Simulation On Fullerenes And Nitrides With First Principle Study

Materials are the foundation and precursor of social development whilenew materials are the milestone of social progress. New materials containing Cor N element are widely applied to chemical, material, electronic and biologicaldomain, such as diamond, fullerenes and GaN. However, during the researchprocessing of these new materials, some properties can not be measuredaccurately due to the restriction by experimental conditions, such as highpressure and high temperature. The appearance of computational materialsscience changed this state. Through theoretical simulation, scientists can obtainthe data which otherwise would not be available experimentally. The maincontents of computational materials science contain two parts. On one hand,confirm and supply the experimental data through mathematical modeling andnumerical calculation. On the other hand, design novel materials with interestingproperties by simulation. Because of the independence of empirical parameters,the first principle study becomes the main research means in condensed matterphysics, quantum chemistry and materials science. In this paper, correspondingto the first part of computational materials science, the structural and electronicproperties of endohedral metallofullerenes were studied by density functional theory. Then from the perspective of material design, first principle study was performed on AlN-based diluted semiconductor and 5d transition metal dinitrides to find and design new functional materials with high Curie temperature and hardness. Research contents in this thesis include geometrical, electronic, magnetic and mechanical properties.Based on the use of numerical atomic orbitals as basis sets, the structural and electronic properties of three kinds of endohedral metallofullerenes (Po@C₆₀, La@C₇₂ and Eu@C₇₂), which have been synthesized but not been well characterized, were studied. The possible positions of metal atoms in carbon cages were discussed. For Po@C₆₀, the ground state is the triple state with Po atom sitting on the center of C₆₀. While for La@C₇₂ and Eu@C₇₂, the most favorable endohedral positions are off-center site along the C₂ axis, pointing to the 5, 5 bond, the fusion of two pentagons. According to the electron density, spin density and frontier orbitals results, there is not charge transfer between Po and C₆₀. The bond type for La@C₇₂ is electrovalent bond. While for Eu@C₇₂, in addition to the charge transfer, the orbital hybridization between the guest metal atoms and fullerenes could also be seen.Using first-principles method based on density functional theory (DFT), the electronic structure and magnetic properties of AlN-based diluted semiconductor with vacancy and doped defects were studied. The results show that Al vacancy in AlN lead to spin-polarized ground states and half-metallic ferromagnetism. The magnetic moment is mainly attributed to the unpaired 2p electrons of the N atoms around the vacancy site. For doped system Al₁₅MN₁₆(M=Mg, Cu, Zn, Pd), the computational results show that all the four structures lead to spin-polarized ground states and half-metallic ferromagnetism, but the magnitude and origin of magnetic moment are different. The magnetic moments for Al₁₅MgN₁₆ and Al₁₅ZnN₁₆ are 1.0(μ_B), mainly ascribed to the unpaired 2p electrons of the N atoms around Mg or Zn atoms. While the magnetic moments for Al₁₅CuN₁₆ and Al₁₅PdN₁₆ are 2.0(μ_B), mainly from the hybridization between p electrons of N atoms and d electrons of Cu or Pd atom. The ferromagnet is stable in Al₁₅MN₁₆ (M=Mg, Cu, Zn, Pd). The Curie temperature ranks in an order of Al₁₅CuN₁₆> Al₁₅PdN₁₆>Al₁₅MgN₁₆>Al₁₅ZnN₁₆, and the values higher than 350K. These results suggested that Al₁₅MN₁₆(M=Mg, Cu, Zn, Pd)may exhibit ferromagnetism at room temperature and present a new promising DMS.First principle study was performed on 5d transition metal dinitrides TMN₂ (HfN₂, TaN₂, WN₂, ReN₂, OsN₂, IrN₂, PtN₂, AuN₂) with five different crystal structures. The heat of formation and elastic constants results show that the dinitrides satisfying both thermodynamic and mechanical stability standards are: HfN₂, WN₂ and PtN₂ with pyrite structure, ReN₂ with fluorite structure, TaN₂, WN₂, ReN₂ and OsN₂ with rutile structure, HfN₂, TaN₂, WN₂, OsN₂, IrN₂ and PtN₂ with marcasite structure, and HfN₂, TaN₂, ReN₂, OsN₂, IrN₂ and PtN₂ with arsenopyrite structure. For the compound at the beginning of this series, HfN₂, the additional valence electron continues to fill the p-d bonding states. This results in an increase in bulk modulus and shear modulus. The bonding state is filled completely in OsN₂ and the antibonding states begin to be filled from IrN₂. The excessive occupation of the p-d antibonding states decreases bulk modulus and shear modulus. The DOS and PDOS results show that there is significant hybridization between TM 5d and N 2p.