Electronic structure of ZrCuSiAs and ZrCuSiP by X-ray photoelectron and absorption spectroscopy

The electronic structures of quaternary pnictides ZrCuSiPn (Pn=P, As) were analyzed by X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES). Shifts in the core-line XPS and the XANES spectra indicate that the Zr and Cu atoms are cationic, whereas the Si and Pn atoms are anionic, consistent with expectations from simple bonding models. The Cu 2p XPS and Cu L-edge XANES spectra support the presence of Cu¹⁺. The small magnitudes of the energy shifts in the XPS spectra suggest significant covalent character in the Zr-Si, Zr-Pn, and Cu-Pn bonds. On progressing from ZrCuSiP to ZrCuSiAs, the Si atoms remain largely unaffected, as indicated by the absence of shifts in the Si 2p_3/2 binding energy and the Si L-edge absorption energy, while the charge transfer from metal to Pn atoms becomes less pronounced, as indicated by shifts in the Cu K-edge and Zr K, L-edge absorption energies. The transition from two-dimensional character in LaNiAsO to three-dimensional character in ZrCuSiAs proceeds through the development of Si-Si bonds within the [ZrSi] layer and Zr-As bonds between the [ZrSi] and [CuAs] layers.     

Effects of rare-earth substitution in the oxyarsenides REFeAsO (RE=Ce, Pr, Nd, Sm, Gd) and CeNiAsO by X-ray photoelectron and absorption spectroscopy

X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES) have been applied to examine the electronic structure of the rare-earth transition-metal oxyarsenides REFeAsO (RE=Ce, Pr, Nd, Sm, Gd) and CeNiAsO. Within the metal-arsenic layer [MAs], the bonding character is predominantly covalent and the As atoms are anionic, as implied by the small energy shifts in the M 2p and As 3d XPS spectra. Within the rare-earth-oxygen layer [REO], the bonding character is predominantly ionic, as implied by the similarity of the O 1s binding energies to those in highly ionic oxides. Substitution with a smaller RE element increases the O 1s binding energy, a result of an enhanced Madelung potential. The Ce 3d XPS and Ce L₃-edge XANES spectra have lineshapes and energies that confirm the presence of trivalent cerium in CeFeAsO and CeNiAsO. A population analysis of the valence band spectrum of CeNiAsO supports the formal charge assignment [Ce³⁺O²⁻][Ni²⁺As³⁻].     

X-ray absorption spectroscopy of nanostructured polyanilines

In this paper, X-ray absorption near edge spectroscopy at the nitrogen K edge (N K XANES) data of polyaniline (PANI) and its derivatives were revisited and expanded. The N K XANES spectra of PANI nanocomposites and PANI nanofibers were also investigated. The analysis of N K XANES spectra were done by the deconvolution of bands and the 1s → π* and 1s → σ* transitions were assigned by a correlation with the N K XANES spectra of smaller organic compounds. The “free” forms of PANI were dominated by bands from 397.7 eV to 399.1 eV attributed to imine and radical cation nitrogen atoms, respectively. Nitrogen bonded to phenazine-like rings can also be seen, mainly for PANI prepared at pH higher than 3.0. The spectra of nanocomposites show sharper bands than the “free” polymers as well as new bands at 398.8 eV and 405–406 eV. These new bands were assigned to phenazine-like rings and azo bonds in the structure of the polymers (polyaniline, polybenzidine, and poly(p-phenylediamine)) within the galleries of the montmorillonite clay. PANI nanofibers doped with HCl or HClO₄ show bands related to phenazine-like rings and/or dication segments of PANI, indicating that these segments are important in the formation of PANI nanofibers.     

Electronic structure of lanthanum copper oxychalcogenides LaCuOCh (Ch=S, Se, Te) by X-ray photoelectron and absorption spectroscopy

X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES) have been applied to examine the electronic structures of lanthanum copper oxychalcogenides LaCuOCh (Ch=S, Se, Te), whose structure has been conventionally viewed as consisting of nominally isolated [LaO] and [CuCh] layers. However, there is evidence for weak La-Ch interactions between these layers, as seen in small changes in the satellite intensity of the La 3d XPS spectra as the chalcogen is changed and as supported by band structure calculations. The O 1s and Cu 2p XPS spectra are insensitive to chalcogen substitution. Lineshapes in the Cu 2p XPS spectra and fine-structure in the Cu L- and M-edge XANES spectra support the presence of Cu⁺ species. The Ch XPS spectra show negative BE shifts relative to the elemental chalcogen, indicative of anionic species; these shifts correlate well with greater difference in electronegativity between the Cu and Ch atoms, provided that an intermediate electronegativity is chosen for Se.     

