Measurements Of X-ray Absorption Spectra Research Articles

Estimation of spin states of cobalt ions in LaCoO[formula omitted] nanoparticles by means of soft X-ray absorption spectroscopy

Spin states of Co3+ ions in LaCoO3 nanoparticles of 20, 25 and 28 nm in sizes were studied by means of soft X-ray absorption spectroscopy at the O K and Co L2,3 edges. Using measurements of oxygen K X-ray absorption spectra, there were estimated the relative numbers of t2g and eg holes in hybridized vacant O 2p–Co 3d states. Based on the results of these studies, the relative amounts of low-spin and high-spin Co3+ ions were estimated. In particular, for LaCoO3 with an average nanoparticle size of 25 nm, approximately 60% and 40% of the Co3+ ions are in the low-spin and high-spin states, respectively. It corresponds to an effective spin Seff=0.8. A slightly different result was obtained using Co L X-ray spectra and multiplet calculations of the spectra of Co3+ ions. According to these data, about 2/3 of trivalent cobalt ions in the specimen with an average nanoparticle size of 25 nm are in the low-spin state. The results of measurements of X-ray absorption spectra can be explained quite well from the assumption that Co3+ electrons are in the mixture of low- and high-spin states.

High-energy-resolution off-resonant spectroscopy with self-seeded x-ray free-electron laser pulses.

This paper presents the implementation of high-energy-resolution off-resonant spectroscopy (HEROS) measurements using self-seeded x-ray free-electron laser (XFEL) pulses. This study systematically investigated XFEL conditions, including photon energy and accumulated shot numbers, to optimize the measurement efficiency for copper foil samples near the K-edge. The x-ray absorption spectra reconstructed using HEROS were compared with those derived from fluorescence-yield measurements. The HEROS-based spectra exhibited consistent line shapes independent of the sample thickness. The potential application of HEROS to high-temperature copper was also explored. HEROS offers distinct advantages including scan-free measurement of x-ray absorption spectra with reduced core-hole lifetime broadening and self-absorption effects. Using self-seeded XFEL pulses, HEROS facilitates single-shot-based pump-probe measurements to investigate the ultrafast dynamics in various materials and diverse conditions.

X-Ray Absorption Co L Spectra for the Determination of the Charge and Spin States of Cobalt Ions in LnBaCo2O5 + δ Cobaltites (Ln = Eu, Gd, Tb)

Co L X-ray absorption spectra were used to determine the charge and spin states of ions in LnBaCo2O5 + δ cobaltites (Ln = Tb, Eu and Gd, 0.08 leqslant delta leqslant 0.55). It is shown that with a decrease in the oxygen content in cobaltites (with a decrease in the average charge state of cobalt ions), the Co {{L}_{3}} maximum shifts linearly towards low photon energies. It was found using Co {{L}_{3}} X-ray spectra that the mechanical action on GdBaCo2O5.5 (uniaxial compression and torsion under pressure) leads to the appearance of divalent cobalt ions, i.e., the CoO phase. The CoO phase appears in the sample under pressure even in the absence of shear deformations. Based on the measurements of X-ray absorption spectra at different temperatures, it was found that at 440 K, which is above the metal–insulator transition point, some of the low-spin Co3+ ions in EuBaCo2O5.52 cobaltite octahedra pass into the high-spin state. No change of the spin state of Co ions in EuBaCo2O5.24 with temperature was found.

Effect of surface oxidation on soft x-ray optical properties of ion beam sputter deposited amorphous AlN thin film

In the present study, structural and compositional analyses of reactive ion beam sputter deposited aluminum nitride (AlN) thin film of thickness 100 Å are carried out using x-ray reflectivity and x-ray photoelectron spectroscopy techniques. Soft x-ray optical response of the film is derived from energy dependent soft x-ray reflectivity measurements performed in photon energy region of 380–1700 eV. Optical constants (δ and β) obtained from the reflectivity spectra show features corresponding to absorption edges of the constituent elements. Observed fine features in the β profile are further confirmed from x-ray absorption (XAS) measurements carried out in the total electron yield mode. The measured XAS spectra are correlated with electronic and compositional properties of the AlN film. The effects of surface oxidation on soft x-ray optical properties of the AlN thin film are discussed.

