Phase transitions of carbon at extreme conditions

Accurate phase diagram of carbon at extreme pressures and temperatures is of critical importance for constructing interior models of carbon-rich exoplanets as well as for design of ablation capsules for inertial confinement fusion experiments. However, the stability of diamond and possible transitions to other high-pressure carbon phases above 1 TPa are still a matter of debate. We investigate the phase diagram and phase transitions of carbon materials in both crystalline and amorphous forms at high temperatures and pressures using machine-learning molecular dynamics (MD) simulations. Our results explain extreme metastability of diamond well beyond the previously predicted-region of its thermodynamic stability. We also predict experimentally viable synthesis pathways of elusive BC8 high pressure phase of carbon.