transformations in a wide range of carbon-based materials.