Edge Extended X-ray Absorption Fine Structure Research Articles

The local environment of cations in Bi 12.5Er 1.5ReO 24.5

The local environments of cations in a recently reported δ-Bi 2O 3-related phase stabilised by erbium and rhenium, and which shows exceptionally high oxide ion conductivity at low temperatures, have been examined by Bi L III-, Er L III- and Re L III- edge extended X-ray absorption fine structure (EXAFS) measurements. The Bi L III- and Er L III- edge data recorded from the compound Bi 12.5Er 1.5ReO 24.5 revealed a high level of oxygen disorder and the adoption by bismuth and erbium of different local environments than those previously observed in erbium-doped δ-Bi 2O 3. The Re L III edge EXAFS recorded from Bi 12.5Er 1.5ReO 24.5 endorsed the highly disordered nature of the oxygen system and showed rhenium to adopt fourfold oxygen coordination and to be significantly different from that of the local environment of rhenium in Bi 28Re 2O 49.

Characterization of stabilized/solidified refinery oily sludge and incinerated refinery sludge with cement using XRD, SEM and EXAFS

Solidification/stabilization (S/S) of refinery oily sludge and incinerated oily sludge (ash) with cement type I42.5 and II42.5 was investigated using, X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) combined with Energy Dispersive Spectroscopy (EDS) and Extended X-ray Absorption Fine Structure (EXAFS). The results showed that delayed ettringite formation (DEF) and major cement hydration reactions occurred. XRD analysis of S/S oily sludge samples revealed cement-bearing solid phases, such as portlandite, calcite, C3S, C2S and C4AF. SEM analysis, confirmed ettringite at solidified oily sludge samples. Solidified ash samples contained ettringite substituted by chromates. However, solid phases found in solidified ash samples with I42.5 cement showed minor variation in type and structure compared to those observed in solidified ash samples with II42.5 cement. Fe K edge EXAFS analysis revealed the presence of iron oxides in both S/S wastes. The comparison between spectra of the S/S resulting materials and the ones of their original components, showed that the first sphere Fe–O distances were longer than in the pure iron oxide thereby providing evidence that the resulting materials were not simple mixtures, but products of a reaction that modified the local environment of iron.

Local order around rare earth ions during the devitrification of oxyfluoride glasses

Erbium L(3)-edge extended x-ray absorption fine structure (EXAFS) measurements were performed on rare earth doped fluorosilicate and fluoroborate glasses and glass ceramics. The well known nucleating effects of erbium ions for the crystallization of cubic lead fluoride (based on x-ray diffraction measurements) and the fact that the rare earth ions are present in the crystalline phase (as indicated by Er(3+) emission spectra) seem in contradiction with the present EXAFS analysis, which indicates a lack of medium range structural ordering around the Er(3+) ions and suggests that the lead fluoride crystallization does not occur in the nearest neighbor distance of the rare earth ion. Molecular dynamics simulations of the devitrification process of a lead fluoride glass doped with Er(3+) ions were performed, and results indicate that Er(3+) ions lower the devitrification temperature of PbF(2), in good agreement with the experimental results. The genuine role of Er(3+) ions in the devitrification process of PbF(2) has been investigated. Although Er(3+) ions could indeed act as seeds for crystallization, as experiments suggest, molecular dynamics simulation results corroborate the experimental EXAFS observation that the devitrification does not occur at its nearest neighbor distance.

Direct Spectroscopic Observation of the Structural Origin of Peroxide Generation from Co-Based Pyrolyzed Porphyrins for ORR Applications

