Local structure of liquid carbon controls diamond nucleation

TL;DR

This study uses simulations to show that increasing pressure in liquid carbon enhances diamond nucleation by promoting four-fold coordination, impacting understanding of crystallization in extreme environments.

Contribution

It reveals how local atomic structure changes under pressure influence diamond nucleation rates in liquid carbon, a novel insight into nucleation mechanisms.

Findings

Nucleation rate increases with pressure at constant supersaturation.

Four-fold coordination facilitates easier diamond nucleation.

Homogeneous nucleation is likely in carbon-rich stars, unlikely in gaseous planets.

Abstract

Diamonds melt at temperatures above 4000 K. There are no measurements of the steady-state rate of the reverse process: diamond nucleation from the melt, because experiments are difficult at these extreme temperatures and pressures. Using numerical simulations, we estimate the diamond nucleation rate and find that it increases by many orders of magnitude when the pressure is increased at constant supersaturation. The reason is that an increase in pressure changes the local coordination of carbon atoms from three-fold to four-fold. It turns out to be much easier to nucleate diamond in a four-fold coordinated liquid than in a liquid with three-fold coordination, because in the latter case the free-energy cost to create a diamond-liquid interface is higher. We speculate that this mechanism for nucleation control is relevant for crystallization in many network-forming liquids. On the basis of our calculations, we conclude that homogeneous diamond nucleation is likely in carbon-rich stars and unlikely in gaseous planets.