Hexagonal diamond: theoretical study of methods of fabrication and experimental identification

Methods for the formation of hexagonal (2H) diamond from diamond polytypes under various types of deformation of the structure have been studied using density functional theory in the generalized gradient approximation. It has been established that the most appropriate method for the formation the structure of the 2H diamond polytype is the application of shear stresses > 102.9 GPa along the [211] direction to the (111) planes of cubic diamond when pressures along the [111], [110], and [211] axes reach 21.6, 21.7, and 69.9 GPa, respectively. Raman and X-ray absorption spectra have also been calculated for various diamond polytypes. The analysis of calculated spectra shows that hexagonal diamond can be unambiguously identified if other diamond polytypes with nonzero hexagonality are absent in the system under study. In addition, Raman spectroscopy data and characteristic electron energy losses have been analyzed in order to determine the presence of 2H diamond in artificial or natural carbon compounds. It has been established that hexagonal diamond in the pure form has not yet been obtained and the structure of synthesized compounds is close to the structure of polytypes with a long lattice period or with a random packing of layers.