Circular dichroism of photoemission of Fe<Subscript>1/4</Subscript>TiTe<Subscript>2</Subscript>

High-resolution ARPES has been employed for examination of angular distribution of valence band photoelectrons emitted by a Fe_1/4TiTe₂ single crystal near the point Γ of the Brillouin zone in the most symmetrical directions ΓK (A-H) and ΓM (A-L) under excitation with synchrotron radiation (21.5 eV, He I) at 50 K. It has been found out that excitation with circular polarized synchrotron radiation results in essential circular dichroism in angular dependence of valence band photoemission of Fe_1/4TiTe₂. Analysis of entire experimental data has revealed that the dichroic effect is also related to the chiral geometry of the experiment. The decrease in symmetry induces dichroism, thus allowing determination of electron states at intercalant atoms.     

LiPN<Subscript>2</Subscript> and NaPN<Subscript>2</Subscript> crystals: Structural features and chemical bonding

The band structure spectra, densities of states, and valence and difference densities of LiPN₂ and NaPN₂ crystals were obtained by DFT self-consistent calculations using the nonlocal pseudopotentials and the localized pseudoorbital basis. Crystal-chemical analysis of these compounds shows that they occupy an intermediate position between the ideal structures of β-cristobalite and chalcopyrite, which manifests itself in the peculiarities of the electronic structure and chemical bonding. The valence band consists of three allowed subbands and differs radically from the typical valence band of chalcopyrite crystals in both subband structure and contributions of the s, p, and d atomic orbitals to the crystal orbitals.     

First Principles Investigations of the Crystal Structure,Electron Spectra,and Chemical Bonding in the Low-Temperature Phase of Lithium Imide

The density functional method in the full-electron approximation is used to calculate optimized structural parameters, band spectrum, density of states, electron density maps and to study chemical bonding in the low-temperature phase of lithium imide with a crystal structure of the space group Ima2. The valence band width is 15.14 eV, and it consists of three subzones (slightly dispersed bottom subzone, medium subzone, and top subzone with a width of ~2.4 eV) due to dispersion cased by hybridization of hydrogen s states and nitrogen p states. The top of the valence band and the bottom of the conduction band are in the center of the Brillouin zone, the width of the band gap is 4.48 eV, the absorption edge is direct. The electron density maps demonstrate chains of N–H complexes bound by pairs of Li atoms. The Li–N bond is predominantly ionic, while the N–H bond is polar covalent.     

Resonant emission of UO<Subscript>2</Subscript>, U<Subscript>3</Subscript>O<Subscript>8</Subscript>, and UO<Subscript>2+<Emphasis Type="Italic">x</Emphasis></Subscript> valence electrons under SR excitation near the O<Subscript>4,5</Subscript>(U) absorption edge

The structure of the resonant electron emission (REE) spectra of UO₂ (REE appears under the excitation with synchrotron radiation near the O_4,5(U) absorption edge at ∼100 eV and ∼110 eV) is studied with regard to the X-ray O_4,5(U) absorption spectrum of UO₂ and a quantitative scheme of molecular orbitals based on the X-ray electron spectroscopy data and the results of a relativistic calculation of the electronic structure of UO₂. The structure of the REE spectra of U₃O₈ and UO_2+x is studied for comparison, and the effect of the uranium chemical environment in oxides on it is found. The appearance of such a structure reflects the processes of excitation and decay involving the U5d and electrons of the outer valence MOs (OVMOs, from 0 to ∼13 eV) and inner valence MOs (IVMOs, from ∼13 eV to ∼35 eV) of the studied oxides. It is noted that REE spectra show the partial density of states of U6p and U5f electrons. Based on the structure of REE spectra, it is revealed that U5f electrons directly participate (without losing the f nature) in the chemical bonding of uranium oxides and are delocalized within CMOs (in the middle of the band), which results in the enhancement of the intensity of the REE spectra of CMO electrons during resonances. The U6d electrons are found to be localized near the bottom of the outer valence band and are observed in the REE spectra of the studied oxides as a characteristic maximum at 10.8 eV. It is confirmed that U6p electrons are effectively involved in the formation of IVMOs, which leads to the appearance of the structure in the region of IVMO electron energies during resonances. This structure depends on the chemical environment of uranium in the considered oxides.     