The bonding in magnesium oxide. A discussion based on X-ray emission and X-ray photoelectron spectra

Previously published X-ray emission spectra (Mg Kβ_1,3; MgL_2,3M and OKα) from magnesium oxide are aligned on a common energy scale using atomic orbital ionisation energies determined by X-ray photoelectron spectroscopy. All three spectra show two principal components which derive from the same two bands of molecular orbitals. The two components of both magnesium spectra have equal intensity; in the oxygen spectrum the high energy peak is the most intense: these observations are rationalised using a simple molecular orbital model based on the MgO₆^10?unit. It is necessary to consider the interactions of this unit with further magnesium and oxygen atoms and also π -type perturbations between Mg—O σ bonds in order to explain the structure of the observed emission spectra (but there is no recourse to forbidden 3d → 1s transitions).     

Covalency in actinide(iv) hexachlorides in relation to the chlorine K-edge X-ray absorption structure

Chlorine K-edge X-ray absorption near edge structure (XANES) in actinide^IV hexachlorides, [AnCl₆]²⁻ (An = Th–Pu), is calculated with relativistic multiconfiguration wavefunction theory (WFT). Of particular focus is a 3-peak feature emerging from U toward Pu, and its assignment in terms of donation bonding to the An 5f vs. 6d shells. With or without spin–orbit coupling, the calculated and previously measured XANES spectra are in excellent agreement with respect to relative peak positions, relative peak intensities, and peak assignments. Metal–ligand bonding analyses from WFT and Kohn–Sham theory (KST) predict comparable An 5f and 6d covalency from U to Np and Pu. Although some frontier molecular orbitals in the KST calculations display increasing An 5f–Cl 3p mixing from Th to Pu, because of energetic stabilization of 5f relative to the Cl 3p combinations of the matching symmetry, increasing hybridization is neither seen in the WFT natural orbitals, nor is it reflected in the calculated bond orders. The appearance of the pre-edge peaks from U to Pu and their relative intensities are rationalized simply by the energetic separation of transitions to 6d t_2gversus transitions to weakly-bonded and strongly stabilized a_2u, t_2u and t_1u orbitals with 5f character. The study highlights potential pitfalls when interpreting XANES spectra based on ground state Kohn–Sham molecular orbitals.

Chlorine K-edge XANES of An(iv) hexachlorides, calculated with multiconfiguration wavefunction theory, is interpreted in terms of similar metal–ligand covalency along the An = Th–Pu series.     

Local structure and oxidation state of uranium in some ternary oxides: X-ray absorption analysis

We investigated the local atomic and electronic structures of two related systematic sets of ternary uranium oxides, NaUO₃-KUO₃-RbUO₃ and BaUO₃-Ba₂U₂O₇-BaUO₄, by measuring the X-ray absorption near edge structure (XANES). The results are compared with calculations based on a self-consistent real space full multiple scattering analysis. We found a very good agreement between measured and calculated spectra, which indicates that the uranium ions are in a pure U⁵⁺ oxidation state in these compounds. The low energy shoulder observed in the U L₃ edge XANES is an intrinsic feature of the uranium unoccupied 6d electronic states of the U⁵⁺ ions within the studied materials. Specific double shoulder features in the higher energy range of the U L₃ edge XANES can be interpreted as indicative of the pure cubic perovskite structure.     

X-ray spectra and electronic energy structure of carboaluminides Ti-Al-C

The electronic energy structure, TiL_2,3 X-ray emission, and TiL2j XANES spectra of diamond and titanium carboaluminides Ti-Al-C are calculated by the local coherent potential method in terms of multiple scattering theory. The cluster version of the MT approximation is used to calculate the crystal potential. The electronic energy structures of diamond and the ternary and binary titanium carbide systems are compared in a single approximation. The broadening of the hybridized band in Ti-Al-C is discussed and compared with the experimental CK_α emission spectra. The specifics of chemical binding in the compounds is explained by delocalization of the carbon sp³ configuration and formation of a metal bond in carboaluminides. Translated fromZhurnal Strukturnoi Khimii, Vol. 41, No. 3, pp. 505-514, May–June, 2000.     

