EXAFS Spectra Research Articles

Magnetic and Dielectric Properties of R0.5Ca0.5MnO3 (R = Eu-Lu)

The magnetic and dielectric properties of perovskite manganites R 0.5 Ca 0.5 MnO 3 (R = Eu-Lu), which show charge ordering of the Mn-3d electrons below room temperature, have been investigated. Their dielectric constants were ∼ 1000–10000 at around room temperature. Inflections or peaks of temperature dependences of the dielectric constants were observed at around the charge-ordering temperatures. Analyses of tan δ for R = Eu-Lu and EXAFS spectra for R = Dy suggested that the dielectric properties of the present system are rooted in a special arrangement and transfer of the Mn-3d electrons. The two-peak structure of tan δ indicates that dielectric measurement can be used to observe homogeneity or inhomogeneity of sample, as mentioned in our previous paper on the ruthenium perovskite oxides.

EXAFS studies of negative thermal expansion materials

AbstractExtended X‐ray absorption fine structure (EXAFS) can give information on the local dynamical behavior of crystals, opening new perspectives for understanding the origin of negative thermal expansion (NTE). In this paper, after a short review on the effects of thermal disorder in EXAFS spectra and on the relation between EXAFS and Bragg diffraction, some recent results on crystals with the zincblende and cuprite structures are presented and critically compared. The tension effect giving rise to NTE can be correlated to the anisotropy of the mean square relative displacements measured by EXAFS. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

In situ XAS study of the Mn(III)(salen)Br catalyzed synthesis of cyclic organic carbonates from epoxides and CO 2

In situ X-ray absorption spectroscopy at the Mn K- and Br K-edge was employed to study the cycloaddition of carbon dioxide to propylene oxide and styrene oxide, catalyzed by Mn(III) salen bromide complexes. Three homogeneous complexes with varying salen ligand structure and one complex immobilized on silica were used and compared. Ex situ X-ray absorption spectroscopy showed that in all cases octahedral complexes in a distorted symmetry were present and that the bromide ligand was coordinated to the Mn central atom. Under reaction conditions the structure of the catalysts strongly changed. In order to investigate this change, a special spectroscopic batch reactor cell was designed that allowed continuous monitoring of catalyst structure under cycloaddition conditions directly at the synchrotron radiation source, i.e. while maintaining the demanding conditions of elevated pressure and temperature. Complementary XAS investigations on the two key elements of the catalyst, the manganese central atom and the bromide ligand, showed that the coordination sphere of the hexacoordinated manganese central atom changed due to the replacement of the bromo-ligand probably by an epoxide molecule under reaction conditions. This was the case for all three homogeneous catalysts with varying salen ligand structure as well as for the heterogeneous catalyst. In case of the heterogeneous catalyst bromide also went into the solution whereas most of the manganese remained on the solid support. The loss of the direct coordination of the bromine neighbors to the Mn central atom was also evidenced by EXAFS spectra, e.g. loss of Br backscattering in the Mn K-EXAFS spectra and the Mn-backscattering in the Br K-edge spectra. In the catalytic studies it was observed that propylene oxide usually reacted much faster than styrene oxide. This seems to be related to a faster coordination of propylene oxide to the Mn atom, which could be followed in a time-resolved manner at the Br K-edge. On the basis of the analysis of the XANES and EXAFS data together with observations from literature a reaction mechanism is proposed.

Local structure and effective pair potential of rock-salt type and disordered rock-salt type AgI under pressure

X-ray absorption fine structure (XAFS) observations on iodine K edge of AgI have been performed under high pressure and high temperature up to 6.0 GPa and 1000K to investigate the local structure and effective potentials in high pressure phases. In EXAFS analysis, we have directly carried out the numerical integration of EXAFS function. The anharmonic effective pair potentials V(u)=au2/2+bu3/3! for I-Ag bond have been determined under pressures. α-AgI and disordered rock-salt type phase have super-ionic conduction behaviors. Ag ions occupy both octahedral and tetrahedral sites in the disordered phase and twenty percent of Ag ions occupy the tetrahedral site as a maximum value at 2GPa. The transition between the rock-salt type and disordered rock-salt type phases is a broad disorder type within the same structure. From the viewpoint of the local structure analyses on XANES and EXAFS spectra, some sudden changes are recognized near the phase transition point. Pressure influences greatly the effective pair potential of anti-bonding side and anharmonicity decreases with increasing pressure. Phonon dispersion relation of the rock-salt type AgI under pressure has been derived from EXAFS experiments. Analysis of EXAFS Debye-Waller factor is useful because the force constant can be decided directly even at high pressure and high temperature.

