Structural transformations in carbon under extreme pressure: Beyond diamond

High-pressure structural transformations of carbon at terapascal pressures are studied using metadynamics and ab initio methods. Diamond transforms to a mechanically stable cubic structure (P4(1)32) at 2.5 TPa and 300 K. At 4000 K and 2 TPa, simple cubic carbon SC1 (Pm-3m) is obtained from cubic diamond. The high-pressure tetrahedrally coordinated BC8 (Ia-3) structure of carbon is obtained by decompression of the SC1 structure at 1 TPa and 5000 K. At 3000 K, with decompression of SC1 carbon to 1 TPa, two new metastable tetrahedrally coordinated structures form, MP8 (P2/c) and OP8 (Pccn) with higher density than that of cubic diamond. The results show the presence of strong kinetic effects and suggest that phase transformations and structures of carbon at extreme pressures are more complex than previously thought.