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

TL;DR

This study uses advanced neural-network simulations to elucidate the nucleation mechanism behind the graphite-to-diamond phase transition, explaining pressure and temperature effects and the preference for hexagonal diamond formation.

Contribution

It introduces a nucleation-based model for the graphite-to-diamond transition, explaining phenomena that previous concerted mechanisms could not.

Findings

Large lattice distortions hinder low-pressure transitions.

Higher pressure favors nucleation of cubic diamond.

The mechanism explains the formation of hexagonal diamond at lower temperatures.

Abstract

Graphite and diamond have comparable free energies, yet forming diamond from graphite is far from easy. In the absence of a catalyst, pressures that are significantly higher than the equilibrium coexistence pressures are required to induce the graphite-to-diamond transition. Furthermore, the formation of the metastable hexagonal polymorph of diamond instead of the more stable cubic diamond is favored at lower temperatures. The concerted mechanism suggested in previous theoretical studies cannot explain these phenomena. Using an ab initio quality neural-network potential we performed a large-scale study of the graphite-to-diamond transition assuming that it occurs via nucleation. The nucleation mechanism accounts for the observed phenomenology and reveals its microscopic origins. We demonstrated that the large lattice distortions that accompany the formation of the diamond nuclei inhibit the phase transition at low pressure and direct it towards the hexagonal diamond phase at higher pressure. The nucleation mechanism proposed in this work is an important step towards a better understanding of structural transformations in a wide range of complex systems such as amorphous carbon and carbon nanomaterials.