【作者】 迟美；

【导师】 许并社； 刘旭光；

【作者基本信息】 太原理工大学 ， 材料加工工程， 2008， 博士

【摘要】 材料是社会发展的物质基础和先导，而新材料则是社会进步的里程碑。由C、N两种非金属轻元素构成的新材料被广泛应用于化工、材料、电子、生物等多个领域，如人们熟知的金刚石、富勒烯、氮化镓等。然而，在对这些材料的研究过程中由于实验条件的限制（如高温、高压等），无法对材料的性能进行精确的测量。计算材料科学的出现使得许多在现有条件下无法完成的实验可以通过理论计算的方式来得以实现。计算材料科学主要包括两个方面的内容：一方面是从已有实验数据出发，通过建立数学模型及数值计算对已有的实验数据进行验证和补充；另一方面是通过理论模型和计算，设计可能具有特殊性能的新材料。基于密度泛函理论的第一性原理方法因其不依赖于经验参数而成为凝聚态物理、量子化学和材料科学中的主要研究手段。本论文首先从计算模拟角度出发，使用密度泛函理论对实验上已经合成的内包金属富勒烯的结构和物性进行了计算。接着从材料设计的角度出发对AlN基稀磁半导体材料和5d过渡金属二氮化物超硬材料进行了理论研究，寻找和设计具有室温居里温度和较高硬度的新型功能材料。研究涉及的内容包括几何构型、电子结构、磁学性能和力学性能等。采用原子轨道数值基组对实验上已经合成但仍未对其结构和性能进行精确分析的三种富勒烯团簇(Po@C₆₀，La@C₇₂和Eu@C₇₂)进行了理论研究。探讨了内包金属在碳笼中的可能位置，Po原子位于碳笼中心，而La和Eu原子均偏离碳笼中心，靠近C₇₂笼五元环-五元环连接处。通过对电荷密度、自旋密度及前沿轨道等性能的分析，得出Po原子与碳笼之间没有发生电荷转移且与碳笼之间不成键，而La原子与碳笼之间的化学键类型为离子键，Eu原子与碳笼之间的化学键为离子键和共价键共存的形式。采用第一性原理计算，以AlN为基体，研究N空位和Al空位两种点缺陷以及Mg、Cu、Zn、Pd四种非磁性离子p型掺杂后对AlN电子结构、磁性的影响，分析磁矩大小及来源。计算结果表明Al空位可以使AlN产生半金属铁磁性，磁矩来源主要来自于空位周围的四个N原子的p轨道。四种非磁性离子掺杂后的体系均具有半金属铁磁性，但磁矩大小和来源有所不同。Al₁₅MgN₁₆和Al₁₅ZnN₁₆的磁矩均为1．0μB，主要来源于Mg或Zn周围的四个N原子的p轨道电子，而Al₁₅CuN₁₆和Al₁₅PdN₁₆的磁矩均为2．0μB，主要来源于Cu或Pd的d轨道电子与N原子的p轨道电子发生的杂化作用。四种体系均会表现出稳定的铁磁状态，它们的居里温度关系为Al₁₅CuN₁₆>Al₁₅PdN₁₆>Al₁₅MgN₁₆>Al₁₅ZnN₁₆，且都大于350K，具有良好的室温铁磁性，有可能成为稀磁半导体的候选材料。采用第一性原理平面波赝势方法系统地对5d过渡金属二氮化物的五种不同晶体结构进行了理论计算。共对8种化合物（HfN₂、TaN₂、WN₂、ReN₂、OsN₂、IrN₂、PtN₂、AuN₂）的40种构型进行了结构优化和性能计算。通过对形成热及弹性常数的计算得到：对于黄铁矿结构既满足热力学稳定标准又满足机械稳定性标准的二氮化物为HfN₂、WN₂和PtN₂。对于萤石结构符合上述两个标准的二氮化物为ReN₂。对于金红石结构符合上述两个标准的二氮化物为TaN₂、WN₂、ReN₂和OsN₂。对于白铁矿符合上述两个标准的二氮化物为HfN₂、TaN₂、WN₂、OsN₂、IrN₂和PtN₂。对于含砷黄铁矿结构，除WN₂和AuN₂外，均满足上述两个标准。弹性常数的变化趋势表明，从HfN₂开始，随着过渡金属氮化物原胞中的价电子数增加，越来越多的成键态被价电子占据，使得化合物中原子间的束缚增加，导致体弹性模量增加。到达OsN₂时，成键态趋于添满，原子间的束缚趋于最强。随着价电子的继续增加，电子将填充反键态，价电子越多，反键态占据得越多。这将削弱化合物中原子间的束缚，导致体弹性模量降低。DOS及PDOS的计算结果表明过渡金属的5d轨道与N原子的2p轨道之间存在强烈的杂化现象，形成了较强的共价键。更多还原

【Abstract】 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.更多还原