【作者】 刘志明；

【导师】 崔田；

【作者基本信息】 吉林大学 ， 凝聚态物理， 2009， 博士

【摘要】 材料中的缺陷行为及其对宿主物理性质的影响是一个具有现实意义和广泛应用价值的重要课题,因而一直是理论和实验研究的热点。本工作采用第一性原理方法较为系统地研究了不同外界条件下的两类典型缺陷体系:金属Nb掺杂氢氦体系和B-C-N超硬多功能材料的缺陷和杂质效应。论文主要包括:掺氢金属铌体系的晶格畸变、电子结构、光学特性和氢原子的绝热晶格动力学行为和压力效应,以及有限温度和压力下氦原子的凝聚机制;提出了一种计算金属晶体弹性性质的方法;预言了两种具有导电性质的立方相超硬B-C-N三元晶体,并且研究了相应的电子结构、基本力学特性和晶格动力学特性;提出了定体焓分析方法,并研究了BCN体系的稳定性与合成的可行性问题,研究了B-C-N体系中的杂质问题。具体的研究工作如下:（一）在1:1,1:2和1:16的浓度下,研究了氢原子在金属铌中的电子结构、宿主晶格响应和氢原子的晶格动力学行为,提出了一种NbH的新结构,对文献报导的低位能带（lower lying band）的成因进行了进一步阐释;采用位置试探的方法,在第一性原理的框架下得到了氢原子在Nb中感受到的周期势场,通过求解氢原子的Schr（o|¨）dinger方程,得到了氢原子量子行为的较为精确表述;采用分子动力学方法在有限温度和压力下,研究了氢、氦动力学效应对宿主金属的影响及其差异性。证明了氦泡形成的局域密度和压力相关性。（二）研究了β-C₃N₄体系的本征缺陷和杂质效应。证明在Ga间隙掺杂下,宽带隙中生成丰富的杂质能级,导致体系在可见光的广谱范围内的光学吸收和发射。（三）研究了多种典型的B-C-N超硬材料体系的力学特性和晶格动力学稳定性,预言了B₃CN₄和B₃C₄N两种具有导电性的BCN三元立方相材料,研究了高压超硬相B₃CN₄体系中的杂质行为。（1）提出了一种基于基本晶格振动模式下的定体焓分析方法,用以研究闪锌矿晶体的宏观稳定性及其合成的可能性,并且藉以给出材料合成压力。研究结果显示,8原子单胞描述下Zinc-blende结构的基本晶格振动模式中,呼吸模式A₁对外界压力的响应最为显著,导致该模式下体系的定体焓（constant volume enthalphy,CVE）变化剧烈。在宽广的压力范围内,金刚石、立方氮化硼晶体的CVE极小点与总能（total energy,ETOT）极小一致,表明体系是稳定的,并且其实验合成的成功具有其热动力学意义上的合理性。（2）证明了B₃CN₄在压力为～80GPa到～120GPa的范围内ETOT和CVE极小所对应的位形一致性,表明该结构在该压力区间内是稳定的,并且能够合成;B₃C₄N稳定于25～45GPa范围内,且其切变模量过小。（3）常压条件下两种BC₂N材料（BC₂N-1和BC₂N-2）的某些振动模式对CVE影响显著,且其极小与ETOT的极小值大幅度偏离,而且原子按照基本振动模的集体移动显示,平衡位形是总能的鞍点,这些证据表明BC₂N-1和BC₂N-2可能是不稳定结构。（4）研究了高压条件下B₃CN₄体系的杂质模型,结果表明金属性替位掺杂将产生多个浅层受主能级,而金属间隙原子形成三条深层杂质能级（带）。三元体系及其杂质相呈现金属性低频高反射。（四）作为材料物性研究的补充,系统地研究了晶格振动动力学和弹性力学量之间的关系,提出了利用Christoffel长波动力学矩阵,通过在倒易空间单位球内求解本征值,籍以判定材料力学稳定性的一般方法;提出了从动力学矩阵出发,计算宏观弹性常量的方法。更多还原

