Pre-edge Structure Research Articles

Designing novel tunable Mn-based inorganic oxyfluoride pigments

A high tunability of green-blue colors in the manganese-doped oxyfluoride Sr2.5A0.5Mn0.1MO4F (A = Ca, Sr, Ba; M = Al, Ga) and anion-deficient Sr2.5A0.5Mn0.1MO4-αF1-δ (A = Ca, Sr, Ba; M = Al, Ga) is reported, and the chromophores responsible for this intense pigmentation are investigated. The hues exhibited by these materials are quantified via diffuse reflectance UV/Vis spectroscopy and measurement of their direct band gaps via Tauc plot. It is shown that choice of A cation (A = Ca, Sr, Ba) and M cation (M = Al, Ga) for as-synthesized phases Sr2.5A0.5Mn0.1MO4F yield a wide range of green colors (band gap range 2.70–2.96 eV). Treatment of these phases under reducing conditions according to Sr2.5A0.5Mn0.1MO4-αF1-δ (A = Ca, Sr, Ba; M = Al, Ga) induces anion non-stoichiometry, shifting the observed colors to a wide range of blue/blue-purple hues (band gaps from 3.31 to 3.66 eV), showing potential as tunable inorganic blue pigments. Density field theory (DFT) calculations support the preferential occupation of the smaller 8-coordinate Sr(2) site by the substituted Mn2+ cation. X-ray absorption near-edge structure (XANES) data reveal more subtle nuances in the interplay between formal manganese oxidation state, crystallographic site and observed hue. In general, for as synthesized (green) Sr2.5A0.5Mn0.1MO4F (A = Ca, Sr, Ba; M = Al, Ga), the edge position in Mn K-edge XANES is consistent with mixed Mn3+-Mn4+ oxidation state whilst a clear pre-edge structure suggesting that Mn is present on a tetrahedral site. This would suggest that during the reduction step, Mn3+/Mn4+ is reduced to entirely Mn2+ and migrates from the tetrahedral to the Sr(2) lattice site.

Application of Ligand Field Theory for Simulation of the Pre-Edge Structure of X-Ray Absorption Spectra of Amorphous Systems

Application of Ligand Field Theory for Simulation of the Pre-Edge Structure of X-Ray Absorption Spectra of Amorphous Systems

Understanding XANES spectra of two-temperature warm dense copper using ab initio simulations

Using ab initio molecular-dynamics simulations combined with linear-response theory, we studied the x-ray absorption near-edge spectra (XANES) of a two-temperature dense copper plasma. As the temperature increases, XANES spectra exhibit a pre-edge structure balanced by a reduction of the absorption just behind the edge. By performing systematic simulations for various thermodynamic conditions, we establish a formulation to deduce the electronic temperature Te directly from the spectral integral of the pre-edge that can be used for various thermodynamic conditions encountered in a femtosecond heating experiment where thermal non equilibrium and expanded states have to be considered.

Comparison of conventional and reactive sintering techniques for Lead–Free BCZT ferroelectric ceramics

Piezoelectric, dielectric and ferroelectric properties of a ceramic material greatly influenced by the synthesis technique and prepared condition. In this work, the efforts have been made to prepare barium calcium zirconium titanate (BCZT) ceramics and used to study the effectiveness of conventional sintering (CS) and reactive sintering (RS) methods. Specific composition of BCZT ceramic (Ba0.85Ca0.15Zr0.1Ti0.9O3) was achieved by suitable and verified sintering condition. Phase and structure of ceramics identified by using X-ray Diffraction (XRD). The result confirms the existence of pure perovskite structure and mixture phase of orthorhombic and tetragonal in both CS and RS sintering techniques. Surface morphology and grain size variations for different dwell times were studied using Scanning Electron Microscopy (SEM). In both CS and RS techniques, the optimal sintering condition (1540 °C and 2 h of dwell time) possess gives relatively high density, better electric and piezoelectric properties. RS technique emerged as an alternative for CS technique. The implementation of sintering conditions, driven effects and their role on the above-mentioned properties were discussed. Pre-edge fine structure (PEFS) and X-Ray absorption near edge spectroscopy (XANES) is employed to elucidate the variation and oxidation state of 4+ of Ti and Zr in BCZT ceramics.

