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Theoretical investigations of the phase transformations of structural modifications of tetragonal L4–8 graphite in diamond-like phases were performed using the density functional theory method in local density approximation (LDA) and generalized gradient approximation (GGA). The varieties of L4–8 graphite were examined with the following packing of graphene layers: AA (P4 / mmm), AB (I4 / mmm), ABCD (I41 / amd). The LA6 (Imma), LA7 (Cmcm), and LA10 (I41 / amd) diamond-like phases can be obtained from these graphites under compression. The LA6 diamond-like phase can be obtained from L4–8 AA graphite at pressure of 44 GPa. Another phase, LA7, can be formed from tetragonal L4–8 AB graphite in the pressure range from 43 to 46 GPa. The LA10 structure can be obtained only from L4–8 graphite with the ABCD layer packing of at pressures from 32 to 40 GPa. It was established that «L4–8 AA graphite → LA6 phase», «L4–8 AB graphite → LA7 phase», and «L4–8 ABCD graphite → LA10 phase» structural transitions are exothermic phase transitions of the first kind, as a result of which an energy of 0.5, 0.5, and 0.3 eV / atom is released, respectively. The energy barriers that need to be overcome to observe reverse «LA6 → L4–8 AA graphite», «LA7 → L4–8 AB graphite» и «LA10 → L4–8 ABCD graphite» phase transitions are 0.38, 0.34, and 0.18 eV / atom for the LDA-calculations or 0.31, 0.28, and 0.13 eV / atom for the GGA-calculations, respectively. Possible ways of synthesizing LA6, LA7, and LA10 phases are a strong compression of graphite along the [001]-axis at low temperatures.

Modeling of the formation of diamond-like phases from structural varieties of tetragonal graphite