Molecular design of new magnetically active copper complexes with heteroaromatic schiff bases and azo compounds

Copper chelates with tridentate ligands containing pyridine or pyrazole ring at the azomethine or azo fragment were synthesized by chemical electrochemical methods, and their structure was characterized by the EXAFS spectra. Thermal magnetochemical analysis of the complexes revealed antiferromagnetic exchange interaction in all complexes. The exchange interaction parameter of the complex containing an N-tosylamino group in the ortho position with respect to the azomethine group is much lesser than that of the corresponding complex having an oxygen atom in the same position. The copper chelate derived from azopyrazole ligand shows low-temperature ferromagnetic phase transition.

EXAFS Study on the Reactions between Iron and Fulvic Acid in Acid Aqueous Solutions

Iron(III) competes with trace metals for binding sites on organic ligands. We used X-ray absorption fine structure (EXAFS) spectroscopy to determine the binding mode and oxidation state of iron in solutions initially containing only iron(III) and fulvic acid at pHs 2 and 4. EXAFS spectra were recorded at different times after sample preparation. Iron was octahedrally configured with inner-sphere Fe-O interactions at 1.98-2.10 A, depending on the oxidation state of iron. Iron(III) formed complexes with fulvic acid within 15 min. Iron(III) was reduced to iron(II) with time at pH 2, whereas no significant reduction occurred at pH 4. No signs of dimeric/trimeric hydrolysis products were found in any of the solution samples (<0.45 microm). However, the isolated precipitate of the pH 2 sample (>0.45 microm) showed Fe...Fe distances, indicating the presence of tightly packed iron(III) trimers and/or clusters of corner-sharing octahedra. It is suggested that the binding mode of iron(III) to fulvic acid at low pH may be phase-dependent: in solution mononuclear complexes predominate, whereas in the solid phase hydrolyzed polynuclear iron(III) complexes form, even at very low pH values. The observed pH dependence of iron(III) reduction was consistent with expected results based on thermodynamic calculations for model ligands.

X-ray Absorption Spectroscopy Investigation of Different Crystallographic Structures of Cobalt Nanoparticles

ABSTRACTThe ultimate goal of nanotechnology is to design successful syntheses to produce nanomaterials with required properties. The properties of nanoparticles mainly depend on their size, shape and crystal structure, hence control over these factors is a major issue in this field. In this study, Co nanoparticles with four different crystallographic structures (hcp, fcc, bcc and epsilon) have been wet-chemically synthesized by varying precursors and surfactants in the reaction. Co K-edge absorption spectra were collected in transmission mode at the XMP beamline at the Center for Advanced Microstructures and Devices (CAMD). The X-ray Absorption Near Edge Structure (XANES) and Fourier Transformed Extended X-ray Absorption Fine Structure (FT EXAFS) spectra were analyzed using the “fingerprint method”, i.e. by comparing them with either standard compounds or with the simulated spectra that were calculated using the FEFF8 code. EXAFS spectra were also analyzed by fitting them with relevant crystallographic models in r-space including higher coordination shells using the IFEFFIT analysis program. All four crystal structure types were conclusively determined by using this approach.

Influence of Anion Coexistence on Crystal Structure of Iron Oxides Deposited from Steel Surfaces

The EXAFS analysis of ferric oxide particles precipitated from ferric aqueous solutions including chloride ion and with/without additional anions such as sulfate or silicate. In the solution reaction process, an akaganeite (β-FeOOH)-like oxyhydroxide with low crystallinity is precipitated and subsequently transforms to hematite (α-Fe2O3). Coexistence of anion species of sulfate or silicate is effective to reduce the rate of structural transformation and influence morphology of final iron oxide products. The radial distribution functions was obtained by Fourier transformation from EXAFS spectra measured at Fe K absorption edge, and Debye-Waller factor related to Fe-Fe correlation was determined. The results indicated that the linkages with face-sharing and double-corner-sharing were influenced by adding anions in corrosion reaction solutions. [DOI: 10.1380/ejssnt.2008.60]

Surface complexation of U(VI) on goethite (α-FeOOH)