Structural Evolution of MoO3 Thin Films Deposited on Copper Substrates upon Annealing: An X-ray Absorption Spectroscopy Study

Structural changes of MoO3 thin films deposited on thick copper substrates upon annealing at different temperatures were investigated via ex situ X-Ray Absorption Spectroscopy (XAS). From the analysis of the X-ray Absorption Near-Edge Structure (XANES) pre-edge and Extended X-ray Absorption Fine Structure (EXAFS), we show the dynamics of the structural order and of the valence state. As-deposited films were mainly disordered, and ordering phenomena did not occur for annealing temperatures up to 300 °C. At ~350 °C, a dominant α-MoO3 crystalline phase started to emerge, and XAS spectra ruled out the formation of a molybdenum dioxide phase. A further increase of the annealing temperature to ~500 °C resulted in a complex phase transformation with a concurrent reduction of Mo6+ ions to Mo4+. These original results suggest the possibility of using MoO3 as a hard, protective, transparent, and conductive material in different technologies, such as accelerating copper-based devices, to reduce damage at high gradients.

Electronic properties of a PrPC-Cu(ii) complex as a marker of 5-fold Cu(ii) coordination.

Human prion protein is a subject of extensive study, related in particular to the molecular basis of neurodegenerative disease development and prevention. This protein has two main domains: the membrane C-terminal, structured domain as well as the unstructured N-terminal domain. While PrPC (23-231) has up to eight Cu(ii) binding sites in the N-terminal domain, it includes a characteristic, conservative octarepeat region PHGGGWGQ, which was studied by means of X-ray absorption near edge spectroscopy. The measurements were conducted at the SuperXAS beamline (SLS, PSI, Villigen). For the initial 1 : 1 protein-to-Cu(ii) ratio, the two main Cu(ii) binding modes were identified using linear combination fitting and ab initio FEFF calculations for X-ray spectra. Their electronic structures indicated that Cu(ii) coordinated by strong π-donors could effectively suppress the pre-edge structure due to the filling of empty Cu(ii) d-states. The suppression was correlated with the charge transfer effect and filling of the virtual electronic Cu(ii) states. What is more, we showed that the 1s → 4p + LMCT (Ligand-to-Metal-Charge-Transfer) multielectron transition relation with the main edge transition could be used as a marker for preliminary comparison of an unknown organic compound to a reference. The presented results permitted a possible explanation of the mechanism of choosing the preferred Cu(ii) modes in PrPC-Cu(ii) coordination processes and of the complex stability from the electronic point of view.

Ca and S K-edge XANES of CaS calculated by different methods: influence of full potential, core hole and Eu doping

Ca and S K-edge spectra of CaS are calculated by the full-potential Green's function multiple-scattering method, by the FLAPW method and by the finite-difference method. All three techniques lead to similar spectra. Some differences remain close to the edge, both when comparing different calculations with each other and when comparing the calculations with earlier experimental data. Here it is found that using the full potential does not lead to significant improvement over the atomic spheres approximation and that the effect of the core hole can be limited to the photoabsorbing atom alone. Doping CaS with Eu will not affect the Ca and S K-edge XANES of CaS significantly but may give rise to a pre-edge structure not present for clean CaS.

Accurate Fe3+/Fetot ratio from XAS spectra at the Fe K-edge

A new approach to extract accurate information on the Fe oxidation state has been tested on three different Fe-rich amphiboles annealed at 194–520 °C. The method is based on the analysis of the pre-edge structure of the X-ray absorption spectra at the Fe-K edge. The pre-edge band is deconvolved into four Gaussian components where the integrated intensities of the pairs of lower- and higher-energy bands are related to the ferrous to ferric ion ratio. These Fe3+/Fetot ratios have been compared with those obtained with independent techniques (X-ray diffraction and Mossbauer spectroscopy) on the same samples. The experimental data allowed establishing a calibration curve that is in good agreement with a recent statistical analysis based on spectroscopic data for the same type of minerals. The method could be extended to the analysis of other complex systems containing both Fe3+ and Fe2+ in octahedral coordination or to study the dynamics of iron under non-ambient conditions.

