Моделирование процесса формирования алмазоподобных фаз из структурных разновидностей тетрагонального графита

Abstract

Theoretical investigations of the phase transformations of structural modifications of tetragonal L4-8 graphite in diamond-like phases was 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.