ELECTRONIC STRUCTURES AND GEOMETRY OF THE Al12Si CLUSTER SOLID

The electronic structures and geometry of a solid assembled out of cubic close packing of Al₁₂Si clusters have been studied with an ab initio pseudopotential method and within the local density functional theory. Both the lattice constant of the solid formed and the atomic geometry of the clusters in it have been optimized through the ab initio method without adjustable variables. Our results show that the crystal composed of Al₁₂Si clusters is a metal rather than a semiconductor. Interactions between Al₁₂Si clusters in the solid are strong and the clusters are no longer inert under crystal field.     

Mechanical Properties and Electronic Structures of Cotunnite TiO2

Based on the first-principles plane-wave basis pseudopotential calculations, we investigate mechanical properties and electronic structures of the hardest known oxide, cotunnite TiO2. The calculated results show that cotunnite TiO2 has the highest bulk modulus （348 GPa） and hardness （32 GPa） among the high-pressure phases of TiO2, but its mechanical properties are not superior to those of c-BN. Moreover, the high hardness of cotunnite TiO2 can be understood from both the dense crystal structure （high valence electron density and short bond lengths） and the unusual mixtures of covalent and ionic bonding of Ti-O.     

Phonon dispersion relations of tetrahedrally-bonded crystals

The lattice dynamics of the tetrahedrally-bonded compounds is formulated from first principle by using a model of binding force proposed by us, which mainly includes covalent interactions in the perturbational treatment based on the pseudopotential formalism and also partly on ionic interactions. Numerical calculations are performed for GaP and ZnS. In spite of our introducing no adjustable parameter except the optical phonon-frequency splitting in the long-wavelength limit, the resulting phonon dispersion curves are qualitatively in good agreement with the observed data. We show that ionic contributions are important for the optical phonon frequencies of these compounds.     

Crystal field interactions in DyFe2Si2 and DyFe2Ge2

Two sets of crystal field (CF) parameters have been proposed for DyFe₂Si₂, none of which could provide a simultaneous explanation of the available experimental data, particularly at low temperatures (below 100 K). The set derived from magnetic studies could not even explain the thermal variation of the magnetic specific heat reported in the same work. Although the set of CF parameters, obtained from a fit to the Mossbauer spectra, could provide a fairly good explanation of the thermal variation of the magnetic susceptibilities along the c-axis, it could not explain the observed thermal variation of other reported experimental findings. In the present work, an appraisal of the CF parameters proposed earlier has been done and a set of CF parameters has been derived, which provide a simultaneous explanation of all the available experimental data. The effect of substitution of Ge for Si on the magnetic properties and the magnetic specific heat of DyFe₂Si₂ has been studied in the framework of one electron crystal field model. The inelastic neutron scattering studies and EPR measurements are required to check the predicted Stark energies and the paramagnetic resonance g-values.     

First-Principles Calculations for Structures and Melting Temperature of Si6 Clusters

We investigate the structures and the melting temperature of the Si6 cluster by using the first-principles pseudopotential method in real space and Langevin molecular dynamics. It is shown that the ground structure of the Si6 cluster is a square bipyramid, and the corresponding melting temperature is about 1923 K. In the heating procedure, the structures of the Si6 cluster change from high symmetry structures containing 5-8 bonds, via prolate structures containing 3-4 bonds, to oblate structures containing 1-2 bonds.     

Ab initio calculations of surface structure and electronic properties caused by adsorption of Ca atoms on a Si(1 1 0) surface

The adsorption of Ca metals onto a Si(1 1 0) surface has been theoretically investigated by first-principle total-energy calculations. We employed a local density approximation of the density functional theory as well as a pseudopotential theory to study the atomic and electronic properties of the Ca/Si(1 1 0) structure. The (1×1) and (2×1) surface structures were considered for Ca coverages of 0.5 and 0.25 ML, respectively. It is found that the (1×1) phase is not expected to occur even for rich Ca regime. It was found that Ca adatoms are adsorbed on top of the surface and form a bridge with the uppermost Si atoms. The most stable structure of Ca/Si(1 1 0)-(2×1) surface produces a semiconducting surface band structure with a direct band gap that is slightly smaller than that of the clean surface. We have observed one filled and two empty surface states in the gap region. These empty surface states originated from the uppermost Si dangling bond states and the Ca 4s states. Furthermore, the Ca-Si bonds have an ionic nature with almost complete charge transfer from Ca to the surface Si atoms. The structural parameters of the ground state atomic configuration are detailed and compared with the available results of metal-adsorbed Si(1 1 0) surface, Ca/Si(0 0 1), and Ca/Si(1 1 1) structures.     

