Valence Electron Distribution Research Articles

Influence of pressure on the valence bond structure of tungsten

The valence bond structure of tungsten is redetermined at ambient pressure and calculated for pressures up to 300 GPa using the energy and shape method. The outer shell valence electronic distribution of tungsten at ambient pressure is e c 3.7408 ( d c 2.9268+ S c 0.2040 p c 0.6100) e f 2.2592. The value of the single-bond radius is 1.2532 Å. Under pressure, the covalent electrons e c ( d c, s c and p c) increase while near free electrons e f decreases with increasing pressure. The number of covalent electrons e c is 3.7408 under ambient conditions and 5.0321 at 300 GPa and the number of near free electrons e f changes from 2.2592 at ambient pressure to 0.9679 at 300 GPa. The bond length and single-bond radius decrease while bond valence increases with increasing pressure. These results indicate that the electron distributions are more localized and the bonding character of metal becomes more covalent bonding with increasing pressure.

Temperature-dependent valence bond structure study of tantalum

Based on energy and shape method for determining the VB structures of crystal, the valence bond structure of tantalum is redetermined at room temperature and extended to the whole temperature range. The outer shell valence electronic distribution of Ta at room temperature is e c 2.9011 (d c 2.2711+s c 0.1580+p c 0.4720) e f 2.0989 in crystal. The single-bond radius value is 1.2812 Å. The temperature-dependent valence structure of tantalum is not a drastic change. The bond length and single-bond radius increases while the bond energy and bond valence decreases with increasing temperature. The valence electrons e c (d c, s c and p c) decrease while e f increases with increasing temperature.

The breakdown of the Born–Oppenheimer approximation for protonated rare gas ions

For a molecular ion, an expression of the rotational constant including corrections to the Born–Oppenheimer approximation was derived. The parameters for Born–Oppenheimer breakdown effects were related to the molecular charge number and to the parameters characteristic of the electron distribution in the molecule. By using these relations, a physical interpretation was given to experimentally obtained breakdown parameters of protonated rare gas ions. It was found that the nonadiabatic correction for an unbalanced distribution of valence electrons dominates the observed trends of the breakdown parameters.

Lone pair effects in trihalo-stannate and -plumbate anions in the crystal structures of Sr(MX 3) 2 · 5H 2O (M = Sn, X = Cl, Br; M = Pb, X = Br): A joint crystallographic and electronic structure calculation approach

The crystal structures of Sr(SnCl 3) 2 · 5H 2O, Sr(SnBr 3) 2 · 5H 2O and Sr(PbBr 3) 2 · 5H 2O are presented along with the predicted structure of Sr(PbCl 3) 2 · 5H 2O. The compounds are essentially isostructural with Sn/Pb in a trigonal pyramidal environment with Cl/Br. Stereochemical activity of the non-bonding valence electrons on Sn/Pb is evident in the crystal structures, with the degree of distortion less pronounced in the two lead containing structures. Valence electron pair distributions have been examined using a combination of density functional theory and localised Wannier functions. The valence electron distribution around M in the [MX 3] − anion (M = Sn, Pb, X = Cl, Br) is compared in the gas phase and solid state. Electronic structure calculations reveal a contraction towards the metal centre of the orbital associated with the non-bonding valence electrons, on substitution of Sn by Pb. Relatively large deviations of the calculated charge centre maxima from the bond centre in the M–X bonds are an indication of the polarity of the M–X bonds and lead to the conclusion that while the [SnX 3] − anion appears to be a discrete entity within the system Sr(SnX 3) 2 · 5H 2O, there is less evidence for the existence of a corresponding discrete [PbX 3] − anion in lead analogues.

La3Cl3BC — Structure, Bonding and Electrical Conductivity.

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La3Cl3BC – Structure, Bonding and Electrical Conductivity

A new rare earth carbide boride halide, La3Cl3BC, has been prepared by heating a mixture of stoichiometric quantities of LaCl3, La, B and C at 1050 °C for 10 days. La3Cl3BC (La3Br3BC type) crystallizes in the monoclinic system with space group P21/m (No. 11), a = 8.2040(16), b = 3.8824(8), c=11.328(2)Å , β =100.82(3)°. In the structure, monocapped trigonal prisms containing B-C units are condensed into chains along the b direction, and the chains are further linked by Cl atoms in the a and c directions. The condensation results in a polymeric anion 1 ∞[BC] with a spine of B atoms in a trigonal prismatic coordination by La, and the C atoms attached in a square pyramidal coordination. The B-B and B-C distances are 2.16 and 1.63 Å , respectively. La3Cl3BC is metallic. The EH calculation shows that the distribution of valence electrons can be formulated as (La3+)3(Cl−)3(BC)5− · e−.

