K-edge X-ray Absorption Measurements Research Articles

Elemental mapping and iron oxidation state measurement of synthetic low-activity waste feeds

During the conversion of nuclear waste feed into glass, iron bearing precursors are incorporated into the melt and affect various properties of the melt such as viscosity and density. Laboratory-scale measurements to determine iron oxidation state could be used in cases where samples are subjected to ex-situ heat treatment and kinetic relationships developed. Typically, measurements of iron oxidation state have been limited to synchrotron X-ray absorption techniques, Mossbauer spectroscopy, or destructive wet chemical techniques. In this work, we present our method development which employs electron probe microanalysis (EPMA) to measure the oxidation state of iron. This method could also serve broader applications, not only in nuclear waste vitrification but also in commercial glass making, geology, and archaeology. Using EPMA, we measured the Fe L-edge X-ray emission spectra of nuclear waste feed samples prepared with a reduced iron precursor and heat treated to various temperatures in air. EPMA based measurements provide a cost-effective and rapid alternative to measure iron oxidation state. Furthermore, the method described in this paper allows for spatially resolved measurements with a minimum step size of 100 µm. We demonstrate that iron oxidation state can be calculated using EPMA, and we compare the calculated iron oxidation state to that measured by synchrotron Fe K-edge X-ray absorption measurements. This work demonstrates that, for nuclear waste feeds prepared with reduced iron, the oxidation state of the iron is influenced by the thermal history. Fast-drying the feed slurry produced more reduced iron in feed than previously observed in feeds prepared by slowly drying and crushing into a powder.

Oxygen K-edge X-ray Absorption Spectra.

We review oxygen K-edge X-ray absorption spectra of both molecules and solids. We start with an overview of the main experimental aspects of oxygen K-edge X-ray absorption measurements including X-ray sources, monochromators, and detection schemes. Many recent oxygen K-edge studies combine X-ray absorption with time and spatially resolved measurements and/or operando conditions. The main theoretical and conceptual approximations for the simulation of oxygen K-edges are discussed in the Theory section. We subsequently discuss oxygen atoms and ions, binary molecules, water, and larger molecules containing oxygen, including biomolecular systems. The largest part of the review deals with the experimental results for solid oxides, starting from s- and p-electron oxides. Examples of theoretical simulations for these oxides are introduced in order to show how accurate a DFT description can be in the case of s and p electron overlap. We discuss the general analysis of the 3d transition metal oxides including discussions of the crystal field effect and the effects and trends in oxidation state and covalency. In addition to the general concepts, we give a systematic overview of the oxygen K-edges element by element, for the s-, p-, d-, and f-electron systems.

Investigating possible kinetic limitations to MgB2 hydrogenation

An investigation is reported of possible kinetic limitations to MgB2 hydrogenation. The role of H–H bond breaking, a necessary first step in the hydrogenation process, is assessed for bulk MgB2, ball-milled MgB2, as well as MgB2 mixed with Pd, Fe and TiF3 additives. The Pd and Fe additives in the MgB2 material exist as dispersed metallic particles in the size range ~5–40 nm diameter. In contrast, TiF3 reacts with MgB2 to form Ti metal, elemental B and MgF2, with the Ti and the MgF2 phases proximate to each other and coating the MgB2 particulates with a film of thickness ~3 nm. Sieverts studies of hydrogenation kinetics are reported and compared to the rate of H–H bond breaking as measured by H-D exchange studies. The results show that H–H bond dissociation does not limit the rate of hydrogenation of MgB2 because H–H bond cleavage occurs rapidly compared to the initial MgB2 hydrogenation. The results also show that surface diffusion of hydrogen atoms cannot be a limiting factor for MgB2 hydrogenation. Instead, it is speculated that it is the intrinsic stability of the B–B extended hexagonal ring structure in MgB2 that hinders the hydrogenation of this material. This supposition is supported by B K-edge x-ray absorption measurements of the materials, which showed spectroscopically that the B–B ring was intact in the material systems studied. The TiF3/MgB2 system was examined further theoretically with reaction thermodynamics and phase nucleation kinetic calculations to better understand the production of Ti metal when TiB2 is thermodynamically favored. The results show that there exist physically reasonable ranges for which nucleation kinetics supersede thermodynamics in determining the reactive pathway for the TiF3/MgB2 system and perhaps for other additive systems as well.

