Theoretical X-ray Absorption Near-edge Structure Spectra Research Articles

Local structure investigations of accumulated damage in irradiated MgAl2O4

AbstractDamaged structures in the MgAl2O4 spinel induced by swift heavy ions were investigated using the X‐ray absorption near edge structure (XANES) and small angle X‐ray scattering (SAXS). Increasing the fluence of 100 MeV Xe ions leads to increased SAXS intensity and XANES spectral changes on both Mg and Al K‐edges. The damaged regions of ion tracks were observed by SAXS to be cylindrical in shape with a diameter of 5 nm. The theoretical XANES spectra indicated that the changes in the experimental spectra were due to the cationic disordering between tetrahedral and octahedral sites. This disordering caused an increase in the inversion degree of the cations. Furthermore, the quantitative analysis of the XANES spectra revealed the preferential occupation of cations at the octahedral sites at high fluence.

Influence of calcination temperature on phase formation and local structure of Co0.6Zn0.4Fe1.6Cr0.4O4 nanoparticles

In this work, Co0.6Zn0.4Fe1.6Cr0.4O4 nanoparticles were synthesized by the solid state combustion technique and calcined at various temperatures from 700 to 900 °C. The phase formation, morphology and local structure of the calcined samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray absorption near edge structure (XANES) techniques. The XRD analysis revealed existence of the single-phase cubic spinel structure in the samples calcined at 900 °C. The average crystallite sizes increased as the calcination temperature increased. The XANES spectra allowed the valence states of Co2+, Cr3+, Fe3+ and Zn2+ ions in all calcined samples to be established. The clear agreement between measured and ab initio theoretical XANES spectra was the strongest evidence of Cr3+ and Zn2+ ions substituting for Co2+ and Fe3+, respectively, in CoFe2O4 inverse spinel structure. XANES results also suggested that the site occupancy of Co2+, Cr3+, Fe3+ and Zn2+ ions was mixed, with the majority Co2+ and Cr3+ ions occupying octahedral sites and the majority of Fe3+ ions occupying tetrahedral sites, while the Zn2+ ions equally occupied octahedral and tetrahedral sites in a ratio of 50:50.

Vibrational Effects in X-ray Absorption Spectra of Two-Dimensional Layered Materials

With the examples of the C $K$-edge in graphite and the B $K$-edge in hexagonal BN, we demonstrate the impact of vibrational coupling and lattice distortions on the X-ray absorption near-edge structure (XANES) in 2D layered materials. Theoretical XANES spectra are obtained by solving the Bethe-Salpeter equation of many-body perturbation theory, including excitonic effects through the correlated motion of core-hole and excited electron. We show that accounting for zero-point motion is important for the interpretation and understanding of the measured X-ray absorption fine structure in both materials, in particular for describing the $\sigma^*$-peak structure.

Application of x-ray absorption spectroscopy on local structure study in Fe-doped BaTiO3 ceramics

ABSTRACTIn this work, Fe-doped BaTiO3 ceramics were prepared by a conventional solid state reaction method. The effects of sintering temperature on the global and local structures of Fe-doped BaTiO3 ceramic (with 10 mole% Fe) were investigated by X-ray diffraction (XRD) and X-ray Absorption Spectroscopy (XAS) techniques, respectively. The XRD measurements indicated the global structure of the prepared ceramic changed from pure tetragonal to hexagonal structure with increasing sintering temperature. From Synchrotron X-ray Absorption Near-Edge Structure (XANES) measurements, the oxidation state of Fe ions in BaTiO3 structure was clearly identified, as +3. The clear agreement between the experimental and theoretical XANES spectra indicated Fe substituting on Ti site in BaTiO3 structure.

Modelling of substitutional defects in the structure of Ti-bearing hibonite

A local atomic structure around titanium positions in Ti-bearing hibonite (CaAl12O19) has been studied. The structural models of substitution of different substitution defects Ti–Al in hibonite by titanium atoms have been considered. Optimization of structural models of hibonite has been done by means of density functional theory calculations using pseudopotential approximation as implemented in VASP 5.3 code. Gibbs free energies analysis has shown that models of substitution of M2 and M4 aluminum positions by titanium atoms are the most probable. For the most probable structural models of Ti-bearing hibonite theoretical X-ray absorption near-edge structure (XANES) spectra near the titanium K edge have been calculated. Significant differences in theoretical XANES spectra calculated for different structural models with non-optimized and optimized atomic structure have been demonstrated. Changes in the intensity of pre-edge structure of TiK XANES spectra for different substitution models of aluminum by titanium have been observed which relate to different titanium coordination in structural models. Energy shift of spectral features towards lower energy for optimized models implies increase of interatomic distances in local surroundings of Ti absorbing atoms.