Pyrolyzed transition metal based porphyrins present an attractive alternative to state of the art Pt-based electrocatalysts for fuel cell applications based on their comparatively low cost. Unfortunately, the large array of precursors and synthetic strategies has led to considerable ambiguity regarding the specific structure/property relationships that give rise to their activity for oxygen reduction. Specifically, considerable debate exists in actual chemical structure of the pyrolyzed reaction centers, and their relationship to membrane-damaging peroxide yield. In this manuscript a comprehensive electrochemical and spectroscopic study of pyrolyzed CoTMPP produced via a self-templating process is presented. The resulting electrocatalysts are not carbon-supported, but are highly porous self-supported pyropolymers. Rotating ring disk electrode measurements showed that the materials pyrolyzed at 700 °C exhibited the highest performance, whereas pyrolysis at 800 °C resulted in a significant increase in the peroxide yield. X-ray photoelectron spectroscopy and Co L and K edge extended X-ray absorption fine structure (EXAFS) studies confirm that the majority of the Co−N4 active site has broken down to Co−N2 at 800 °C. Application of Δμ analysis (an X-ray absorption near-edge structure difference technique) to the in situ Co K edge EXAFS data allowed for direct spectroscopic observation of the geometry of Oads on the pyropolymer active sites. The specific geometrical adsorption of molecular oxygen with respect to the plane of the Co−Nx moieties highly influences the oxygen reduction reaction pathway. The application of the Δμ technique to other transition metal based macrocycle electrocatalyst systems is expected to provide similarly detailed information.

Local structure aroundEr3+inSiO2−HfO2glassy waveguides using EXAFS

${\mathrm{Er}}^{3+}$-doped $\mathrm{Si}{\mathrm{O}}_{2}\text{\ensuremath{-}}\mathrm{Hf}{\mathrm{O}}_{2}$ glassy waveguides with $\mathrm{Hf}{\mathrm{O}}_{2}$ concentrations ranging from $10\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}50\phantom{\rule{0.3em}{0ex}}\mathrm{mol}\phantom{\rule{0.2em}{0ex}}%$ were prepared using the sol-gel route and deposited on $v\text{\ensuremath{-}}\mathrm{Si}{\mathrm{O}}_{2}$ substrates using the dip-coating technique. The local environment around ${\mathrm{Er}}^{3+}$ ions was determined from Er ${L}_{3}$-edge extended x-ray-absorption fine-structure (EXAFS) measurements. The first coordination shell around ${\mathrm{Er}}^{3+}$ ions is composed of oxygen atoms. Hafnium is the main constituent of the second coordination shell of ${\mathrm{Er}}^{3+}$, differing from the cases of pure $\mathrm{Si}{\mathrm{O}}_{2}$ and $\mathrm{Si}{\mathrm{O}}_{2}\text{\ensuremath{-}}\mathrm{Ti}{\mathrm{O}}_{2}$ glassy hosts, in which silicon is the main atomic species. The local structure around ${\mathrm{Er}}^{3+}$ ions has been found to be independent on $\mathrm{Hf}{\mathrm{O}}_{2}$ concentration within the studied composition range. This fact implies that ${\mathrm{Er}}^{3+}$ ions are preferentially dispersed in $\mathrm{Hf}{\mathrm{O}}_{2}$-rich regions of the glassy waveguide, even at the lowest $\mathrm{Hf}{\mathrm{O}}_{2}$ concentration. For all samples, no ${\mathrm{Er}}^{3+}\text{\ensuremath{-}}{\mathrm{Er}}^{3+}$ coordination shell has been detected by EXAFS. The presented structural results allow us to understand some spectroscopic properties typical of ${\mathrm{Er}}^{3+}$-doped $\mathrm{Si}{\mathrm{O}}_{2}$ glassy waveguides co-doped with $\mathrm{Hf}{\mathrm{O}}_{2}$.

An EXAFS study of photographic development in thermographic films

Silver K edge extended X-ray absorption fine structure (EXAFS) spectroscopy of films containing silver behenate (AgBeh) in the unprocessed, fully processed, and step-processed states has been performed. The results of the EXAFS analysis indicate that the intensity for the real-space peak for the Ag-O distance (∼2.3 Å) decreases while the real-space peak for the Ag-Ag distance (∼2.9 Å) grows with increasing thermal processing of the film. The changes observed in the real-space EXAFS signal indicate the growth of metallic silver at the expense of AgBeh. The X-ray absorption near-edge spectroscopy (XANES) portion of the signal shows that the absorption edge position varies stepwise, with unprocessed films and pure AgBeh having an edge location at 25 506 eV, films processed from steps 1 through 10 have an absorption edge at 25 508 eV, and the fully processed film has an edge location at 25 512 eV.