Lithium adsorption byTiSe2of varying concentration via density functional theory

Alkali adsorption on a transition metal dichalcogenide substrate has been the subject of many investigations. However, a theoretical description of its coverage dependence is still missing. In a first attempt we considered Li as an alkali prototype at a specified selection of coverages, $\ensuremath{\Theta}=0.11$, 0.25, 0.33, 0.5, and 1 ML, adsorbed on $\mathrm{Ti}{\mathrm{Se}}_{2}$ (0001). By means of the density functional theory we obtained through structural optimization the coverage dependence of the physical properties as adsorption energy, work function, and valence electron distribution. From the knowledge of the preferred adsorption sites, an optimum coverage for the most stable structure could be inferred. The work function curve shows a behavior similar to that found for alkali adsorption on metals. The redistribution of the charge density at adsorption is presented.

Coincidence Doppler broadening study on hydrocarbons with pi and sigma valence electrons: positronium correction

The coincidence Doppler broadening (CDB) technique was applied to study the electron momentum distribution in anthracene, diphenyl, naphthalene, and polystyrene. A method for separation of the positron and positronium (Ps) components from the Doppler-broadened annihilation line (DBAL) was developed further to be applicable to hydrocarbons with different π and σ valence electron distributions. This method allows extraction of the electron momentum distribution (EMD) from DBAL for samples when Ps formation occurs. The annihilation on π valence electrons was detected as broadening of the EMD compared to that obtained for a polymer sample only with σ valence electrons. The broadening appeared as a significant change in the shape of the CDB ratio of the corresponding positronium-corrected curves: a slight enhancement above the unity line in the low-momentum region and a drop in the momentum region, 10–20×10 −3 m oc .

Beyond locating the atoms towards addressing the electronic structure of crystals using advanced transmission electron microscopy

With the present focus on nanostructures, the role of the transmission electron microscope (TEM) as a research tool is becoming increasingly important. Among themajor strengths of TEM are the capability of obtaining diffraction patterns and electron energy loss spectra (EELS) from nanometer sized areas. In addition, advanced TEMmay serve as a supplement to synchrotron x-ray diffraction and absorption spectroscopy in efforts to study bonding and electronic structure of bulk materials. This is in particular the case when only small-grained polycrystalline samples are available, but TEM may also provide valuable input in studies of single crystals. For example, the near edge fine structure at EELS absorption edges was used to locate different valence states of Fe within the unit cell of a mixed valence crystal already two decades ago [1], and more recently precision convergent beam electron diffraction (CBED)was used in combination with x-ray diffraction to study the spatial distribution of valence electrons in Cu2O [2]. In this presentation, we report our recent study of the MgB2 where we use both EELS [3] and CBED [4] to gain insight into the electronic structure of this new superconductor with Tc=39K. Using angle-resolved EELS we studied the fine structure at the K-edge of boron in order to probe the empty boron pxy and pz states near the Fermi level in this anisotropic hexagonal crystal. Similar information can be obtained using polarized soft x-rays in absorption spectroscopy (XANES) if sufficiently large single crystals become available. Using quantitative CBED we determine the structure factors at short g-vectors. These structure factors at short g-vectors, which are strongly influenced by the valence electron redistribution, can be determined with very high accuracy using electron diffraction. For example, after converting to x-ray structure factors, we measure F001 of MgB2 to be 2.15±0.03 electrons while the calculated structure factor for the procrystal differs 20 standard deviations from this value (2.75 electrons).We also did DFT calculations, and found good agreement between experimental and theoretical structure factors.

Relativistic component of chemical shift of Uranium X-ray emission lines

Twelve most intensive hard X-ray emission lines belonging to two multiplets (2p-3d and 2p-4d) and three doublets (2s-3p, 2s-4p, and 2p-3s) of the uranium L-series were studied by using a precise crystal-diffraction X-ray spectrometer. Chemical shift (CS) values were obtained for the first time for uranium (VI) oxide and uranium fluoride with respect to uranium (IV) oxide. The first attempt was made to separate CS of hard X-ray lines of an actinide atom into two components: the non-relativistic CS part characterizing the charge distribution of valence electrons between the atom and its partners in chemical bonding and the relativistic part reflecting the effects which are not related to electron removal from the atom. A method for calculating the relativistic CS component is proposed. It is concluded that it is the relativistic component that provides information about the changes in the valence state of the actinide atom in a compound. On the basis of model Dirac–Hartree–Fock calculations, it is shown that in the case of the uranium atom the relativistic component of CS is a direct consequence of intra-atomic electrons redistribution between 5f 7/2- and 5f 5/2-split levels of the uranium atom fine structure without changes in its charge state.

