Mean-square atomic displacements in f.c.c. crystals

The mean-square displacements of volume atoms, surface atoms and atoms near a vacancy are calculated for f.c.c. metals and rare gas solids, using a simple series expansion of the Wallis formula. We employ a model with rotationally invariant bond bending forces, unlike the de Launay model used by Masri and Dobrzynski. For atoms near a vacancy and for rare gas solids this method has been applied for the first time. The calculations are done to the R⁴ term in the expansion. The inclusion of the R⁴ term together with temperature-dependent elastic constants give results which are in better agreement with experiment. For rare gas solids Lindemann ratios calculated by this method agree closely with the values obtained from elaborate computations as well as those deduced from entropy data. The present calculation also supports the view that the melting of solids is initiated at the crystal surfaces and in the neighbourhood of vacancies.     

A model for the lattice dynamics of b.c.c. transition metals

Recent ideas proposed by Fielek for the lattice dynamics of f.c.c. noble and transition metals have been extended to b.c.c. transition metals. Phonon dispersion curves, calculated on the basis of this model, for chromium are in fairly good agreement with experimental ones and are far better than those reported so far.     

Structural,elastic, electronic and optical properties of the cubic perovskites CaXO3 (X=Hf and Sn)

The structural, elastic, electronic and optical properties of CaXO₃ compounds with the cubic perovskites structure have been investigated, by employing a first principles method, using the plane wave pseudo potential calculations (PP-PW), based on the density functional theory (DFT), within the local density approximation (LDA). The elastic constants and their pressure dependence are calculated using the static finite strain technique. We derived the bulk, shear and Young’s moduli for ideal monocrystalline and for polycrystalline CaXO₃ aggregates which we have classified as ductile in nature. Band structures reveal that these compounds are indirect energy band gap (R-G) semiconductors; the analysis of the site and momentum projected densities, valence charge density bond length, bond population and Milliken charges, shows that bonding is of covalent–ionic nature. We have found that the elastic constants C₁₁, C₁₂, C₄₄ are in good correlation with the bonding properties. The optical constants, including the dielectric function, optical reflectivity, refractive index and electron energy loss, are calculated for radiation up to 20 eV.     

Lattice dynamics of b.c.c. alkaline-earth metals: Barium

Summary The attractive short-range component of the interatomic potential screens the conventional Born-Mayer potential in the framework of the resonance pseudopotential model. The elastic constants are evaluated at the long-wavelength limit of the phonon spectrum and the obtained results are compared with previous experimental values. The numerical calculations show that the attractive component of the potential explains the soft modes in this body-centred cubic (b.c.c.) alkaline-earth metal barium.     

Atomic chemisorption on simple metals: Chemical trends and core-hole relaxation effects

We have obtained essentially exact numerical solutions for a simple model of atomic chemisorption on simple metals. The approximations constituting the model are the semi-infinite jellium simulation of the metal substrate and the self-consistent local density theory of exchange and correlation. The solutions provide a detailed picture of the electronic charge making up the chemisorption bond. The variation of this picture with the valence of the adatom exhibits in a direct and microscopic way the roles of electronegativity, charge transfer, and covalency. Predicted bond energies, bond lengths, and dipole moments are consistent with measurements and independent theoretical considerations.     

高压下RhB的相变、弹性性质、电子结构及硬度的第一性原理计算

采用密度泛函理论中的赝势平面波方法系统地研究了高压下RhB的结构相变、弹性性质、电子结构和硬度.分析表明,RhB在25.3 GPa时从anti-NiAs结构相变到FeB结构,这两种结构的弹性常数、体弹模量、剪切模量、杨氏模量和弹性各向异性因子的外压力效应明显.电子态密度的计算结果显示,这两种结构是金属性的,且费米能级附近的峰随着压强的增大向两侧移动,赝能隙变宽,轨道杂化增强,共价性增强,非局域化更加明显.此外,硬度计算结果显示,anti-NiAs-RhB的金属性比较弱,有着较高的硬度,属于硬质材料.     

The elastic moduli,the volume contribution and the Cauchy ratio ford andf shell metals

The elastic constants of nine transition metals and four rare-earths and actinides are calculated using the ion-ion interaction defined by us recently. The volume contribution to elastic moduli is calculated by exploiting the density dependence of the screening function. The calculated volume contribution to bulk modulus is found to vary between 17.1% and 62.4% for a number of metals, which is quite significant and play an important role for describing quantitatively the violation of the Cauchy ratio for these metals.     