晶体电光和非线性光学效应的离子基团理论(Ⅰ)——利用氧八面体畸变模型计算BaTiO3晶体电光及倍频系数

本文对钙钛矿型晶体的电光及倍频效应的机理提出了一个(MO₆)离子基团模型。并从(TiO₆)离子基团的准分子轨道及晶格场位能作用下的(TiO₆)离子基团的离子键轨道出发,利用ABDP理论,分别计算了BaTiO₃的各个电光及倍频系数。计算结果表明,在没有引入任何可调整参量的情况下,用(TiO₆)离子基团的晶格场理论所得到的电光及倍频系数的计算值和实验值符合得相当好,而准分子轨道对这些效应的贡献很小。由     

Structural, electronic, elastic and thermal properties of Mg2Si

First-principles calculations of the lattice parameter, electron density maps, density of states and elastic constants of Mg₂Si are reported. The lattice parameter is found to differ by less than 0.8% from the experimental data. Calculations of density of states and electron density maps are also performed to describe the orbital mixing and the nature of chemical bonding. Our results indicate that the bonding interactions in the Mg₂Si crystal are more covalent than ionic. The quasi-harmonic Debye model, by means of total energy versus volume calculations obtained with the plane-wave pseudopotential method, is applied to study the elastic, thermal and vibrational effects. The variations of bulk modulus, Grüneisen parameter, Debye temperature, heat capacity C_v, C_p and entropy with pressure P up to 7 GPa in the temperature interval 0-1300 K have been systemically investigated. Significant differences in properties are observed at high pressure and high temperature. When T<1300 K, the calculated entropy and heat capacity agree reasonably with available experimental data. Therefore, the present results indicate that the combination of first-principles and quasi-harmonic Debye model is an efficient approach to simulate the behavior of Mg₂Si.     

Two-parameter pseudopotential

A new physical model for determining the parameters of the two-parameter pseudopotential based on the Heine-Abarenkov approach is proposed, which in fact made it possible to pass to the one-parameter pseudopotential. The pseudopotential parameters are determined for atoms and ions of the second and third periods and Sr II ion. It is shown that the proposed two-parameter pseudopotential with determined parameters allows calculation of the position of excited one-electron levels of ions with good accuracy.     

Self-consistent pseudopotential calculations of the equilibrium properties of bulk and surface Si

A momentum-space formalism is used study the equilibrium properties of bulk and surface Si based on the self-consistent pseudopotential method. The calculated equilibrium lattice constant and the crystal energy are in agreement with experiment. By minimizing the total energy of the system, the equilibrium relaxation for the Si (111) surface was found to be 0.15 Å, in excellent agreement with measurements on the impurity-stabilized relaxed surface.     

Application of a structure factor-potential relationship to the electrical resistivity of the liquid alkali metals

In the Ziman formulation for the electrical properties of liquid metals, the resistivity depends on an average of the product of the structure factor and the pseudopotential. Ascarelli, Harrison and Paskin have derived a relationship for small wave vector between the structure factor and the pseudopotential for liquid metals such as the alkali metals. This formulation has been used over the entire range of wave vector (k = 0 to 2k _F). The resistivities of Na, K, Rb and Cs calculated with no adjustable parameters are within 25% of the observed values, while Li is underestimated by about a factor of five. The temperature dependencies of all but Li (which is anomalously non-linear) are in similar agreement with experiments made at constant volume.     

Relevance of non-linear screening in covalent semiconductors

The electronic response of a Si crystal to both long- and short-range external periodic perturbatations is calculated by the SCF pseudopotential method. The decomposition of the total response into linear and higher order contributions shows that non-linear terms are quite important and that their relevance increases with the localization of the perturbation.     