Sorption of U(VI) to goethite is a fundamental control on the mobility of uranium in soil and groundwater. Here, we investigated the sorption of U on goethite using EXAFS spectroscopy, batch sorption experiments and DFT calculations of the energetics and structures of possible surface complexes. Based on EXAFS spectra, it has previously been proposed that U(VI), as the uranyl cation UO22+, sorbs to Fe oxide hydroxide phases by forming a bidentate edge-sharing (E2) surface complex, >Fe(OH)2UO2(H2O)n. Here, we argue that this complex alone cannot account for the sorption capacity of goethite (α-FeOOH). Moreover, we show that all of the EXAFS signal attributed to the E2 complex can be accounted for by multiple scattering. We propose that the dominant surface complex in CO2-free systems is a bidentate corner-sharing (C2) complex, (>FeOH)2UO2(H2O)3 which can form on the dominant {101} surface. However, in the presence of CO2, we find an enhancement of UO2 sorption at low pH and attribute this to a (>FeO)CO2UO2 ternary complex. With increasing pH, U(VI) desorbs by the formation of aqueous carbonate and hydroxyl complexes. However, this desorption is preceded by the formation of a second ternary surface complex (>FeOH)2UO2CO3. The three proposed surface complexes, (>FeOH)2UO2(H2O)3, >FeOCO2UO2, and (>FeOH)2UO2CO3 are consistent with EXAFS spectra. Using these complexes, we developed a surface complexation model for U on goethite with a 1-pK model for surface protonation, an extended Stern model for surface electrostatics and inclusion of all known UO2–OH–CO3 aqueous complexes in the current thermodynamic database. The model gives an excellent fit to our sorption experiments done in both ambient and reduced CO2 environments at surface loadings of 0.02–2.0wt% U.

Nanocrystalline Formation in Immiscible Cu-Mo System Subjected to Mechanical Alloying

The mechanical alloying process has been studied on the Cu-Mo system, the atomic pair of which is characterized by a positive heat of mixing of +19 kJ/mol. The EXAFS and X-ray diffraction measurements have been employed to analyze the structural changes taking place during milling. Two phases mixture of nanocrystalline fcc-Cu and bcc-Mo with a grain size of 10 nm has been formed by MA of Cu30Mo70 powders for 200 hours. The structural analysis based on the EXAFS spectra revealed that bcc and fcc crystal structure clearly do not change around Mo and Cu atoms up to 200 h of milling, respectively. Studies of the thermodynamical considerations by DSC analyses confirmed that the alloying does not occur even after 200 hours of MA in Cu-Mo system.

A Mononuclear Cyclopentadiene–Iron Complex Grafted in the Supercages of HY Zeolite: Synthesis, Structure, and Reactivity

The reaction of ferrocene with the acidic hydroxy groups in the supercages of zeolite HY dehydrated at 673 K and the reactivity of the resultant surface species towards CO and O(2) were investigated by temperature-programmed decomposition (TPD) and reduction (TPR) and IR, X-ray absorption fine structure analysis (XAFS), and X-ray photoelectron (XP) spectroscopy. In situ FTIR, TPD, TPR, and chemical analysis reveal that the Cp(2)Fe molecule adsorbed on the zeolite surface loses one cyclopentadienyl group under vacuum at 423 K, which leads to the formation of a well-defined mononuclear surface Fe-C(5)H(6) complex grafted to two acidic sites and one ([triple bond]Si-O-Si[triple bond]) unit, as confirmed by the lack of Fe-Fe contributions in the EXAFS spectra. Each iron atom is coordinated, on average, to three oxygen atoms of the zeolite surface with a Fe--O distance of 2.00 A and to five carbon atoms with a Fe--C distance of 2.09 A. IR spectra indicate that the cyclopentadiene-iron species grafted on the surface of the zeolite is quite stable in vacuo or under an inert or hydrogen atmosphere below 423 K, and is also relatively stable under oxygen at room temperature. However, the cyclopentadiene ligand readily reacts with CO to form a compound containing carbonyl at 323 K, and even at room temperature. The single carbonyl band in the IR spectra provides evidence for the nearly uniform formation of a cyclopentadiene-iron species on the surface of the zeolite.

Use of the variational principle with the aim of determining oscillations in the extended X-ray absorption fine structure

A method of determining the smooth and oscillating components of extended X-ray absorption fine structure spectra is developed. The form of a functional that is minimized by the desired smooth function is suggested, and a series of EXAFS spectra is analyzed. A way of solution optimization is proposed. The results of applying the method to experimental spectra are reported.

