First-principles Calculations For Mechanical Properties Of Super Hard Materials IrN₂ And BC₅

As the development of science, super hard material research has been one of the hot topics. Because of unique properties, such as high elastic module and hardness, good thermal conductivity, anti-planning, surface durability and stability et al, super hard material has been used in industry extensively. In search for new potential candidate super hard materials as to diamond and c-BN, two groups of materials are powerful candidates for super hard materials generally: (i) partially covalent heavy transition metal boride, carbide, nitride, and oxide;(ii) strong covalent compounds formed by C,B,N,O etc light elements.In this paper, plane-wave pseudo potential density functional theory method were performed to investigate the crystal structure, elastic property of marcasite IrN₂ under 0⁶0GPa equivalent hydrostatic pressure, then give a brief analysis of the electronic structure. The calculated elastic constants are mechanically stable and in good agreement with other theoretical studies and experimental. Under 0GPa, the calculated B is 400GPa, so we can indirectly predict marcasite IrN₂ is candidate super hard material. The anisotropic factors suggest that IrN₂ is anisotropic and well ductile and metallic.Then the same calculation was carried on BC₅ belonging to hexagonal and tetragonal crystal system. The lattice parameters, elastic constants modulus and anisotropic were calculated. We investigated Poisson's ratio and the Cauchy violation, which character the central force field. The results show that two structures were stable at high pressure, and the incompressibility increases with pressure. The compression wave anisotropy with structure P3m1 weakens as pressure. However, shear wave anisotropy increases. As investigation to hardness, the same conclusion was found between theory and experiment. It shows that ours investigations are practical significance. However, the results about equilibrium structure and hardness are same incompressibility. Only from Yong's modulus, we can find incompressibility of I-4m2 stronger than P3m1. The obtained Cauchy condition and Poisson's ratio indicate a decreasing role of non-central forces. What is more, the electron structure, total and partial density of states at same conditions were calculated. It showed that BC₅ is metallic and has band gap, which disagree with other B-C compounds. All these show that the BC₅ is unusual super hard material, and it may be potential candidate for diamond at high temperature. The given density of states indicates the covalent hybridization between B and C atom in this compounds. The pressure slightly influences density of states and band gap, indicating that BC₅ is stable at high pressure. We also found that the pressure has no influence on the density of states near Fermi level and the band gap, only makes the band slightly excurse. Therefore, we can predict that BC₅ has good stability at high pressure.