Elastic phase transitions in metals at high pressures

The elastic phase transitions of cubic metals at high pressures are investigated within the framework of Landau theory. It is shown that at pressures comparable with the magnitude of the bulk modulus the phase transition is connected with the loss of stability relative to uniform deformation of the crystalline lattice. Discontinuity of the order parameter at the transition point and its equilibrium value are expressed through the second-?to fourth-order elastic constants. The second-,third-?and fourth-order elastic constants and phonon dispersion curves of vanadium under hydrostatic pressure are obtained by first-principles calculations. Structural transformation in vanadium under pressure is studied using the obtained results. It is shown that the experimentally observed at P?≈?69?GPa phase transition in vanadium is the first-order phase transition close to a second-order phase transition.     

Variable charges at govalent bonds and phonon dispersion in silicon

A model of variable charges at covalent bonds is proposed to describe the effective atomic interaction in a crystal lattice. It is shown that, in the region of elastic deformations, it is sufficient to take account only of the linear term in the expansion of the charge with respect to the relative deformation of the bond. Expressions are obtained for the dynamic matrix on the basis of the pseudopotential model and the model of variable charges at the bonds. The results of calculating the phonon spectrum of silicon are in good agreement with experimental data and reflect the characteristic features of the spectrum.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 95–98, July, 1988.     

Angular forces in the lattice dynamics of b.c.c. metals

The lattice dynamics of the b.c.c. lattice has been investigated using the central and the angular forces which are in general rotationally invariant. It has been observed that the four force constants are related to three elastic constants only. Dispersion curves for sodium were calculated and were found to be in excellent agreement with experiment.     

The density of states of a single tight-binding band for a b.c.c. element

Summary An exact expression has been derived for the density of statesn(ε) of a single tight-binding band of a b.c.c. element, within the two-centred approximation with nearest-neighbour contribution. Using this expression, a simple approximate relation is suggested forn(ε), which can be useful in deriving various properties of transition metals. Supporting organization: Shahid Beheshty University.     

YPd3X（X=B,C）晶体结构、弹性性质与电子结构的第一性原理研究

在广义梯度近似（GGA）和GGA+U（在位库仑势）下,采用第一性原理方法系统地研究了三元过渡金属硼碳化合物YPd3X(X＝B,C)的晶体结构、弹性性质、电子结构和成键特性．计算的晶格参数和体弹性模量均与报道的实验结果吻合,而YPd3X(X＝B,C) 的弹性参数计算值则表明YPd3C的硬度大于YPd3B．根据晶体机械稳定标准得到YPd3B和YPd3C的失稳临界压强分别约为16.5GPa和23GPa．由Pugh经验关系可知YPd3X(X=B,C)均属于韧性材料,且YPd3B的韧性略高于YPd3C．电子能带结构分析表明YPd3B和YPd3C均具有金属特性,且导电能力相当．由态密度和电荷密度分析得知,X与Pd之间形成较强的共价键,而Y与Pd3X之间形成离子键,化学键键能的不同是两种材料的弹性参数存在差异的内在原因．上述的研究结果为YPd3X(X=B,C)的力电材料的设计和应用提供了一定的理论依据．     

Orientation dependence of screw dislocation motion in b.c.c. transition metals

A tight-binding type electronic theory is used to calculate the Peierls stress of the screw dislocation in b.c.c. transition metals. The repulsive core-core interaction energies are simulated by Born-Mayer type potentials as well as the modified Born-Mayer potentials. It is shown that there are essential differences in the orientation dependence of the screw dislocation motion among the transition metals, in agreement with experiments.     

First principles study of structural,phonon, optical,elastic and electronic properties of Y3Al5O12

Structural, phonon, optical, elastic and electronic properties of Y₃Al₅O₁₂ have been investigated by means of the first principles method with the Cambridge Serial Total Energy Package (CASTEP) code based on the density functional theory. The calculated lattice parameters, valence charge density, bond length and single crystal elastic properties at zero pressure are in good agreement with the available experimental data. The close agreement with the experimental values provides a good confirmation of the reliability of the calculations. Optical, elastic and phonon properties of Y₃Al₅O₁₂ under pressures are performed. The results that are obtained show the changes of optical and elastic properties under the influence of applied pressure, and proving the dynamical stability of YAG are destructed when applied pressure up to 7 GPa. Moreover, polycrystalline elastic moduli are deduced according to the Reuss assumption. Those elastic constants provide important parameters that describe reliability of both physical model and engineering application at the atomistic level. The result of the density of states explains the nature of the electronic band structure.     