Valence-electron distribution inMgB2by accurate diffraction measurements and first-principles calculations

We use synchrotron x-ray and precision electron-diffraction techniques to determine accurately the structure factors of reflections that are sensitive to the valence-electron distribution in the superconductor ${\mathrm{MgB}}_{2}.$ These values deviate significantly from those calculated using the scattering factors of free (or neutral) atoms, but agree well with our calculated structure factors based on density-functional theory. Having experimentally established the reliability of our first-principles-based structure factors, we present electron-density maps of the redistribution of the valence electrons that takes place when hypothetical free atoms of Mg and B in ${\mathrm{MgB}}_{2}$ interact to form the real crystal.

Analysis of Electron Spectra of Carbon Allotropes (Diamond, Graphite, Fullerene) by Density Functional Theory Calculations Using the Model Molecules

X-ray photoelectron, emission, and Auger electron spectra of diamond, graphite, and fullerene have been analyzed by deMon density-functional theory (DFT) calculations using the model molecules adamantane derivative (C 1 0 H 1 2 (CH 3 ) 4 ), pyrene (C 1 6 H 1 0 ), and C 6 0 , respectively. The theoretical valence photoelectron, C Kα X-ray emission, and Auger electron spectra for the allotropes are in good accordance with the experimental ones. The combination analysis of the valence X-ray photoelectron and C Ka X emission spectra enables us to divide the valence electronic distribution in the individual contributions for pa- and pπ-bonding MOs of the carbon allotropes, respectively. The experimental Auger electron spectra of the allotropes can be classified in each range of ls-2p2p, ls-2s2p, and 1s-2s2s transitions for C KVV spectra, and in individual contributions of the chemically different carbon atoms from the theoretical analysis.

Bonding character of SiO 2 stishovite under high pressures up to 30 Gpa

The charge density and bond character of the rutile-type structure of SiO2 (stishovite) under compression to 30 GPa were investigated by X-ray diffraction study using synchrotron radiation and AgKα rotating anode X-ray generator through a newly devised diamond-anvil cell. The valence electron density was determined by least-squares refinement including the κ parameter and the electron population in the X-ray atomic scattering parameters. The oxygen κ-parameter of SiO2 is 0.94 under ambient conditions and 1.11 at 29.1 GPa and the silicon valence changes from +2.12(8) at ambient pressure to +2.26(15) at 29.1 GPa. These values indicate that the electron distributions are more localized with increasing pressure. The difference Fourier map shows the deformation of the valence electron distribution and the bonding electron population in residual electron densities. The bonding electron observed from the X-ray diffraction study is interpreted by molecular orbital calculations. The deformation of SiO6octahedra and the bonding electron density of stishovite structures are elucidated from the overlapping electron orbits. The O–O distances of shared and unshared edge of SiO6 octahedra change with the cation ionicity. The repulsive force between the two cations in the adjacent octahedron makes its shared edge shorter. The pressure changes of the apical and equatorial Si–O interatomic distances are explained by the electron density of state (DOS) of Si and electron configuration.

Electronic and structural properties of micro-and nanometre-sized crystallinecopper monoxide ceramics investigated by positron annihilation

Electronic and structural properties of copper monoxide (CuO) sintered as acommon ceramic and nanoceramic are studied by positron annihilationspectroscopy. A CuO nanoceramic with crystallite size ranging from 15 to 90 nmwas prepared from a common one by shock-wave loading. It is foundthat the momentum distribution of valence electrons in CuO is shifted,as compared with metallic copper, towards higher momentum values.This result is related to the effect of the Cu 3d–O 2p hybridization inthe Cu–O ionic covalent bond formation. It is found that open volumes,identified mainly as small agglomerates of oxygen vacancies, appear at thenanoceramic crystallite interfaces. The degree of the Cu–O bond covalencydecreases locally at the crystallite interfaces because of an oxygen deficit. Thenanocrystalline state in CuO is shown to be thermally stable up to 700 K.

