Electron Density Functional Method Research Articles

Zinc glycolate Zn(OCH2CH2O): Synthesis and structure, spectral and optical properties, electronic structure and chemical bonding

This article presents new findings obtained from the study of formation conditions, crystal structure, thermal, spectral, optical properties and electronic band structure of zinc glycolate Zn(OCH2CH2O). This compound was synthesized by heating the solutions of zinc formate Zn(HCOO)2·2H2O in ethylene glycol (A) or in a mixture of ethylene glycol and distilled water (B). The crystal structure of Zn(OCH2CH2O) has been studied using the X-ray powder diffraction method. It is shown that the crystal structure is built via zigzag joining of [Zn4O12C8H16] tetracycles with the tetrahedrally coordinated zinc (ZnO4). Zinc atoms inside the tetracycles and the tetracycles themselves are interconnected with oxygen bridges. Complex anions OCH2CH2O2- are bonded to zinc atoms by chelation. The unit cell parameters of Zn(OCH2CH2O) are as follows: the tetragonal structure, space group I41/a (88−2), Z = 16, a = b = 11.08673(9) Å, c = 11.5902(1) Å, V = 1424.62(2) Å3. The IR and Raman spectra of Zn(OCH2CH2O) correlate fully with the results of structural analysis. Under UV excitation, the luminescence spectra of Zn(OCH2CH2O) samples synthesized following the methods (A) and (B) are characterized by emission maxima at 460 nm (blue luminescence) and 540 nm (yellow-green luminescence), respectively. Yellow-green luminescence is due to the presence of an admixture of zinc oxide nanoparticles of size 10 nm in the sample. The electron density functional method is employed to study the electronic band structure and chemical bonding in Zn(OCH2CH2O). It is shown that the 3dZn orbitals are covalently bonded to 2pO orbitals so that an octagon is formed, where the zinc atoms of four neighboring ZnO4 tetrahedrons are linked through their vertices. The feasibility of synthesizing a layered structure of Zn(OCH2CH2O) is analyzed on the basis of ab initio calculations and Voigt-Reuss-Hill theory.

Протонпроводящая керамика на основе гафната и церата бария, легированных оксидами циркония, иттрия и иттербия для электролитов топливных элементов

Nanopowders of the compositions BaHf1 – xYbxO3 – δ (x = 0.04; 0.08; 0.10) and BaCe0.9 – xZrxY0.1O3 – δ (x = 0; 0.5; 0.6; 0.7 and 0.8) were synthesized by the combined crystallization and nitric acid salts using the citrate-nitrate method. Those nanopowders were used to produce ceramic materials with a cubic crystal structure of the perovskite type, with a grain size of ~ 20 – 70 nm. The study of electrophysical properties revealed that they have a proton type of conductivity in the temperature range of 500 – 700 °C; σ = 10–2 – 10–5 Cm/cm. Type and mechanism of electrical conductivity of ceramics of the composition BaHf1 – xYbxO3 – δ (x = 0.04; 0.08; 0.10) have been studied both experimentally and using theoretical calculations-by computer modeling using the electron density functional method; the results are in good agreement. The research shows the prospects of using the obtained ceramic materials as proton-conducting electrolytes of solid-oxide fuel cells.

Comparative Study of Internal Mechanical Stresses in the Structures of Montmorillonite and Halloysite

Obtaining nanoscale tubes and scrolls is of fundamental and practical interest for science. In this study, the ability of montmorillonite and halloysite monolayers to curl into nanoscale scrolls is investigated using the electron density functional theory method on the example of studying the internal mechanical stresses in them. As a result of the study, it is revealed that mechanical stresses are present in both structures; however, they can be caused by the twisting of only the halloysite layers, due to the peculiarities of the structure. The mechanical stresses in montmorillonite are roughly double those in halloysite. However, the symmetrical structure of montmorillonite does not favor the formation of nanoscale scrolls and tubes, in contrast to the asymmetric structure of halloysite.

Peculiarities of the Electron Structure of Pseudobinary Alloys (GeTe)m–(Sb2Te3)n

The electron band structure of GeTe, Sb2Te3, GeSb2Te4, and Ge2Sb2Te5 compounds has been calculated by the electron density functional method using the WIEN2k software package. These compounds belong to the class of (GeTe)m–(Sb2Te3)n pseudobinary alloys, which pass from the crystalline to the amorphous state and vice versa under laser irradiation or an electric pulse for extremely short times on the order of 1–100 ns. A detailed analysis of the parameters of critical points (maxima, minima, and inflection points) in the electron density distribution, located at the high-symmetry points of crystal structure, is performed. Characteristic values of the parameters of critical points in the electron density distribution are found for this class of materials.

