Theoretical investigation of phase transition of tetragonal $\mathrm{L}_{4-8}$ graphene into $\mathrm{LA7}$ diamond polymorph

In this paper, the study of phase transition of tetragonal $\mathrm{L}_{4-8}$ graphene into base-centered orthorhombic $\mathrm{LA7}$ diamond polymorph is carried out using the density functional theory method. Analysis of the possible formation methods of $\mathrm{LA7}$ phase showed that its structure can be obtained as a result of strong uniaxial compression of tetragonal graphite with packing of AB at a pressure of $42,5$ GPa. The pressure at which $\mathrm{LA7}$ phase can be synthesized is the lowest in comparison with the pressures at which other diamond polymorphs can be obtained. The calculations also showed that the process of this structural transformation should be accompanied by energy release of $0,52$ eV/atom. The polymorphic modification of the diamond can stably exist under normal conditions, since the potential barrier separating states corresponding to graphite $\mathrm{L}_{4-8}$ and $\mathrm{LA7}$ phase is $0,34$ eV/atom. The theoretical X-ray diffraction patterns of the "graphite $\mathrm{L}_{4-8}$–$\mathrm{LA7}$" phase transition were calculated for the experimental identification of $\mathrm{LA7}$ phase. The calculated powder X-ray diffraction pattern of the orthorhombic polymorph of diamond differs greatly from the X-ray diffraction patterns of graphite or cubic and hexagonal diamond polytypes.