Determination of structures of cobalt(II)-chloro complexes in hydrochloric acid solutions by X-ray absorption spectroscopy at 298 K

The anion exchange reaction is fundamental to the adsorption and desorption of a specific species from a solution phase to an extracting phase, and it is widely used for separation and waste fluid treatment in industrial fields. However, the details of the anion exchange reaction are poorly understood. Quantitative thermodynamic analysis needs a precise solution condition before and after the exchange reaction. Identification of species adsorbed on the anion exchanger is also necessary because there are multiple species in the solution phase in general. Cobalt is a base metal that is widely used in modern society. One of the authors determined the distribution of cobalt-chloro complexes in hydrochloric acid solutions. It is necessary to know what species are adsorbed on anion exchangers for the thermodynamic analysis of the anion exchange reaction. The comparison in structures between the species in the solution phase and adsorbed on anion exchangers reveals what species are adsorbed. Therefore, the determination of the structures of cobalt-chloro complexes in the solution phase is the next step for quantitative analysis. X-ray absorption spectroscopy (XAS) was used for the structure analysis. Factor analysis can decompose extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectra consisting of multiple species into individual spectra of single species using the distribution determined using UV-Vis absorption spectra. Fitting EXAFS theoretical models to the decomposed individual spectra determined the structures of three cobalt-chloro complexes: an octahedron of [Co^II(H₂O)₆]²⁺, a distorted octahedron of [Co^II(H₂O)₅Cl]⁺, and a tetrahedron of [Co^IICl₄]^2?. The XANES spectra showed us that the Cl ligand in [Co^II(H₂O)₅Cl]⁺ was attracted to the center atom of Co^II by an electrostatic force, and the bonding system between Cl ligands and Co^II in [Co^IICl₄]^2? involved covalency.

     

X-ray spectral and photoelectron study of the electronic structure of copper phthalocyanine and its fluoro-substituted analog

A complex experimental study of Cu(2p _3/2), Cu(2p _1/2) photoelectron and Kα_1,2 and Lα_1,2 X-ray emission spectra of copper in copper phthalocyanine CuPcH₁₆ and its fluoro-substituted analog CuPcF₁₆ is carried out. A charge transfer model is used to interpret the spectra. It is shown that Kα₁ and Kα₂ lines of the spindoublet of copper have a complex structure due to the processes of metal-to-ligand charge transfer. The role of a satellite in the formation of emission lines is revealed.     

X-ray emission and X-ray photoelectron studies of electron density distribution in Cu(II) complexes with 2-imidazoline nitroxides

Electron density distribution in Cu(II) complexes with 2-imidazoline nitroxides was investigated by X-ray emission and X-ray photoelectron spectroscopy. The structure of the highest occupied molecular orbital (HOMO) of the compounds is determined mainly by the interaction of the metal with the donor atoms of the ligands. Coordination of the nitroxide by the Cu(hfac)₂ acceptor matrix changes the shape of the CuL _α spectral line. The 3s X-ray photoelectron spectra provide evidence about the transfer of electron density to the copper atom in the complexes.     

X-ray diffraction, magnetochemical, and quantum chemical study of the structure and properties of binuclear copper(II) complexes

The binuclear chelates of copper(II) based on the tridentate azomethine ligands were synthesized and characterized by elemental analysis, IR, ¹H NMR spectroscopy, X-ray absorption spectroscopy (XANES and EXAFS), and magnetic measurements. The quantum-chemical study of the structure and calculation of magnetic properties of the obtained metal-chelates was performed using the density functional theory with the broken symmetry technique. The performed magnetochemical studies in the temperature range 2?C300 K suggest the existence of antiferromagnetic exchange interaction in all obtained complexes. The parameters of the exchange interaction ?2J > 260 cm^?1 were determined experimentally for all compounds, the experimental data is in a good agreement with the results of quantum-chemical calculations.     