Influence of film thickness on the structural transition cubic/hexagonal within Ti0.38Al0.62N films

Metastable Ti1−xAlxN films have the more outstanding functional properties in the range of composition around x = 0.6 for their use as protective coatings. They exhibit complex texture because of the phase transition between TiN-like rocksalt and AlN-like wurtzite structures. The competitive growth between cubic and wurtzite domains is studied in metastable Ti0.38Al0.62N thin film deposited by reactive magnetron sputtering for thicknesses between 1 and 3.7 μm. Long-range order is analysed within the film by X-ray diffraction and cross-sectional TEM imaging. The study is complemented by using Ti K-edge diffraction anomalous spectroscopy to obtain differentiated information on short-range order around Ti atoms incorporated in cubic and wurtzite domains. The growth along the [111]c direction has the effect of promoting the appearance of crystallites with wurtzite symmetry oriented along [001]h. In the middle of the layer, cubic and wurtzite domains become coherently oriented. Lattice distortions, which affect long- and short-range order, are revealed by the anisotropy of broadening of the diffraction lines and by spectroscopic data collected on the main diffraction lines. The fine pre-edge structure indicates a strong distortion of the tetrahedral sites occupied by Ti atoms along the c axis, which minimizes the in-plane lattice mismatch. Near the film surface, a dramatic change in growth directions is observed. A better understanding of the structural transition mechanism in the (Ti,Al)N system could help improve the mechanical performances of these coatings in the future.

1s2p Resonant Inelastic X-ray Scattering Magnetic Circular Dichroism as a probe for the local and non-local orbitals in CrO2

We have determined the magnetic ground state of the half-metal CrO2 based on 1s2p Resonant Inelastic X-ray Scattering Magnetic Circular Dichroism (RIXS-MCD) experiments. The two-dimensional RIXS-MCD map displays the 1s X-ray absorption spectrum combined with the 1s2p X-ray emission decay, where there is a large MCD contrast in the final state involving the 2p core hole.Our measurements show that the Cr K pre-edge structure is dominated by dipolar contributions and the quadrupole peak is invisible in direct K pre-edge absorption. Using RIXS-MCD, we reveal that the quadrupole 1s3d pre-edge has a large MCD contrast, which appears at lower energy with respect to the K pre-edge maximum.We use crystal field multiplet calculations to model the excitonic RIXS-MCD spectral shape in tetragonal (D4h) symmetry. The RIXS-MCD is strongly sensitive to the ground state distortion of the Cr4+ sites. The calculations of the RIXS-MCD maps suggest that the 3d spin–orbit interaction is fully quenched (ζ3d=0meV) and the ground state electron configuration must contain a 3B2g (D4h) contribution, which is required to explain the appearance of the Magnetic Circular Dichroism (MCD) in the Cr K pre-edge. This is in apparent contrast with the compressed tetragonal distortion.

Femtosecond time-resolved X-ray absorption spectroscopy of anatase TiO2 nanoparticles using XFEL.

The charge-carrier dynamics of anatase TiO2 nanoparticles in an aqueous solution were studied by femtosecond time-resolved X-ray absorption spectroscopy using an X-ray free electron laser in combination with a synchronized ultraviolet femtosecond laser (268 nm). Using an arrival time monitor for the X-ray pulses, we obtained a temporal resolution of 170 fs. The transient X-ray absorption spectra revealed an ultrafast Ti K-edge shift and a subsequent growth of a pre-edge structure. The edge shift occurred in ca. 100 fs and is ascribed to reduction of Ti by localization of generated conduction band electrons into shallow traps of self-trapped polarons or deep traps at penta-coordinate Ti sites. Growth of the pre-edge feature and reduction of the above-edge peak intensity occur with similar time constants of 300–400 fs, which we assign to the structural distortion dynamics near the surface.

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.

Observation of the flexoelectricity of a SrTiO3 single crystal by x-ray absorption and emission spectroscopies

Flexoelectricity, defined as the spontaneous electric polarization in a dielectric material induced by a strain gradient, is investigated from the microscopic viewpoint by x-ray spectroscopy. A single crystal SrTiO3 sample was used as a test system in order to reveal the appearance of the electric dipole moment by simple bending of the crystal. The spectral change characteristic of ferroelectric transition in SrTiO3 was not observed in the Ti K-edge absorption spectra. Instead, the gradual decrease (increase) of the post-edge feature (pre-edge structure) by bending was qualitatively explained using theoretical calculations that assumed the presence of oxygen vacancies and a slight crystal distortion. This assumption is also supported by the broadening of a tiny charge-transfer peak in the Ti Kβ resonant emission spectra. Therefore, it was revealed that the flexoelectric effect in SrTiO3 is easily drowned out through local imperfection induced by crystal deformations and cracks.

