Structure and properties of a chiral polymorph of diamond with a crystal lattice of the SA3 type

An ab initio study of a chiral polymorphic type of diamond (SA3), in which all atoms are in crystallographically equivalent states, was carried out. The calculations of the structure and properties were performed using the density functional theory method in the generalized gradient approximation. The crystal structure of the SA3 diamond polymorph can be formed during the polymerization of close-packed chiral carbon nanotubes (5, 4). The SA3 phase has a hexagonal unit cell with parameters a = 0.40696 nm and c = 0.24779 nm, which contains six carbon atoms. The crystal lattice of the SA3 diamond polymorph belongs to the space symmetry group P6122 (P6522). The cohesive energy of the SA3 phase is 0.525 Rydberg / atom, which is only 9 % less than the cohesive energy of cubic diamond. Molecular dynamics modeling showed that the structure of the SA3 phase should be stable under normal conditions. The chiral diamond polymorph can exhibit the properties of a wide-gap semiconductor, since its minimum direct band gap is 19 % less than the corresponding value for diamond. The SA3 diamond polymorph can be unambiguously identified experimentally using diffraction and spectral analysis methods. It is found that the calculated powder X-ray diffraction pattern of this phase is characterized by the five most intense maxima, which correspond to the following interplanar distances: 0.35244, 0.20309, 0.17622, 0.14361, and 0.11740 nm. The X-ray absorption spectrum of the SA3 phase differs significantly from similar spectra of diamond and graphite in the photon energy range from 290 to 315 eV. The calculated Raman spectrum of the chiral phase contains five peaks in the range of wavenumbers from 660 to 1210 cm−1; therefore, the identification of the SA3 phase should not cause difficulties.

Structure and properties of diamond-like phase obtained from tetragonal graphene layers

V.A. Greshnyakov, E.A. Belenkov