X-ray absorption spectroscopy set-up for unstable gases: A study of 5p hydrides

An absorption cell is constructed for x-ray absorption spectroscopy of reactive, unstable or hazardous gases at room temperature. In conjunction with in-situ micro-synthesis technique relying on handling the gas in syringes it enabled a first measurement of x-ray absorption spectra in the region of K and L edges for the series of hydrides of 5p elements (SnH4, SbH3, TeH2, HI). The signal-to-noise ratio above 103 was achieved, whereby fine detail is discerned in the spectra, in particular the small sharp features above each absorption edge, testifying of coexcitations of outer electrons in the core photoeffect.

Magnetite magnetosome biomineralization in Magnetospirillum magneticum strain AMB-1: A time course study

Magnetotactic bacteria are a highly studied group of diverse prokaryotes that biomineralize chains of magnetosomes, single domain, single crystal magnetic nanoparticles of magnetite or greigite, enclosed by a lipid bilayer membrane whose synthesis is under strict genetic control. In addition to characterizing the genetics and physicochemical properties of both cultured and uncultured environmental species, there have been a number of investigations using a time course approach to determine the chemical pathway of magnetite biomineralization in these organisms. In time course studies, cells of MTB are typically grown in the absence of iron so they cannot make magnetite, and then provided with iron in culture medium which initiates the biomineralization of magnetosome chains over a subsequent time period. Results from previous time course studies are not consistent with one another, differing with regard to the nature of chemical intermediates and the rate of establishment of magnetosome chains. In this work we report a time course study of Magnetospirillum magneticum strain AMB-1 over a 48h (hour) period, using transmission electron microscopy (TEM) and soft X-ray scanning transmission X-ray microscopy (STXM) at the Fe L-edge. STXM provides capability to measure X-ray absorption spectra (XAS) and map chemical species with ∼25nm spatial resolution and thus gives detailed results on the chemistry of individual particles in single cells. An evolution of the iron oxide speciation, from a more Fe(III)-rich species, possibly α-hematite (Fe2O3), to magnetite (Fe3O4), was observed in the early stages, with evidence for the presence of the Fe(III)-rich character persisting up to 24 h. The spectromicroscopy (X-ray absorption, XAS and X-ray magnetic circular dichroism, XMCD) and TEM results show that biomineralization occurs in a stepwise fashion. First, very small particles, with no measurable magnetization, are produced at different sites in the cell without significant chain formation. The Fe L3 spectra of these early stage particles typically differ from magnetite with an additional signal at 708.4 eV that is consistent with α-hematite. By 6–8h the particles are more numerous, partial chain formation is evident, and the L3 spectrum is very similar to that of magnetite. By 24 h particles-in-chains are the dominant motif and magnetism with the moment along the chain is established. By 48 h the cells are essentially the same as cells grown in Fe-rich medium.

Direct observation of electronic structure change by resistance random access memory effect in amorphous alumina

We measured X-ray absorption spectra of amorphous alumina with vacancy-type oxygen defects (AlOx) which exhibits the resistance random access memory (ReRAM) effect. We were able to detect changes in the electronic structure owing to the ReRAM effect. A major difference in the spectra near the O K-absorption edge was observed between a low resistance state (LRS) and a high resistance state (HRS). The subpeak profile within the band gap appeared in the LRS, while it was suppressed in the HRS. By contrast, the spectra near the Al K-absorption edge in the LRS and HRS appeared almost identical, indicating that no byproducts are generated. These findings imply that the distribution of charged electrons primarily changes near oxygen sites from the HRS to the LRS. The features of the subpeak profile are analogous to those of the mid-gap profile, as speculated by the first-principles calculation [Momida et al., Appl. Phys. Lett. 98, 042102 (2011)]. The LRS was mainly detected near the surface of the thin film.

