First Principles Studies Of Zr_nCo Clusters And Transition-Metal Boride/Nitride

In the thesis we have studied the structural, electronic, magnetic and elastic properties of transition-metal and doped transition-metal clusters as well as transition-metal nitrides and borides by using the first-principles calculation. On the basis, we account for all the simulated results and predict more general results.Cluster and supper hard material are attracting great attentions due to their particular physical properties and importance of both fundamental science and industrial applications. In the thesis, some basic concepts, general properties and the studied review of cluster science have been briefly summered. Then, we have introduced the research background and meaning of superhard material.The density function theory (DFT) is given rigorous theoretical proof by Hohenberg and Kohn in 1964. They proved that all properties of many-electron systems are a unique functional with electron density. It is not only the theoretical foundation that the many-electronic problem will be more simplified single-electron issues, but also an effective tool that can calculate the total energy and the electronic structure of atom, molecule and solids. We have introduced the first-principle calculations some correlated method and simulation computer program based on DFT in the thesis, including the density functional theory, the local density approximation, the generalized gradient approximation, Gaussian03 program, Dmol3 program, VASP program.By using the first-principles calculatin, we have studied the structural, electronic, and magnetic properties of ZrnCo ( n = 1~13) as well as the structural, phase transtion, electronic, and elastic properties of OsB2, RuB2, and RuN2. The results show that the relative stabilities of Zr4Co, Zr7Co, Zr9Co, Zr12Co are stronger than other sized clusters, indicated that these are the magic number clusters, especially, the ground state for Zr12Co cluster is icosahedral structure with Ih symmetry, moreover the stability of Zr12Co is strongest among all the investigated clusters. The magnetic moment of ZrnCo clusters mainly comes from the localed d electron, the system magnetic moment to be allowed to be divided three stages along with the size change: n = 1~3 have the stable magnetic moment, the magnetic moment of ZrnCo clusters starts to appear the vibration quenching from n = 4 until the n≥8 magnetic moments completely quenched. The charge transfer and the strong hybridization between s, p and d states might be one major reason for quenching the magnetic moment of ZrnCo clusters. Meanwhile, the clusters which are composed of the transition metal doping in the different characteristic material is worth further studying, for example TMX12, Zr13TM clusters,their structure , stability and magnetism are extremely interesting similar. The experimentally synthesized osmium diboride as an ultraincompressible hard material has the orthorhombic symmetry under ambient pressure. Here, we firstly report that OsB2 will transfer to the hexagonal phase with increasing pressure. The results of the first-principles calculation predict that the transition pressure is 10.8 GPa with GGA. Such pressure induced phase transition has never been reported in recently synthesized 5d transition-metal nitrides and borides. OsB2 keeps the metallic behavior even under extreme high pressure. The origin of the orthorhombic-hexagonal phase transition were investigated by analysing their density of states. The calculated elastic moduli suggest that RuB2 and RuN2 are also compressibility and hard materials. Based on the calculated electronic density of states and valence charge density distribution, the bonding nature of RuB2 and RuN2 is examined to obtain a deeper insight into the physical origin of the mechanical properties. The metallicity and highhardness of RuB2 and RuN2 might suggest their potential application as hard conductors.