五种SiO_2晶体原子和电子结构的第一性原理研究

α-石英、β-石英、α-方石英、β-方石英和超石英是自然界中SiO_2常见的五种晶型.本文利用第一性原理平面波赝势方法,系统计算了五种SiO_2晶体的体弹性模量、电荷密度、电子态密度,能带结构,并和可获得的实验进行了比较.α-英、β-石英和具有空间群F d3m结构的β-方石英显示间接的带隙,而α-方石英,超石英和具有空间群P2_13和I42d结构的β-方石英显示直接的带隙.五种晶型SiO_2的带隙宽度均大于5.5 eV,都是绝缘体.     

The influence of impurities on the electric and thermoelectric properties of CdSb single crystals

The influence of impurities on the electric and thermoelectric properties of CdSb single crystals prepared by the modified Czochralski method and the method of zone melting was studied. It was found which impurities are electrically active and which give rise to electron and hole conductivity and a schematic model of their substitutions in the CdSb lattice was proposed. The temperature dependence of the mobility was found for both electrons and holes and the density effective masses were determined.Reported by L. toura at the Conference on Thermoelectricity, 10–12. 7. 1961, Durham, England [26].     

Ground-state structures of atomic metallic hydrogen

Ab initio random structure searching using density functional theory is used to determine the ground-state structures of atomic metallic hydrogen from 500 GPa to 5 TPa. Including proton zero-point motion within the harmonic approximation, we estimate that molecular hydrogen dissociates into a monatomic body-centered tetragonal structure near 500 GPa (r(s)=1.23) that remains stable to 1 TPa (r(s)=1.11). At higher pressures, hydrogen stabilizes in an …ABCABC… planar structure that is similar to the ground state of lithium, but with a different stacking sequence. With increasing pressure, this structure compresses to the face-centered cubic lattice near 3.5 TPa (r(s)=0.92).     

Study of the atomic and electronic structures of liquid gold

The electronic and atomic structures of liquid gold has been studied theoretically and experimentally using X-ray photoelectron spectroscopy (XPS) and a combination of classical molecular dynamics and ab initio band calculations. The results obtained showed good agreement between the XPS data and the results of calculations of the total density of states. It is shown that the bonding of atoms in liquid gold clusters is covalent.     

Engineering electronic lifetimes in artificial atomic structures

By means of atomic manipulation, 51 Ag atoms have been precisely positioned to form a triangle with a base length of 245 A on a Ag(111) substrate. The scattering of the surface electrons at these adatoms results in a complex interference pattern. Spectroscopic data and dI/dV maps taken inside the triangle have been quantitatively evaluated by multiple scattering calculations of the wave pattern. Adjustment of the scattering parameters to the data yields the properties of the scatterers and the electron lifetimes. The experimental results for the electron lifetimes deviate from a (E-E(F))(-2) dependence and reflect the electronic band structure at the surface as well as the local influence of the triangle.     

The influence of the static lattice distortion on the localized modes of atomic impurities in alkali halides

We have studied the influence of static distortion on the localized modes of a Li⁺ impurity in KCl and KBr using the molecular model. For off-center equilibrium positions with the impurity on a symmetry axis of the cubic host lattice the concept of structure elements has been set up to derive the symmetry properties of the vibrating molecular region. In the frame-work of a rigid ion model with only central forces the 111-off-center position turns out to be the only possible stable off-center position. For the system KCl:Li⁺ the measured resonant modes in the region of 40 cm^–1 can only be interpreted in this model as cluster vibrations. In addition we have calculated defect induced Einstein-modes with frequencies near the upper band edge.     

Nb原子链的结构稳定性和电子性质

使用基于密度泛函理论的第一性原理平面波赝势法，研究了Nb原子链的结构稳定性和电子结构性质.计算表明，Nb原子可以形成线性链，平面之字形，二聚化以及梯子形等一系列的一 维链式结构.结果也显示，其中之字形结构最为稳定，其他结构均为亚稳的.通过第一性原理 计算的电子结构和Jahn-Teller效应，讨论了这些结构的相对稳定性以及各链式结构的电子 能带、态密度和电荷密度等性质. 关键词： Nb原子链 结构稳定性 电子结构 从头计算     

