Nucleation of diamond from liquid carbon under extreme pressures: Atomistic simulation

The stable solid form of carbon is graphite; diamond is thermodynamically unstable at atmospheric pressure. High pressure and high temperature must be applied to enable diamond crystal growth. Cubic diamond grows when hydrostatic pressure is applied, whereas hexagonal diamond (which is another form of ${\mathrm{sp}}^{3}$-hybridized carbon) has been reported to grow when uniaxial pressure is applied. In the present study, we simulate the precipitation and growth of diamond clusters inside an amorphous carbon network by rapid quenching of the compressed liquid phase, followed by volume expansion. The simulations are carried out under both hydrostatic (in all three directions) and uniaxial pressure, exposing the samples to different initial pressures (densities) as well as to different cooling rates. At fast cooling rates $(500--1000\phantom{\rule{0.3em}{0ex}}\mathrm{K}∕\mathrm{ps})$ and high densities $(3.7--3.9\phantom{\rule{0.3em}{0ex}}\mathrm{g}∕\mathrm{cc})$, large diamond crystallites (containing up to 120 atoms) are formed. We find that the probability of precipitation of diamond crystallites increases with density and with cooling rate. Uniaxial compression of the samples does not lead to nucleation of the hexagonal form of diamond; all uniaxially compressed ordered ${\mathrm{sp}}^{3}$ clusters were identified to be cubic diamond, with random orientation relative to the compression direction. At slower cooling rates $(200--500\phantom{\rule{0.3em}{0ex}}\mathrm{K}∕\mathrm{ps})$, some samples transform to graphite with an interplanar distance smaller than that of perfect graphite. Graphite formed under hydrostatic pressure has planes with random oreintation, whereas the planes of graphite formed under uniaxial pressure were oriented in parallel with the direction of compression.