Reduction and oxidation annealing effects on Cu K-edge XAFS for electron-doped cuprate superconductors

Reduction and oxidation annealing effects on the electronic states around the copper sites for Pr2-xCexCuO4 and Nd2-xCexCuO4 with x = 0 and x = 0.15 were investigated by Cu K-edge x-ray absorption measurements. Cu K near-edge spectra were changed by the reduction annealing in a manner similar to the case of Ce substitution for both x = 0 and x = 0.15. This means an increase of electron density at the copper sites, indicating the aspect of electron doping in the reduction annealing. This reduction annealing effect on the near-edge spectra are reverted by the additional oxidation annealing. The amount of electron density around the copper sites is varied by the reduction and oxidation annealing, reversibly, corresponding to the reversible variation of the physical property from insulating to superconductivity.

Mono- and binuclear tris(3-tert-butyl-2-sulfanylidene-1H-imidazol-1-yl)hydroborate bismuth(III) dichloride complexes: a soft scorpionate ligand can coordinate to p-block elements.

Tris(pyrazolyl)hydroborate ligands have been utilized in the fields of inorganic and coordination chemistry due to the ease of introduction of steric and electronic substitutions at the pyrazole rings. The development and use of the tris(pyrazolyl)hydroborate ligand, called a `scorpionate', were pioneered by the late Professor Swiatoslaw Trofimenko. He developed a second generation for his ligand system by the introduction of 3-tert-butyl and 3-phenyl substituents and this new ligand system accounted for many remarkable developments in inorganic and coordination chemistry in stabilizing monomeric species while maintaining an open coordination site. Bismuth is remarkably harmless among the toxic heavy metal p-block elements and is now becoming popular as a replacement for highly toxic metal elements, such as lead. Two bismuth(III) complexes of the anionic sulfur-containing tripod tris(3-tert-butyl-2-sulfanylidene-1H-imidazol-1-yl)hydroborate ligand were prepared. By recrystallization from MeOH/CH2Cl2, orange crystals of dichlorido(methanol-κO)[tris(3-tert-butyl-2-sulfanylidene-1H-imidazol-1-yl-κS)hydroborato]bismuth(III), [Bi(C21H34BN6S3)Cl2(CH4O)], (I), were obtained, manifesting a mononuclear structure. By using a noncoordinating solvent, red crystals of the binuclear structure with bridging Cl atoms were obtained, namely di-μ-chlorido-bis{chlorido[tris(3-tert-butyl-2-sulfanylidene-1H-imidazol-1-yl-κS)hydroborato]bismuth(III)}, [Bi2(C21H34BN6S3)2Cl4], (II). These complexes show {BiIIIS3Cl2O} and {BiIIIS3Cl3} coordination geometries with average BiIII-S bond lengths of 2.73 and 2.78 Å in (I) and (II), respectively. The overall BiIII coordination geometry is distorted octahedral due to stereochemically active lone pairs. The three BiIII-S bond lengths are almost equal in (I) but show considerable differences in (II), with one long and two shorter distances that also correlate with changes in the UV-Vis and 1H NMR spectra. For direct measurements of the Bi-S/Cl coordination, ligand K-edge X-ray absorption measurements were carried out in combination with ground and excited-state electronic structure analyses. For p-block elements, these sulfur-containing ligands are useful for preparing the appropriate complexes due to their flexible coordination geometry.

Enhanced Rate Capability in Two Phase Reaction in LiFePO4 By Interfacial Modification Between Cathode and Electrolyte

Olivine-type LiFePO4 is considered as a promising cathode material for high power density lithium-ion battery. Lithium-ion intercalation proceeds thorough a two-phase reaction divided into LiFePO4 and FePO4. During the phase transition between LiFePO4 and FePO4, the nucleation of a new phase occurs at the electrode / electrolyte interface, which is reported to be the rate determining reaction.1 To improve the rate capability of LiFePO4, Park et al performed the surface modification by nitrogen on LiFePO4 particles.2 However, their improved mechanism is not clearly understood due to the difficulty in the observation of electrode / electrolyte interface. In order to analyze the electrode / electrolyte interface, thin-film electrodes and surface sensitive X-ray absorption spectroscopy are powerful tool.3 In this study, we prepared LiFePO4thin-film model electrodes to clarify the reaction kinetics and applied the interfacial modification of nitrogen. Their reaction mechanism is discussed with the information of X-ray absorption spectroscopy. LiFePO4 thin-film was prepared on a polycrystalline Au substrate by pulsed laser deposition method. The deposition was conducted under Ar atmosphere (0.0001 Pa) at 923K for 30 min. The prepared thin films were annealed under ammonia gas at 873 K. The obtained thin-film was characterized by X-ray diffraction and TEM. X-ray absorption spectroscopy at Co K-edge was performed at BL37XU, Spring-8. N K-edge X-ray absorption measurements were performed in Ritsumeikan SR-center. For the electrochemical analysis, three-electrode cells employing Li metal as a counter and a reference electrode, and 1 M LiClO4in propylene carbonate liquid electrolyte were prepared. Annealing of ammonia gas introduces nitrogen into LiFePO4 surface, which is confirmed by N K-edge X-ray absorption spectra. The rate capability of the nitrogen modified LiFePO4 thin-film electrodes is improved. The surface sensitive Fe K-edge X-ray absorption spectra show the increasing of oxidation state in iron, which implies the lattice mismatch is decreased in surface of LiFePO4. Therefore, nucleation reaction is enhanced during two phase reaction between LiFePO4 and FePO4, which is the origin of surface modification effect.

