First-principles Study Of The Structural And Elastic Properties Of Os_xW_1-xB₂, Re_xW_1-xB₂ Alloys, C₁₄N And C₁₃N₂

Superhard materials (Hv≥40 GPa) are of great importance in defense, national economy, science, and technology. They are widely used as cutting and polishing tools, coatings and abrasives, etc. Diamond is the traditional superhard material and the hardest and most incompressible single-phase material so far. But the diamond is very rare in the nature and requires extremely high cost of synthetic. Meanwhile, the study results show that its thermal stability and chemical inertia is poor, and it is easy to react with ferrous metals in high temperature. CBN which synthesized later is the second hard material, comparable to the hardness of diamond, and it has higher chemical stability. But it must be synthesized under high pressure and high temperature conditions. So it is very expensive to synthesize CBN. As a result, basic research focuses on the understanding of structure-property relationships, which yields hints for the design of new superhard and ultra-incompressible materials. Attempts to synthesize or theoretically predict new superhard and ultra-incompressible materials are the subject of intensive current research activities.Traditionally, it is commonly accepted that superhard materials are those strongly covalent bonds compounds formed by light elements (LEs), namely, C, N, and O. such as B4C, BC2N, etc. they are all good superhard materials for their excellent stability. In the past few decades, plenty of theories and experiments are reported to study the synthesized compounds with light elements. Among various searching approaches, inserting light elements (B, C, N, and O) into a dense transition metal (TM) with high valence electron density is an efficient way to form superhard materials. Recently, several 5d TM nitrides (PtN₂, IrN₂, and OsN₂) and borides (ReB₂, OsB₂, WB₄, and TaB₂) have been synthesized. These materials show high bulk and shear modules, indicating their potential high hardness.It is useful to use computer to simulate the real materials and it is low coat and high efficiency to test proprieties of the materials. Recently, with the development of the first-principles based on density function theory(DFT), hardware and software are developing fast in computed materials simulation. As a result, many researchers are beginning to use computer to search and design new superhard materials. The superhard materials are developing fast in experiments and theories. In this letter, we first introduce the development of the superhard materials and the theory hardness models, and then introduce the developing process of DFT. Finally, we introduce the application of software of the first-principles calculations based DFT.The calculations in this paper were conducted by using the VASP package, based on the projector augmented-wave (PAW) method and CASTEP package. The exchange-correlation energy was treated in the local density approximation (LDA) and the generalized gradient approximation (GGA). The elastic stiffness constants were calculated by stress–strain method. The calculated bulk modulus B and shear modulus G were obtained from the Voigt-Reuss-Hill approximation.We calculated and studied the ternary boride Os_xW_1-xB₂ and Re_xW_1-xB₂ with the structure of ReB₂, and then studied their elastic properties. The results show that the structure of optimized Os_xW_1-xB₂ and RexW1-xB2 are the same to the structure of ReB2. Then we further studied the elastic constants and electronic properties for Os_xW_1-xB₂ and RexW1-xB2 with the variable x from 0.1 to 0.9. The studies suggest that Os_0.5W_0.5B₂ and Re_0.4W_0.6B₂ have the best properties among the compounds. The C₁₁ and C₃₃ of them are much higher, 600 GPa and 1000 GPa, respectively. The C₄₄ is also to 300 GPa, indicating that the ability of pressure and shear is very strong. From the electron structure, we find that the metal elements and boron elements form stronger covalent bonds, suggesting they are potential superhard materials. Therefore, tit is worthy to further study them.We also study the C₁₃N₂ and C₁₄N with the structure of B₁₃C₂, B₁₃C₂ is a traditional superhard materials and its hardness is very high exceeding 58 GPa. We replace the position of B by higher valence electron C and N. We find that the length of bonds become shorter and the volume become smaller, indicating better structure and high valence electron density. With the first-principles method, we find that C₁₃N₂ and C₁₄N have better elastic properties, and estimate their theory hardness with the Gao's hardness model by using their Mullikan overlap population. Their hardness is very high exceeding 60 GPa. In a word, they are good superhard materials.