Electronic transitions induced by short-range structural order in amorphousTiO2

Several promising applications of amorphous titanium dioxide, $a{\mathrm{TiO}}_{2}$, have appeared recently, but the correlation between electronic properties and atomic short-range structural order is poorly understood. Herein we show that structural disorder yields local undercoordinated ${\mathrm{TiO}}_{x}$ units which influence electronic hybridization of $\text{Ti-}[4p]$ and $\text{Ti-}[3d]$ orbitals with a low crystal-field splitting $[E({e}_{g})\ensuremath{-}E({t}_{2g})=2.4\ifmmode\pm\else\textpm\fi{}0.3\phantom{\rule{0.16em}{0ex}}\mathrm{eV}]$. The short-range order and electronic properties of $a{\mathrm{TiO}}_{2}$ thin-film oxides are described through an integrated approach based on x-ray-absorption experiments and ab initio computational simulations where the energy splitting of the electronic levels in the $\text{Ti-}[4p\ensuremath{-}3d]$ manifold are analyzed. Structural disorder provides enough $p\ensuremath{-}d$ orbital mixing for the hybridized electronic transitions from the $\text{Ti-}[1s]$ core level into the $[\mathrm{Ti}\ensuremath{-}{t}_{2g}]$ and $[\mathrm{Ti}\ensuremath{-}{e}_{g}]$ bands [$1s\ensuremath{\rightarrow}4p\ensuremath{-}3d$ excitations], to be allowed. This yields an intense pre-edge structure in the Ti $K$-edge x-ray-absorption near-edge structure spectrum of $a{\mathrm{TiO}}_{2}$, which is consistent with the projected density of states on the photoabsorbing Ti atoms.

X-ray absorption spectra of graphene and graphene oxide by full-potential multiple scattering calculations with self-consistent charge density

The x-ray absorption near-edge structure of graphene, graphene oxide, and diamond is studied by the recently developed real-space full potential multiple scattering (FPMS) theory with space-filling cells. It is shown how accurate potentials for FPMS can be generated from self-consistent charge densities obtained with other schemes, especially the projector augmented wave method. Compared to standard multiple scattering calculations in the muffin-tin approximation, FPMS gives much better agreement with experiment. The effects of various structural modifications on the graphene spectra are well reproduced. (1) Stacking of graphene layers increases the peak intensity in the higher energy region. (2) The spectrum of the C atom located at the edge of a graphene sheet shows a prominent pre-edge structure. (3) Adsorption of oxygen gives rise to the so-called interlayer-state peak. Moreover, O K-edge spectra of graphene oxide are calculated for three types of bonding, C-OH, C-O-C, and C-O, and the proportions of these bondings at 800 degrees C are deduced by fitting them to the experimental spectrum.

Electronic Structure of Nitrogen-Doped Graphene in the Ground and Core-Excited States from First-Principles Simulations

We have calculated the N 1s near-edge X-ray absorption fine structure (NEXAFS) spectra of nitrogen-doped monolayer graphene (NG) using density functional theory (DFT) with the equivalent core hole approximation. The hexavacancy (6V) defect and its dependence on the nitrogen-doping concentration have been analyzed in detail via both N 1s → π* and N 1s → σ* transitions. The NEXAFS spectra are sensitive to the doping concentration of N in the π* region: diluted doping weakens the main π* peak and smears the oscillations in this region. The vacancy defect leads to a red-shift in both the π and σ spectra. A pyridinic nitrogen at the 6V defect center exhibits a sharp π* peak at 398.4 eV, which agrees well with the experimental pre-edge structure at 398.6 eV. The σ* peak is split in two, which can serve as the fingerprint to reveal the nature of the defect. A structural change from pyridinic to pyrrolic NG results in a distinctive difference in the spectral shape. The ground-state band structure has also been si...

X-Ray Absorption Fine Structure Investigation of Copper(II) Mixed Ligand Complexes with Pyridinedicarboxylic Acid as Primary Ligand

The X-ray absorption fine structure (XAFS) spectra at the K-edge of the copper complexes Cu(PDC)(Mim)3 H2O (1) and Cu(PDC)2(EA)2H2O (2) (where PDC – Pyridine-2,3-dicarboxylic acid, Mim – 2-methylimidazole, and EA – ethyl acetate) have been investigated. The experimental extended X-ray absorption fine structure data of complex 1 have been analyzed by fitting the theoretical model generated from its own crystallographic data. The crystallographic data for complex 2 are not available. It has been found by comparing the intensity of the pre-edge peaks and X-ray absorption near edge structure features of complexes 1 and 2 that both complexes possess square pyramidal geometry around the copper centers and thus complex 2 is analogous to complex 1. Hence, the theoretical model generated for complex 1 has been fitted to the experimental EXAFS data of complex 2 to determine the structural parameters of complex 2. The coordination geometry of both complexes has been depicted. Further, the chemical shifts have been used to determine the oxidation state as well as to estimate the effective nuclear charge on the copper atom.

