Electronic structure of binary zirconium compounds: Cluster calculations using different molecular orbital methods

Cluster calculations of the electronic structure and charge distribution in the refractory compounds ZrX(X = C, N, O) and ZrO₂ have been performed using different molecular orbital methods; a semiempirical LCAO model based on the Mulliken-Wolfsberg-Helmholz approximation, and the Hartree-Fock-Slater model in both the Discrete Variational X_α and Multiple Scattering X_α versions. The main features of chemical bonding are discussed and illustrated by contour level diagrams of some bonding molecular orbitais of the valence band. Finally, the results are compared with X-ray emission and X-ray photoelectron spectra, and also with band structure calculations.     

Electronic structure and X-ray photoelectron spectra of TiC and ZrC: Cluster and band structure approaches

The electronic structure for titanium and zirconium monocarbides have been calculated with the SCF MS X_α-cluster method. The results are compared with the experimental X-ray photoelectron spectra and augmented plane wave (APW) calculations.     

Theoretical calculation of electronic structure and X-ray absorption near-edge structure of cathode materials for Li ion batteries

First principles calculations were performed on the electronic structure, chemical bonding and X-ray absorption near edge structure (XANES) of various lithium transition metal oxides. In the electronic structure using the discrete variational Xα method (DV-Xα), chemical bonding is changed by Li deintercalation. Li is found to be nearly ionized in LiMO₂ and strong covalent bonding between M and O is noted. The larger the difference of covalency between cation and nearest neighbor anion when Li intercalated/deintercalated is, the lower the voltage calculated by Vienna Ab Initio Simulation Package (VASP) is. By calculations of transition state, we reproduce the characteristics of the spectra as well as the chemical shifts and the origin of peaks appearing in the experimental XANES spectra is interpreted in terms of orbital interactions using bond overlap population diagrams.     

Study of the electronic structure and spectral features of TiS2,ZrS2 and HfS2 by the Xα discrete variational method

In order to study the electronic structure of TiS₂, ZrS₂ and HfS₂, selfconsistent charge X_α discrete variational calculations of clusters MeS₆^8- (Me = Ti, Zr, Hf) have been performed. The results obtained are consistent with band structure calculations. The covalency of the chemical bonding is shown to increase in the series TiS₂-ZrS₂-HfS₂, which corresponds to the atomization energy in the series. The results are applied in the interpretation of X-ray emission, photoelectron and optical spectra.     

On the electronic properties of the sulfur nitride polymer (SN)x

Results of an ab initio LCAO Hartree-Fock crystal orbital calculation are reported for (SN)_x using a double-zeta type atomic basis set. In contrast with previous minimal basis calculations the width of the metallic half-filled band is only ～ 4 eV in this study. The calculated effective mass (1.7m_e), electron state density [0.14/(ev spin molecule)] and transferred charge (～0.4e from sulfur to nitrogen) are also in good agreement with experiment.     

AB initio energy band structure of polysulfur nitride, (SN)x

All electron energy band structure is reported for an infinite one-dimensional model of polysulfur nitride, (SN)_x, using the ab initio LCAO Hartree-Fock method. The calculated values of the effective mass and density of states at the Fermi level are ?0.72 m_e and 0.06 states/(eV spin molecule), respectively. An appreciable amount of charge transfer (0.30 e) from sulfur to nitrogen was obtained. Finally, comparison is made with the results of a semi-empirical version of the same method.     

Angle-resolved photoelectron spectroscopy of benzene and hexafluorobenzene as a function of photon energy

Angle-resolved photoelectron spectra of benzene have been recorded using synchrotron radiation as the photon source. From these results the angular dis the first three orbitals; 1e_1g(π), 3e_2g (σ) and 1a_2u(π). Calculations of β employing the MS Xα method have also b and theory for the first two orbitals is excellent. Poor agreement in the core of the third orbital is believed to be due to overlap of the second and has been verified by data obtained for perfluorobenzene. The universality of the energy dependence of β for π orbitals, noted previously for unsatu aromatic systems. The importance of the results for benzene for the future application of angle-resolved photoelectron spectroscopy to complex molecule     