Effect of Nitrogen and Phosphorus Dopants on the Electronic Properties of ZrO<Subscript>2</Subscript> Nanotubes

The electronic structure of hexagonal ZrO₂ nanotubes, pure and doped with N and P atoms, has been calculated by the linearized augmented cylindrical wave method. The calculated band structures and densities of states demonstrate that the substitution of nitrogen or phosphorus for a part of the oxygen atoms leads to a decrease in the optical gap from 4 to 1.95 and 1.9 eV, which makes such nanotubes candidate materials for creation of electrodes for electrochemical photolysis of water.     

Electronic structure of Ti<Subscript>4</Subscript>Fe<Subscript>2</Subscript>O as determined from <Emphasis Type="Italic">ab initio</Emphasis> calculations and X-ray spectroscopy studies

The TiL _α, FeL _α and OK _α ultrasoft X-ray emission bands obtained in experiment reflect, respectively, the energy distribution of mainly the Ti3d, Fe3d and O2p electronic states in Ti₄Fe₂O compound, which is an efficient hydrogen absorber for energy cells. Full and partial densities of electronic states for all atoms constituting the indicated oxide were calculated by a modified method of associated plane waves (APW) using the WIEN2k software package. The APW calculation data for Ti₄Fe₂O compound as well as superposition of TiL _α, FeL _α and OK _α ultrasoft X-ray emission bands on a single energy scale indicate that O2p states in the oxide are localized mainly near the bottom of the valence band, the major contribution near the ceiling of the valence band belonging to Fe3d and Ti3d states. According to the APW calculation, the major contribution to the bottom of Ti₄Fe₂O conduction band is made by Fe3d* and Ti3d* states. The APW data for Ti₄Fe₂O are supported by the cluster calculation performed for this compound using a FEFF82 software package.     

Electronic structure and chemical bond in KN<Subscript>3</Subscript> and KSCN crystals

Crystal structure parameters, band spectra, distribution maps of the toal and partial electron densities of KN₃ and KSCN are calculated from the first principles using the density functional method and the general properties and distinctions in the band spectra are analyzed. The groups of balance subbands are qualitatively similar; in the upper valence band of KN₃ there is weak hybridization of p states of the complex anion and metal, which is not observed in KSCN. KSCN crystals are direct band gap materials and KN₃ are indirect band gap materials. The existence of weak covalent bonding of complex anions with metal is found from the electron density distribution maps in both crystals and also between metal atoms in KSCN, which is absent in KN₃.     

Electron microscopy, spectroscopy, and first-principles calculations of Cs2O

Oxides of cesium play a key role in ameliorating the photoelectron emission of various opto-electronic devices. However, due to their extreme reactivity, their electronic and optical properties have hardly been touched upon. With the objective of better understanding the electronic and optical properties of Cs₂O in relationship to its structure, an experimental and theoretical study of this compound was undertaken. First-principles density functional theory calculations were performed. The preferred structural motif for this compound was found to be anti-CdCl₂. Here three Cs-O-Cs molecular layers are stacked together through relatively weak van-der-Waals forces. The energy bands were also calculated. The lowest transition at 1.45 eV, was found to be between the K point in the valence band to the Γ point in the conduction band. A direct transition at 2 eV was found in the center (Γ) of the Brillouin zone. X-ray powder diffraction, transmission electron microscopy and selected area electron diffraction were used to analyze the synthesized material. These measurements showed good agreement with the calculated structure of this compound. Absorption measurements at 4.2 K indicated two optical transitions with somewhat higher energy (indirect one at 1.65 and a direct transition at 2.2 eV, respectively). Photoluminescence measurements also showed similar transitions, suggesting that the lower indirect transition is enhanced by three nearby minima at 1.5 eV in the Brillouin zone.     