The Fe/NiO interface studied by polarization dependent X-ray absorption spectroscopy

In order to provide a structural basis for the modelling of the electronic and magnetic properties of the Fe/NiO(001) interface we have performed polarization dependent Fe K-edge X-ray absorption measurements. A multi-shell fit of the data is presented and discussed. We find that a 2ML Fe film exhibits a complete tetragonal distortion of the unit cell and demonstrate the formation of a planar FeO layer with expanded Fe–O distances perpendicular to the growth plane. We discuss a model for the interface with the FeO layer at the oxide–metal interface. At 10ML thickness the tetragonal strain of the Fe film is partially released.

New perovskite related-compounds of Ru(V) in K2NiF4 type structure

The preparation of the new K2NiF4-type compounds, Sr2Co0.5Ru0.5O4, Sr2Ni0.5Ru0.5O4 and Sr2Ga0.5Ru0.5O4 by direct solid state synthesis and the subsequent characterization by powder X-ray diffraction measurements are described. From Rietveld refinements, we found that the compounds have a K2NiF4-type structure with the symmetry of space group I4/mmm. It was found that in these compounds, the substituted trivalent metal ions (M=Co, Ni, and Ga) and the Ru ions are disordered over the octahedral B sites of the K2NiF4 structure. The octahedral Ru–O environment is less elongated in the apical direction in these compounds than in the Sr2RuO4 compound. We also found that the octahedron in Sr2Ga0.5Ru0.5O4 is more symmetric than that of Sr2Co0.5Ru0.5O4 and Sr2Ni0.5Ru0.5O4. The Ru K-edge X-ray absorption measurements showed that Ru ions were in a pentavalent oxidation state.

Silicon clustering in Si–GaAs δ-doped layers and superlattices

To probe the development of Si–GaAs superlattices, we performed Si K-edge x-ray absorption measurements on periodic Si δ-doped structures in GaAs, for equivalent Si thicknesses of 0.02, 0.2, 0.5, 1, 2, and 4 monolayers (ML). We detected the presence of both Si–Si and Si–As (or Si–Ga) bonds and followed the variation of the coordination numbers as a function of the equivalent thickness of the Si layers. The Si–Si coordination number was found to gradually increase with increasing thickness, but Si–Si bonds were always detected, even at equivalent Si coverages of 0.02 ML. This indicates that the preferred growth conditions for Si–GaAs superlattices lead to widespread Si clustering and self-compensation, and suggests that lateral growth of such clusters to achieve coalescence may be the main mechanism for Si quantum well development.

In situ X-ray absorption spectroscopy study of lithium insertion in a new disordered manganese oxi-iodide

Mn K-edge X-ray absorption measurements were carried out on an amorphous manganese oxi-iodide (Li 0.60Na 0.16MnO 2.33I ≈0.05) as cathode material in plastic lithium batteries. X-ray absorption spectroscopy experiments were performed in situ for every lithium intercalation increment Δ x≈0.10 per formula unit along a discharge with 0≤ x≤0.67. The X-ray absorption near edge structure (XANES) spectra demonstrate a smooth decrease of Mn valence with in-situ reduction, confirmed by a monotonous increase in Mn–O distance from extended X-ray absorption fine structure (EXAFS). The first-shell octahedral coordination of manganese is hardly affected by lithium insertion, while the evolution of the Mn–Mn coordinence shows a trend towards the formation of MnO n polyhedra chains on deep discharge. The material discharged to an average manganese valence 3.25+ showed negligible static Jahn–Teller effect. Together with the inherent flexibility of disordered structures towards intercalation and bond length changes, this is probably a major cause of the stability of this oxi-iodide on cycling in lithium batteries.