Cluster calculation of electronic structure and hyperfine interactions for α-Fe2O3

The MSX_α cluster technique has been used to study the electronic structure of hematite α-Fe₂O₃, where iron is formally in a 3d₅⁶S state. The calculated energy levels are compared with X-ray emission and photoelectron spectra. The wave functions have been used to compute the charge distribution, as well as hyperfine parameters such as quadrupole coupling constant, isomer shift and magnetic hyperfine field. The results indicate a considerable influence of chemical bonding on these parameters due to charge transfer and covalency. From the calculated field gradient and the measured quadrupole coupling constant a nuclear quadrupole moment for ^57mTe of about 0.11b is deduced. This value is smaller than the most recent estimates of 0.15b based on the ionic model but not as small as the value of 0.082b obtained from first principles calculations on iron dihalides.     

The electronic structures of some aromatic sulfinylamines

He(I) and He(II) photoelectron spectra of some sulfinylamines are reported and assignments made on the basis of CNDO/S LCAO MO calculations. Insight into the effects of several substituents on the electronic structures of the compounds is obtained. The difference between the actual electronic structure of o,o′, p-trimethyl sulfinylaniline and that interpolated from other molecules in the series is assumed to reflect a different conformation for this compound.The results of the MO calculations agree fairly well with the experimental data, although some discrepancies occur, concerning especially the π-donating capacity of a Cl substituent, and ionizations from chlorine lone pair orbitals.     

The use of ionization energies of core electrons for determination of the geometrical structure in inorganic salts

Based on the results of calculations on the electronic structure of the LiNO₃ and TINO₃ molecules by the Discrete Variational X_α-method (DVM—X_α) it is concluded that experimental X-ray photoelectron spectra (XPS) may be used in the determination of the equilibrium structure of inorganic salts. This may be done in those cases where peaks corresponding to ionization from the core levels of bridge and terminal ligand atoms of an anion are resolved in XPS. It is shown that the splitting energy itself is determined mainly by the deformation of the anion geometry.     

X-ray emission study of the electronic structure of nanocrystalline Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

Valence states of metal ions and the phase composition of nanocrystalline Al₂O₃ (of the original oxide and the oxide irradiated by high-energy Fe⁺ ions) are studied by using x-ray emission Al L_{2, 3} and O Kα spectra. It is established that the shape of the Al L_{2, 3} spectra strongly changes as one goes from the original (bulk) Al₂O₃ to nanocrystalline oxide, while the O Kα spectra remain practically unchanged. Moreover, irradiation by high-energy Fe⁺ ions results in slight additional changes in the x-ray spectral characteristics of the aluminum oxides under study. The obtained experimental data are compared with the results of theoretical calculations of the electronic structure of α and γ phases of Al₂O₃ performed using the LDA formalism. Using the results of x-ray spectral studies, electronic structure calculations, and x-ray diffraction analysis, it is shown that the revealed spectral differences between the nanocrystalline state of aluminum oxide and the bulk material can be interpreted as a phase transition from the α phase to the γ phase of Al₂O₃ with an addition of bayerite.     

A theoretical study of the electronic structure of Pd(111) clean surface

A detailed investigation of the electronic structure of Pd(l 11) clean surface is presented in terms of a LCAO band model. Results including surface bands and local densities of states are given and a comparison with recently performed photoemission experiments is presented. For hω = 21.2 eV we find that the angle resolved distribution curves can be explained in terms of initial state density and k_∥ conserving transitions. For lower photon energies a mixture of bulk transition calculations and surface density of states seems more appropriate. Comparison with previous theoretical work is also presented.     

The electronic structure of NbO: Theory and experiment

Experimental photoelectric- and X-ray emission spectra obtained from NbO are compared with the molecular orbital calculations of NbO₆^10? cluster using X_α discrete variational method. The agreement between theory and experiment establishes the validity of the cluster approach to determine the electronic structure of the 4d transition metal chalcogenides.     