Electronic structure and quadrupole interactions in triple molybdates Li<Subscript>2</Subscript>M<Subscript>3</Subscript>Al(MoO<Subscript>4</Subscript>)<Subscript>4</Subscript>, M = Cs,Rb

Within the density functional theory the electronic structure of triple molybdates Li₂M₃Al(MoO₄)₄, where M = Cs, Rb, is studied for the first time. It is found that all molybdates studied belong to wide band insulators with a band gap of ~4 eV. Quadrupole frequencies and asymmetry parameters of the electric field gradient near magnetic ⁷Li, ²⁷Al, ⁸⁷Rb, and ¹³³Cs nuclei are calculated and experimental NMR spectra are interpreted.     

Optical transitions,XPS, electronic states in NiPS3

We have measured the optical absorption below the fundamental threshold, the normal-incidence reflectivity between 1.5 and 30 eV and the X-ray photoemission spectra of NiPS₃. Shake-up satellites present at the Ni 2p and 3p core levels are strong evidence for the ionicity of the NiS bonds. We have also derived a qualitative molecular orbital model of NiPS₃ in which the trigonal crystal field splits the P and S 3p_xp_y-3p_a states, and strong covalent hybridization between P and S p_xp_y orbitals leads to covalent electronic bonding. Ni is envisaged as a divalent ion which plays little role in the electronic bonding and its 3d levels are localized, lying near the top both of the valence states. This model accounts well for both the valence band XPS data and the low energy optical transitions. Our model should represent, at the center of the Brillouin zone but not at the boundaries, the energy level sequence in NiPS₃ and other related MPX₃ layer-type compounds where M Co²⁺, Mn²⁺, Fe²⁺, Zn²⁺ and X is sulfur or selenium.The XPS spectra and optical properties of NiPS₃ have been obtained and interpreted on a qualitative molecular orbital model in which the Ni is a divalent positive ion which plays little role in the bonding. Evidence for such ionicity appears in the optical properties and XPS satellite structures, as well as in the magnetic properties. The model should represent qualitatively the band structure at the center of the Brillouin zone, but not at the boundaries. It should also be valid for other compounds similar to NiPS₃, i.e. those with other metals in place of Ni and those with Se in place of S.     

First-principles study of the pressure effects on the structural and electronic properties of crystalline organic azide C<Subscript>10</Subscript>H<Subscript>8</Subscript>N<Subscript>6</Subscript>O<Subscript>4</Subscript>

Within the density functional theory with regard to the dispersion interaction the crystal structure parameters of organic C₁₀H₈N₆O₄ azide are determined. The pressure effect in the range 0-20 GPa on its structural and electronic properties is studied. Parameters of the equation of state in the Vinet and Birch–Murnaghan models are determined. Within the quasi-particle method (G ₀ W ₀) the energy band structure is calculated. It is shown that the hydrostatic pressure of 20 GPa results in the approach of planes of C₁₀H₈N₆O₄ molecules and their shift relative to each other. This is accompanied by a broadening of the upper valence bands and a decrease in the band gap from 5.07 eV to 3.97 eV.     

Quantum-chemical study of ytterbium fluorides and of complex F<Subscript>2</Subscript>YbF<Subscript>2</Subscript>CeF<Subscript>2</Subscript>

The structural parameters of the (²Σ⁺//C_∞v)-YbF, (¹A₁//C_2v)-YbF2, (²A₂^′//D_3h)-YbF₃, (¹Ag//D_2h)-YbF₂Yb, (¹Ag//C_2h)-FYbF₂YbF, (¹A₁//C_2v)-FYbF₂YbF, (¹A₁//C_2v)-YbF₂YbF₂, (³B_3u//D_2h)-F₂YbF₂YbF₂, (²A′//C_s)-FYbF₂YbF₂, and (³B₂//С_2v)-F₂YbF₂CeF₂ molecules have been determined. Disproportionation of ytterbium monofluoride (2YbF → YbF₂ + Yb + 0.46 eV) is less exothermic than dimerization (2YbF → YbF₂Yb + 2.10 eV). The bond energy of the ytterbium difluoride molecules in the trans dimer (2.93 eV) exceeds those in the cis dimer (2.86 eV) and the coaxial dimer (1.66 eV). Ytterbium trifluoride dimerizes exothermically (2.95 eV) without spin pairing. The dipole and quadrupole moments of the molecules as well as the charges and spin populations of the atoms and the valence electron configurations of the lanthanides have been calculated.     