【Abstract】 Booming advances of computational technology make it possible to explore physical properties of materials, even design new materials. It is of vast importance to know deep into the composition, interactions and response to environment of a material via numerical calculations, since many body problems are impossible to be solved analytically. This paper systematically studied problems in solid state matter in three aspects using first principles method: the behavior of H and He in niobium, the existence of BCN ternary compounds and elastic stability of crystals.The problem of impurities in metals is continuous attractive, because not only the improvements on mechanical and electrical properties of a metal, but also the failures or malfunctions, especially for light elements as hydrogen and helium. Embrittlement and excessive ion implantation in metals will make the first wall of nuclear reactor shorten its mechanical lifetime. Recently, there is a trend to use hydrogen as a medium to make the full use of reproducible energy resources applicable. All these promote the researches on the behavior of hydrogen/helium in metals. On the other hand, thermonuclear reactors promote both the behavior of H and He in metals.The typical transition metal niobium is selected as the host. Different density of hydrogen is doped into Nb till niobium monohydride is formed. The interactions between Nb and H are studied, focusing mainly on the phase of NbH, the quantium behavior of H in Nb and the thermodynamic properties of both H and He doped Nb.The result shows that hydrogen has less effect on host lattice even for 1:1 dopant, because of the strong nd metallic bonding of transition metals. Since the mass of H is by far smaller than Nb, there exhibit considerable quantum effects. H is easier to be excited up, hovering in Nb lattices.Four types of NbH derived fromβ-phase is studied and all structures are shown to be dynamically stable.β-phaseis the lowest in total enthalpy, next D2d-1, D2d-9 and D4h-7. The electronic structure （DOS） of any structure is not in consistent with experimental EDC data. The DOS of NbH governed not only by nearest H-H distance, but also the local geometry. This implies that NbH may be the mixture of the four structure phase.Conventional cell of Nb₂H is used to calculate the potential experienced by H in Nb by directly evaluate the total energy at different H configurations. Schrodinger equation of H is solved numerically to obtain the energy levels and eigenstates of H, the 4T and 6T ring excited states are identified. The low density impure Nb lattice of 1/16 H dopant under pressures are studied, the lattice relaxation, electronic structure and optical absorption spectra are obtained.The behavior of helium in niobium is simulated using Car-Parrinello molecular dynamics methods. Two types of local He concentration are considered. The result shows that He behaves vast electronic stable such that the space occupied by He is a forbidden area for near free electrons of Nb. The existence of denser He damages the metallic bonding of Nb, producing vacancy/interstitial defects. Under ambient pressure, helium bubble formation depends on the local concentration and the existence of native vacancy. In perfect host lattice, high He content results in dislocations of neighboring Nb, lattice distortion and production of vacancy/interstitial of Nb and the bubble formation at the vacancy; a helium bubble will form direct at a nearer native vacancy with no new vacancy produced and less host lattice distortion. Under high pressures, no bubbles are formed due to the strong background interactions between Nb atoms and the relatively weakened He effects.Another type of impure systemβ-C₃N₄ is also studied. Before constructiong impure system, cubic C₃N₄ andβ-C₃N₄ are pre-studied. Cubic phase C₃N₄ is shown to be conductive, whileβ-C₃N₄ is semiconductive with a band gap of about 3.4eV（LDA）. The chemical potential of cubic C₃N₄ relative to raw materials is by far larger than that ofβ-C₃N₄, resulting in the difficulty in the synthesis of cubic C₃N₄, although it may be the only one expected to be harder than diamond. The two structures ofβ-C₃N₄ suggested by Cohen’s group and Hemley’s Group are different in symmetry but similar in electronic structure.Defected system ofβ-C₃N₄ including host vacancy, inter-substitution, native interstitial and group III metallic interstitial are studied. The interstitial gallium is found to have the most doping effect on the electronic structure and optical response ofβ-C₃N₄. Three deep impurity energy levels are observed in the main gap, with one of which stands on the Fermi Level. The impurity induced electronic bands enables electron transitions in almost all impurity band transition models. The doped system is active to electromagnetic field from far infrared to ultraviolet, with a reflect window of about 0.2eV at frequency 1.5eV. The doped system may be a potential optic material with high optical efficiency.Thirdly, the existence, stability and the probability to be synthesized of zinc-blende crystal are studied using constant volume enthalpy, in addition to total energy, atomic forces and lattice vibration calculations.Up to date, the probability to obtain a new material depends mainly on the optimized structure （atom coordination and lattice parameters）, the relative enthalpy and lattice vibration properties for stability investigation. But not all materials predicted by theory are successfully synthesized, although they are confirmed to be stable and probable to be synthesized at certain conditions by the methods above mentioned. On the other hand, considerable phase synthesized experimentally are not well interpreted by theory.In the present work we have gone further. The properties of a superhard material （B₃CN₄） are studied using ab initio pseudopotential methods within density functional framework. In addition to total energy, electronic structure and lattice dynamics, the normal vibration mode dependency of constant volume enthalpy （CVE） is systematically investigated in a wide pressure range up to about 200GPa. The new compound is shown to be conductive and comparable to BC₂N and cBN in basic mechanical properties, but dynamically stable in a narrow range from～80GPa to～120GP. The instability is confirmed by the acoustic mode softening around q-vector [0.5 0.5 0.0] and monotonic variation of CVE at normal vibration mode A₁. The lattice vibration mode dependence of CVE is expected to filter out the most stable structure of a material from the predictions based on total energy, atomic forces and lattice vibration calculations. The CVEs of kinds of zinc-blende materials such as diamond, cBN, ZnS etc. are systematically investigated and A₁ mode is found to play the dominant role in the crystallization. But for BC₂N-1 and BC₂N-2, negative results are obtained, showing that they are not stable at ambient pressures and BC₂N may not be described by 8-atom cell. This scheme is expected to clarify the most stable phase from those predicted by ab initio total energy, lattice vibration calculations.After the stable pressure range of B₃CN₄, impurity effects on its electronic structure are studied. Vacancy of C, C substuted by N, B substituted by Ga and interstitial Ga are all calculated though. The band distortion is strongly C content dependent, reflected in the optical response spectra. While C content goes down to 1/64, almost no optical response below ⁵eV is observed, while any of the other impurity systems has strong low frequency reflection/absorption just as metals. The interstitial dopant of Ga is found to produce deep impurity bands in the main gap, resulting in the blue-shift of metallic reflection edge.Additionally, the lattice dynamics and elastic properties of typical elastic crystal are studied in order to filter the abinitio calculated elastic constants and obtain the information of lattice stability. Since more than 3 independent elastic constants are needed to describe elastic crystals except cubic ones, it is difficult to confirm if the elastic constants are reasonable, or if the calculated crystal is stable. This may be handled out by solving eigenvalue problems of Christoffel dynamics matrices in a unit reciprocal sphere. A new method of calculating elastic constants is also proposed.更多还原