Theoretical total-energy study of the transformation of graphite into hexagonal diamond

Variations in the structural energy of solid carbon along a transformation path from a graphitic structure with . . .[AA] . . . stacking to hexagonal diamond are calculated using the first-principles pseudopotential local-orbital total-energy method. The transformation path is that defined in an earlier study of the transformation of rhombohedral graphite into cubic diamond. The energy barrier along the transformation path is found to be only slightly higher than that of the corresponding path in the earlier study. Thus the major features of the energetics of the transformation are found to be relatively insensitive to the stacking of the graphite. However, it is found that in the region of metastability of graphite, the . . .[AA]. . . stacking never has lower energy than the rhombohedral stacking, and so we do not expect a transition from rhombohedral to . . .[AA]. . . stacking under static pressure. While the energy of hexagonal diamond is higher than that of cubic diamond in their final formation, nevertheless, as the ${\mathrm{sp}}^3$ bonds are forming during the transition from graphite, it is found that the hexagonal diamond has approximately the same energy as cubic diamond. The implications of these results for the synthesis of diamond from graphite are discussed.