First-principles Calculations For Mechanical Properties Of PtN₂ And ReB₂ Superhard Materials

In recent years, the study of superhard materials has been a hot topic in the experiment and theory.The superhard material is not only used in grinding, polishing and coating, but also used to produce tools which play an important part in the field of modern industry. In addition to having high degree of hardness, they also have special optics, electricity and magnetics characteristic, so a considerable number of theoretical and experimental research works are committed to looking for new superhard materials. One of the ways to produce superhard materials is to search for metals which are"incompressible"and try to harden them. The new materials synthesized from transition metal which is much harder and nitride or boron attracts much attention. Thus the noble metal nitride and metal boron have been researched in this article.In this thesis, the equilibrium lattice parameters and state structures of PtN2 and ReB2 are investigated by the pseudopotential plane-wave method based on the density functional theory after the system of PtN2 and ReB2 is optimized. We use the the norm-conserving pseudopotential introduced by Hamann and the LDA is used to describe the exchange and correlation function.The elastic properties of the pyrite structure and STAA structure of PtN2 are calculated based on the optimation of geometric structure. The energy vs volume curve shows the energy of STAA structure is lower than that of pyrite strcture and this result is identical with the opinion given by Daniel ?berg etc., that the PtN2 stays on metastable state under the ambient condition.According to the experimental criterion raised by Pugh, we could come to the conclusion that the PtN2 is very brittle and its brittleness turns into malleability gradually with the pressure increasing. From the density of states and the band structure, we could draw a conclusion that the pyrite structure of the PtN2 is a semiconductor while the STAA structure of the PtN2 is a conductor.Comparing with other hard materials, we conclude that ReB2 is a sort of low-compressible material. The results of the compressional wave anisotropy (Δp ) and the shear wave anisotropy (Δs 1,Δs 2) at both zero pressure and high pressure are away from 1, showing the compressional anisotropy and the shear elastic anisotropy. At any pressure, c3 3 is bigger than c1 1, indicating rigidity of ReB2 in z direction is harder than that in x, y direction. The density of states and the band structure of ReB2 indicate that it is metal and holds strong covalent bond.Finally, in the article, the calculated results of PtN2 and ReB2 show that the bulk modulus and shear modulus of superhard materials are big and covalent bonds are strong. The conclusions of this article pay an important assistant role in researching superhard materials in future.