Structural stabilities and electronic structures of Ti atomic chains

The present first principles density-functional calculations reveal that titanium can form one-dimensional chains in linear, dimer, zigzag and ladder structures. The most stable structure is a zigzag chain with a unit cell rather close to an equilateral triangular geometry with four nearest neighbors. Two intermediary chains between the linear and zigzag ones have the ladder and dimer structure, respectively. Titanium can also form a metastable zigzag structure with an obtuse bond angle. It is important and interesting to find that during the elongation of the zigzag chain, the bond angle will shift abruptly from a sharp angle to an obtuse one at a critical point, and the bonding character also transforms from mainly metallic to more covalent. This is the first time that such a structural transition is discussed in the atomic chain system. The electronic structures of these one-dimensional titanium chains are also discussed.     

KDP和尿素晶体电子结构的第一性原理研究

基于第一性原理的平面波超软赝势法对KDP(KH₂PO₄)和尿素(CH₄N₂O)晶体的能带结构、电子态密度、电荷差分密度以及布局分析进行了计算讨论.结果表明:尿素晶体中的C1-O1、C1-N1、N1-H2和N1-H1键都具有共价键特性,带隙值为4.636 eV,价带顶主要由H-1s与N、O的2p态贡献,导带底主要是H-1s与C、N、O的2p态贡献;KDP晶体的H1-O1键具有离子性而P1-O1则具有共价性,带隙宽度为5.713 eV,价带顶主要由O-2p以及P-3p贡献,导带底主要由H-1s、P-3s和3p以及K-4s和3p态贡献.     

The influence of cascade on the polarization of radiation produced in electronic or atomic collisions

A study is presented, in which we consider the influence of cascade effects on the polarization of light emission produced in electronic or atomic collisions. Cascade transitions are shown to cause a polarization transfer which may result in a total falsification and even in a sign reversal of the polarization. Closed expressions are derived for some special cases. A practical application is given in which the derived formulas are used to understand a serious discrepancy between theoretical predictions and experimental data in the case of the line polarization due to some 3p-3s transitions in neon induced by proton impact.     

The influence of hydrogen on the atomic distribution function and electrical resistance of amorphous chromium films

Electrical resistivity of amorphous chromium films (20–37 nm thick) deposited in a hydrogen atmosphere (P _{H 2}=8·10^–6–2·10^–4 hPa) onto a substrate cooled by liquid helium down to 2 K is measured, and electron-diffraction studies have been performed immediately after the quench condensation and after annealing to different temperatures up to 300 K. The preparation method employed permits a considerable hydrogen enrichment of the films to be reached. The maximum hydrogen concentration corresponds approximately to a stoichiometric composition of CrH. It is found that as the maximum concentration is approached the atomic distribution functionG(r) changes remarkably. The interatomic distances increase considerably (by 10%) and the atomic densities decrease. It is quite possible that amorphous chromium hydride is a final state with the maximum hydrogen concentration. For films with intermediate concentration,G(r) is found to vary substantially under annealing up to 90 K. The electron-diffraction and electronmicroscopic data, as well as the variations in resistivity due to annealing, suggest that with annealing up to 90 K, a hydrogen redistribution occurs in the amorphous films, initially homogenous in concentration. We observe also distinct indications of separation into phases with increased and reduced hydrogen contents.     

Stereo-PEEM for three-dimensional atomic and electronic structures of microscopic materials

The principle and design of a new photoemission electron microscope (PEEM), which is called Stereo-PEEM, is described here. Stereo-PEEM can display not only the image of microscopic materials but also the angular distribution of high-energy photoelectrons up to about ±60°, which is about 100-fold the acceptance angle of usual PEEM. This wide angle acceptance for high-energy photoelectrons enables the three-dimensional (3D) display of atomic structure as well as the 3D electronic structure of individual micromaterials. The 3D atomic structure of a sample can be observed directly by taking stereophotographs using photons with angular momentum.     

Infrared spectra of hydrogen impurities in BaTiO3 crystals

The IR spectra in the 2.8 μm region of the OH^? impurities in thick single domain crystals have been measured for different polarization geometries and various temperatures. Simple calculations of the Coulomb interactions between hydrogen and lattice ions show that these interactions are much more important than the hydrogen bonding ones for determining the positions of the hydrogen ions. The appearance of only one strong non-dichroic OH^? stretching line is due to an almost equal population of the proton axial sites. The side bands are interpreted as combinations of the stretching mode with a low-frequency translational mode. The large broadening of the main line just above T_c supports the model which attributes a considerable order—disorder character to the ferroelectric phase transition.     