Stability of Defectless Structures of Titanium Monoxide at High Pressures

Titanium monoxide with the basis crystal structure B1 is of interest because of an anomalously high concentration of vacancies and diverse effects of atom–vacancy ordering. It was previously believed that the application of relatively low pressures to such systems does not change the crystal structure type, and the crystal structure is densified through a decrease in the equilibrium concentration of defects. The genetic algorithm to search for optimal structures and calculations by the electron density functional method have demonstrated that phases with a structure derivative of the B1 structure should be metastable in a wide pressure range from 0 to 100 GPa. Two defectless hexagonal modifications—the e-TiO and H-TiO phases—are thermodynamically stable at P 28 GPa, respectively. These phases demonstrate a pronounced pseudogap at the Fermi level and, thereby, have a low electrical conductivity.

Study of the Chemical Bond in Li2 – yFe1 – xMnxSiO4 (x = 0.0, 0.5, 1.0; y = 0.0, 2.0) by the Method of Computer Simulation

The electron structure of Li2MnSiO4 and Li2FeSiO4 in a layered orthorhombic crystal structure of Pmn21 is studied by the electron density functional method. Using the analysis of the density of crystal orbital Hamilton populations (COHPs), the features of chemical bond formation in these substances are studied. Anisotropy of the chemical bond of Mn with oxygen atoms is observed for Li2MnSiO4 with the complete extraction of lithium atoms from the structure. The formation of anisotropy of the chemical bond can indicate that Mn is trying to change the coordination and the beginning of the restructuring of the compound structure and its reduced stability.

X-Ray Spectral and Theoretical Study of Electronic Structure and Features of Interatomic Interactions in Phenylaminosilanes HnSi(NHC6H5)4–n (n = 3–0)

Electronic structure and features of interatomic interactions in a series of phenylaminosilanes HnSi(NHC6H5)4–n (n = 3–0) have been studied by means of computer-based simulation (electron density functional method) in comparison with experimental (X-ray and photoelectron spectroscopy). Distributions of density of electronic states of the valence band, correlation diagrams of the energy levels, and visualizations of the isosurfaces of molecular orbitals have been obtained. Major types of interatomic electronic interactions providing the Si–NH–C6H5 bonds in tetraphenylaminosilane have been elucidated.

Magnetic Properties of Vacancies and Doped Chromium in a ZnO Crystal

The electronic and magnetic properties of ZnO containing Cr doped atoms and Zn and O vacancies in its crystal structure are theoretically investigated. Calculations are performed using Atomistix Tool Kit and Vienna Ab-initio Simulation Package software implementing the electron density functional theory method with the Hubbard correction. It is shown that the magnetic moment of a defect supercell strongly depends on the impurity concentration and presence of vacancies. The doping of an oxygen atom increases the probability of zinc-vacancy formation.

Магнитные свойства вакансий и внедренного хрома в кристалле ZnO

AbstractThe electronic and magnetic properties of ZnO containing Cr doped atoms and Zn and O vacancies in its crystal structure are theoretically investigated. Calculations are performed using Atomistix Tool Kit and Vienna Ab-initio Simulation Package software implementing the electron density functional theory method with the Hubbard correction. It is shown that the magnetic moment of a defect supercell strongly depends on the impurity concentration and presence of vacancies. The doping of an oxygen atom increases the probability of zinc-vacancy formation.

Mathematical modelling of convective processes in a weld pool under electric arc surfacing

The authors develop the mathematical model of convective processes in a molten pool under electric arc surfacing with flux-cored wire. The model is based on the ideas of how convective flows appear due to temperature gradient and action of electromagnetic forces. Influence of alloying elements in the molten metal was modeled as a non-linear dependence of surface tension upon temperature. Surface tension and its temperature coefficient were calculated according to the electron density functional method with consideration to asymmetric electron distribution at the interface “molten metal / shielding gas”. Simultaneous solution of Navier-Stokes and Maxwell equations according to finite elements method with consideration to the moving heat source at the interface showed that there is a multi-vortex structure in the molten metal. This structure gives rise to a downward heat flux which, at the stage of heating, moves from the centre of the pool and stirs it full width. At the cooling stage this flux moves towards the centre of the pool and a single vortex is formed near the symmetry centre. This flux penetration is ∼ 10 mm. Formation of the downward heat flux is determined by sign reversal of the temperature coefficient of surface tension due to the presence of alloying elements.

Змочування графену метанолом або водою

The spatial distributions of the valence-electron density and the total energy reliefs for water (or methanol) migration on the free surface of graphene are obtained, by using the electron density functional and ab initio pseudopotential methods. Water and methanol molecules are found to migrate along the surface of graphene with an energy relief with barriers and wells. The interaction of water molecules located on the opposite sides of the graphene plane through the regions in the graphene plane with a low electron density is detected. The hovering of molecule over the graphene plane is found to locally change plane’s conductivity. The estimate of energy costs during the propagation of adsorbent molecules over the graphene surface testifies to the graphene hydrophobicity.