Exploring the Valence Electron Distribution in High Temperature Superconductors with a Focused Electron Probe

Exploring the Valence Electron Distribution in High Temperature Superconductors with a Focused Electron Probe

Spatial distribution of valence electrons in metallocenes studied by Penning ionization electron spectroscopy

Penning ionization electron spectra resulting from thermal collisions of He*(1s2s, 23S) metastable atoms with gaseous metallocenes, M(C5H5)2 (M = V, Cr, Mn, Fe, Co, Ni, Ru, and Os), were measured to directly probe the spatial electron distribution of the metal d-derived orbitals. Owing to the strong antibonding d–π interactions the metallic e1″ orbitals in Mn(C5H5)2, Ni(C5H5)2, and especially Co(C5H5)2, show rather diffuse electron distributions. In contrast, the Cp π-derived e1″ orbitals exhibit compact distributions due to the bonding d–π interactions, which is clearly seen in the cases of Ru(C5H5)2 and Os(C5H5)2. For the metallic e2′ and a1′ orbitals with essentially nonbonding character, the radial extent of the d orbitals increase on going from Fe(C5H5)2 to Ru(C5H5)2 and Os(C5H5)2, as expected. Mn(C5H5)2 is anomalous among the 3d metallocenes, because the nonbonding e2′ and a1′ orbitals are significantly contracted in space, reflecting their highly ionic character.

The ( [formula omitted]) surface electronic structure of FeTi, CoTi, and NiTi

We present a self-consistent calculation of the electronic structure for the (1 1 0) surface of equiatomic XTi (X=Fe, Co, and Ni) alloys in the B2 (CsCl) structure. The results were obtained using the film linearized augmented-plane-wave method. We have analyzed the change of the local density of states, the valence electronic charge distribution and the calculated work function in the series FeTi, NiTi, and CoTi alloys. It has been found that the bond ionicity at the surface is enhanced and the work function is reduced from FeTi to NiTi. The trend to surface segregation and chemical activity for the (1 1 0) surface of the studied alloys is discussed.

On Relativity, Bonding, and Valence Electron Distribution

The scalar relativistic contributions to bond or atomization energies of homo- or hetero-polar s−p-bonded atoms, ΔrelE(bond), correlate well with the changes on bond formation of the electron density, integrated throughout the spatial K-shell region of the heavy nucleus, ΔbondρK-region, times Z4, where Z is the nuclear charge. The “bond density changes” in the innermost atomic core regions, however, have no direct simple relationship to the electron density reorganizations in the outer atomic valence shell regions, which determine the nonrelativistic main contribution to the bond energy, ΔnonrelE(bond). Also, scalar relativistic bond energy changes and bond length changes, ΔrelRe, are not directly correlated. Namely, ΔrelE(bond) correlates with the difference ΔbondρK-region, whereas ΔrelRe correlates with the derivative (dρK-region/dR). Reducing the internuclear distance from the separated to the united atom limit through the equilibrium distance value, Re, the density around the nucleus at first decreases in many cases, goes through a minimum, and finally increases again. Therefore (dρK-region/dR) may be positive or negative at Re.

Description of the distribution of valence electrons in simple metals by the density amplitude

A satisfactory solution for some many-body problems, in particular electron–positron interaction in metals, could not be obtained because of the complication connected with different electron momentum states each interacting in a different way. A possibility to achieve progress in the theory of e + –e − interaction in metals is connected with presenting the distribution of conduction electrons by means of a single density amplitude obeying an Euler–Lagrange equation of the Kohn–Sham type. This approach to band structure allows to avoid the muffin-tin assumption as well as the iteration method of reaching self-consistency, replacing the latter by the linearization procedure. Calculations of the density amplitude are performed for different BCC metals i.e. alkalis and barium. The distribution of conduction electrons in the lattice obtained in this way is reasonable in the case of alkalis. The method to benefit from the density amplitude formalism in calculations of the effect of electron–positron interaction is indicated.

Ab initio determination of the valence electron distribution in the average structure of the incommensurately modulated calaverite AuTe2.

The valence-electron density distribution of the average structure of incommensurately modulated calaverite, AuTe2, has been computed using density-functional theory. High-density regions, centered around the Au and Te atoms, are not spheric, but present charge concentrations along the Au-Te and Te-Te bonds. The electronic band structure and its corresponding density of states reveal the presence of three electronic band groups, constituted mainly by Te 5s, Au 5d and hybrids of Te 6p + Au 6s + Au 5d orbitals. The electrons belonging to the last block are responsible for the chemical bonds.