EXAFS Analyses on Local Structure of Gallium in Amorphous Selenide Optical Materials Doped with Rare Earths

Ga K-edge EXAFS spectra have been analyzed to elucidate the local coordination structure of Ga in two representative selenide Ge-As-Se and Ge-Sb-Se glasses all doped with Pr. Gallium turned out to be coordinated with 4 Se atoms in its first neighboring shell. This implies that Ga does not follow the 8-N rule associated with the short-range order structures of typical covalent glasses, further indicating there being more ionic-bond nature in the Ga-Se bonds compared to other heteropolar chemical bonds in the selenide glasses. This is decisive for the Pr3+ ions to be incorporated in the selenide glasses. In this case, the GaSe4 units can be electrically neutralized by the doped Pr3+ ions that act as a charge compensator. As such, inside the selenide glasses, distributions of Pr3+ ions and the Ga tetrahedral units are closely correlated. Spectroscopic properties of rare earths embedded in these Ga-containing selenide glasses thus can be explained in connection with the proposed role of Ga.

EXAFS spectroscopy of perovskite-type superconducting oxides

On the basis of the investigation of the temperature dependences of EXAFS spectra of the Ba1−xKxBiO3 superconducting oxide, a model of relationship between the local crystalline and electronic structures is proposed. It is found that oxygen ions vibrate in a double-well potential and their vibrations are correlated with the local electron pair transfer. Analogous specific features are also observed in the EXAFS spectra of La2−xSrxCuO4, which gives grounds to extend the proposed model to Cu-based superconductors. The model based on the unified approach makes it possible to explain the ground antiferromagnetic state, the insulator-metal transition, and the occurrence of superconductivity upon doping of La2CuO4 with strontium.

Cation occupancies in Mg, Co, Ni, Zn, Al ferrite spinels: a multi-element EXAFSstudy

The distribution of cations between tetrahedral (A) sites and octahedral(B) sites in ferrite spinels has been studied using K-edge x-ray absorptionspectroscopy. The samples include natural and synthetic end-member magnetites(Fe3O4), a naturalMn- and Zn-rich magnetite (franklinite) and synthetic binary, ternary and quaternary ferrites of stoichiometryM2+M23+O4, whereM2+ = Mg, Co, Ni,Zn and M3+ = Fe, Al. XAS data were obtained for all metals. Complete, unfiltered, EXAFS spectra were refinedto determine the percentage distribution of each element over the A and B sites and thesedata were combined with microprobe analyses to quantify the tetrahedral occupancy foreach element in each sample. Measured site occupancies and an internally consistent set of(M–O)A and (M–O)B bond lengths were used to calculate unit-cell parameters, which show excellentagreement with measured values, pointing to the reliability of the measured occupancyfactors. The average occupancies determined for the tetrahedral sites in ferritesare (atoms per formula unit) Mg 0.44, Co 0.24, Ni 0.11, Zn 0.76, Al 0.11 andFe3+ 0.92–0.19. The widerange found for Fe3+ is consistent with it playing a relatively passive role by making good any A-site deficit leftby the other competing cations.

Investigation of the structure of impurity defects in alkali halide crystals by the method of fluorescence EXAFS

It has been shown by experiment that the structure of impurity centers in NaCl–Sr crystals depends on impurity concentration and storage time of crystals. At low concentration and in quenched crystals the impurity is in site of the lattice. At concentrations higher than 0.15 wt% Sr, the impurity forms SrCl 2 phase. For the crystals containing Cu + impurity, it has been shown that an increase in the impurity concentration leads to the formation of clusters with CuCl structure. At the same time the existence of isolated copper ions shifted from the sites of the lattice is obvious. This leads to asymmetry of the environment of copper, which is manifested both in the absorption spectra in the ultraviolet region and in the EXAFS spectra. Heating of the crystals leads to destruction of complex clusters and formation exclusively of centers containing isolated copper ions.

Ruthenium sulfide clusters in acidic zeolites: In situ XAS characterization during sulfidation and reaction

Abstract In situ XAS at the Ru K edge was used to study the reductive sulfidation (15% H 2 S/H 2 ) of Ru(NH 3 ) 6 3+ supported on HYd zeolite, and the evolution of the sulfided sample under working conditions (lower H 2 S partial pressure, addition of toluene to the feed). Sulfidation starts at room temperature and is complete at 373 K. At that temperature, the sample is fully sulfided (5.9 S neighbours per Ru). Above 423 K a partial surface reduction (4.3 S neighbours per Ru), and the formation of metallic ruthenium (3.3 Ru neighbours per Ru) are observed. The evolution under working conditions shows that the local structure is not significantly modified when the H 2 S partial pressure is decreased and especially that it does not lead to an increased reduction. Upon addition of toluene the intensity of the Ru–Ru peak in the FT EXAFS spectra is modified, whereas the Ru–S peak remains unchanged. This is indicative of the chemisorption of toluene on the metallic sites and points out the prominent role of these sites for aromatics hydrogenation.

