First-principles Study Of The Structural, Elastic And Electronic Properties Of Several Transition Metal Borides, Nitrides, And BC₅

Superhard material has a wide range of industrial uses and has been widely used in metallurgy, oil drilling, construction, mechanical processing, devices, instruments, electronics, and aerospace, etc. modern cutting edge science. Diamond is the hardest material available in nature, however, natural diamond is very rare. CBN is the second hard material, just behind synthetic diamond, however, there are not natural CBN.Thus, the research projects of looking for new materials with high hardness are very concerned by the experimental and theoretical workers. In the initial process of search for superhard material, people pin high hopes on the B-C-N compounds. This promoted the remarkable headway of the preparation and theoretical research of the B-C-N compounds. Recently, under high pressure and temperature, OsB₂, ReB₂, PtN₂, OsN₂ and IrN₂ have been synthesized experimentally, the common features of which are that they all have large bulk modulus (B≥350 GPa) and shear modulus (G≥200 GPa). It is well known that the transition metal's hardness is relatively low despite the large bulk modulus. For example, the bulk modulus of pure Os is 410 GPa (the bulk modulus of diamond is just 442 GPa), but the hardness of pure Os is only one thirtieth of that of diamond. Thus, people started to pay attention to the transition metal borides and nitrides in recent years.The calculations presented in this study were performed within the density functional theory, using the project-augmented wave (PAW) method as implemented in Vienna Ab initio simulation package (VASP). Both the local density approximation (LDA) and the generalized gradient approximation (GGA) were used with the PAW potential. The structure was optimized with the conjugate-gradient algorithm method. A plane wave cutoff energy of 500 eV was employed throughout. According to the specific lattice structure, appropriate Monkhorst-Pack mesh was chosen to guarantee the sufficient k-points in the irreducible Brillouin zone. The bulk modulus and its pressure derivative are obtained by fitting pressures and cell volumes with the third-order Birch-Murnaghan equation of state. The strain-stress method was used to obtain the elastic constant. From the calculated cij, the polycrystalline bulk modulus B, shear modulus G, Young's modulus E and Poisson's ratioνwere further estimated using the Voigt-Reuss-Hill approximation.Using first-principles method, this paper has investigated the structural, mechanical and electronic properties of several materials. This paper first introduces the theoretical calculation method used here and, the density functional theory, elastic mechanics and the knowledge of VASP. We predict the existence of orthorhombic structure ReN₄ (Pbca) and investigate its structural, elastic and electronic properties systematically. On the basis of the calculated results, we show that ReN₄ (Pbca) is a metallic hard material. This paper predicts the existence of orthorhombic structure OsN₄ (Pbca). And we have systematically studied the structural, elastic and electronic properties of OsN₄. The calculated results show that the orthorhombic structure OsN₄ is semiconducting superhard material with a band gap of 0.7 eV. Several transition metal borides, among which TaB₂ has been synthesized experimentally, have been investigated. For TaB₂, the calculated lattice parameters, shear modulus 219 GPa as well as Young's modulus 531 GPa are in good agreement with the available experiment data (G=228 GPa, E=551 GPa). Moreover, the calculated results show that the hardness of the several transition metal borides is essentially proportional to their respective constant c44. Two phases of Ta3N5 have been investigated in this paper. The Cmcm phase i.e. the most stable modification of Ta3N5 up to 9 GPa has been synthesized. The Pnma phase has not been synthesized but is predicted to be the new high pressure modification at about 9 GPa. This paper systematically studied the structural, elastic and electronic properties of the two phases of Ta3N5. The calculated results show that both phases of Ta3N5 are not superhard materials, the hardness of Cmcm phase is only 14.25 GPa, and the hardness of Pnma one is 26.67 GPa. This paper has theoretically studied the crystal structure of the recently synthesized diamondlike BC5, and systematically investigated its elastic properties, our calculations confirm that BC5 is a metallic superhard material, and we systematically analyzed the origin of it high hardness. Our calculated bulk modulus B (377-407 GPa), shear modulus G (398-419 GPa), elastic constant c44 (374-389 GPa), and theoretical hardness H (84 GPa) confirm that BC5 is an ultraincompressible and superhard material. Also, it exhibits mechanical stability and metallic feature. With these physical properties, such as high hardness and metallic behavior, the recently synthesized diamondlike BC5 is attractive for advanced abrasive and superhard materials. Electronic structure analysis shows that the sp3 hybridization still exists in BC5 between B and its neighboring C atoms. The strong directional covalent C-C bonds in diamond are only partly substituted by the B-C bond in BC5. There are still three dimensional covalent network of high symmetry in BC5, which result in excellent mechanical properties and high hardness.