X-ray emission spectra of vanadium atoms in a new series of (Cu,V)-based high-Tc superconductors

Non-resonant V L_2,3 and OKα X-ray emission spectra of a new series of (Cu,V)Sr₂Ca_n−1Cu_nO_y (n=3-7) superconductors are presented. We have found that oxygen atoms surround V-atoms in the given compounds, form a (VO₄)³⁻ tetrahedrons and have a pentavalent state. This induces holes due to the substitution of divalent copper by pentavalent vanadium ions providing a hole-doping mechanism of superconductivity in this series of compounds. A spectral estimation of the oxygen concentration shows that the oxygen content under high-pressure/high-temperature synthesis conditions is not changed considerably from the start to the final product.     

X-ray spectroscopy of lanthanum manganites: Nature of doping holes,correlation effects,and orbital ordering

The Mn3s X-ray photoelectron spectra of manganites were studied. It was shown that for the formal valence of manganese from 3+ to 3.3+, the doping holes are O2p in character; as the valence of manganese increases further, the Mn3d states acquire holes. For La_0.7Sr_0.3MnO₃, the Mn3p-3d resonance spectra provided information about the occupied and unoccupied Mn3d states, and the correlation energy U = 6.7 eV was determined experimentally. An analysis of X-ray dichroism on the L absorption spectra of three-dimensional La_7/8Sr_1/8MnO₃ showed that the cooperative Jahn Teller distortion of the orthorhombic phase at 240 K was related to (x ² ? z ²)/(y ² ? z ²) type orbital ordering.     

Synthesis,structure, and X-ray absorption spectra of LixNbO2 and NaxNbO2 (x ≤ 1)

The two series of compounds, Li_xNbO₂ and Na_xNbO₂, were synthesized from LiNbO₂ and NaNbO₂ with oxidizing agents. From neutron diffraction, it is concluded that the Li position is fully occupied for x = 1. Even though the long-range crystal symmetry does not seem to change for x < 1, as monitored by Guinier X-ray powder patterns, the Nb? L_III XANES spectra exhibit significant spectral variations with x which reflect changes in the unoccupied d-density of states.     

Contribution de la spectrometrie X à l'étude du manganèse dans les oxydes mixtes de structure spinelle: Analyse du spectre MnKβ et du seuil d'absorption K

Chemical shifts of Kβ_1,3 and Kβ₅ emission bands and X-ray absorption spectra near the K edge have been measured in several manganese spinel oxides with the metal in the formal oxidation states +2, +3, and +4. The position of line MnKβ_1,3 is determined mainly by the valence of manganese. The relative intensity of Kβ′ satellite with respect to the Kβ_1,3 line gives qualitative information about the presence of Mn(II) in mixed oxides. Mn(IV) oxides are characterized by a small chemical shift of the Kβ₅ band unlike Mn(II) and Mn(III) compounds. The first high resolution XANES spectra for these materials were performed at the DCI storage ring at LURE (Orsay, France). The chemical shifts ΔE (K absorption discontinuty) and ΔE_max (main peak) are correlated with the oxidation state of metal. Spectra of Mn³⁺ and Mn⁴⁺ ions in the octahedral environment are characterized by the splitting of 1s → 3d transitions (2 eV). In mixed oxides, the first Mn(II) 1s → 4s-4p transition is observed as a peak (or shoulder) located at 7 eV above the 1s → 3d transition. The study of the X-ray absorption fine structure in the near edge region can be used for qualitative solid-state analysis of mixed oxides such as NiMn₂O₄ or CuMn₂O₄.     

An SCF XαSW study of the electronic structure and X-ray and photoelectron spectra of Fe(II) and Fe(III) hexacyano complexes in a cluster approach

The electronic structure of the hexacyano complexes [Fe(CN)₆]^4– and [Fe(CN)₆]^3– as clusters of the cyanide complex salts has been calculated by the SCF XSW method. Theoretical photoelectron, X-ray emission and absorption spectra have been constructed. The contribution of the resonance emission to the X-ray emission spectra has been estimated. On the basis of detailed comparison of the theoretical and experimental spectra an assignment of the fine structure of the spectra has been proposed.     

