X-ray Absorption Data Research Articles

A spectroscopic analysis code for spatially resolved x-ray absorption data from the COAX platform.

Sophisticated tools such as computer vision techniques in combination with 1D lineout type analyses have been used in automating the analysis of spectral data for high energy density (HED) plasmas. Standardized automation can solve the problems posed by the complexity of HED spectra and the quantity of data. We present a spectroscopic code written for automated and streamlined analysis of spatially resolved x-ray absorption data from the COAX platform on Omega-60. COAX uses radiographs and spectroscopic diagnostics to provide shock position and density information. We also obtain the more novel spectral-derived spatial profile of the supersonic radiation flow into a low-density foam. Considerable effort has been spent modernizing our previous spectroscopic analysis method, including the development of new tools characterized by a faster runtime and minimal user input to reduce bias and a testing suite for verifying the accuracy of the various functions within the code. The new code analyzes our spectroscopic images in 1-2min, with added uncertainty and confidence.

The role of Cu doped Ba0.5Sr0.5Fe0.95Zr0.05O3-δ: Local structure distortion, conductivity, and magnetization

The composition Ba0.5Sr0.5Fe1-xCuxZr0.05O3-δ (BSFCZ-x, x = 0–0.20) was successfully synthesized by the sol-gel self-combustion method using citric acid as fuel combustion. Structure, local structure, electrical, and magnetic properties are investigated. The X-ray diffraction revealed the oxides exhibit a cubic structure (Pm3 m), and the lattice parameter increases with x. The X-ray absorption data are analyzed to determine the oxidation state of B site metal transition Fe and Cu through edge-energy E0 evaluation. Fe performs the oxidation states Fe3+ and Fe4+, similarly to Cu2+ and Cu3+. Furthermore, the pre-edge was analyzed to determine the spin state configuration of the 3d subshell. The local lattice distortion was evaluated through EXAFS, resulting in a distorted FeO6 octahedron elongated to the z-axis, inducing the Jahn-Teller effect. The nonstoichiometric δ-value (3-δ) measurement leads to oxygen vacancy VO⦁⦁ defect. The electrical conductivity of semiconducting behavior increases significantly after Cu is dissolved in BSFZ up to 60 S/cm at 500 °C, from initially 5 S/cm. Furthermore, the magnetic properties show a slight magnetic saturation Ms for BSFZ related to the low spin state configuration of Fe 3d subshell, then significantly increased on BSFCZ samples due to Cu provoking spin state dragged partly to high spin state configuration.

The Role of the Lowest Excited Triplet State in Defining the Rate of Photoaquation of Hexacyanometalates.

Photosolvation is a type of ligand substitution reaction started by irradiation of a solution with light, triggering the replacement of a ligand with a molecule from the solvent. The excited state is created through many possible pathways. For the class of hexacyanides of groups 8 and 9 of the periodic table, irradiation in the ligand field band is followed by intersystem crossing to the lowest excited triplet state, which we propose to mediate the photoaquation reaction in this class of complexes. In this study, we present time-resolved X-ray absorption data showing indications of the triplet intermediate state in the cobalt(III) hexacyanide complex and we discuss general aspects of the photoaquation reaction in comparison with reported data on the isoelectronic iron(II) hexacyanide. Quantum chemical calculations are analyzed and suggest that the nature of the lowest excited triplet state in each complex can explain the drastically different rate of reactions observed.

Strong enhancement of magnetic ordering temperature and structural/valence transitions in EuPd3S4 under high pressure.

