X-ray Absorption Near-edge Spectroscopy Data Research Articles

Chemical state mapping of simulant Chernobyl lava-like fuel containing material using micro-focused synchrotron X-ray spectroscopy.

Uranium speciation and redox behaviour is of critical importance in the nuclear fuel cycle. X-ray absorption near-edge spectroscopy (XANES) is commonly used to probe the oxidation state and speciation of uranium, and other elements, at the macroscopic and microscopic scale, within nuclear materials. Two-dimensional (2D) speciation maps, derived from microfocus X-ray fluorescence and XANES data, provide essential information on the spatial variation and gradients of the oxidation state of redox active elements such as uranium. In the present work, we elaborate and evaluate approaches to the construction of 2D speciation maps, in an effort to maximize sensitivity to the U oxidation state at the U L3-edge, applied to a suite of synthetic Chernobyl lava specimens. Our analysis shows that calibration of speciation maps can be improved by determination of the normalized X-ray absorption at excitation energies selected to maximize oxidation state contrast. The maps are calibrated to the normalized absorption of U L3 XANES spectra of relevant reference compounds, modelled using a combination of arctangent and pseudo-Voigt functions (to represent the photoelectric absorption and multiple-scattering contributions). We validate this approach by microfocus X-ray diffraction and XANES analysis of points of interest, which afford average U oxidation states in excellent agreement with those estimated from the chemical state maps. This simple and easy-to-implement approach is general and transferrable, and will assist in the future analysis of real lava-like fuel-containing materials to understand their environmental degradation, which is a source of radioactive dust production within the Chernobyl shelter.

Inside black pearls

The quality of natural and cultured pearls depends on the size, shape and properties of the surface. The inner structure of the pearl is not known, and not taken into account. Indeed, it is assumed that the cultured pearl is a nucleus covered by a thin organic layer, a thin prismatic layer and a thick nacreous layer. The examination of spherical and pear shape samples resulting from the grafting of the black lip pearl oyster (Pinctada margaritifera) shows that this assumption is, in most cases, erroneous. Optical, electron and atomic force microscopic observations, infrared and Raman analyses, as well as micro X-ray fluorescence and micro X-ray absorption near-edge spectroscopy data show that the inner structure of a pearl does not depend on its shape. Both circle or pear shape and spherical pearls show a large variability of the inner structures and composition. There are no two identical samples. Moreover, the arrangement is irregular within a single pearl, and some structures (aragonitic pseudo-prisms) do not exist in the shell.

Structure rearrangements induced by lithium insertion in metal alloying oxide mixed spinel structure studied by x-ray absorption near-edge spectroscopy

Carbon-coated ZnFe2O4 (ZFO-C) spinel ferrite nanoparticles can be used in electrodes for Li-ion batteries and are known to show capacities larger than those calculated for an ideal spinel structure. In this work, the local structure evolution and reordering of this material upon lithium insertion are studied using K-edge and L-edge x-ray absorption near edge spectroscopy (XANES). XANES simulations corresponding to different lithiation stages are performed using full multiple scattering (Fe, Zn K-edge, Zn L-edge) and ligand field multiplet (LFM) calculations (Fe L-edge). XANES simulations are compared with experimental spectra obtained on ZFO-C nanoparticles previously characterized by electrochemical measurements. It is shown that a satisfactory agreement for the XANES Fe and Zn K-edges of pristine ZFO-C bulk nanoparticles can be obtained introducing a mixed spinel structure with Fe and Zn partially occupying tetrahedral and octahedral sites. Upon lithiation, changes in the XANES spectra are interpreted introducing displacements of the cations as an effect of occupation of Li into empty lattice sites. In particular, comparison of the simulations with the XANES data indicates that reversible Li insertion is accompanied by a migration of the Zn and Fe atoms from tetrahedral to octahedral sites. Furthermore, by studying L-edge XANES spectra, we show that the relocation and valence change of metal ions occur at earlier lithiation stages at the surface of the active material, gradually extending to the bulk for larger Li uptakes.

On the Oxidation State of Manganese Ions in Li-Ion Battery Electrolyte Solutions.

