Atmospheric-pressure stability of energetic phases of carbon

The stability of various energetic phases of carbon is investigated with the use of ab initio pseudopotential total-energy methods. In particular, we examine the atmospheric-pressure stability of the fourfold-coordinated bc8 phase of carbon against transformations to graphitic and diamond structures lying at lower energy. A group-theoretical analysis is used to determine high-symmetry transformation paths to these low-energy structures. Ab initio total-energy calculations are then carried out along those paths characterized by minimal bond breaking to identify minimum-energy configurational transformations. In all cases we find energy barriers inhibiting transformation from the bc8 structure to the lower-energy phases, with the smallest barrier being approximately 0.2 eV/atom and occurring along specific paths to the graphitic forms of carbon. Consequently, it appears that bc8 carbon should be metastable at atmospheric pressure against spontaneous transformation to the lower-energy observed phases. In contrast, we find three metallic high-coordination forms of carbon (face-centered cubic, hexagonal close-packed, and body-centered cubic) to be mechanically unstable against spontaneous transformation to the cubic diamond structure.