XRD and EXAFS studies of [formula omitted] crystallisation in [formula omitted] films

This paper reports a detailed structural study on the nucleation of t- HfO 2 nanocrystals in thin films of 70 SiO 2 – 30 HfO 2 prepared by sol–gel route on v- SiO 2 substrates. Thermal treatment was performed at different temperatures ranging from 900 to 1200 °C for short (30 min) or long (24 h) time periods. Crystallisation and microstructure evolutions were traced by X-ray diffraction (XRD). The local structure around hafnium ions was determined from Hf L 3 -edge extended X-ray absorption fine structure (EXAFS) measurements carried out at the BM08-GILDA Beamline of ESRF (France). XRD shows the nucleation of HfO 2 nanocrystals in the tetragonal phase after heat treatment at 1000 °C for 30 min, and a partial phase transformation to the monoclinic phase (m- HfO 2 ) starts after heat treatment at 1200 °C for 30 min. The lattice parameters as well as the average crystallites size and their distributions were determined as a function of the heat treatment. EXAFS results are in agreement with the XRD ones, with hafnium ions in the film heat treated at 1100 °C for 24 h are present in mixed phases.

Local distortion in LaCoO3 and PrCoO3: extended x-ray absorption fine structure, x-ray diffraction and x-ray absorption near edgestructure studies

Room temperature Co K edge extended x-ray absorption fine structure(EXAFS), x-ray absorption near edge structure (XANES), including thepre-edge structure, and x-ray diffraction (XRD) studies are carried out onLaCoO3 and PrCoO3. The Co–O, Co–La/Pr and Co–Co bond lengths are obtained from EXAFS analysis and comparedwith those obtained from XRD study. The EXAFS analysis of the data indicates that theCoO6 octahedron isdistorted in both LaCoO3 and PrCoO3.There are two Co–O bonds with bond length 1.863 (1.886) Å and four Co–O bonds with bond length 1.928(1.942) Å for LaCoO3 (PrCoO3). Such distortion is expected in orthorhombicPrCoO3 but not inrhombohedral LaCoO3. Thisdistortion in the CoO6 octahedron is attributed to Jahn–Teller activeCo3+ ions in an intermediate spin state in these compounds. Higher shell studiesreveal that Debye–Waller (DW) factors of Co–Pr and Co–Co bonds inPrCoO3 are greater in comparison with those of Co–La and Co–Co bonds inLaCoO3, indicating that these bonds are structurally more disordered inPrCoO3. The comparison of Co–Co bond lengths and corresponding DW factors indicates that thestructural disorder plays an important role in deciding the insulating properties of thesecompounds. XANES studies have shown changes in the intensities and positions ofdifferent near edge features. The comparison of experimental spectra with thecalculated ones—using the Co 4p density of states obtained from local densityapproximation calculations and matrix elements calculated using an atomic likecore state as the initial state and a confluent hypergeometric function as thefinal state—indicates that for orthorhombic structure, the intensities of differentfeatures are lower as compared to those for the cubic structure. The pre-edge peaksattributed to and transitions show the effects of hybridization of theeg orbitals with O 2p orbitals, and their relative intensities inPrCoO3 and LaCoO3, can be explained by using the average Co–O bond length obtained from theEXAFS.

Comparative EXAFS Investigation of Uranium(VI) and -(IV) Aquo Chloro Complexes in Solution Using a Newly Developed Spectroelectrochemical Cell

The coordination of the U(IV) and U(VI) ions as a function of the chloride concentration in aqueous solution has been studied by U L(III)-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. The oxidation state of uranium was changed in situ using a gastight spectroelectrochemical cell, specifically designed for the safe use with radioactive solutions. For U(VI) we observed the complexes UO2(H2O)5(2+), UO2(H2O)4Cl+, UO2(H2O)3Cl2(0), and UO2(H2O)2Cl3- with [Cl-] increasing from 0 to 9 M, and for U(IV) we observed the complexes U(H2O)9(4+), U(H2O)8Cl3+, U(H2O)(6-7)Cl2(2+), and U(H2O)5Cl3+. The distances in the U(VI) coordination sphere are U-Oax = 1.76+/-0.02 A, Oeq = 2.41 +/- 0.02 A, and U-Cl = 2.71 +/- 0.02 A; the distances in the U(IV) coordination sphere are U-O = 2.41 +/- 0.02 A and U-Cl = 2.71 +/- 0.02 A.