We demonstrate herein that Mn3+ and not Mn2+, as commonly accepted, is the dominant dissolved manganese cation in LiPF6-based electrolyte solutions of Li-ion batteries with lithium manganate spinel positive and graphite negative electrodes chemistry. The Mn3+ fractions in solution, derived from a combined analysis of electron paramagnetic resonance and inductively coupled plasma spectroscopy data, are ∼80% for either fully discharged (3.0 V hold) or fully charged (4.2 V hold) cells, and ∼60% for galvanostatically cycled cells. These findings agree with the average oxidation state of dissolved Mn ions determined from X-ray absorption near-edge spectroscopy data, as verified through a speciation diagram analysis. We also show that the fractions of Mn3+ in the aprotic nonaqueous electrolyte solution are constant over the duration of our experiments and that disproportionation of Mn3+ occurs at a very slow rate.

Valence modulations in CeRuSn

CeRuSn exhibits an extraordinary room temperature structure at 300~K with coexistence of two types of Ce ions, namely trivalent Ce$^{3+}$ and intermediate valent Ce$^{(4-\delta)+}$, in a metallic environment. The ordered arrangement of these two Ce types on specific crystallographic sites results in a doubling of the unit cell along the $c$-axis with respect to the basic monoclinic CeCoAl-type structure. Below room temperature, structural modulation transitions with very broad hysteresis have been reported from measurements of various bulk properties. X-ray diffraction revealed that at low temperatures the doubling of the CeCoAl type structure is replaced by a different modulated ground state, approximating a near tripling of the basic CeCoAl cell. The transition is accompanied by a significant contraction of the $c$ axis. We present new x-ray absorption near-edge spectroscopy data at the Ce L$_{3}$ absorption edge, measured on a freshly cleaved surface of a CeRuSn single crystal. In contrast to a previous report, the new data exhibit small but significant variations as function of temperature that are consistent with a transition of a fraction of Ce$^{3+}$ ions to the intermediate valence state, analogous to the $\gamma \rightarrow \alpha$ transition in elemental cerium, when cooling through the structural transitions of CeRuSn. Such results in a valence-modulated state.

Theoretical X-ray absorption near-edge structure signatures of solid and liquid phases of iron at extreme conditions

X-ray absorption near-edge spectroscopy (XANES) spectra of solid and liquid iron at pressure and temperature obtainable at the Earth's mantle conditions have been calculated. The spectra have been obtained using the supercell with an electron core-hole approach, in an ab initio scheme based on the continued-fraction approach and norm-conserving pseudo-potentials. The atomic structures for the spectra calculations were obtained from classical molecular dynamics simulations performed using an optimized modified embedded-atom potential. Comparisons to experimental XANES data are made for both solid and liquid phases of iron. For the liquid configuration, we find excellent agreement. Calculated spectra obtained for the solid configurations are also in good agreement with the experiment, and those obtained via other theoretical methods. We discuss the electronic origin of the XANES features.

Nature of Pd and Ti Metals in the Structure of CMAS Glass and Ceramics

The structure of electrically conductive CMAS-TiO2-Pd glass and ceramics was investigated by transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge spectroscopy (XANES), and extended X-ray absorption fine structure spectroscopy (EXAFS). The XANES spectra of Ti do not show any significant difference between the glasses ceramized in air or in a reducing “forming” gas, as well as between Pd-containing versus Pd-free samples, nor between surface versus bulk of the glass-ceramic samples. However, EXAFS and XANES data recorded at the Pd K-edge show significant dependences on whether the glass-ceramic was ceramized in air or in “forming” gas. The XPS spectra of Ti 2p core-level electrons for glasses ceramized in air or “forming” gas also show a strong difference depending on whether the samples did or did not contain Pd. STEM mapping confirms the existence of grains in the form of main crystalline phases identified with XRD, and also reveals the existence of Pd nanoparticles in glasses ceramized in both air and in “forming” gas.

Designing new n=2 Sillen–Aurivillius phases by lattice-matched substitutions in the halide and [Bi2O2]2+ layers

Abstract A new n=2 Sillen–Aurivillius compound Bi3Sr2Nb2O11Br has been synthesised based on Bi3Pb2Nb2O11Cl by simultaneously replacing Pb2+ with Sr2+ and Cl− with Br−. Rietveld refinements against X-ray and neutron powder diffraction data revealed a significant relative compression in the stacking axis (c-axis) of the new compound. Sr2+ doping reduces the impact of the stereochemically active 6s2 lone pair found on Pb2+ and Bi3+, resulting in a contraction of the c-axis by 1.22% and an expansion of the ab plane by 0.25%. This improves the inter-layer compatibility with the larger halide Br−. Analysis of X-ray absorption near-edge spectroscopy data show that the ferroelectric distortion of the B-site cation is less apparent in Bi3Sr2Nb2O11Br compared to Bi3Pb2Nb2O11Cl, and variable-temperature neutron diffraction data show no evidence for a ferroelectric distortion.

