Evolution of local atomic structure in a melt-spun Ni25Ti50Cu25 shape memory alloy during crystallization

The local atomic environment of a melt-spun Ni₂₅Ti₅₀Cu₂₅ amorphous alloy and bond evolution during crystallization were studied by extended X-ray absorption fine structure (EXAFS) spectroscopy and differential scanning calorimetry. In the amorphous alloy, the interatomic distances of Ni–Ti and Cu–Ti are distinct from Ti–Ti and can be indicative of the formation of two types of dominant polyhedra or distorted polyhedral clusters centered with Ni and Cu, with the majority of shell atoms being Ti. The overall increase in the coordination numbers of Ni, Ti, and Cu by crystallization and evidence for structural relaxation suggest that the melt-spun ribbon contains a combination of ordered structures and free volume prior to the heat treatment. Copper and nickel are co-located as their absorption spectra are similar. Although crystallization occurs rapidly (within 4?min at 500 °C), the local atomic environment change persists at longer annealing durations (up to 10?min). An increase in the Ti–Ti and Cu–Cu homo-bond fractions at short and intermediate annealing times suggests that these species segregate between Cu-rich and Cu-poor phases. Crystallization continues through a short-range Ti and Cu diffusion-dominated process, as the near-neighbor structures of Ti and Cu change considerably more than for Ni during annealing. This homogenizes the microstructure followed by possible precipitation of a TiCu compound.     

Structural study of highly dispersed cerium dioxides

Cerium dioxide samples of different dispersivity, obtained by different methods, have been investigated by X-ray diffraction (XRD) analysis, full-profile analysis of diffraction patterns, calculation of radial distribution function (RDF) of electron density, and extended X-ray absorption fine-structure (EXAFS) spectroscopy. According to the XRD data, the samples studied can be identified in the fluorite structure with conventional unit-cell parameters. However, the local structure of highly dispersed samples, determined by the RDF and EXAFS methods, changes: the coordination spheres for the Ce-O distances are represented by two distances.     

Local structure study of vanadium pentoxide 1D-nanostructures

Vanadium pentoxide (V₂O₅·nH₂O) 1D-nanostructures as nanowires and nanorods have been obtained by decomposition of vanadium peroxide in hydrothermal conditions. Electron microscopy, Raman spectroscopy, and X-ray absorption spectroscopy (XAS) were employed to characterize the morphology and the local structure of as-obtained samples. Scanning transmission electron microscopy (STEM) revealed that the diameter of the nanowires and nanorods were found to be 10–20 and 30–40 nm, respectively. The results demonstrated that a combination of Raman and XAS techniques allowed the accurate characterization of the local structure of V₂O₅ 1D-nanostructures which are related to different morphologies. Analyses of X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra reveals that the local structure of V in the as-obtained samples is similar to the bulk V₂O₅ (in orthorhombic phase), except for a higher degree of local symmetry within the structure of the VO₅ square pyramid. Additionally, the nanostructures prepared by this technique present a single crystalline nature and could emit visible light at room temperature which is related to the local order of V atoms of the studied samples.     

Study of local environment of Cu atoms in Fe73.5Cu1Nb3Si13.5B9 alloy with different annealing temperature

The local environment of Cu atoms in Fe73.5Cu1Nb3Si13.5B9 alloy was investigated by extended X-ray absorption fine structure(EXAFS).Cu clusters began to order when the annealing temperature was around 733 K from the results of the Fourier transform curves.The fitting results showed that the first shell of the near fcc(face-centered cubic)Cu clusters only contained Cu atoms.The coordination number increased with the annealing temperature.Subsequently,the occupancy rate increased from 33.3%(annealed at 733 K)to 100% (annealed at 853 K).This local structural change of Cu atoms could probably affect the distribution of the bcc(body-centered cubic)α-Fe in Fe73.5Cu1Nb3Si13.5B9 alloy.     

