Valence Electron Charge Research Articles

Electronic structure of thec(2×2)O/Cu(001)system

The locally self-consistent real space multiple scattering technique has been applied to calculate the electronic structure and chemical binding for the c(2x2)O/Cu(001) system, as a function of $d_{O-Cu1}$ -- the height of oxygen above the fourfold hollow sites. It is found that the chemical binding between oxygen and copper has a mixed ionic-covalent character for all plausible values of $d_{O-Cu1}$. Furthermore, the electron charge transfer from Cu to O depends strongly on $d_{O-Cu1}$ and is traced to the variation of the long-range electrostatic part of the potential. A competition between the hybridization of Cu1-$d_{xz}$ with O-$p_x,p_y$ and Cu1-$d_{x^2-y^2}$ with O-$p_z$ states controls modification of the electronic structure when oxygen atoms approach the Cu(001) surface. The anisotropy of the oxygen valence electron charge density is found to be strongly and non-monotonically dependent on $d_{O-Cu1}$.

POSITRON ANNIHILATION IN SiC

Valence electron and positron charge densities in SiC are obtained from wave functions derived in a model pseudopotential bandstructure calculation. It is observed that the positron density is maximum in the open interstices and is excluded not only from the ion cores but also, to a considerable degree, from the valence bonds. Electron–positron momentum densities are calculated for the (001–110) plane. The results are used to analyze the positron effect in large gap semiconductors.

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.

Chemical bonding, elasticity, and valence force field models: A case study forα−Pt2Siand PtSi

We have carried out a detailed study of the chemical bonding for two room-temperature stable platinum silicide phases, tetragonal alpha-Pt_2Si and orthorhombic PtSi. An analysis of the valence electronic charge density reveals surprising evidence of covalent three-center bonds in both silicide phases, as well as two-dimensional metallic sheets in alpha-Pt_2Si. These elements of the bonding are further analyzed by constructing valence force field models using the results from recent first principles calculations of the six (nine) independent, non-zero elastic constants of alpha-Pt_2Si (PtSi). The resulting volume-, radial-, and angular-dependent force constants provide insight into the relative strength of various bonding elements as well as the trends observed in the elastic constants themselves. The valence force field analysis yields quantitative information about the nature of the chemical bonding which is not easily discernable from the more qualitative charge density plots. More generally, this study demonstrates that the detailed variations in the elastic constants of a material contain useful information about the chemical bonds which can be extracted using valence force field models. Inversely, these models also allow identification of specific elements of the chemical bonding with particular trends in the elastic constants, both within a given material and among a class of related materials.

The ionicstructure and the electronic states of liquid Li-Pb alloys obtainedfrom ab initio molecular dynamics simulations

Ab initio molecular dynamics simulations are carried outfor liquid Li0.8Pb0.2 and Li0.5Pb0.5alloys to investigate the ionic structure and the electronic states.In our simulation, the existence of the `chemical complex' Li4Pb is not found; rather, a salt-like ordering of Pb ionsis seen in the liquid Li0.8Pb0.2 alloy. It is foundfrom the calculated partial and total structure factors that thisordering leads to the characteristic behaviour of the totalstructure factor, which agrees well with the results of a neutrondiffraction experiment. The composition dependence of theelectronic states is explained on the basis of the ionic configuration.The tendency towards ionicity or charge transfer is seen in both liquidalloys, though the valence-electronic charge distribution is not solocalized around the ions.

Electronic structure and bonding in the Cmca phases of Si and Cs

The crystal structures of the high-pressure phases Cs-V and Si-VI have recently been solved and assigned both to the Cmca space group with nearly identical axial ratios and atomic coordinates. In order to investigate the bonding properties of these two phases we have performed self-consistent density-functional linear muffin-tin orbital calculations of the electronic structure. We find for Si-VI that the band structure and density-of-states (DOS) are nearly perfectly free-electron-like, although features reminiscent of σ-type bonds made up of sp orbitals are evident from the valence electron charge density. In Cs-V the pressure-driven electronic s→d transition results in a strongly structured DOS. The charge distribution in Cs-V shows features which can be interpreted in terms of multi-center d-electron bonding.