Sublattice effect on the formation of the band structure of crystals with the chalcopyrite lattice: B<Subscript>2</Subscript>CN,BC<Subscript>2</Subscript>N,BCN<Subscript>2</Subscript>

Three possible models of the ordered arrangement of B, C, and N atoms in chalcopyrite sublattices are considered. Crystal lattice parameters are obtained from the first principles, band spectra are calculated, and the role of B, C, and N atoms in the formation of chemical bonds and the valence band in the B₂CN, BC₂N, and BCN₂ crystals is revealed.     

Modeling of half-Heusler crystals with the chalcopyrite structure: Li<Subscript>2</Subscript>MgZn<Emphasis Type="Italic">X</Emphasis><Subscript>2</Subscript> (X = N,P, As,Sb)

A model of Li₂MgZnX ₂ half-Heusler compounds with the chalcopyrite structure is considered. The electronic structure is studied from first principles, showing that Li₂MgZnX ₂ are direct-gap crystals, except for pseudo-direct-gap Li₂MgZnP₂, with a band gap of 2.7 eV, 2.2 eV, 3.3 eV, and 2.5 eV for X = N, P, As, and Sb, respectively. The band structure and chemical bonding in the model crystals are found to be similar to those in LiMgX and LiZnX half-Heusler crystals. Total electron density and deformation electron density distributions are obtained. It is found that Mg–X and Zn–X ionic-covalent bonds are stronger than Li–X ionic bonds in Li₂MgZnX ₂ crystals, which allows Li atoms to move in the space between MgX ₄ and ZnX ₄ cation tetrahedra.     

Electrodeposition of CuGaSe<Subscript>2</Subscript> and CuGaS<Subscript>2</Subscript> thin films for photovoltaic applications

CuGaSe₂ and CuGaS₂ polycrystalline thin film absorbers were prepared by one-step electrodeposition from an aqueous electrolyte containing CuCl₂, GaCl₃ and H₂SeO₃. The pH of the solution was adjusted to 2.3 by adding HCl and KOH. Annealing improved crystallinity of CuGaSe₂ and further annealing in sulphur atmosphere was required to obtain CuGaS₂ layers. The morphology, topography, chemical composition and crystal structure of the deposited thin films were analysed by scanning electron microscopy, atomic force microscopy, energy dispersive spectroscopy and X-ray diffraction, respectively. X-Ray diffraction showed that the as-deposited CuGaSe₂ film exhibited poor crystallinity, but which improved dramatically when the layers were annealed in forming gas atmosphere for 40 min. Subsequent sulphurization of CuGaSe₂ films was performed at 400 °C for 10 min in presence of molecular sulphur and under forming gas atmosphere. The effect of sulphurization was the conversion of CuGaSe₂ into CuGaS₂. The formation of CuGaS₂ thin films was evidenced by the shift observed in the X-ray diffraction pattern and by the blue shift of the optical bandgap. The bandgap of CuGaSe₂ was found to be 1.66 eV, while for CuGaS₂ it raised up to 2.2 eV. A broad intermediate absorption band associated to Cr and centred at 1.63 eV was observed in Cr-doped CuGaS₂ films.     

Low temperature synthesis of nanosized ZnNb<Subscript>2</Subscript>O<Subscript>6</Subscript> photocatalysts by a citrate complex method

Nanosized ZnNb₂O₆ photocatalysts (band gaps ~4.0 eV) were successfully synthesized via a citrate complex method. Their particle sizes ranged from 50 to 150 nm. The result of Mott–Schottky measurement revealed that the flat-band potential of ZnNb₂O₆ was ca. −1.3 V versus Ag/AgCl at pH 6.6. The photocatalytic activities of the samples for the degradation of methyl orange were evaluated under UV-light (λ = 254 nm). It was found that the sample obtained at 850 °C showed the highest photocatalytic activity due to its opportune crystallinity and surface area. Furthermore, ·OH radicals were detected as the major oxidation agents responsible for the decomposition of methyl orange.     

