Thermodynamically stable phases of carbon at multiterapascal pressures.

Phases of carbon are studied up to pressures of 1 petapascal (PPa) using first-principles density-functional-theory methods and a structure searching algorithm. Our extensive search over the potential energy surface supports the sequence of transitions diamond → BC8 → simple cubic under increasing pressure found in previous theoretical studies. At higher pressures we predict a soft-phonon driven transition to a simple hexagonal structure at 6.4 terapascals (TPa), and further transitions to the face centered cubic electride structure at 21 TPa, a double hexagonal close packed structure at 270 TPa, and the body centered cubic structure at 650 TPa.