EXAFS study of rubidium-doped single-wall carbon nanotube bundles

The local structure around the rubidium ions inserted in single-wall carbon nanotube bundles (Rb-doped SWCNT) is studied by Rb $K$-edge extended x-ray-absorption fine structure (EXAFS). The dependence of the local order around the rubidium ions is investigated as a function of the time of doping (i.e., as a function of the stoichiometry of the sample). The first coordination shell of the rubidium ions, related to the distance between rubidium and the first nearest-neighboring carbon atoms, has a clear time doping dependence. Comparison between ab initio simulations of the EXAFS spectra and experimental data questions the interstitial site (between three tubes) as the preferential insertion site in SWCNT bundles. The results indicate that the rubidium ions are mainly located inside the tubes and around the bundles. The results are in good agreement with combined x-ray and neutron diffraction experiments performed on the same samples.

Np(IV)/Np(V) valence determinations from Np L3 edge XANES/EXAFS

X-ray absorption near edge structure (XANES) spectroscopy for in situ metal valence determination has become a powerful analytical tool in heterogeneous systems. This is in part because it is applicable without prior separation procedures. For some systems, however, determining the oxidation state from XANES spectra is not straightforward and caution must be used. We show that the analysis of L3,2 edge EXAFS (extended X-ray absorption fine structure) spectra is better suited to distinguish between Np(IV) and Np(V) than from their XANES spectra. Whereas evidence for the oxidation of Np(IV) in solution samples from their Np L3 XANES is unclear, their EXAFS data unequivocally reveals Np(V) formation in the solutions.

Extended X-ray absorption fine structure and time differential perturbed angular correlation study of hafnium(IV) sorbed onto amorphous silica

Sorption of Hf(IV) onto amorphous silica at pH values between 1 and 7 is investigated by extended X-ray absorption fine structure (EXAFS) and time differential perturbed angular correlation (TDPAC) spectroscopies. Hafnium(IV) is studied as a chemical analogue for understanding the speciation of tetravalent actinides. In addition, 181Hf is a frequently investigated TDPAC probe nuclide. Spectra obtained from both techniques show that there is a change in Hf-species with sample preparation pH. Results from TDPAC measurements show that two similar surface binding sites exist for Hf(IV) on the silica surface at low pH values. At a pH above 3, a third, unspecific surface binding site for Hf(IV) begins to dominate. EXAFS investigations do not detect the presence of polynuclear surface species. Both TDPAC and Hf L3 edge EXAFS spectra of the sorption samples differ from that of a Hf(IV) solution or a Hf(IV) hydrolysis product. This indicates that the sorbed species are different from solution or simple hydrolysis species. Short Hf-O distances are obtained from theoretical fits to the EXAFS oscillations for all samples. This suggests that the primary interaction of Hf(IV) with the silica surface occurs via silanol groups. Differences in the electric field gradient surrounding sorbed Hf(IV) cations as a function of pH, observed in the TDPAC spectra, must result from changes in overall species geometry rather than a change in the primary Hf-silica surface interaction.

Pressure-induced bond buckling inYBa2Cu3O7−δ

Cu {ital K}-edge extended x-ray-absorption fine-structure (EXAFS) measurements of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}h({delta}=0.05) at atmospheric pressure and at 3.4, 11.4, 31.8, and 65 kbar are presented. The experiments were performed isothermally at 80 K. In the EXAFS region, the Cu(1)-O(4)-Cu(2) path is enhanced by focused multiple scattering. Analysis indicates a buckling of this path due to out-of-line movement of the O(4) atom. The forward scattering angle {beta} shows a rapid increase at low pressures and then saturates at 65 kbar to an angle (as large as) 12{degree} depending upon the buckling model used in the EXAFS analysis. Analysis of x-ray-absorption near-edge structure data taken for the {delta}=0.9 and {delta}=0.05 compounds shows a decrease in the amplitude of the spectral feature associated with the Cu(1) atom in the {delta}=0.9 compound indicating that the amount of pressure-induced charge transfer from Cu(1) to Cu(2) is of the order of 0.0007 holes/kbar {copyright} {ital 1999} {ital The American Physical Society}