MoS<Subscript>2</Subscript> single slab as a model for active component of hydrodesulfuration catalyst: a quantum chemical study 1. Molecular and electronic structure of Mo<Subscript>12</Subscript>S<Subscript>24</Subscript> macromolecule and its adsorption complex with H<Subscript>2</Subscript>S

The molecular and electronic structure of Mo₁₂S₂₄ macromolecule as the MoS₂ single slab structure was calculated by the density functional theory (DFT) method with the B3P86 hybrid exchange-correlation functional. The results of calculations point to slight relaxation of coordinatively unsaturated Mo and S atoms, which is consistent with the published data. The calculated width of the forbidden band (0.85–0.98 eV) is comparable with the experimental value (1.30 eV) and similar to that obtained from DFT calculations with periodic boundary conditions (0.89 eV). The surface Mo centers in the Mo₁₂S₂₄ macromolecule are more reduced than the internal (Mo^IV) atoms. In order to characterize the adsorption capacity of coordinatively unsaturated Mo centers, a Mo₁₂S₂₄·6H₂S adsorption complex was calculated. The structure and energy characteristics of the adsorption complex point to a weak donor-acceptor interaction of the π-lone pair of H₂S molecule with the surface (reduced) Mo centers. The active center of thiophene hydrodesulfuration catalysts is formed as a result of the oxidative addition of hydrogen followed by occlusion of hydrogen into the MoS₂ matrix. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2189–2193, October, 2005.     

Electronic energy structure of As<Subscript>2</Subscript>S<Subscript>3</Subscript>, AsSI,AgAsS<Subscript>2</Subscript>, and TiS<Subscript>2</Subscript> semiconductors

Quantum-mechanical calculations with the FEFF8 code were used to study the electronic energy structure of 200-atomic clusters of As₂S₃, AsSI, AgAsS₂, and TiS₂ semiconductor compounds. The calculated local partial densities of electronic states are compared with the sulfur K and L X-ray emission spectra and sulfur K absorption spectra for fine powders of these compounds. Good agreement between theory and experiment has been obtained.     

A Systemic DFT Study on Several 5<Emphasis Type="Italic">d</Emphasis>-Electron Element Dimers: Hf<Subscript>2</Subscript>, Ta<Subscript>2</Subscript>, Re<Subscript>2</Subscript>, W<Subscript>2</Subscript>, and Hg<Subscript>2</Subscript>

Eleven kinds of density functionals in conjunction with three different basis sets are employed to investigate the homonuclear 5d-electron dimers: Hf₂, Ta₂, Re₂, W₂ and Hg₂. The computed bond lengths, vibrational frequencies and dissociation energies of these molecules are used to compare with available experimental data to find the appropriate combination of functional and basis set. The different functionals and basis sets favor different ground electronic state for Hf₂ and Re₂ molecules, indicating that these two dimers are sensitive to the functionals used. The molecular properties of Hg₂ dimer depend strongly on both functionals and basis sets used. It is found that the BP86 and PBEPBE functionals are generally successful in describing the 5d-electron dimers. For the ground states of these dimers, the bonding patterns are determined by natural bond orbital (NBO) analysis. Natural electron configurations show that the 6s and 5d orbitals in the bonding atoms hybrid with each other for the studied dimers except for Hg₂.     

Electron densities and chemical bonding in TiC,TiN, and TiO derived from energy band calculations

The chemical bonding in the interesting class of refractory transition metal compounds is illustrated for TiC, TiN, and TiO. Self-consistent augmented plane wave (APW ) calculations are already available for these compounds. Using the respective potentials we have repeated the band calculations on a finer k grid with the linearized APW method to obtain accurate densities of states (DOS ). These DOS can be divided into local partial contributions to characterize the bonding. Further information can be obtained from a decomposition of the metal d DOS into t_2g and e_g symmetry components. These partial local DOS are compared with the LCAO counterpart and give a first picture of the chemical bonding in these compounds. The electron densities corresponding to the occupied valence states are obtained from the LAPW calculations. They provide further insight into characteristic trends in the series from TiC to TiO: around the nonmetal site the density shows increasing localization; around the metal site the deviation from spherical symmetry changes from e_g to t_2g. These effects can be traced back to the three types of valence bands. Electron density plots of characteristic band states (all energies of a selected k point in the Brillouin zone) will be shown. These plots can describe the different types of bonding occurring in these systems.