Band structure and spatial charge distribution in AlxGa1−x N

The band structure of AlxGa1−xN substitutional solid solution is calculated by the method of local model pseudopotential in the modified virtual-crystal approximation. This provides an opportunity to explain both the dependence of the energy gap value (E g ) on AlN concentration (x) and on temperature and the E g (x) bending. The dynamics of chemical bonds in this compounds is studied by analyzing the spatial distribution of valence electron charge. The results of calculations are in a good agreement with the experimental data.

Interpretation of the119SnMössbauer isomer shifts in complex tin chalcogenides

The main observed trends in the variations of the ${}^{119}\mathrm{Sn}$ M\"ossbauer isomer shift \ensuremath{\delta} for tin chalcogenides are interpreted in terms of the valence electronic populations, and are related to changes in the Sn local environment. First, the values of the valence electron charge density at the nucleus ${\ensuremath{\rho}}_{v}(0)$ have been evaluated from a tight-binding calculation for a number of binary compounds in order to check the accuracy of the theoretical approach. A linear correlation between the experimental values of \ensuremath{\delta} and the calculated values of ${\ensuremath{\rho}}_{v}(0)$ is obtained, providing a mean square radius of the nucleus in the M\"ossbauer transition $\ensuremath{\Delta}〈{r}^{2}〉$ in correct agreement with the previously published values. The same approach is applied to tin chalcogenides with complex structures, and a correct linear correlation between ${\ensuremath{\rho}}_{v}(0)$ and \ensuremath{\delta} is obtained. Different series of chalcogenides have been distinguished considering the Sn oxidation state, the type of chalcogen, and the Sn site symmetry. These series correspond to different ranges of experimental values for \ensuremath{\delta} which have been correlated to the calculated values of ${\ensuremath{\rho}}_{v}(0)$ and the Sn valence electron populations. Finally, a molecular model is proposed which gives simple analytical expressions of the numbers of Sn $5s$ and Sn $5p$ electrons as a function of the Sn site symmetry, the type of chalcogen, and the tin-chalcogen interatomic distance. This model provides a rather simple interpretation of the changes in the values of \ensuremath{\delta} for the different series of chalcogenides.

Study of the Molecular Geometry, Electronic Structure, and Thermal Stability of Phosphazene and Heterophosphazene Rings with ab Initio Molecular Orbital Calculations.

Ab initio molecular orbital calculations at the MP2/6-31G level of theory have been used to study the molecular geometry, electronic structure, and the thermal stability of six-membered phosphazene and heterophosphazene rings. The studies included the phosphazene ring [NPCl(2)](3), the carbophosphazene ring [(NCCl)(NPCl(2))(2)], and three thionylphosphazene rings [(NSOX)(NPCl(2))(2)] (X = F, Cl) and [(NSOF)(NPF(2))(2)] and their cations [(NPCl)(NPCl(2))(2)](+), [(NC)(NPCl(2))(2)](+), and [(NSO)(NPY(2))(2)](+) (Y = F, Cl). The ring skeleton of the phosphazene ring, the carbophosphazene ring and of all cation rings adopt a planar conformation; the ring skeletons of the thionylphosphazene rings adopt an envelope conformation. The valence electron charge density of the molecules indicates strong charge separations along their skeleton and is in agreement with Dewar's island delocalization model. The electrostatic potential in the vicinity of the neutral heterophosphazene rings which results from their electronic structure, and the position of the HOMO indicate that a heterolytic cleavage of a ligand and the opening of the ring involving a reaction with a electrophilic cation will most likely occur at the nitrogen atoms close to the heteroatom. The thermal stability of the phosphazene ring with respect to a cleavage of chlorine from phosphorus and the thermal stability of the heterophosphazene rings with respect to the cleavage of the halogen ligand bonded to the heteroatom were studied with several model reactions. Most of the reactions are exothermic. A comparison of isodesmic reactions shows that the thionylphosphazenes molecules are the least thermally stable rings with respect to ionization and that the carbophosphazene molecules are the most thermally stable rings with respect to ionization. The energy gains during the ionization reaction of the rings correlate well with the conformational changes which occur during the reactions.