Study of phase transformation and elastic properties of ThX (X = N,P, As and Sb) under high-pressure

An improved interaction potential model (IIPM) has been formulated to theoretically predict the pressure induced phase transition, elastic properties and thermophysical properties of thorium monopnictides (ThX; X = N, P, As and Sb). The phase transition pressures and volume drop obtained from this model show a better agreement with the available experimental than theoretical results. We have achieved elastic moduli, anisotropy factor, Poisson's ratio, Kleinman parameter, shear and stiffness constants on the basis of the calculated elastic constants. To know the anharmonic properties, we have also computed the third-order elastic constants, first-order pressure derivatives of second-order elastic constants and thermophysical quantities. Our results are in reasonable agreement with available measured and others reported data which supports the validity of model.     

A first-principles study of the structural, electronic and elastic properties of solid nitromethane under pressure

The structural,electronic and elastic properties of solid nitromethane are investigated under pressure by performing first-principles density functional theory(DFT)calculations within the generalized gradient approximation(GGA)and the local density approximation(LDA).The obtained ground state structure properties are found to be consistent with existing experimental and theoretical results.The pressure-induced variations of structure parameters(a,b,c and V)indicate that the solid nitromethane has an anisotropic compressibility,and the compression along the c direction is more difficult than along a and b directions.From the vibration curves of intermolecular bond length and bond angle,we find that the C—N bond is the most sensitive among these bonds under pressure,suggesting that the C—N bonds may be broken first under external loading.The influence of pressure on the electronic properties of solid NM has been studied,indicating that solid NM is an insulating compound with a large indirect band gap and tends to be a semiconductor with increasing pressure.Finally,we predict the elastic constants and their pressure dependence for the solid NM with the bulk modulus,Young’s modulus,shear modulus and the Poisson’s ratio derived.     

立方钙钛矿RbZnF3电子、弹性和光学性质的第一性原理研究

基于密度泛函理论和赝势平面波方法研究了立方钙钛矿RbZnF3的电子结构和光学性质;利用静水有限应变技术计算研究了RbZnF3弹性常数Cij、体积弹性模量B和剪切模量G随压力的变化关系。基态下,RbZnF3晶格常数a和体积弹性模量B0计算值与实验值以及其他理论值一致。根据能带结构、总态密度以及分波态密度分析可知：基态下立方钙钛矿结构RbZnF3为间接带隙半导体材料,带隙为3.57eV,与其他计算结果比较,本文计算结果偏低,这是由于局域密度近似(LAD)或广义梯度近似(GGA)交换关联函数的局限性所致。基态下RbZnF3的Mulliken电荷分布和集居数说明：RbZnF3属于共价键和离子键所形成的混合键化合物;RbZnF3的电荷总数主要来源于Rb 4s和4p轨道,Zn 3d轨道,以及F 2s和2p轨道。电荷主要从Rb, Zn原子向F原子转移。同时,本文还计算研究了RbZnF3的光学介电函数、吸收系数、复折射率、能量损失谱和反射系数等光学性质。     

3d过渡金属在NiAl中的占位及对键合性质的影响

通过赝势平面波方法系统地研究了3d过渡金属元素在B2-NiAl中的占位以及对键合性质的影响.通过形成能得出Sc,Ti,V 和Zn元素优先取代NiAl中的Al位,而Cr,Mn,Fe,Co和Cu优先取代Ni位.通过分析晶格常数变化量、电荷聚居数、交叠聚居数以及价电荷密度分布, 讨论了晶格畸变和键合性质的变化.结果表明: 取代Al的Sc,Ti,V和Zn元素掺杂使NiAl中晶格发生畸变,这对NiAl键合性质的变化起着重要作用,这些掺杂元素与第一近邻Ni原子产生强烈的排斥作用,形成反键,同时它们之间发生较大的电荷转 关键词： NiAl金属间化合物 3d过渡金属 第一性原理 键合性质     

Topological catastrophe and isostructural phase transition in calcium

We predict a quantum phase transition in fcc Ca under hydrostatic pressure. Using density functional theory, we find, at pressures below 80 kbar, the topology of the electron charge density is characterized by nearest neighbor atoms connected through bifurcated bond paths and deep minima in the octahedral holes. At pressures above 80 kbar, the atoms bond through non-nuclear maxima that form in the octahedral holes. This topological change in the charge density softens the C' elastic modulus of fcc Ca, while C44 remains unchanged. We propose an order parameter based on applying Morse theory to the charge density, and we show that near the critical point it follows the expected mean-field scaling law with reduced pressure.     

Highly charged ions in field evaporating 4d-transition metals

An energy-focused time-of-flight atom-probe has been employed in a search for high ionic charges obtained in field-evaporating Zr, Nb, Mo, Rh, Pd and Ag, the 4d-transition metals. Although no new higher charges than the already established ones are found for these metals the maximum abundance of the highest charge is determined in each case. An empirical rule predicting the limits of the charge of field-evaporating ions is proposed.