Beamline-induced chromium structure in carbon K-edge absorption spectra

We show that Cr subcoatings in beamline optical elements can introduce an additional structure in C K-edge X-ray absorption measurements, which somewhat resembles the well-known losses produced by carbon contamination. The photon throughput variations induced by Cr and C require different treatments when correcting absorption spectra. The potential difficulties in unravelling the two effects could be avoided with efficient filtration of higher-order light.

Gallium K-edge x-ray absorption study on Mg-doped GaN

Ga K-edge x-ray absorption measurements were employed to investigate Mg-doping effects in GaN samples. Strong polarization-dependent x-ray absorption near-edge structures become less pronounced with increasing doping concentration, indicating the formation of a mixing-phase structure of cubic and hexagonal phases. Analysis of the extended x-ray absorption region of the spectra revealed doping-related defects such as vacancies, substitutions, and interstitial occupations. They were formed anisotropically in the crystal c axis direction and its perpendiculars. Disorderliness arising from phase mix and defects is believed to have lowered the Debye temperature of the doped GaN films and caused the destructive interference of the absorption fine-structure oscillation functions. These effects were taken into account for the observed large coordination number reductions in our samples.

Spectroscopic and Electronic Structural Studies of Blue Copper Model Complexes. 1. Perturbation of the Thiolate−Cu Bond

A tris(pyrazolyl)hydroborate triphenylmethylthiolate Cu(II) model complex (1) that reproduces structural and spectroscopic features of active sites of blue Cu proteins is characterized using low-temperature absorption, magnetic circular dichroism (MCD), X-ray absorption (XAS), and resonance Raman (rR) spectroscopies combined with DFT calculations to define its electronic structure. The electronic structure of 1 is further related to the oxidized Cu site in plastocyanin. The key spectral differences relative to plastocyanin include an increase in the intensity of the S pπ → Cu CT band and a decrease in the absorption intensity at ∼450 nm. The energies of d → d transitions in 1 decrease relative to plastocyanin, which reflects the more tetrahedral geometry of 1. S K-edge XAS measurements demonstrate a more covalent thiolate interaction in the HOMO of 1 (52% S p) than in plastocyanin (38% S p). The effects of the high thiolate covalency on the absorption and Raman spectral features for 1 are evaluated. Addit...

LOCAL STRUCTURE OF THE CHARGE ORDERED La0.5Ca0.5MnO3

Local lattice of the charge ordered La 0.5 Ca 0.5 MnO 3 has been studied by temperature dependent Mn K-edge x-ray absorption measurements, using fluorescence detection method, to explore how the local lattice distortions are associated with the charge ordering in this system. The x-ray absorption near edge structure (XANES) reveals an abrupt change in the local MnO 6 octahedral geometry across the charge ordering temperature. The extended x-ray absorption fine structure (EXAFS) analysis shows largely distorted MnO 6 octahedra with coexisting Q 3-type and Q 2-type of Jahn-Teller (JT) modes at low temperature in the commensurate charge ordered phase. We find that small JT distortions persist also in the charge disordered phase at high temperature.

Nitrogen K-edge X-ray absorption measurements on N- and O-implanted GaN

Nitrogen K-edge X-ray absorption measurements are used to study the effect of N+ and O+ implantation in the microstructure of GaN. In the as-grown sample, the central N atom is four-fold coordinated with 3.35 Ga atoms at the expected distance of 1.93 Å, and 0.65 displaced to a distance longer by about 0.33 Å. This distortion in the nearest neighbor distances is attributed to thermal strain and/or to the presence of N vacancies. Implantation with either N or O ions enhances the distortion in the microstructure and the number of the displaced Ga atoms increases from 0.65 to 1. In addition to that, implantation causes a reduction in the nearest neighbor distances by about 2% and an increase in the Debye–Waller factors. Finally, implantation induces states in the gap that are detectable in the near edge X-ray absorption fine structure (NEXAFS) spectra, where they introduce a characteristic transition at about 1.4 eV below the absorption edge.