On the role of tannins and iron in the Bogolan or mud cloth dyeing process

We have investigated the chemistry of the Bogolan or mud cloth dyeing process, a traditional technique of coloring cotton cloths deeply rooted in Mali. Textiles produced by the traditional Bogolan process, using tannin-rich plant extract and iron-rich clay-based mud, were compared using infrared (IR) spectroscopy, scanning electron microscopy (SEM) and X-ray absorption near-edge spectroscopy (XANES) with cotton fibers that were impregnated with tannin and iron salt solutions. IR spectroscopy in both reflective mode on the cloth and cotton and in transmission mode on single fibers, together with SEM, showed that gallic and tannic acid adsorb and precipitate onto the cotton fiber surface. IR spectroscopy and comparison with tannin and iron solution-impregnated cotton showed that the black color of the traditional Bogolan cloth is dominated by the formation of iron-tannin complexes. The presence of iron in the Bogolan cloth was confirmed using XANES data, supporting the notion that iron has been transferred from the iron-rich clay-based mud to the cloth. The chemistry of Bogolan cloth is not only historically and culturally significant and of importance in textile conservation, but may also inspire future research on sustainable dyeing and processing techniques based on natural products.

Electronic structure of the chlorinated fullerene C60Cl30 studied by quantum chemical modeling of X‐ray absorption spectra

AbstractElectronic structure of the chlorinated fullerene D3d‐C60Cl30 has been studied using X‐ray photoelectron spectroscopy (XPS), X‐ray absorption near‐edge spectroscopy (XANES), and quantum chemical B3LYP calculations. The calculated shift of the core C 1s lines corresponding to the chlorinated carbon atoms and bare carbon atoms was shown to well agree with the experimental value obtained from the XPS data. This shift was taken into account in the construction of the theoretical XANES spectra near the C K‐edge of the molecule. Ground‐state calculation of D3d‐C60Cl30 and calculations of C59NCl and C60Cl29Ar+ ions simulating the excited states of the chlorinated fullerene (Z + 1 approximation) were used for modeling of C K‐edge and Cl L‐edge XANES data. A satisfactory agreement between experiment and theory was obtained in the case of Z + 1 approximation. Distinctions between the structure of the lowest unoccupied levels in the ground state and the C 1s excited state of D3d‐C60Cl30 have been revealed. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Intense blue and green photoluminescence emissions at room temperature in barium zirconate powders

Intense blue and green photoluminescence (PL) emissions were observed at room temperature in barium zirconate (BaZrO3) powders prepared by chemical method. The powders were analyzed by X-ray diffraction (XRD), X-ray absorption near edge spectroscopy (XANES), extended X-ray absorption spectroscopy (EXAFS) and ultraviolet–visible (UV–vis) absorption spectroscopy. The XRD patterns indicated the presence of a secondary phase of BaCO3 in BaZrO3 powders heat treated at 773 K. K-edge XANES data revealed that Zr atoms presents in at least two Zr environments. EXAFS investigations showed strong bond length dispersion in the first coordination shell around Zr atoms. EXAFS analysis indicated that Zr atoms are coordinated by 6 oxygen in BaZrO3 powders heat treated at 973 K. UV–vis measurements suggested the presence of intermediary energy levels into the band gap of BaZrO3 powders heat treated at 773 K. The intense PL emission in BaZrO3 powders can be attributed to the two Zr environments or ZrO5–ZrO6 clusters. © 2008 Elsevier B.V. All rights reserved.

Structural study of thin films prepared from tungstate glass matrix by Raman and X-ray absorption spectroscopy

Thin films were prepared using glass precursors obtained in the ternary system NaPO 3 BaF 2 WO 3 and the binary system NaPO 3 WO 3 with high concentrations of WO 3 (above 40% molar). Vitreous samples have been used as a target to prepare thin films. Such films were deposited using the electron beam evaporation method onto soda-lime glass substrates. Several structural characterizations were performed by Raman spectroscopy and X-ray Absorption Near Edge Spectroscopy (XANES) at the tungsten L I and L III absorption edges. XANES investigations showed that tungsten atoms are only sixfold coordinated (octahedral WO 6) and that these films are free of tungstate tetrahedral units (WO 4). In addition, Raman spectroscopy allowed identifying a break in the linear phosphate chains as the amount of WO 3 increases and the formation of P O W bonds in the films network indicating the intermediary behavior of WO 6 octahedra in the film network. Based on XANES data, we suggested a new attribution of several Raman absorption bands which allowed identifying the presence of W O − and W O terminal bonds and a progressive apparition of W O W bridging bonds for the most WO 3 concentrated samples (above 40% molar) attributed to the formation of WO 6 clusters.