We present a comprehensive study of the inhomogeneous mixed-valence compound, EuPd3S4, by electrical transport, X-ray diffraction, time-domain 151Eu synchrotron Mössbauer spectroscopy, and X-ray absorption spectroscopy measurements under high pressure. Electrical transport measurements show that the antiferromagnetic ordering temperature, TN, increases rapidly from 2.8 K at ambient pressure to 23.5 K at ~19 GPa and plateaus between ~19 and ~29 GPa after which no anomaly associated with TN is detected. A pressure-induced first-order structural transition from cubic to tetragonal is observed, with a rather broad coexistence region (~20 GPa to ~30 GPa) that corresponds to the TN plateau. Mössbauer spectroscopy measurements show a clear valence transition from approximately 50:50 Eu2+:Eu3+ to fully Eu3+ at ~28 GPa, consistent with the vanishing of the magnetic order at the same pressure. X-ray absorption data show a transition to a fully trivalent state at a similar pressure. Our results show that pressure first greatly enhances TN, most likely via enhanced hybridization between the Eu 4f states and the conduction band, and then, second, causes a structural phase transition that coincides with the conversion of the europium to a fully trivalent state.

Ab Initio Calculations of XUV Ground and Excited States for First-Row Transition Metal Oxides

Transient X-ray spectroscopies have become ubiquitous in studying photoexcited dynamics in solar energy materials due to their sensitivity to carrier occupations and local chemical or structural dynamics. The interpretation of solid-state photoexcited dynamics, however, is complicated by the core–hole perturbation and the resulting many-body dynamics. Here, an ab initio, Bethe–Salpeter equation (BSE) approach is developed that can incorporate photoexcited state effects for solid-state materials. The extreme ultraviolet (XUV) absorption spectra for the ground, photoexcited, and thermally expanded states of first row transition metal oxides─TiO2, α-Cr2O3, β-MnO2, α-Fe2O3, Co3O4, NiO, CuO, and ZnO─are calculated to demonstrate the accuracy of this approach. The theory is used to decompose the core–valence excitons into the separate components of the X-ray transition Hamiltonian for each of the transition metal oxides investigated. The decomposition provides a physical intuition about the origins of XUV spectral features as well as how the spectra will change following photoexcitation. The method is easily generalized to other K, L, M, and N edges to provide a general approach for analyzing transient X-ray absorption or reflection data.

Monomeric Fe Species in Square Planar Geometry Active for Low Temperature NH3-SCR of NO

Monomeric Fe species in zeolites are considered to be the active sites for the low temperature activity toward ammonia-assisted selective catalytic reduction of nitrogen oxides (NH3-SCR) in exhaust gases. Herein, we report on a preparation method to synthesize single-site Fe/ZSM-5 by combination of dealumination with the use of a bulky iron complex to introduce Fe. Transient in situ XAS experiments at the Fe K-edge under dynamic NH3-SCR reaction conditions demonstrated the involvement of all iron atoms in the redox cycle and, with that, suggest that Fe species in the studied Fe/ZSM-5 catalyst are monomers. Simulation of experimental X-ray absorption data revealed that the Fe species in the sample obtained by this approach adopt a square planar geometry, which changes into fivefold coordination at low temperature by strongly binding either NO or NH3 as the ligand. The presence of monomeric Fe species in this Fe/ZSM-5 conveys molecular level insights into the temperature-dependent NH3-SCR activity and might prove useful in the study of other reactions over monomeric Fe species, such as methane partial oxidation or dehydroaromatization reactions.

Evidence for metabolic diversity in Meso-Neoproterozoic stromatolites (Vazante Group, Brazil)

Deciphering the evolution of ecological interactions among the metabolic types during the early diversification of life on Earth is crucial for our understanding of the ancient biosphere. The stromatolites from the genus Conophyton cylindricus represent a datum for the Proterozoic (Meso to Neoproterozoic) on Earth. Their typical conical shape has been considered a result of a competition between microorganisms for space, light and nutrients. Well-preserved records of this genus from the “Paleontological Site of Cabeludo”, Vazante Group, São Francisco Craton (Southern Brazil) present in situ fossilized biofilms, containing preserved carbonaceous matter. Petrographic and geochemical analyses revealed an alternation between mineral laminae (light grey laminae) and fossilized biofilms (dark grey laminae). The dark grey laminae comprise three different biofilms recording a stratified microstructure of microbial communities. These three biofilms composing the dark grey laminae tend to be organized in a specific pattern that repeats through the stromatolite vertical section. Iron and manganese are distributed differently along the dark and light grey laminae; X-ray absorption and luminescence data showed possible different areas with authigenic iron and iron provided from diagenetic infiltration. Cryptocrystalline apatite in the lowermost biofilms in each dark grey laminae may suggest past metabolic activity of sulfide-oxidizing bacteria. These findings suggest that the microorganisms reached a complex metabolic diversification in order to maintain an equilibrium situation between the three different biofilms along the vertical section of the structures, thus benefiting the whole microbial community. This means that the stromatolites from the Conophyton genus may have formed as a result of a greater complexity of interactions between microorganisms, and not only from competition between photosynthesizers.