Electrical resistivity of the Ti\mathsf{_4}O\mathsf{_7} Magneli phase under high pressure

The electronic and positronic properties of the pentanary semiconductor alloys Ga_xIn_1-xP_ySb_zAs_1-y-z lattice matched to GaSb have been studied. The electron wave function is calculated semiempirically using the pseudopotential band model under the virtual crystal approximation. The positron wave function is evaluated under the point core approximation for the ionic potential. Electronic and positronic quantities namely, electronic structure and band gaps, positron band structure, effective mass and affinity, and electron-positron momentum densities have been predicted and their dependence on the phosphorus composition has been discussed. Received 30 August 2002 / Received in final form 12 February 2003 Published online 24 April 2003     

用赝势方法计算某些简单金属的状态方程

本文提出了一种确定简化的Heine-Abarenkov模型赝势参数的新方法,使用零温、零压下晶体的体积模量和晶格常数的实验值来确定赝势参数γ_c以及电子-离子平均相互作用能的修正系数H。用这种模型势和二阶微扰理论计算了十一种简单金属的状态方程。在293K计算出的压力-体积关系与实验数据进行了比较。除了Zn在不同实验数据之间本身就偏离较大外,理论与实验的符合都比较好。 关键词：     

晶体电光和非线性光学效应的离子基团理论(Ⅲ)——利用畸变氧八面体的离子基团模型计算LiNbO3,LiTaO3,KNbO3,BNN晶体的电光和倍频系数

本文用文献[1]中所提出的(MO₆)离子基团模型,计算LiNbO₃,LiTaO₃,KNbO₃,BNN等晶体的电光和倍频系数。假设在O_h对称时,这些晶体中的氧八面体具有相同的能级和对称波函数,则通过群表示理论就可得到在C_3ν,C_2ν对称性时氧八面体的能级和对称波函数,并进而用ABDP理论计算它们的电光和倍频系数。计算结果和实验符合得相当好。本文并从理论上解释了这些晶体的倍频系数大小、符号和氧八面体畸变间的关系,由此可以得到以下两个结论:(1)畸变氧八面体的离子基团模型不但适用于钙钛矿型材料,同时也适用于钨青铜型、LiNbO₃型材料。(2)在钨青铜型、LiNbO₃型的材料中,仍是“离子键”对电光和倍频效应作出主要贡献,同时由于LiTaO₃的共价键成份比LiNbO₃大,因而LiTaO₃的非线性光学效应比LiNbO₃小。     

Structural phase transitions in Si and Ge under pressures up to 50 GPa

The crystal structures of Si and Ge were studied by energy dispersive X-ray diffraction at room temperature and pressures up to 50 GPa. Si transforms to a primitive hexagonal (Si-V) structure around 16 GPa, to an intermediate phase Si-VI between 35 and 40 GPa, and to hcp (Si-VII) around 40 GPa. In contrast, Ge remains in the β-tin structure up to at least 51 GPa. The pseudopotential method reproduces these differences in the high-pressure behavior of Si and Ge.     

Bulk modulus and its pressure derivatives of cuprous halides

The ab initio pseudopotential approach to the total crystal energy is presented using local DF formalism. The expressions for bulk modulus, its first and second pressure derivatives for group I-VII semiconductor binary compounds are derived. The expression for the second pressure derivative of the bulk modulus for four-fold crystal structures is derived for the first time within the pseudopotential framework. The computed results of the bulk modulus for cuprous halides are very close to the available experimental data.     

Semi-empirical APW calculation of the band structure of silicon

We propose a modification to the usual augmented-plane-wave muffin-tin approximation approach which fits exactly the measured value of the band gap of Si. The method involves one adjustable parameter which is introduced by a uniform shift of the s-like logarithmic derivatives. The results are in good agreement with those of the empirical pseudopotential method not only for the gap but also for an energy range of approximately 3eV from the top of the valence band.     

First principles vibrational dynamics of magnesium telluride

The lattice dynamics of large-gap semiconductor MgTe compound at various crystallographic phases; rocksalt (B1), zincblende (B3), NiAs (B8₁) and wurtzite (B4), has been investigated from first principles calculations based on density functional theory (DFT) within plane-wave pseudopotential method and generalized gradient approximation (GGA) of the exchange-correlation functional. The static equation of states of the compound has been studied with Vinet equation of states. The ground state of the compound is a fourfold coordinated wurtzite structure, which is consistent with experiments and recent theoretical calculations. Full phonon dispersion spectra of all related phases of the MgTe have been calculated using density functional perturbation theory within the linear-response approach. In view of the total energy calculations and the obtained vibrational spectra, it can be emphasized that the MgTe polymorphs with tetrahedral coordination (zincblende and wurtzite structures) are of covalent character rather than ionic. The large TO-LO splitting of phonon branches of rocksalt and NiAs phases reflect the high ionicity of these phases.