Metal nanocluster formation in silica films prepared by rf-sputtering: an experimental study

Composite silica films containing metal nanoclusters were prepared by the rf- sputtering technique, in which SiO₂ was co-deposited with gold+copper, gold+silver, or copper+silver. The formation of either pure or alloy clusters was studied by extended X-ray absorption fine structure spectroscopy and transmission electron microscopy. For all systems, the presence of alloy aggregates was evidenced. Moreover, small amounts of pure metal aggregates as well as dispersed or oxidized dopants were observed. 61.46.+w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals – 61.10.Ht X-ray absorption spectroscopy: EXAFS, NEXAFS, XANES, etc. – 81.05.Pj Glass-based composites, vitroceramics Received 29 June 2001     

Local coordination geometry around Cu and Cu ions in silicate glasses: an X-ray absorption near edge structure investigation

We present an X-ray absorption near edge structure (XANES) study on Cu⁺ and Cu²⁺ ions in silicate glasses at the Cu K-edge, aimed to determine the geometry of the local structure around the metal. This study is based on the comparison between experimental data and theoretical calculations made in the framework of multiple scattering theory. The XANES signals relative to several clusters are simulated on the basis of known crystalline structures involving Cu⁺ and Cu²⁺ ions in silicate matrices. Concerning the Cu²⁺ in glass, the simulations suggest the presence of a square coordination of oxygen atoms around the absorber, with a possible presence of metal ions in the second shell. As for the Cu⁺ ions, the metal clustering is excluded and a linear O-Cu-O coordination is evidenced. Received 30 April 1999     

X-ray absorption study of Cu,Zn SOD under high pressure

Abstract

We have investigated Cu, Zn Superoxide Dismutase (Cu, Zn SOD) metal sites at high pressure using X-ray absorption. XAS (X-ray Absorption Spectroscopy) gives information on local structure and it is particularly suited to metal site investigation. To the best of our knowledge, this is the first time that protein conformational states have been investigated using the high pressure XAS technique. Cu, Zn SOD catalyses the dismutation of toxic oxygen radicals produced in cells; this reaction occurs at the copper metal site. Structural changes around the copper, induced by pressure, can be directly related to protein substates. Their characterisation is thus important in the understanding of protein activity.

The high-pressure device was a Paris-Edinburgh large volume cell.

Experiments were performed on lyophilised Cu, Zn SOD between 0 and 48 kbar at the copper and zinc K-edges. The two metal local atomic environments have a different behaviour as pressure increases: copper exhibits a more flexible environment; on the contrary, zinc shows small structural modifications. We have identified a state, formed between 3 and 8 kbar, which is stable up to 48 kbar.     

X‐ray absorption spectroscopy analysis of Ru in La2RuO5

The compound La₂RuO₅ was examined by the x‐ray absorption spectroscopy (XAS) methods, x‐ray absorption near edge structure (XANES) and extended x‐ray absorption fine structure (EXAFS). XANES technique was used to probe directly the average valence of Ru atoms in the compound. The energy shift of the Ru K‐edge in the XANES signal gave the average Ru valence state as 4.0 ± 0.1. EXAFS analysis provided, by yielding directly the interatomic distances and coordination numbers, the first information on the Ru atom neighborhood, on which the model for the Rietveld refinement of the unit cell of the new compound was devised. Finally, the local structure around the Ru atoms from the refinement was used in the FEFF6 code for a model EXAFS spectrum. The very good quantitative agreement with the measured spectrum proves that the refined crystal structure contains no systematic defects in the vicinity of Ru atoms. This result, together with the valence obtained from XANES, strongly confirms the proposed La₂RuO₅ stoichiometry. Copyright © 2007 John Wiley & Sons, Ltd.     