Ab Initio Prediction of Excited-State and Polaron Effects in Transient XUV Measurements of α-Fe2O3.

Transient X-ray and extreme ultraviolet (XUV) spectroscopies have become invaluable tools for studying photoexcited dynamics due to their sensitivity to carrier occupations and local chemical or structural changes. One of the most studied materials using transient XUV spectroscopy is α-Fe2O3 because of its rich photoexcited dynamics, including small polaron formation. The interpretation of carrier and polaron effects in α-Fe2O3 is currently carried out using a semi-empirical method that is not transferrable to most materials. Here, an ab initio, Bethe-Salpeter equation (BSE) approach is developed that can incorporate photoexcited-state effects into arbitrary material systems. The accuracy of this approach is proven by calculating the XUV absorption spectra for the ground, photoexcited, and polaron states of α-Fe2O3. Furthermore, the theoretical approach allows for the projection of the core-valence excitons and different components of the X-ray transition Hamiltonian onto the band structure, providing new insights into old measurements. From this information, a physical intuition about the origins and nature of the transient XUV spectra can be built. A route to extracting electron and hole energies is even shown possible for highly angular momentum split XUV peaks. This method is easily generalized to K, L, M, and N edges to provide a general approach for analyzing transient X-ray absorption or reflection data.

Photoluminescence intensity of Cu-doped ZnO modulated via defect occupancy by applying electric bias

Discovering multifunctional materials is of paramount importance for advancing the science and technology. Herein, we report on an optical phenomenon modulated by an electrical process that happened at the metal–ZnO:Cu junction, for which the light emission intensity from the photoluminescence is tuned reversibly by applying electric bias to the junction. Importantly, these observations were correlated with the x-ray absorption measurements, detecting prominent flips in Cu+/Cu2+ oxidation state occupations in ZnO:Cu film as a function of the resistive switching. Moreover, further analysis of the x-ray absorption data revealed an additional prominent correlation—the signals interpreted as the Zn–O bond fingerprints also exhibited the modulations. By considering the whole set of data, we propose a scenario explaining the modulation phenomena.

Transformation of reduced graphene aerogel-supported atomically dispersed iridium into stable clusters approximated as Ir6 during ethylene hydrogenation catalysis

Transformation of atomically dispersed reduced graphene aerogel (rGA)-supported complexes, IrI(C2H4)2+, with an iridium loading of 9.9 wt%, to form low-nuclearity clusters was investigated during ethylene hydrogenation catalysis. Continuous-scan X-ray absorption spectra demonstrate the formation of clusters well approximated as Ir4 during reaction at 100 °C in flowing equimolar ethylene and H2. The Ir4 clusters transformed into clusters well approximated as Ir6 when the feed molar ratio was switched to H2:C2H4 = 2 and remained stable in pure H2 at 100 °C. Catalyst performance data show that hydrogenation activity increased with metal nuclearity in the order of atomically dispersed iridium/rGA≪ Ir4/rGA < Ir6/rGA. Continuous scan X-ray absorption data, complemented with aberration-corrected scanning transmission electron microscopy images, demonstrate that the supported clusters approximated as Ir6 are stable even in H2 at atmospheric pressure and 100 °C. These supported iridium clusters are among the ones having the highest metal loadings reported for a supported metal cluster catalyst.