Influence of sample charging on satellite spectra during particle induced X-ray emission analysis

We demonstrate how the fine structures of particle induced X-ray emission spectra are influenced by build up of charge in insulating materials. We have measured silicon Kα satellite spectra emitted from fused quartz (SiO₂) that had been bombarded with 1.5 MeV He⁺ ions under various conditions of electron shower. For the insulating fused quartz, the diagram line (KL⁰) appears much stronger than the KLⁿ multiple-vacancy satellites (n ⩾ 1) making the satellites difficult to discern. However, when an electron shower is applied to the fused quartz, the diagram line is lowered according to shower conditions and the KL¹, KL² and KL³ satellites appear prominently. Emission of a strong diagram line from insulating fused quartz is principally due to the charging of the sample and its subsequent discharge.     

Structure of Aqueous Gallium(III) Bromide Solutions Over a Temperature Range 80–333 K by Raman Spectroscopy, X-ray Absorption Fine Structure, and X-ray Diffraction

The structure and complex formation of concentrated aqueous gallium(III) bromide (GaBr₃) solutions have been investigated over a temperature range 80–333 K by Raman spectroscopy, X-ray absorption fine structure (XAFS), and X-ray diffraction. The Raman spectra obtained at various [Br^?]/[Ga³⁺] molar ratios and temperatures have shown that complex formation between Ga³⁺ and Br^? occurs as a predominant species, with [GaBr₄]^? at [Ga³⁺] as high as 1～2 M (M = mol?dm ^?3) and [Br^?]/[Ga³⁺] ratios > ～2, and that cooling of the solutions favors the formation of the aqua Ga³⁺. The intermediate species were not seen in the Raman spectra. The XAFS data have revealed that the aqua complex has a sixfold coordination as [Ga(H₂O)₆]³⁺ with a Ga³⁺–H₂O distance of (1.96 ± 0.02) Å, whereas the [GaBr₄]^? complex has a Ga³⁺–Br^? distance of (2.33± 0.02) Å, and that vitrification of the aqueous GaBr₃ solution at liquid nitrogen temperature shifts the equilibrium toward the aqua complex. The X-ray diffraction data at different subzero temperatures have shown a tendency of decreasing Ga³⁺–Br^? and increasing Ga³⁺–H₂O interactions with lowering temperature, confirming the preference of aqua Ga³⁺ in the supercooled liquid state as well as in the glassy state. The Ga³⁺–H₂O distance of ～1.8 Å for the tetrahedral coordination was found in a 2.01 M gallium(III) bromide solution with a [Br^?]/[Ga³⁺] ratio of 3.7 and gradually increased to a value of 1.92 Å for octahedral geometry with decreasing temperature, suggesting that equilibrium shifts from [GaBr₄]^? to [Ga(H₂O)₆]³⁺ through intermediate species, [GaBr _n ]^(3?n)+ (n = 2 and 3). The Ga³⁺–Br^? and Br^?–Br^? distances within [GaBr₄]^? with an almost tetrahedral symmetry are (2.35± 0.02) and (3.82± 0.03) Å, respectively. The Ga³⁺ has the second hydration shell at (4.03± 0.03) Å and the hydration of Br^? is characterized with a Br^?–H₂O distance of (3.35± 0.02) Å at all temperatures investigated.     

Luminescence characteristics and electronic levels of Eu(III) in the N,N-dimethyl-diphenyl-phosphinamide (DDPA) adduct of europium perrhenate

Optical absorption and emission spectra (1200-200 nm) of Eu(ReO₄)₃ · 2DDPA are measured between 77 and 650 K. Based on a C_s symmetry of the Eu(III) ion, a group theoretical analysis has been carried out and tentative assignments of ⁵D_J and ⁷F_J′ energy levels and their Stark splitting are made. The emission spectra measured with various Ar⁺ laser lines exhibit prominent fluorescence exclusively from ⁵D₀ to ⁷F_J′ levels. The relative intensities of the ⁵D_J emissions exhibit a temperature dependence showing a continuously decreasing intensity for the J ≥ 1 states. The ⁵D₀ state remains only a fluorescence state at higher temperatures (above ∼450 K). For J ≥ 1, the excited Eu(III) ions in these states decay by a combination of radiative transitions to the ⁷F_J′ levels and nonradiative processes operative within the ⁵D_J manifold.