Characterization of Cu1.4Mn1.6O4/PPy composite electrodes

In this work, we have studied the multilayered polypyrrole(PPy)/oxide composite electrode on glassy carbon (GC) having the structure GC/PPy/PPy(Cu_1.4Mn_1.6O₄)/PPy using X-ray Photoelectron Spectroscopy and Mn K-edge and Cu K-edge XANES and EXAFS. The mixed oxide particles have been incorporated into the PPy matrix simultaneously to the electropolymerization of Py from a solution containing 0.1 M Py + 0.15 M KCl + Cu_1.4Mn_1.6O₄. The XPS data have shown that, prior to the incorporation of the oxide into the PPy matrix, it contains Cu⁺, Cu²⁺, Mn³⁺ and Mn⁴⁺. The XPS, XANES and EXAFS results have shown that when the oxide is incorporated into the PPy matrix, the Cu⁺ present in the original oxide suffers dismutation to give Cu²⁺ and metallic Cu. The metallic Cu is segregated out of the spinel structure. The Mn K-edge XANES and EXAFS data show that, after the incorporation into the PPy matrix, Mn is present as Mn³⁺ and Mn⁴⁺ occupying octahedral sites in a spinel-related structure while the Cu K-edge XANES and EXAFS data indicate that copper occupies tetrahedral sites predominantly in that structure but having a large degree of disorder in the second and higher coordination shells.     

X-ray absorption spectroscopy study and photocatalyst application of CuO and Cu0.9Ti0.1O nanoparticles

We report detailed investigations on the electronic structure and photocatalyst application of CuO and Cu_0.9Ti_0.1O nanoparticles (NPs). The NPs were prepared by co-precipitation method and subsequent annealing. Crystal structure and morphology of the NPs were investigated by synchrotron X-ray diffraction and high resolution transmission electron microscope, respectively. The local atomic structure around the Cu atoms was investigated by the extended X-ray absorption fine structure (EXAFS) at the Cu K-edge. Electronic structure determination was done using near edge X-ray absorption fine structure (NEXAFS) at the O K-edge, Cu L-edge, Cu K-edge and Ti L-edge. From the structural and electronic structure investigations, it is inferred that the Ti substitutes the Cu in CuO lattice without forming any secondary phases and the valence state of Cu is not affected by the Ti substitution; however the Cu – O bond length is found to be shorten in the Ti doped sample. As prepared NPs exhibit excellent photocatalyst application toward the degradation of methyl orange (MO) and potassium dichromate (PD) pollutant dyes under the visible light irradiation. The mechanism of the photodegradation of MO and PD pollutants, by the smaller sized CuO and larger sized Cu_0.9Ti_0.1O NPs, is briefly discussed.     

X-ray absorption spectroscopy in YBa2Cu3O7−x superconductors with variable oxygen content

We report the results of XANES and EXAFS measurements performed on YBa₂Cu₃O_7–x compounds withx ranging from 0 to 1. Data for Cu and YK-edges are presented and definite trends in the behaviour of the main near edge structures are singled out, which allow to address the question of the Cu and Y oxidation states and to elucidate the role of the chemical environment in determining the shape of the absorption lines. The inter-atomic distances as obtained by EXAFS generally agree with the results of X-ray diffraction data with the exception of the Y–O distances. Possible sources of this discrepancy are indicated.Sadly, Prof. G.M. Antonini died before the appearance of this paper     

Comparison among the local atomic order of amorphous TM-Ti alloys (TM = Co,Ni, Cu) produced by mechanical alloying studied by EXAFS

We have investigated the local atomic structure of amorphous TM-Ti alloys (TM = Co, Ni, Cu) produced by Mechanical Alloying by means of EXAFS analyses on TM and Ti K-edges. Coordination numbers and interatomic distances for the three alloys where found and compared. EXAFS results obtained indicated a shortening in the unlike pairs TM-Ti as the difference between d electrons of TM and Ti atoms increases, suggesting an increase in the chemical short range order (CSRO) from TM = Co to Cu.Received: 12 October 2003, Published online: 9 April 2004PACS: 61.43.Dq Amorphous semiconductors, metals, and alloys - 61.10.Ht X-ray absorption spectroscopy: EXAFS, NEXAFS, XANES, etc. - 81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation     

Alternative difference analysis scheme combining R‐space EXAFS fit with global optimization XANES fit for X‐ray transient absorption spectroscopy