Electronic characterization of the single-wall carbon nanotubes: a XANES study

The electronic structure of two different samples of single wall carbon nanotubes (SWCNs) have been investigated using x-ray absorption near edge spectroscopy (XANES). The XANES data, detected around the C K edge, have been compared with that of a pure HOPG sample used as structural standard. The measurements have been carried out at the photoemission 4B9B beamline (Synchrotron Radiation Facility of the Institute of High Energy Physics of Beijing) using a surface-sensitive total electron yield technique. The samples were previously characterized using Raman spectroscopy, electron microscopy (SEM), and tunneling microscopy (STM). The bonding properties of C atoms (π* and σ* bands of sp2-coordination) and the structural characteristics are identified by looking at the changes in both pre-edge and multiple-scattering regions of the XANES spectra.

Electronic, Magnetic, and Magnetoresistance Properties of the n=2 Ruddlesden–Popper Phases Sr3Fe2−xCoxO7−δ (0.25≤x≤1.75)

A series of oxygen-deficient n=2 Ruddlesden–Popper phases, Sr3Fe2−xCoxO7−δ (0.25≤x≤1.75), were prepared by solid-state reactions. Temperature-dependent susceptibility and field-dependent magnetization data indicate that for x≥0.25 the dominant magnetic interactions are ferromagnetic. The onset of strong ferromagnetic interactions is evident at ∼200 K, and a transition to a cluster-glass state is observed for all compositions below ∼45 K. The temperature variation of resistivity for all the compounds shows variable-range hopping behavior with two different localization energy scales: one for T<40 K and another for T>80 K. Large negative magnetoresistance (the largest MR ∼−65% for x=0.25) is observed for all phases. The magnetic susceptibility, Mössbauer and X-ray absorption near-edge spectroscopy data indicate that the formal oxidation state of Fe is close to 4+. The key role of d delocalization in the Sr3Fe2−xCoxO7−δ system is compared to the Sr3Fe2−xMnxO7−δ series, where d localization dominates the properties.

X-ray photoelectron spectroscopy and x-ray absorption near-edge spectroscopy study of SiO2/Si(100)

X-ray photoelectron spectroscopy (XPS) and Si K-edge x-ray absorption near-edge spectroscopy (XANES) have been used to characterize 0.5–20 nm thick SiO2 films on Si(100). In XPS measurements, the chemical shift of the Si(-O) 2p peak increases with oxide thickness. However, by neutralizing the surface with low-energy electrons, and using the C 1s as a reference, a chemical shift of 3.9±0.1 eV is found for neutralized oxide films, and the energy difference between the O 1s and Si(-O) 2p is independent of oxide thickness. XANES measurements confirm an invariant chemical shift with oxide thickness, the Si K edge from SiO2 being 1845.50±0.05 eV for all samples. XPS and XANES data then clearly show that any change in chemical shift of the Si(-O) 2p with oxide thickness is mainly due to a surface charging rather than microscopic strain induced by the molar volume mismatch between the oxide and the substrate, as has been previously believed.

The Adsorption of Ordered Polyimide Monolayers

AbstractOrdered polyimide monolayers can be prepared on solid surfaces using the Langmuir‐Blodgett technique of Kakimoto and co‐workers. In the present study we have investigated the adsorption of two different monolayers (PMDA‐ODA and PMDA‐PDA) on both graphite and oxidised silicon surfaces. Scanning tunneling microscopy indicates that on graphite both the rod‐like PMDA‐PDA and the zig‐zag PMDA‐ODA are characterized by a high degree of two‐dimensional order. X‐ray absorption near edge spectroscopy (XANES) at the carbon, nitrogen and oxygen K‐edges has been used to obtain information on the orientation of the different moieties in the polymer chain. These XANES data are compared to the corresponding spectra of pyromellitic diimide and diphenyl ether, which may be regarded as the building blocks of PMDA‐ODA polyimide, in order to obtain more insight into the origin of the resonances.