The electronic structures of Mg,Al and Si in octahedral coordination with oxygen from SCF Xα MO calculations

Molecular orbital calculations performed using the SCF Xα Scattered Wave Cluster method are presented for the octahedral oxyanions MgO₆^?10, AlO₆^?9 and SiO₆^?8. The AlO₆^?9 results are used to assign and interpret the X-ray photoelectron spectra (XPS), X-ray emission (XES) and u.v. spectra of Al₂O₃. Agreement between calculation and experiment is good for valence band and fair for conduction band orbitais. The SCF Xα results for MgO₆^?10 are also in good agreement with observed valence band energies in MgO, but in this case the lowest energy features in the u.v. spectrum are not assignable in terms of either the calculations or the X-ray spectral results. The substantial increase in covalency expected between the Mg and Si oxides is evidenced in the calculations by an increase in valence region width from 2.6 to 5.3 eV and an increase in valence-conduction band separation from 5.2 to 10.0 eV. The calculated trends are in reasonable agreement with u.v. spectral data and with absolute valence orbital binding energies derived from X-ray spectra. A comparison of the SiO₆^?8 calculation with the analogous tetrahedral SiO₄^?4 calculation shows the valence band in the octahedral oxyanion to be much simpler in structure and somewhat narrower than that in the tetrahedral oxyanion. Using the orbital structure calculated for the valence bands of tetrahedral and octahedral oxides, a method is presented for calculating atomization energies directly from X-ray spectral data for SiO₂, Al₂O₃ and MgO. Results are in good agreement with experiment but the method involves an empirical parameter which is not presently understood in detail. Studies of trends in p-type bonding orbital binding energies derived from experimental data provide a qualitative explanation for the preferred coordination numbers in the Mg, Al and Si oxides.     

Systematics of diatomic molecular transition moments: Encouraging progress—II

Literature absolute transition moments for A-X(0-0) bands have been compiled and used to obtain “best” values of R_e for some 90 transitions. Correlations were sought between R_e and various molecular parameters such as multiplicity, average internuclear separation, etc. None was found. The A-X (and related) R_e have been plotted for isoelectronic molecules against the nuclear charge difference and fitted with simple curves which seem to approach the R_e axis with zero slope. Both of these results are in accord with theory. The concept of isovalence has been clarified to allow for the orbital structure of molecules. It appears that the heavier of two isovalent hydrides has the greater R_e. Graphs of R_e for groups of isovalent molecules can be fitted with simple curves.     

Calculations of photoelectron spectra for tetrahedral oxyanions by the Hartree—Fock—Slater model

The electronic structures of typical oxyanions are studied using the discrete variational (DV)—Xα cluster method. The level structures calculated for the clusters correspond very well with the valence-state levels found in experimental XPS spectra. Theoretical intensities of the XPS peaks are evaluated, using the orbital populations obtained with the DV—Xα method together with atomic subshell photoionization cross-sections obtained in previous work with the same electron model. The results are in good agreement with experiment.     

X-ray emission spectra and electronic structure of VO,VN, VC

The experimental K_β5 and L_α emission spectra for vanadium and K_α spectra for nonmetal are studied for vanadium monoxide, nitride and carbide. Using the spectra unfolding the experimental molecular orbital diagrams have been obtained. By the semiempirical Mulliken-Wolfsberg-Helmholz method with the self-consistency on charges and configurations the calculations of electronic structure of clusters [VO₆]^10?, [VN₆]^15? and [VC₆]^20? have been carried out. The results of calculations are compared with the X-ray data. The effects of chemical bonding and charge densities on metal atoms in VO, VN, VC are discussed.     

Alternate interpretation of final state relaxation for chemisorbed molecules

Ab initio Xα electronic spectra have been calculated for CO before and after chemisorption onto Ni(001) in the observed c(2 × 2) symmetry. The electronic spectra of a monolayer of C₂H₂ has also been calculated and referenced to E_F of Ni. The theoretical eigenvalue spectra of these two molecules agrees colosely with ionization potentials measured by UPS when these molecules are chemisorbed onto Ni. Charge density calculations for CO indicate that the molecular orbitais are delocalized by the COCO and CONi bonds. These results suggest that final state relaxation effects are small and possibly zero for the chemisorbed molecule and that the relaxation energy of chemisorbed molecules is approximately the correlation energy.