MO961: Mineral Metabolism Parameters and Bone Density During The First Year of Kidney Transplantation

Abstract BACKGROUND AND AIMS We evaluated retrospectively in a cohort of kidney-transplanted patients (KTxp), the variations of mineral metabolism (MM) parameters, femoral and vertebral bone density during the first year of kidney transplantation (KTx). METHOD 383 KTxp (M = 232), up to the 650 transplanted in our Department (2004–2017) were studied. At 1st(T1) and 12th(T2) mth of KTx biochemical, femoral and vertebral dual-energy X-ray absorption (DEXA) data were recorded. T-score (Ts) -1 &amp;gt;Ts &amp;gt; -2.5 was considered Osteopenia (F-OPN/V-OPN) and Ts &amp;lt; -2.5 osteoporosis (F-OPS/V-OPS). RESULTS The KTxp age and dialysis vintage (DV) were 48 ± 12 years and 53 ± 25 months; 67% of KTxp had a history of hemodialysis. In 82.5% of cases, a donor deceased KTx (DD) was performed and in 40% of KTxp had a previous history of steroid therapy. During the first year of KTx 91% and 93% of KTxp received respectively calcineurin inhibitors and mycophenolate, and the steroids cumulative dose (SCD) was 2683 ± 926 mg. At T1, cholecalciferol and calcifediol were supplemented in 5% and 8% of KTxp, and AT T12 in 12% and 15% of KTxp. An increase in BMI (23 ± 3 versus 24 ± 3 kg/m2 P &amp;lt; 0.0001), Ca (9.82 ± 0.83 versus 9.9 ± 0.73 mg/dL, P &amp;lt; 0.0001), P (2.4 ± 0.96 versus 3.1 ± 0.61 mg/dL, P &amp;lt; 0.0001), 25OH-D (14.4 ± 6.81 versus 18.8 ± 9.7 ng/mL, P &amp;lt; 0.0001), Hb (11.0 ± 1.4 versus 12.8 ± 1.5 g/dL, P &amp;lt; 0.0001), albumin (4.2 ± 0.38 versus 4.46 ± 0.35 g/dL, P &amp;lt; 0.0001) was observed. Serum creatinine (1.41 ± 0.4 versus 1.37 ± 0.3 mg/dL, P = 0.03), PTH (87 ± 73 versus 79 ± 86 pg/mL, P = 0.06 and ALP (107 ± 92 versus 93 ± 58 U/L, P &amp;lt; 0.0001) had a reduction. Femoral bone mineral density (F-BMD) at T1 and F-Ts-T1 were 0.749 ± 0.17 g/cm2 and -1.55 ± 1.06. F-OPS-T1 was present in 17.5% and F-OPN-T1 in 53% of KTxp. F-BMD-T1 correlated directly with BMI-T1 (P &amp;lt; 0.0001) and inversely with 25OH-D-T1 (P &amp;lt; 0.004). F-Ts-T1 was inversely correlated with DV (P = 0.04), BMI-T1 (P &amp;lt;0.0001) and PTH-T1 (P = 0.02). KTxp with F-OPS-T1 had longer DV and lower BMI (P = 0.02). At T12, F-BMD-T12 and F-Ts-T12 were 0.77 ± 0.67 g/cm2 (P = 0.17 versus F-BMD-T1) and −1.4 ± 0.9 (P = 0.002 versus F-Ts-T1).F-OPS-T12 was present in 13.2% of patients and F-OPN-T1 in 55.2%. F-BMD-T12 correlated directly with BMI-T12 (P &amp;lt; 0.04). F-Ts-T12 correlated inversely with DV (P = 0.004), Ca at T1 (P = 0.001) and T12 (P = 0.0005) and with [SCD at T12 (P = 0.02)]. Patients with F-OPS-T1 had longer DV (P = 0.004) and higher PTH at T1 and T12 (P = 0.04 and P = 0.08). F-DEXA category worsened in 3.5% of KTxp. At T1, V-BMD-T1 and V-Ts-T1 were 0.92 ± 0.19 g/cm2 and −1.5 ± 1.58. V-OPS-T1 was present in 30% of KTxp and V-OPN-T1 in 34.5%. V-BMD-T1 correlated directly with BMI-T1 (P &amp;lt; 0.0001) and Ca-T1 (P = 0.04). V-Ts-T1 was inversely correlated with DV (P = 0.04), PTH-T1 (P = 0.02), Ca-T1 (P = 0.03), ALP-T1 (P = 0.01). A direct correlation between V-Ts-T1 and BMI-T1 (P &amp;lt; 0.0001) and 25OHD-T1 (P = 0.02) was present. V-OPS-T1 was more prevalent in males (P = 0.01).They had lower BMI (P &amp;lt; 0.0001), albumin (P = 0.02) and higher Ca-T1 (P = 0.008) and ALP-T1 (P = 0.03). At T12, V-BMD-T12 and V-Ts-T12 were 0.90 ± 0.22 g/cm2 and −1.5 ± 1 .33 (both NS versus T1). V-OPS-T12 was present in 27.7% of KTxp and V-OPN-T12 in 37.2%. V-BMD-T12 correlated directly with BMI-T1 and T12 (P &amp;lt; 0.0001 and P = 0.005) and inversely with Ca-T12 (P = 0.03) and SCD-T12 (P = 0.04). V-Ts-T12 was inversely correlated with DV (P = 0.004), SCD-T12 (P = 0.02) and with Ca at T1 (P = 0.001) and T12 (P = 0.0005). A direct correlation was found with BMI-T1 and T12 (P = 0.0002 and P = 0.001). V-OPS-T12 was more prevalent in DD (P = 0.001). They were older (P = 0.01), had longer DV (P = 0.01). BMI-T1 and T12 were lower in V-OPS-T12 (P = 0.002 and P = 0.03) and Ca-T12 higher (P = 0.01). SCD-T12 was higher in V-OPN-T12 and V-OPS-T12 (P = 0.05). V-DEXA category worsened in 32% of KTxp. CONCLUSION In the first year of KTx several modifications of MM are present. Both femoral and vertebral DEXA seem to be strongly related to the pre-KTx status, in particular, nutritional status and dialysis vintage are related to bone status at T1. Strong importance on T12 evaluation is taken by the SCD.