Time‐resolved X‐ray absorption spectroscopy (TR‐XAS), based on the laser‐pump/X‐ray‐probe method, is powerful in capturing the change of the geometrical and electronic structure of the absorbing atom upon excitation. TR‐XAS data analysis is generally performed on the laser‐on minus laser‐off difference spectrum. Here, a new analysis scheme is presented for the TR‐XAS difference fitting in both the extended X‐ray absorption fine‐structure (EXAFS) and the X‐ray absorption near‐edge structure (XANES) regions. R‐space EXAFS difference fitting could quickly provide the main quantitative structure change of the first shell. The XANES fitting part introduces a global non‐derivative optimization algorithm and optimizes the local structure change in a flexible way where both the core XAS calculation package and the search method in the fitting shell are changeable. The scheme was applied to the TR‐XAS difference analysis of Fe(phen)₃ spin crossover complex and yielded reliable distance change and excitation population.     

ODE: a new beam line for high-pressure XAS and XMCD studies at SOLEIL

X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) under pressure are probes of local order and microscopic magnetic properties. XMCD is a selective probe that has access to a large variety of elements. The dispersive extended X-ray absorption fine structure (EXAFS) station at SOLEIL (ODE beam line) provides the possibility to perform numerous pressure XAS and XMCD experiments with an excellent statistic. The main advantages of dispersive XAFS are the focusing optics, the short acquisition time (few μs) and great stability during the measurements due to the absence of any mechanical movement. These advantages allow the study of small samples, 70 μm at SOLEIL, which is mandatory in the case of high-pressure studies. We present the new ODE beam line at SOLEIL and its first high-pressure XMCD results.     

Applications of X-ray absorption spectroscopy in materials science: Status and new trends

X-ray absorption spectroscopy (XAS) is a local probe of the geometric and electronic structure of individual atomic species in condensed matter. With the availability of intense, polarized and highly collimated X-rays from storage rings the technique has found a widespread application in physics, chemistry, biology and geology. Multiple scattering of the photoelectron in the edge region and in shadowing configurations makes it possible to deduce higher correlations in the atomic arrangement besides the pair correlation obtained from standard EXAFS. This gives radial and angular information on the geometric structure. A promising new application is XAS under total external reflection with detection of the absorption by fluorescence. This allows in-situ investigations of solid-solid, solid-liquid and solid-gas interfaces. New dedicated storage rings with high brilliance will have a major impact on XAS in dispersive mode and for very dilute systems.     

In-situ XAS study on the Cu and Ce local structural changes in a CuO-CeO2/Al2O3 catalyst under propane reduction and re-oxidation

The redox behaviour of a CuO-CeO₂/Al₂O₃ catalyst is studied under propane reduction and re-oxidation. The evolution of the local Cu and Ce structure is studied with in-situ transmission X-ray absorption spectroscopy (XAS) at the Cu K and Ce L₃ absorption edges.CuO and CeO₂ structures are present in the catalyst as such. No structural effect on the local Cu structure is observed upon heating in He up to 873 K or after pre-oxidation at 423 K.Exposure to propane at reaction temperature (600-763 K) fully reduces the Cu²⁺ cations towards metallic Cu⁰. Quick EXAFS spectra taken during reduction show a small amount of intermediate Cu¹⁺ species. Parallel to the CuO reduction, CeO₂ is also reduced in the same temperature range. About 25% of the Ce⁴⁺ reduces rapidly to Ce³⁺ in the 610-640 K temperature interval, while beyond 640 K a further slower reduction of Ce⁴⁺ to Ce³⁺ occurs. At 763 K, Ce reduction is still incomplete with 32% of Ce³⁺.Re-oxidation of Cu and Ce is fast and brings back the original oxides.The propane reduction of the CuO-CeO₂/Al₂O₃ catalyst involves both CuO and CeO₂ reduction at similar temperatures, which is ascribed to an interaction between the two compounds.     