EXAFS investigation of the local atomic structure of Fe-Ge nanocrystalline disordered alloys

This paper reports on the results of EXAFS investigations (Fe K and Ge K absorption edges) of binary supersaturated nanocrystalline solid solutions Fe100−x Ge x (x = 15–40 at % Ge) prepared by mechanical alloying. The nanocrystalline disordered structure of the single-phase alloys is characterized using X-ray diffraction, Mossbauer spectroscopy, and magnetic measurements. The EXAFS spectra are analyzed by solving the inverse binary problem with the use of combined data on the Fe K and Ge K absorption edges. The parameters of the partial pair correlation functions obtained indicate that the crystal lattice is characterized by strong local static distortions and short-range chemical order increasing with an increase in the germanium content. Moreover, the formation of a hexagonal-type macrostructure in the body-centered cubic (bcc) lattice occurs through a “local” stage.

Structure of the Extracted Complex in the Ni(II)‐LIX84I System and the Effect of D2EHPA Addition

The structure of the Ni(II) complex extracted with the commercial hydroxyoxime, LIX84I, and the effect of adding bis(2‐ethylhexyl) phosphoric acid (D2EHPA) to LIX84I on the extraction rate and the coordination of Ni(II) were investigated by solvent extraction and XAFS methods. The XANES spectrum and the curve fits of the EXAFS spectrum of the Ni‐LIX84I complex showed that the complex is four‐coordinate square‐planar with a 1:2 stoichiometry. In the Ni(II)–D2EHPA–LIX84I system, the coordination geometry changes from square‐planar to six‐coordinate octahedral with an increase in the D2EHPA concentration. Although the rate of Ni(II) extraction from the model spent electroless nickel plating bath with LIX84I is significantly accelerated by adding a small amount of D2EHPA ([LIX84I]: 0.5 M, [D2EHPA]: 0.05 M), most of the Ni(II) complexes extracted with this organic solution remain square‐planar. This indicates that the increase in the extraction rate does not depend on the change in the coordination structure of the extracted complex.

Refining the formation of nanocrystals in soft magnetic amorphous alloy via EXAFS spectroscopic analyses

The amorphous metallic alloy with the composition Fe73.5 Nb3Cu1Si13.5B9 is now quite well known as FINEMET, mainly owing to its excellent magnetic properties that are supposed to be due to the formation of small nanocrystals consisting of Fe and Si, i.e., DO3 phases, during simple heat treatment [1–3]. The evolution of such nanocrystals and their effects on the magnetic properties were comprehensively described in previous reports [4–6]. With using mainly the Fe K-edge and Cu K-edge EXAFS spectra, Ayers et al. carefully studied nucleation kinetics of samples that were heat-treated at various durations [7, 8]. In particular, the role of Cu on nucleation processes in this noncrystalline metal was largely proven by experimental observations of a series of Fourier-transformed EXAFS spectra taken at the Cu K-edge of differently heat-treated samples. A key point in their nucleation model for this amorphous metal is that Cu clusters act as nucleation sites of the DO3 nanocrystallites [8]. It is noteworthy that not all Cu atoms are involved in the generation of the DO3 nanocrystals. Nonetheless, Ayers et al.’s analyses were generally qualitative since they explained the local structural evolution of Cu based on the fingerprint comparison between the partial radial distribution functions of the samples. Thus, though successful, their analyses did not give a quantitative fraction of Cu participating in the nucleation processes of DO3 nanocrystals in their samples. This study sought to come up with a quantitative refinement of the local structures of copper atoms in the amorphous metals. Specifically, the Cu K-edge EXAFS spectra of three representative specimens, i.e., single crystalline Cu (designated SC hereafter), as-quenched Fe73.5Nb3Cu1Si13.5B9 (AQ) and the other one heattreated for 1 h (HT), were recorded. The fraction of the Cu atoms that remained amorphous without playing a role in the nucleation of the Fe-rich DO3 precipitates was then determined.