Atomic and electronic structure of free niobium nanoclusters: Simulation of the M4,5-XANES spectrum of Nb13+

The atomic and electronic structure of free niobium nanoclusters has been studied on the basis of X-ray absorption near-edge structure (XANES) spectroscopy and density functional theory. M4,5-XANES spectra have been calculated for several structural models of the 13-atomic niobium cluster. The calculations have been done on the basis of both full multiple scattering theory within the muffin-tin approximation for a potential and full-potential finite difference method. The comparison of the experimental M4,5-edge XANES spectrum (Peredkov et al., J. Electron Spectros. Relat. Phenomena 184 (2011) 113–118) with the simulated X-ray absorption spectra of Nb13+ hints to a highly-symmetric icosahedral structure of the cluster. An internuclear distance of 2.2±0.1Å between neighboring “surface” atoms of the icosahedron and 2.09Å between the central “bulk” atom and “surface” atoms, respectively, has been found upon comparison of the experimental and theoretical XANES spectra.

Direct determination of Mn occupations in Ga1−xMnxN dilute magnetic semiconductors by x-ray absorption near-edge structure spectroscopy

X-ray absorption near-edge structure (XANES) spectroscopy is used to study the features of occupation sites of Mn dopants in the Ga1−xMnxN dilute magnetic semiconductors (DMSs) with zinc-blende structure. Theoretical XANES spectra are calculated for representative structure models of Mn atoms in the GaN lattice. It is shown that the substitutional Mn in GaN is characterized by a preedge peak at 2.0eV and a postedge multiple-scattering peak at 29.1eV. The peaks shift in position and drop in intensity dramatically for the interstitial MnI and MnGa–MnI dimer, and then disappear completely for Mn clusters. The experimental spectrum of Ga0.990Mn0.010N is almost reproduced by the calculated XANES spectrum of GaMnN with substitutional Mn.

XANES Mn K edge in NaNbO3 based ceramics doped with Mn and Bi ions

Sodium niobate based ceramics doped with Mn atoms have been sintered from oxides. The Bi co-dopants were introduced to increase the solubility limit for Mn atoms. Two series of samples were heated at lowered air pressure to produce oxygen nonstoichiometry. X-ray Absorption Near Edge Structure (XANES) spectra at Mn K-edge were measured for samples with and without Bi co-dopants. Theoretical XANES spectra were calculated using the Real Space Multiple Scattering method (FEFF 8 code). The simulation showed that ceramics samples have Pbcm symmetry and all of them have Mn atoms located in a Nb sublattice.

Nickel site distribution and clustering in synthetic double-chain silicates by experimental and theoretical XANES spectroscopy

The crystal-chemical role of Ni in synthetic potassium-richterites within the join ${\mathrm{K}(\mathrm{CaNa})\mathrm{Mg}}_{4}{\mathrm{Ni}[\mathrm{Si}}_{8}{\mathrm{O}}_{22}]{(\mathrm{OH})}_{2}{\ensuremath{-}\mathrm{K}(\mathrm{CaNa})\mathrm{Ni}}_{5}[{\mathrm{Si}}_{8}{\mathrm{O}}_{22}]{(\mathrm{OH})}_{2}$ has been investigated by XAS (x-ray-absorption spectroscopy). From a structural point of view these materials are classified as double-chain silicates and belong to the amphiboles group. X-ray absorption near-edge structure (XANES) spectra recorded at the $\mathrm{Ni}K$ edge show variations that can be related to the Ni-Mg substitution at the octahedral $M$ sites of these amphi-boles. Theoretical XANES spectra, calculated on the basis of the multiple-scattering theory, reproduce very well the experimental spectra, and allow us to understand the relationships between XANES features and the chemical and structural environments around the photoabsorbing ion. The combination of XAS experiments and theoretical calculations provides information on the Ni partitioning between the three octahedral sites and on the local chemistry around the absorber.

A NEW EVALUATION FOR X-RAY ABSORPTION SPECTRA IN THE KANES REGION

The electronic states of copper(II) ions containing the square-planar tetraaza and macrocyclic copper(II) complexes were measured in solid state by means of X-ray absorption near edge structure(XANES) The peaks in the measured XANES spectra shifted to lower energy side with increasing electron density of a central copper(II) ion. The molecular orbital calculations for the complexes was carried out by the DV-Xa method, and the theoretical XANES spectra were estimated. The chemical shift using XANES spectra was evaluated by this work.

A Xanes Study of Square Copper(II) Complexes

AbstractThe XANES (X-ray absorption near edge structure) spectra of copper(II) ions in solid state and in solution of the square-planar copper(II) complexes with tetraaza macrocycles were measured. The peaks in the measured XANES spectra shifted to lower energy side with increasing the electron density of central copper(II) ions. The molecular orbital calculations for the complexes were carried out by the DV-Xα method, and the theoretical XANES spectra were estimated. The clear chemical shift obtained by this XANES study is evaluated and leads to a new concept of π-back donation between the copper(II) complexes and counter anion in aqueous solution.