Oxygen nonstoichiometry and phase transitions of the neodymium-rich Nd 1+ xBa 2− xCu 3O z solid solution

On the basis of chemical, thermal analysis and Cu K-edge X-ray absorption measurements, oxygen content in the Nd 1+ x Ba 2− x Cu 3O z solid solution was determined between 1000°C in air and 400°C in oxygen for x=0.05–0.9 compositions. It has been observed that the oxygen nonstoichiometry Δ z of the Nd 1+ x Ba 2− x Cu 3O 7+ x/2−Δ z solid solution decreases 2–2.5 times for a large substitution (Δ z≈0.3–0.33 for x=0.9), despite of the acclaimed higher total oxygen content. The difference in nonstoichiometry is explained by a higher average value of the copper oxidation state (ACV), which is vital for the solid solution with large x even at elevated temperatures (ACV≈2–2.05 for x>0.3 at 1000°C, PO 2=0.21 atm). On the contrary, the ACV after complete oxygenation is almost constant (about 2.25–2.3) for the whole series. The x-dependence of the oxygen content is not monotonous and structural phase transitions can be observed at x=0.3 and x=0.6, as confirmed by the X-ray diffraction and the Raman scattering spectroscopy. The first well-known transition is connected with the oxygen disorder due to the Nd substitution for Ba at random Ba-sites. In the present work, it is proved by the apical oxygen mode broadening in Raman spectra. Ordering of the Nd and Ba atoms with a subsequent orthorhombic distortion of the lattice may occur even at 1000°C in air due to the second transformation at x≈0.6. The invariable orthorhombicity of the Nd-rich solid solution with x>0.6 is not caused by the oxygen absorption as in the x=0.05 case. Existence of high- and low-temperature orthorhombic modifications of this solid solution has been observed for the first time. Finally, a tentative 3D ( z– x– T) diagram is suggested for the Nd 1+ x Ba 2− x Cu 3O z solid solution up to 1000°C in air, including the new x=0.6–0.9 region.

X-ray absorption of bromonaphthalene dissolved in supercritical fluid xenon.

The results of Br K-edge X-ray absorption measurements of 1-bromonaphthalene dissolved in supercritical fluid Xe are reported. As the pressure of Xe confined in a high-pressure cell is increased, the absorption spectrum of bromonaphthalene gradually appears, showing that Xe in the supercritical fluid state solvates the bromonaphthalene molecule. The spectrum of the dissolved sample shows a remarkable difference from that of the pure liquid sample in the near-edge region of the X-ray absorption spectrum, reflecting the interaction of the solute material with solvent Xe. Ab initio full multiple-scattering calculations can reproduce the spectral features qualitatively. A large value of the Debye-Waller factor must be introduced to give the best fit, corresponding to a large degree of spatial disorder and violent thermal vibration of the Xe atoms around the Br atom.

Magnetic Circular Dichroism And X-Ray Absorption Near Edge Structure Studies Of Magnetoresistive Oxides

AbstractWe report the results of temperature dependent, Mn K-edge X-ray Absorption measurements: Magnetic X-ray Circular Dichroism (MXCD) for local magnetic properties and X-ray Absorption Near Edge (XANES) for electronic and structural properties of Mn-based oxide magnetoresistive compounds La1−xSrxMnO3. This is the first report of an MXCD signal of the Mn K-edge in Mn oxide systems. By studying the integrated peak intensity of the first peak in the MXCD signal, it can be determined that the magnetic properties of the system are independent of temperature in the range 94 to 223K.

Structure and conductivity of copper oxide based compounds

The primary crystal structure of cuprate compounds containing CuO 2 planes derives from an A-O rock-salt layer separated by a number of ACuO 3 perovskite layers. The general composition is A n + 1 Cu n O 3 n + 1 , where A is a divalent or trivalent ion with ionic radius greater than or equal to 1 Å. There is a superimposed secondary structure resulting from various types of oxygen vacancy ordering. The relation between composition and structure is illustrated for compounds in the sub-solidus phase diagrams of the systems La 2O 3-SrO-CuO and La 2O 3-BaO-CuO. The difference between superconducting and non-superconducting compounds is not because of a different type of charge carrier. As concluded from oxygen K-edge X-ray absorption measurements, the conductivity gap is of the charge transfer type. Doping produces holes located on oxygen ions. The electrical conductivity of cuprate compounds is related to the structure of the oxygen sub-lattice. The electrical conductivity of compounds containing holes outside the CuO 2 planes or in imperfect CuO 2 planes is attributed to thermally activated hopping between localized states. Only itinerant holes in perfect CuO 2 planes can yield superconductivity. Also, the difference between superconducting and non-superconducting compounds is related to the distribution of holes between the CuO 2 planes and other structural sub-units. The key factor in determining this charge distribution is the electrostatic interaction as expressed by the Madelung energy. Specifically, this explains the dependence of the T c of Nd(Ba 2 − x Nd x )Cu 3O 7 + δ on the neodymium content.