A near edge X-ray absorption fine structure study of oxidized single crystal and polycrystalline diamond surfaces

The influence of oxidizing environments on single crystal diamond and polycrystalline chemical vapor deposited CVD diamond films was studied using the near edge X-ray absorption fine structure (NEXAFS) pre-edge region in both bulk and surface sensitive modes. The NEXAFS of (100) oriented single crystal diamond was measured following (i) exposure to a microwave (MW) hydrogen plasma, (ii) annealing to 1000 °C, (iii) exposure of the as annealed surface to H2O, and (iv) exposure of the as annealed surface to O2. From these measurements particular surface bonding configurations have been assigned to features in the pre-edge structure. The NEXAFS of microcrystalline CVD diamond films was studied following different oxidative treatments using (i) a thermal atomic oxygen (AO) environment, (ii) a hyperthermal (5 eV) AO source, and (iii) an RF oxygen plasma exposure. The nature of the surface layer was found to be different for differently oxidized surfaces. These treatments were carried out as part of a study of CVD diamond durability in the low Earth orbit space environment.

Selective Probing of the OH or OD Stretch Vibration in Liquid Water Using Resonant Inelastic Soft-X-Ray Scattering

High-resolution O 1s resonant inelastic x-ray scattering spectra of liquid H2O, D2O, and HDO, obtained by excitation near the preedge resonance show, in the elastic line region, well-separated multiple vibrational structures corresponding to the internal OH stretch vibration in the ground state of water. The energy of the first-order vibrational excitation is strongly blueshifted with respect to the main band in the infrared or Raman spectra of water, indicating that water molecules with a highly weakened or broken donating hydrogen bond are correlated with the preedge structure in the x-ray absorption spectrum. The vibrational profile of preedge excited HDO water is well fitted with 50%±20% greater OH-stretch contribution compared to OD, which strongly supports a preference for OH being the weakened or broken H-bond in agreement with the well-known picture that D2O makes stronger H-bonds than H2O. Accompanying path-integral molecular dynamics simulations show that this is particularly the case for strongly asymmetrically H-bonded molecules, i.e., those that are selected by preedge excitation.

First-Principles Study of X-ray Absorption Spectra of FeS2

It is well known for a long time that FeS2 has two stable crystal phases, cubic pyrite (space group Pa 3) and orthorhombic marcasite (Pnnm) structures. Surprisingly, local coordinations around Fe and S atoms are quite similar between the two phases. For example, Fe is almost octahedrally coordinated by six S atoms at a distance of 2.26 A in pyrite and at 2.23–2.25 A in marcasite (see Table I). Dumbbell structure of the nearest neighbor S atoms with a bond distance of about 2.2 A is commonly seen in the both phases. X-ray absorption spectroscopy (XAS) is a useful tool to investigate the element-specific electronic structure of condensed matter systems. In particular, local geometry and symmetry around a given site are also analyzed by combining electronic structure calculations with experiments. XAS spectra at S and Fe K-edges have been measured for NiAs-type FeS and pyrite FeS2 and the observed first peak at the S K-edge and the pre-edge structure at the Fe K-edge are attributed to a transition to the empty anti-bonding states formed by hybridization of Fe 3d and S 3p orbitals. Although several first-principles density-functional-theory (DFT) calculations have been reported for Fe sulfides including pyrite and marcasite FeS2 since then, no theoretical analysis on XAS spectra has been performed for Fe–S systems. Discharge and charge reactions in Li/FeS2 and Na/FeS2 were experimentally studied for ion-battery applications. Among them, Kitajou et al. have measured XAS spectra at Fe K-edge for pyrite FeS2 and Na discharged FeS2 and found some variations in the main peak region while the pre-edge structure retains its shape by discharge. In this note, we present some theoretical results of XAS spectra of FeS2 in the pyrite and marcasite phases by performing firstprinciples DFT calculations. Electronic-structure calculations are carried out within the generalized gradient approximation (GGA) by Perdew, Burke, and Ernzerhof (PBE). One-electron Kohn–Sham equations are self-consistently solved by the all-electron full-potential linearized augmented plane wave method (FLAPW) in a scalar relativistic scheme. Experimental crystal structure including lattice constants and internal parameters is assumed throughout the calculation. XAS spectrum can be computed by using Fermi’s golden rule for the transition probability from an initial state jii to a final state j f i by x-ray with the photon energy h ! as Ið!Þ 1⁄4 2 h X