Fe-Doped Zirconium Oxide Produced by Self-Sustained High-Temperature Synthesis: Evidence for an Fe−Zr Direct Bond

The local structure of iron in Fe-doped cubic ZrO2 produced by combustion synthesis was studied by Mössbauer spectroscopy and Fe-K edge extended x-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES). Iron was found to be in its II oxidation state and to occupy two different sites of the fluorite ZrO2 structure, both associated with some amount of disorder. One of the sites has been identified with the regular Zr position in 0,0,0, thus giving rise to substitutional (i.e., Fe atoms occupying the regular Zr position, with a net 2−charge, with respect to the lattice) defects, while the other site has been identified with the normally empty position at 1/2, 1/2, 1/2, thus giving rise to interstitial (i.e., Fe atoms occupying an interstitial position, with a net 2+ charge, with respect to the lattice) defects. For this last site, there is a short Fe−Zr distance (2.64 Å). This result, coupled to the quite small value of the Debye−Waller factor for this distance, gives evidence of a direct Zr−Fe metal-to-metal bond.

Short-range structural properties ofNd1.85Ce0.15CuO4andNd2CuO4studied by multiple-scattering EXAFS data analysis

An accurate multiple-scattering Cu $K$-edge extended x-ray-absorption fine-structure (EXAFS) study of the average local structure of ${\mathrm{Nd}}_{1.85}{\mathrm{Ce}}_{0.15}{\mathrm{CuO}}_{4}$ and ${\mathrm{Nd}}_{2}{\mathrm{CuO}}_{4}$ is presented in the range of temperature 5--150 K. Clear differences between experimental EXAFS signals of doped and undoped compounds recorded at the same temperature are found. Shortening of 0.015 \AA{} of the average Cu-Nd/Ce distance is unambiguously detected in the doped-superconducting compound. We show that the contraction of the unit cell along the $c$ direction can be assigned to the shift of the Nd/Ce atoms toward the adjacent ${\mathrm{CuO}}_{2}$ plane. No evidence for anomalies of the in-plane Cu-Cu and Cu-O distance distribution is found as a function of temperature and doping. The strong multiple-scattering signal associated with the quasilinear Cu-O-Cu chains in the ${\mathrm{CuO}}_{2}$ planes is used to evaluate the angle distribution at the oxygen site, using a suitable model describing a realistic three-atom vibrating system. Differences between the doped and the undoped compound are detected showing that the angle distribution is broader below ${T}_{c}$ in the doped compound. The increased width of the angle distribution is consistent with the existence of local modes or permanent structural distortions involving the in-plane oxygens.

Short-range structure in solid and liquid CuBr probed by multiple-edge x-ray-absorption spectroscopy

X-ray-absorption measurements of solid and liquid CuBr in the $T=295--793\mathrm{K}$ range of temperature are presented. Accurate short-range structural parameters are extracted using simultaneous Cu and Br $K$-edge extended x-ray-absorption fine-structure (EXAFS) refinement in the framework of the multiple-scattering GNXAS method for data analysis. In solid CuBr we find an increasing asymmetry of the first-neighbor ${g}_{\mathrm{CuBr}}(r)$ distribution function as a function of temperature. The foot and the most probable value of the first-neighbor distribution are found to shift up to about 0.09 \AA{} toward shorter distances from 295 to 670 K, while the long-range part of distribution broadens at high temperatures. Results are discussed taking into account previous neutron diffraction and EXAFS results. A complete account of our EXAFS experiment on liquid CuBr is also reported. The short-range Cu-Br distribution is extracted showing evidence for nearly covalent bonding and improving previous neutron diffraction results. The short-range CuBr liquid structure is found to be quite similar to that of the high-temperature solid. The excellent short-range EXAFS sensitivity and its complementarity with usual diffraction techniques for studying disordered systems are discussed.