The effects of germanium on the configurations and magnetic moments of nickel clusters

We report ab initio calculations of the structures, binding energy and total spin moment of NinGe (n=2–12) clusters using all electron density-functional method. Our results show that the doped Ge makes the stability of NinGe clusters weakened as compared with pure Nin+1 clusters. The Ni6Ge and Ni9Ge clusters possess relatively higher stability. Compared with the magnetism of pure Nin+1 clusters, the magnetism of NinGe (n=2–10) clusters is reduced, which is attributed mainly to the nor-magnetism Ge element and the weakening of spin polarization of the Ni atoms. For Ni11Ge and Ni12Ge, the spin polarization of Ni atoms is enhanced. Furthermore, it can be interpreted that the size effect which should contribute to the increase of the magnetic moment for Ni12Ge, and the surface enhance effect should be responsible for the increase of the magnetic moment of Ni11Ge.

Electron-phonon interaction in short-period (GaAs) m (AlAs) n (001) superlattices

The deformation potentials of electron scattering at short-wavelength phonons for intervalley transitions in the conduction band of short-period (GaAs)m(AlAs)n (001) (m, n = 1, 2, 3) superlattices are determined by the electron density functional method. The dependences of the electron and phonon states and deformation potentials on the layer thickness in the superlattices are analyzed. The results of ab initio calculations are in good agreement with the data of empirical calculation of the deformation potentials integrated over phonons, but differ from data on the corresponding potentials for partial scattering channels because of approximations of the phenomenological model of interatomic binding.

Effect of Zr on the mechanical properties of BaTiO3

Values of the bulk modulus of BaTi1 − xZrxO3 (x = 0.000, 0.250, 0.375) have been calculated using the electron density functional method. It has been shown that incorporation of zirconium results in the induction of local distortions in the structure that have an impact on the decrease in the bulk modulus.

Physical and mathematical model of the surface segregation in binary alloys of transition metals

The surface segregation in binary alloys of transition metals based on platinum and palladium with different surface-face orientations are simulated via the electron-density functional method. The concentrations of the surface-active components of the binary alloys and displacements of the surface ion planes thereof are calculated self-consistently. The influence of surface segregation and lattice relaxation on the surface energy and electron work function on the surface of alloys of transition metals with various concentrations is investigated. The simulation results are compared with experimental data.

Elastic constants of solids at high pressures

The isothermal and adiabatic nth-order (n ≥ 2) elastic constants of a loaded crystal are defined. These constants fully determine the behavior of solids at an arbitrary load and are controlled by both an interatomic interaction and an applied load. Expressions that relate these constants (of the second, third, and fourth order) to Brugger elastic constants of the corresponding order, which are only determined by an inter-atomic interaction, are found for cubic symmetry crystals under hydrostatic pressure. These expressions are used to calculate the equation of state and the second- and third-order elastic constants of bcc tantalum at T = 0 K over a wide pressure range (0–600 GPa) using an electron density functional method. The results of calculating the equation of state and the second-order elastic constants agree with available experimental data and the calculation results obtained in other works.

Generalized model of metal adsorption with inclusion of the effects of substrate lattice relaxation: Adsorption of gold atoms onto aluminum

The geometric and energy characteristics of Au atom adsorption onto an Al(100) surface have been calculated self-consistently in the framework of the electron density functional method. These calculations take into account the lattice relaxation of two near-surface substrate layers and the first Au adlayer. It is shown that the interstitial regions on the Al(100) surface are the most advantageous adsorption regions.

Adhesion of niobium films to differently oriented α-Al2O3 surfaces

The atomic and electronic structures of the Nb/α-Al2O3 interface are studied by the electron density functional method. The structural and electronic properties of three corundum surfaces, as well as chemical bonds produced by metallic niobium films at variously oriented interfaces, are discussed. Relations between the electronic structure, geometry, and mechanical properties of the interfaces are analyzed. It is shown that the adhesion of niobium films to a great extent depends on the type of oxide surface.

Calcium coated B80 fullerene: A study on various coating configurations of B80

We investigate the adsorption of calcium atoms on the surface of novel boron fullerenes and the α-boron sheet using an all electron density functional method within the generalized gradient approximation. Recent theoretical works show that the B80 fullerene coated with one Ca atom on each pentagonal ring is capable of storing up to 60 H2 molecules, yielding a gravimetric density of 8.2wt.%. We have performed a detailed investigation of Ca adsorption on the B80 fullerene. At variance with recent results, our calculations show that a single Ca atom prefers to occupy a hexagonal site rather than a pentagonal site. The calculations on Ca12B80, Ca20B80 and Ca32B80 indicate that Ca atoms prefer to uniformly coat the whole surface over selectively occupying the pentagonal or hexagonal sites.

Calculation of the temperature-dependent energy characteristics of adsorption systems based on transition metals

Self-consistent calculations of the temperature-dependent absorption energy and electron work function on the surface of absorption systems based on transition metals have been performed via the electron-density functional method. The influence of substrate surface relaxation on the energy characteristics of adsorption systems is investigated. Gradient corrections to kinetic and exchange-correlation energies are estimated to describe a strong inhomogeneity arising in the electron system of the subsurface region. The influence of electron-ion interaction on energy characteristics is calculated with the use of the Ashcroft pseudopotential. The temperature-dependent formation of various adsorption structures from transition metal atoms is discussed.