锂原子修饰碳原子链团簇的结构和储氢性能研究

利用密度泛函理论研究锂原子修饰线型碳原子链团簇Li2Cm(m=2—8)的结构及其储氢性能. 结果表明, Li原子可键合于碳链团簇的两端,Li原子本身不发生团聚,氢在Li2Cm (m=2—8)中能以分子形式吸附,每一个Li原子最多可吸附5个氢分子,氢分子的平均吸附能为0.460 ~ 2.276 kcal.mol-1. 其中Li原子修饰C2团簇的质量储氢分数最大,为34.72 wt%,表明了它在常温常压条件下作为储氢材料的可行性.     

锂原子修饰硼原子链团簇的结构和储氢性能研究

采用密度泛函方法对锂原子修饰线型硼原子链团簇Li_2B_n(n=2~8)的结构及其储氢性能进行理论研究.结果显示,Li原子可键合于硼链团簇的两端,氢能以分子形式吸附在Li原子周围,每一个Li原子最多可吸附4个氢分子,氢分子的平均吸附能为2.020~2.832 kcal.mol~(-1).其中Li原子修饰B2小团簇的质量储氢分数最大,为31.24 wt%,表明在常温常压条件下它有可能成为一种潜在的储氢媒介.     

锂原子修饰硼原子链团簇的结构和储氢性能研究

采用密度泛函方法对锂原子修饰线型硼原子链团簇Li2Bn(n=2~8)的结构及其储氢性能进行理论研究. 结果显示, Li原子可键合于硼链团簇的两端,氢能以分子形式吸附在Li原子周围, 每一个Li原子最多可吸附4个氢分子, 氢分子的平均吸附能为2.020 ~ 2.832 kcal.mol-1. 其中Li原子修饰B2小团簇的质量储氢分数最大,为31.24 wt%,表明在常温常压条件下它有可能成为一种潜在的储氢媒介.     

Quasi-localized representation of electronic band structures in cubic crystals

Tabulated results and formulae are given for the parametrization of electronic band structures in simple cubic, body-centered cubic and face-centred cubic systems. This technique is based on the decoupling transformation and does not involve least squares fitting.A band labelling scheme is introduced which allows the decoupling transformation to be used to define the energy matrix of Wannier states in copper. This approach is used to re-examine the significance of the tight binding approximation for the d-states and to relate Fermi surface parameters directly to calculated band energies.     

The influence of substitutional imperfections on the electronic structure of refractory compounds. SP-element impurities in titanium nitride

Using the SCF MO LCAO (self-consistent MO LCAO method in the Mulliken-Wolfsbergelmholz approximation) cluster method and the Green's function method in the ASA-KKR (Korringa-Kohn-Rostoker method in the atomic spheres) approximation, the effect of substitutional impurities (O, C, B, Be, He) on the electronic structure of titanium nitride has been studied. The distributions of the local electronic states of impurity centers and the location of impurity levels in the total energy spectrum of the crystal have been calculated. A detailed discussion is presented of how the entire system of chemical bonding near a defect and the partial interatomic interactions vary as a function of the nature of the impurity atom. The results obtained are employed to interpret the available X-ray emission spectra of ternary systems, and also in treating problems of the mutual solubility of refractory binary phases and heterogeneous catalysis with their participation.A quantum chemical simulation of possible hydrogen atom positions in the nitride structure has been carried out. Hydrogen has been found to occupy preferentially tetrahedral interstices of the TiN lattice.     

The influence of atomic impurities on the orientational transitions in localized monolayer films of diatomic molecules adsorbed on crystalline surfaces

The orientational ordering in localized films consisting of a mixture of diatomic and mono-atomic particles and formed on the (100) face of a cubic crystal is considered. It is demonstrated that the presence of atomic impurities influence the formation of orientationally ordered states. In particular, it is shown that even a simple mean-field theory leads to a qualitatively correct picture of changes in orientational properties in the film when the molar ratio of spherically symmetric particles increases.