Tuning the electronic properties of epitaxial strained CaFeO3−δ thin films

Strain engineering of transition metal oxides due to their desirable properties has long been a focal point in both physics and material sciences. Here, we investigate the strain dependence of electronic and optical properties of the high valence iron-based perovskite CaFeO3−δ. Using substrates with various lattice constants, we achieve a wide range of tunable epitaxial strain states in CaFeO3−δ thin films ranging from compressive −0.37% to tensile 3.58%. Electrical transport and optical absorption measurements demonstrate a distinct strain-dependent behavior, in which larger tensile strain leads to higher electrical resistivity and a larger optical bandgap. We attribute these modulations to tensile strain suppressed p-d hybridization in CaFeO3−δ, as evidenced by soft X-ray absorption spectra measurements.

NEXAFS C1s-spectra apparatus distortion study

The results of the comparison of TEY and transmission methods of X-ray absorption spectra measurements as well as “thickness effect” modeling in the linear absorption coefficient spectral dependences of the C60-films in the NEXAFS C1s-spectra are represented. The calculations were performed with using linear absorption coefficient spectra obtained in TEY mode as the true (undistorted) data. The modelling results are in good agreement with the experiment. All NEXAFS C1s-spectra was carried out at the BESSY II using synchrotron radiation from the Russian-German dipole beamline.

Nanoscale spatial analysis of clay minerals containing cesium by synchrotron radiation photoemission electron microscopy

A synchrotron radiation photoemission electron microscope (SR-PEEM) was applied to demonstrate the pinpoint analysis of micrometer-sized weathered biotite clay particles with artificially adsorbed cesium (Cs) atoms. Despite the insulating properties of the clay, we observed the spatial distributions of constituent elements (Si, Al, Cs, Mg, and Fe) without charging issues and clarified reciprocal site-correlations among these elements with nanometer resolution. We found that Cs atoms were likely to be adsorbed evenly over the entire particle; however, we identified an occupational conflict between Cs and Mg atoms, implying that Cs sorption involves ion exchange processes. Spatially resolved X-ray absorption spectra (XAS) of the Cs4,5 M-edge region showed Cs to be present in a monocation state (Cs+) as typically observed for Cs compounds. Further pinpoint XAS measurements were also performed at the Fe L2,3-edge to determine the chemical valence of the Fe atoms. The shapes of the spectra were similar to those for Fe2O3, indicating that Fe in the clay was in a 3+ oxidation state. From these observations, we infer that charge compensation facilitates Cs adsorption in the vicinity of a substitution site where Si4+ ions are replaced by Fe3+ ions in SiO4 tetrahedral sheets. Our results demonstrate the utility of SR-PEEM as a tool for spatially resolved chemical analyses of various environmental substances, which is not limited by the poor conductivity of samples.

Van der Waals Contribution to the Relative Stability of Aqueous Zn(2+) Coordination States.

Many properties of aqueous cations depend on their coordination state. However, the lack of long-range order and the dynamic character of aqueous solutions make it difficult to obtain information beyond average coordination parameters. A thorough understanding of the molecular-scale environment of aqueous cations usually requires a combination of experimental and theoretical approaches. In the case of Zn2+, significant discrepancies occur among theoretical investigations based on first-principles molecular dynamics (FPMD) or free-energy calculations, although experimental data consistently point to a dominant hexaaquo-zinc complex (Zn[H2O]6)2+ in pure water. In the present study, the aqueous speciation of zinc is theoretically investigated by combining FPMD simulations and free-energy calculations based on metadynamics and umbrella-sampling strategies. The simulations are carried out within the density functional theory (DFT) framework using for the exchange-correlation functional either a standard generalized gradient approximation (GGA) or a nonlocal functional (vdw-DF2) which includes van der Waals interactions. The theoretical environment of Zn is confronted to experiment by comparing calculated and measured X-ray absorption spectra. It is shown that the inclusion of van der Waals interactions is crucial for the correct modeling of zinc aqueous speciation, whereas GGA incorrectly favors tetraaquo- (Zn[H2O]4)2+ and pentaaquo-zinc (Zn[H2O]5)2+ complexes, results obtained with the vdW-DF2 functional show that the hexaaquo-zinc complex is more stable than the tetraaquo and pentaaquo-zinc complexes by 13 and by 4 kJ mol-1, respectively. These results highlight the critical importance of even subtle interactions for the correct balance of different coordination states in aqueous solutions. However, for a given coordination state, GGA leads to a reasonable description of the geometry of the aqueous complex.