Icosahedral order in Cu-Zr amorphous alloys studied by means of X-ray absorption fine structure and molecular dynamics simulations

We have used the X-ray absorption fine structure method and molecular dynamics (MD) simulations to characterize atomic order in Cu-Zr metallic glasses (MGs). The microstructure of these MGs is described in terms of interconnected icosahedral-like clusters (superclusters) which are basic building units reproducing the stoichiometry of the system. The equilibrium MD configurations are used as an input for ab initio calculations of the extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectra. The theoretical EXAFS and XANES spectra are compared with those measured for rapidly quenched glassy Cu-Zr alloys. We demonstrate that the experimental results are well reproduced by EXAFS modeling of the population of the superclusters derived from the MD configuration. The average local structural motif can be approximated by Cu-centered icosahedral-like cluster satisfying the condition of maximal local packing efficiency and approximating the system stoichiometry. The simulated XANES exhibits good agreement with experiment, indicating that the atomic order of the MD configuration is consistent with that of the real alloy structure over distances of about 1?nm.     

High Pressure X-Ray Absorption and Diffraction Study of InAs

We have performed X-ray absorption (XAS) and diffraction (XRD) measurements at high pressure on samples of powdered InAs, up to 50 and 80 GPa, respectively. In the lower pressure range, our data are consistent with the following structural sequence: Zincblende M NaCl M Cmcm . The first order transition from the semiconducting Zincblende phase to the metallic NaCl phase is clearly seen by the shift in the absorption onset at the As K-edge and the strong modifications of the extended X-ray absorption fine structure (EXAFS) due to the changes in the local structure from a 4-fold to a 6-fold coordinated environment. XAS shows the high pressure phase to be locally site-ordered. The diffraction data, analized by Rietveld fitting, gives a volume discontinuity of j V/V 0 ～0.18 for the first order transition. There is no apparent volume discontinuity associated to the NaCl M Cmcm transition.     

Cu(L-His)2溶液配位结构的X射线吸收精细结构分析

采用X射线吸收精细结构分析(XAFS)方法分析Cu(His)₂复合物的配位模式,通过测定不同pH值下的铜的K边XAFS吸收谱,研究Cu(His)₂第一配位壳层的结构．为了更准确地验证Cu(His)₂配位结构,对组氨酸的羧基和咪唑分别进行了化学修饰．研究结果明确了在不同pH环境下组氨酸的羧基、氨基和咪唑的配位情况．对于争议最大的在生理pH值下组氨酸的配位方式,其结果更支持六配位模式,同时可能有少量的五配位模式配合物平衡存在.     

Calcium in ancient glazes and glasses: a XAFS study

Ceramic tiles used to manufacture artistic panels during the XVI to the XVIII centuries were decorated with high-lead soda-lime glazes, incorporating a diversity of chromophore cations, as ascertained by SRXRF (synchrotron radiation X-ray fluorescence). Previous X-ray absorption spectroscopy (XAS) studies have shown that sodium and lead are hosted by the glassy matrix in those glazes. However, the possible role of calcium as a modifier of the tetrahedral silica network is not fully clarified, despite being recognized that calcium cations alter some fundamental properties of glazes, namely transparency. An X-ray absorption fine structure (XAFS) study of glazes with varied colorings was therefore undertaken at Ca K- and L-edges. Well crystallized oxide minerals were used to model distinct coordination environments by oxygen atoms – close to octahedral geometry in calcite and dodecahedral in gypsum – while fluorite was chosen to mimic ideal cubic coordination. A first XAS approach suggested a minor variation in the energy separation between L₂–L₃ absorption edges when comparing blue and yellow glazes, irrespective of the period of manufacture. A further study on the X-ray absorption near-edge structure (XANES) carried out at the K-edge corroborated this difference and, along with the theoretical spectra modeling performed with the FEFF code, allowed interpreting of the Ca 1s absorption spectra of glazes as arising from a non-regular high-coordination environment within the silica matrix. PACS  61.43.Fs; 41.60.Ap; 61.10.Ht