First-principles Study Of Several Transition Metal Borides And Nitrides

Superhard materials are often used as cutting tools and superhard coatings in industry and national defense because of its high hardness, wearing resistance, thermal stability and other excellent features. Traditional superhard materials are normally synthesized by boron, carbon, and nitrogen. The synthesis of diamond in 1955 and the synthesis of cubic boron nitride in 1957 are two milestone in the history of superhard materials. In recent years, transition metals has attracted extensive attention because of their high valence electron density. However, the hardness of the transition metals are not high enough. People insert light elements, boron, carbon, and nitrogen, into the lattice of transition metals to increase their hardness. For example, Re B2(synthetized under normal pressur), Os B2(synthetized under normal pressur) and Pt N2(synthetized under high pressure) all have high bulk modulus and shear modulus, and were considered to be potential superhard materials. Therefore, the study of transition metal compounds have become a new hot spot of superhard materials research.Some transition metal borides can be synthetised under atmospheric conditions, which can greatly reduce the cost of production. Therefore, transition metal borides are gaining in popularity with researchers. We use package CALYPSO based on particle swarm optimization algorithm and combine with first-principles calculation to predict and study structures of Re B3 and Ir B3. By prediction we found some new structures: Re B3 with symmetry of P-6m2, P63/mmc and P-3m1 and Ir B3 with symmetry of Amm2, P63/mmc, P-6m2 and P-3m1. Re B3 with symmetry of P-6m2 and Ir B3 with symmetry of Amm2 are always ground state structures of Re B3 and Ir B3 under 100 GPa. By calculating the phonon spectra of these structures we found no imaginary phonon frequency appearing in the whole Brillouin zone of them, illustrating that they are dynamically stable. Calculations of elastic properties show that they are elastic stable. Density of states across the Fermi level indicates that they have metallic properties. Theoretical hardness of P63/mmc-Re B3 and P-6m2-Re B3 are 37 and 30 GPa, respectively. By analyzing the electronic structure of these two structures we found that strong B-B bonding and Re-B bonding in these structures is the origin of their high hardness. The high theoretical hardness of P63/mmc-Re B3 and P-6m2-Re B3 can make them to become novel superhard materials.In transition nitrides, tantalum nitride has always been the focus of researchers, because of its excellent properties( such as chemical stability, high hardness, high melting point, good heat and electrical conductivity, and superconducting). We use CALYPSO based on particle swarm optimization to predict six kinds compounds of Ta-N system. The predicted structures including: P-6m2(187)-Ta N, P-6m2(189)-Ta N, C2/m(12)-Ta N2, P4/mmm(123)-Ta N3, P6cn(60)-Ta2 N, P-4m2(115)-Ta2N3, and P63cm(185)-Ta3N5. Then, we adopt the first principles to study their structure, phase stability, dynamic stability, elastic properties, and electronic structure. P-6m2(187)-Ta N is same as the previous experimental structure of δ-Ta N in structure. P-6m2(189)-Ta N is same as the previous experimental structure of ε-Ta N in structure. Formation enthalpy of these predicted structures under pressure 0 to 50 GPa are negative, indicat that they are all stable. We calculated the phonon spectrum to determine the dynamic stability of the structure and found that except P4/mmm(123)-Ta N3, other structures all have no imaginary frequency, shows that they are dynamic stability. The elastic constants conform to the mechanical stability condition show that they are all elastic stability. Except Ta N3, other predicted structures have larger bulk modulus(more than 260 GPa), indicating that they have strong ability to counteract volume deformation. P-6m2(187)-Ta N has high shear modulus, it has strong ability to counteract shear deformation. The density of states of P63cm(185)-Ta3N5 has a band gap at the Fermi level, indicat that it is a semiconductor material. For other predicted structurals, the finite electronic DOS at the Fermi level indicates that they are metallic.Transition compounds, WB3 and Os B3, are widely concerned by the researchers. The ground state structures of them is still an open question. We also used the CALYPSO software package to predicted the structures of WB3 and Os B3, the results are: R-3m-WB3, P63/mmc-WB3, P-3m1-WB3, P-6m2-WB3, P-6m2-Os B3, P-3m1-Os B3, P6/mmm-Os B3. In these structures, P-6m2-Os B3 and R-3m-WB3 are the ground state structure of Os B3 and WB3, respectively. Phonon dispersion curves show that, except P6/mmm-Os B3, other structures have no imaginary frequency in whole Brillouin zone. That is to say other structures are kinetically stable. R-3m-WB3 and P63/mmc-WB3 have great theoretical hardness(37GPa and 38GPa) close to the standard 40 GPa of superhard material, which shows that R-3m-WB3 and P63/mmc-WB3 can be used as superhard material candidates. The electronic structure analysis found that R-3m-WB3 and P63/mmc-WB3 have strong W-B and B-B bonding in their structures. Which can explain their stabilities and high hardness.