The origin of the first sharp diffraction peak in liquid Na-Pb alloys: ab initio molecular-dynamics simulations

Ab initio molecular-dynamics simulations are carried out for liquid Na0.8Pb0.2 and Na0.5Pb0.5 alloys to investigate the ionic structure, the electronic states and especially the existence of the so-called `Zintl ion' Pb44- and the chemical complex Na4Pb. In our simulation, the existence of these chemical units is not confirmed, but rather the intermediate-range ordering of Pb ions is seen in the liquid Na0.8Pb0.2 alloy. It is found from the calculated partial and total structure factors that this ordering of Pb ions leads to the first sharp diffraction peak of the total structure factor, which agrees well with the results of the neutron diffraction experiment. The composition dependence of the electronic states is explained on the basis of the ionic configuration. A tendency towards ionicity or charge transfer is seen in both liquid alloys, though the valence-electronic charge distribution is not so localized around the ions.

The Effects of M2O3 on Stabilizing Monocopper over the Surface of Cu‐ZnO‐M2O3 Catalysts for Mathanol Synthesis

AbstractThe effects of M3O3 (M = Al, Sc etc.) in Cu‐ZnO‐M2O3 catalysts on methanol synthesis at low pressure were studied with ESR, XPS and TPR spectroscopy. The results of ESR showed that the generation of monovalent cationic defects was because the valence state and electronic charge on the ZnO lattice lost their balance as M3+ doped into ZnO. The induced effect by Sc3+ is stronger than that by Al3+. The results of XPS and TPR indicated that the amount and stabilization of Cu+ on the surface of reduced copper‐based catalyst and its catalytic activity were affected by the monovalent cationic defects on the surface of ZnO.

Electronic structure and bonding of ultralight LiMg

Electronic structure calculation using the Tight Binding Linear Muffin Tin Orbital (TB-LMTO) method has been performed for ultralight LiMg alloys. The band structure, density of states, valence electron charge density and Fermi surface have been computed. The lattice constant, cohesive energy, heat of formation at the equilibrium lattice constant, and bulk modulus are in agreement with available experimental and theoretical results. The calculation shows charge transfer from Mg to Li site in agreement with positron annihilation experiment of Y. Tsuchiya et al. (J. Phys. F8, L29 (1978)).

A theoretical study of the local electronic structures of SiC polytypes

The electronic structure of four SiC polytypes has been computed by means of the LMTO (ASA+CC) method. It is shown that the results of the calculations are very sensitive to the choice of the atomic sphere radii, if the combined corrections are not taken into account in the ASA. The local electronic structure shows a different behaviour between the states in the cubic bilayers and those in the hexagonal ones. It is found that the interaction of c and h bilayers plays a major role, giving rise to changes in the polarity of the Si - C bonds and to charging of bilayers. The effect of the lattice distorsion on the local valence electron charge has been investigated and discussed.

Photoisomerization of sterically hindered merocyanine dyes

A small series of merocyanine dyes has been synthesized in which substituents are incorporated into the polymethine backbone. The substituent perturbs the local structure in the polyenic bridge, as evidenced by changes to the bond order, the valence electronic charge on certain bridging carbon atoms, the bond length alternation, and the torsion angle. There are corresponding variations in the rates of internal conversion and reverse isomerization that permit identification of the double bond around which rotation occurs. A good correlation exists between the rates of rotation and the torsion angle that permits a quantitative description of the overall photophysical properties of these compounds in propan-1-ol at room temperature.