Microscopic Examinations of Co Valences and Spin States in Electron-Doped LaCoO3

We studied the Co valences and spin states in electron-doped LaCo$_{1-y}$Te$_{y}$O$_3$ by measuring x-ray absorption spectra and electron spin resonance. The low-temperature insulating state involves the low-spin Co$^{3+}$ ($S=0$) and the high-spin Co$^{2+}$ state, which is described by $g=3.8$ and $j_{\rm eff}=1/2$. The results, in concurrence with the electron-hole asymmetry confirmed in electrical resistivity, coincide with a spin-blockade phenomenon in this system. Further, we discuss the $g$ factor in terms of the strong covalent-bonding nature and consider multiple origins of this phenomenon.

A prototype system to measure X-ray absorption spectra for diagnosis in vivo

We presented a new non-destructive test system for recording of X-ray absorption spectra. Its purpose was to investigate X-ray absorption spectral characteristics of organic and inorganic compounds and to explore the potential for biomedical pathological examination in vivo. An X-ray source with a maximum voltage of 80 kV and maximum current of 1 mA was selected for both good penetration and low radiation dose. A cadmium telluride (CdTe) detector was chosen for its wide energy range. Pulse-height analysis of the output of this sensor yielded energy spectra of X-rays that had passed through the sample. We calibrated the sensor with varying X-ray tube voltages and used tungsten X-ray lines to verify its accuracy. Different kinds of powders, aqueous solutions, aluminium alloys, and plastics were used as experimental samples to show their X-ray spectral absorption differences. The instrument had good prospects for further diagnostics combining radiology and tissue optics.

XAS spectroelectrochemistry: reliable measurement of X-ray absorption spectra from redox manipulated solutions at room temperature.

The design and operation of a low-volume spectroelectrochemical cell for X-ray absorption spectroscopy (XAS) of solutions at room temperature is described. Fluorescence XAS measurements are obtained from samples contained in the void space of a 50 µL reticulated vitreous carbon (sponge) working electrode. Both rapid electrosynthesis and control of the effects of photoreduction are achieved by control over the flow properties of the solution through the working electrode, where a good balance between the rate of consumption of sample and the minimization of decomposition was obtained by pulsing the flow of the solution by 1-2 µL with duty cycle of ∼3 s while maintaining a small net flow rate (26-100 µL h(-1)). The performance of the cell in terms of control of the redox state of the sample and minimization of the effects of photoreduction was demonstrated by XAS measurements of aqueous solutions of the photosensitive Fe(III) species, [Fe(C2O4)3](3-), together with that of the electrogenerated [Fe(C2O4)3](4-) product. The current response from the cell during the collection of XAS spectra provides an independent measure of the stability of the sample of the measurement. The suitability of the approach for the study of small volumes of mM concentrations of protein samples was demonstrated by the measurement of the oxidized and electrochemically reduced forms of cytochrome c.

Multiferroic properties and local structure of spinel (Co,Mn)3O4

A room-temperature structural phase diagram for the spinel Co3-xMnxO4 (0 ≤ x ≤ 3) solid solution system was examined to study the cubic-tetragonal phase transition. The temperaturedependent magnetization was measured to identify the ferrimagnetic ordering at temperatures below TC An enhanced magneto-capacitance effect was observed around TC without any appreciable hysteretic behavior due to a variation in the applied magnetic field. The directional averaged complex atomic form factors of both the Co and the Mn ions at two different crystallographic sites of the oxide spinel structure near the K-absorption edges were extracted from the measured X-ray absorption spectra. Energy dispersive atomic form factors can be used to analyze X-ray diffraction data near the absorption edges to explore the detailed features coming from the local environmental effects

Influence of tungstate ions on transformation of green rust to ferric oxyhydroxide via aqueous solution investigated by in situ X-ray absorption spectroscopy

Influence of tungstate ions on the transformation of chloride-containing green rust (GR(Cl−)) to fine goethite (α-FeOOH) particles due to the oxidation reaction via aqueous solution at about 300K was investigated by in situ measurements of X-ray absorption spectra. Results showed that the transformation rate of GR(Cl−) in the suspension containing 5mol% W was lower than that in the suspension without tungstate ions. Almost all tungstate ions in the suspension were adsorbed on GR(Cl−) and α-FeOOH. It is probable that the adsorption of tungstate ions reduces the transformation rate of GR(Cl−) and also leads to the precipitation of fine particles.