Temperature-dependent vibrational properties of NiAl, CoAl, and FeAl β-phase alloys

Temperature-dependent vibrational properties of \ensuremath{\beta}-phase NiAl, CoAl, and FeAl alloys have been studied by the extended x-ray-absorption-edge fine-structure (EXAFS) technique. From the $K$-edge EXAFS of the transition-metal element, the temperature dependence of the mean-squared relative displacements has been obtained, where these relative displacements are between the absorbing atom and the first, second, or third coordination shell. For NiAl, excellent agreement is obtained with calculations we have carried out, which are based on input from neutron-scattering data obtained by others. We find that the changes with temperature of the mean-squared relative displacements of corresponding atomic pairs are similar for NiAl and FeAl, but CoAl is significantly different from the other two alloys in this regard. In particular, the temperature dependence of the second- and third-neighbor Co-Co mean-squared relative displacement in CoAl is significantly less than the temperature dependence of the corresponding Ni-Ni and Fe-Fe mean-squared relative displacements in NiAl and FeAl, respectively.

Polarized x-ray-absorption spectroscopy of the uranyl ion: Comparison of experiment and theory.

The x-ray linear dichroism of the uranyl ion (${\mathrm{UO}}_{2}^{2+}$) in uranium ${\mathit{L}}_{3}$-edge extended x-ray-absorption fine structure (EXAFS), and ${\mathit{L}}_{1}$- and ${\mathit{L}}_{3}$-edge x-ray-absorption near-edge structure (XANES), has been investigated both by experiment and theory. A striking polarization dependence is observed in the experimental XANES and EXAFS for an oriented single crystal of uranyl acetate dihydrate [${\mathrm{UO}}_{2}$(${\mathrm{CH}}_{3}$${\mathrm{CO}}_{2}$${)}_{2}$\ensuremath{\cdot}${2\mathrm{H}}_{2}$O], with the x-ray polarization vector aligned either parallel or perpendicular to the bond axis of the linear uranyl cation (O-U-O). Single-crystal results are compared to experimental spectra for a polycrystalline uranyl acetate sample and to calculations using the ab initio multiple-scattering (MS) code FEFF 6. Theoretical XANES spectra for uranyl fluoride (${\mathrm{UO}}_{2}$${\mathrm{F}}_{2}$) reproduce all the features of the measured uranyl acetate spectra. By identifying scattering paths which contribute to individual features in the calculated spectrum, a detailed understanding of the ${\mathit{L}}_{1}$-edge XANES is obtained. MS paths within the uranyl cation have a notable influence upon the XANES. The measured ${\mathit{L}}_{3}$-edge EXAFS is also influenced by MS, especially when the x-ray polarization is parallel to the uranyl species. These MS contributions are extracted from the total EXAFS and compared to calculations. The best agreement with the isolated MS signal is obtained by using nonoverlapped muffin-tin spheres in the FEFF 6 calculation. This contrasts the ${\mathit{L}}_{1}$-edge XANES calculations, in which overlapping was required for the best agreement with experiment. \textcopyright{} 1996 The American Physical Society.

Determination of the local lattice distortions in the CuO2 plane of La1.85Sr0.15CuO4.

The measurement of the Cu-O distances by a local and fast probe, polarized Cu {ital K}-edge extended x-ray absorption fine structure (EXAFS) in La{sub 1.85}Sr{sub 0.15}CuO{sub 4} crystal shows two different conformations of the CuO{sub 6} octahedra below 100 K assigned to two types of stripes with different lattice. This experiment supports a model of {open_quote}{open_quote}two components{close_quote}{close_quote} spatially separated in a superlattice of quantum stripes for the anomalous properties of cuprate superconductors. {copyright} {ital 1996 The American Physical Society.}

Low temperature total electron yield EXAFS study of Co xAg 1− x granular alloys

Total electron yield (TEY) Co K edge extended X-ray absorption fine structure (EXAFS) experiments have been performed at low temperature on as deposited and annealed Co x Ag 1− x granular alloys with Co volume concentrations x = 0.2 and 0.35. A new design for a total electron yield detector operating close to liquid nitrogen temperature has been tested with success at ESRF and used for the experiments reported here. A very interesting feature of this study concerns the evolution of the neighbourhood of the Co atoms in these alloys as a function of x and of the annealing temperature. For x = 0.2, at the as deposited stage, one observes clearly the presence of an important fraction of Co in the substitutional sites of the Ag f.c.c. matrix, in addition to Co agglomerates. For x = 0.35 the Co precipitation process seems more advanced compared to x = 0.2. From the evolution of the intensity of the different peaks of the radial distribution, one can follow the agglomeration process of Co in the Ag matrix.