An ab initio and force field study on the conformation and chain flexibility of the dichlorophosphazene trimer

AbstractAb initio molecular orbital calculations have been used to study the conformation, valence electron charge density, and chain flexibility of a dichlorophosphazene trimer (CH3[NP(Cl2)]3CH3). The calculations were carried out at the restricted Hartree‐Fock level with the 6–31 G* basis set. The dichlorophosphazene trimer adopts a planar transcis conformation. The valence electron charge distribution indicates strong charge separations along the backbone of the molecule, and is in agreement with Dewar's island delocalization model for bonding in linear and cyclic phosphazenes. In order to determine the height of the torsional barrier (2,5 kcal/mol), the torsional potential of a central PN bond of the trimer was studied with a rigid rotor scan and geometry optimizations of selected rotamers. The flexibility of the PNP bond angle contributes significantly to the chain flexibility. Based on the results of the ab initio calculations, an empirical force field for the dichlorophosphazene trimer was developed. The energy expression includes bond stretch, angle bend, electrostatic, van der Waals, and torsional potential terms. A relaxed scan with the force field achieves good agreement with the ab initio results for the torsional potential in the vicinity of the stable conformation, and an excellent agreement with the ab initio results on changes in the P2N2P3 bond angle and the N1P2  N2P3 dihedral angle during a full rotation around the N2  P3 bond.

Simple empirical formulas and good quality estimations for electron correlation energies of molecules and molecular clusters: First-row atom molecules

Using the ab initio coupled cluster method, correlation energies were calculated for a number of molecules composed of first-row atoms. The results of computations can be fitted rather well with simple analytic formulas. The main result of the present investigation is that intraatomic part of the correlation energy is proportional to sum of squares of valence electron charges on atoms composing the molecule. The proportionality coefficient depends on the basis set used. Independently, the approximations were introduced which allow for good estimation of intraatomic correlation energy by using M øller-Plesset second-order perturbation calculations only. © 1996 John Wiley & Sons, Inc.

Cohesive, electronic and magnetic properties of the transition metal aluminides FeAl CoAl and NiAl

Electronic structure calculations using the tight-binding linear muffin tin orbital (TB-LMTO) method have been performed for three transition metal aluminides, viz. FeAl, CoAl and NiAl. The band structures and density of states (DOS), valence electron charge density contours and Fermi surfaces have been obtained and compared with the available experimental results as well as with existing theoretical calculations. The lattice constants, cohesive energies and heat of formation at equilibrium lattice constants and bulk moduli agree with the experimental values. The calculations show varying degrees of charge transfer from Al site to the transition metal (TM) sites as one goes from FeAl to CoAl to NiAl. The magnetism of pure elements Fe, Co, Ni is mostly quenched in the stoichiometric phases, with only FeAl retaining a magnetic moment of about 0.7 mu B/atom within the framework of the LMTO.

Electronic and Positronic Structure of Diamond under Normal and High Pressure

AbstractEmpirical nonlocal pseudopotentials of diamond which can describe the electronic energy structure over a wide energy range of more than 20 eV from the bottom of the valence band are determined. The nonlocality of the potential is described by the Gaussian model. The optimized nonlocal pseudopotential reproduces the energy band structure within 5%. The valence electron and positron charge densities in diamond are obtained from wave functions derived from this model at normal and under hydrostatic pressure. It is found that the positron density is maximum in the open interstices and is excluded not only as usual, from the ion cores but also to a considerable degree from the valence bonds.

Band Structure of Layered Semiconductorα-In2Se3by the Numerical-Basis-Set LCAO Method

The electronic band structure of layered semiconductor α -In 2 Se 3 is investigated by using the self-consistent numerical-basis-set LCAO method with a potential based on the local density approximation.The resulting energy band gap occurs between Γ 5 - and Γ 1 + and its value is 0.99 eV. The effective masses in conduction and valence are 0.13 m 0 and 3.2 m 0 , respectively. The self-consistent valence of each ion is as follows: In 0.95+ In 1.23+ Se 1.52- Se 0.54- Se 0.12- . Thus the compound has a considerably ionic character though the interlayer bonding is found to be partly covalent from the valence electron charge distribution.

Positron annihilation in narrow‐gap semiconductors

AbstractThe valence electron and positron charge densities in InAs and InSb are obtained from wave functions derived in a model pseudopotential band‐structure calculation. It is found that the positron density is maximum in the open interstices and is excluded not only, as usual, from the ion cores but also to a considerable degree from the valence bonds. Electron‐positron momentum densities are calculated for the (001–110) plane. The results are used to analyze the positron effect in narrow‐gap semiconductors.