In situ Measurements of X-ray Absorption Spectra during Transformation of Green Rust to Ferric Oxyhydroxide Via Aqueous Solution

In situ measurements of X-ray absorption spectra at an Fe K absorption edge were carried out during the transformation of chloride-containing green rust (GR(Cl)) to oxyhydroxide (that is, lepidocrocite (γ FeOOH)), by oxidation via aqueous solution. Results showed that the Fe K absorption edge in X-ray absorption near-edge structure (XANES) spectrum shifted toward the higher energy side with increasing oxidation time. The factor analysis of the XANES spectra, using reference data, implied that dissolution of GR(Cl) and precipitation of γ -FeOOH occur simultaneously. The fraction of γ -FeOOH in the suspension increased continuously with increasing oxidation time, in response to the continuous decrease of the fraction of GR(Cl). The results lead us to conclude that GR(Cl) transforms to γ -FeOOH during oxidation under the present experimental conditions.

Spectroscopic analysis of phase constitution of high quality VO2 thin film prepared by facile sol-gel method

VO2 thin films with large-area were prepared on Al2O3 substrates by a simple sol-gel method. After an annealing treatment under low vacuum condition, all the VO2 films showed a preferred growth direction and exhibited excellent semiconductor-metal transition (SMT) characteristics. The structure and electrical properties of the obtained VO2 films were investigated systematically. Raman spectra, X-ray diffraction and X-ray absorption spectra measurements pointed out that the VO2 film on ${\rm A}{\rm l}_{\rm 2} {\rm O}_{\rm 3} ( {{\rm 10}\overline {\rm 1} {\rm 0}})$Al2O3(101¯0) substrate showed a M1 phase instead of M2 phase as reported in previous studies. Based on the experiment results, it was suggested that the strained structure of oriented VO2 films could be a mechanism for the formation of the intermediate M2 phase, whereas it is difficult to access the pure M2 phase of undoped VO2 films. VO2 film on ${\rm A}{\rm l}_{\rm 2} {\rm O}_{\rm 3} \left( {{\rm 10}\overline {\rm 1} {\rm 0}} \right)$Al2O3101¯0 substrate showed a lower SMT temperature compared to VO2 film on Al2O3 (0001), which can be mostly attributed to the differences of both lattice mismatch and thermal stress. The present results confirm and make clear the relevance of the substrate orientation in the growth of VO2 film and their different contributions to the SMT characteristics in vanadate systems.

Phase evolution of magnetite nanocrystals on oxide supports via template-free bismuth ferrite precursor approach

This report investigates the phase evolution pathway of magnetite nanocrystal synthesis on oxide-supported substrates. A template-free phase separation approach, which exploits the thermodynamic instability of ternary perovskite BiFeO3 and inherent volatility of bismuth oxide in low oxygen pressure and high temperature is presented. The formation of an intermediate hematite nanocrystal phase is found as a key step that controls the eventual size and morphology of the magnetite nanocrystals. X-ray absorption spectra measurements and X-ray magnetic circular dichroism confirm that the spectral fingerprints of the magnetite nanocrystals match with that of bulk crystals. Magnetic measurements show that magnetic anisotropy is directly attributed to the nanocrystal morphology.

Anisotropy in Borrmann spectroscopy

In this paper we introduce Borrmann Spectroscopy as a method for measuring X-ray absorption spectra under conditions of an exotic wave field, namely, a coherent superposition of two plane waves. The essential features of the Borrmann Effect (also known as anomalous transmission) are outlined. We show that the Borrmann Effect can lead to a very strong relative enhancement of quadrupole absorption. After describing some early results in this field, and some general considerations of multiple-wave absorption, we contrast recent results on anisotropy in Borrmann spectroscopy with normal absorption. Despite the qualitative success of a simple model for quadrupole enhancement, temperature dependence and anisotropy, a general theory of the Borrmann Effect is required which includes anisotropic and non-dipolar scattering. We outline some first steps towards such a theory.