Rietveld Refinement and X-ray Absorption Study on the Bonding States of Lanthanum-Based Perovskite-Type Oxides La1−xCexCoO3

Metal-oxygen bonding of the Ce-doped LaCoO3 system remains largely unexplored despite extensive studies on its magnetic properties. Here, we investigate the structure and local structure of nanoscale La1−xCexCoO3, with x = 0, 0.2, and 0.4, using the Rietveld refinement and synchrotron X-ray absorption techniques, complemented by topological analysis of experimental electron density and electron energy distribution. The Rietveld refinement results show that LaCoO3 subject to Ce addition is best interpretable by a model of cubic symmetry in contrast to the pristine LaCoO3, conventionally described by either a monoclinic model or a rhombohedral model. Ce4+/Co2+ are more evidently compatible dopants than Ce3+ for insertion into the main lattice. X-ray absorption data evidence the partially filled La 5d-band of the pristine LaCoO3 in accordance with the presence of La–O bonds with the shared-type atomic interaction. With increasing x, the increased Ce spectroscopic valence and enhanced La–O ionic bonding are noticeable. Characterization of the local structures around Co species also provides evidence to support the findings of the Rietveld refinement analysis.

Ranks underlie outcome of combining classifiers: Quantitative roles for diversity and accuracy

SummaryCombining classifier systems potentially improves predictive accuracy, but outcomes have proven impossible to predict. Classification most commonly improves when the classifiers are “sufficiently good” (generalized as “accuracy”) and “sufficiently different” (generalized as “diversity”), but the individual and joint quantitative influence of these factors on the final outcome remains unknown. We resolve these issues. Beginning with simulated data, we develop the DIRAC framework (DIversity of Ranks and ACcuracy), which accurately predicts outcome of both score-based fusions originating from exponentially modified Gaussian distributions and rank-based fusions, which are inherently distribution independent. DIRAC was validated using biological dual-energy X-ray absorption and magnetic resonance imaging data. The DIRAC framework is domain independent and has expected utility in far-ranging areas such as clinical biomarker development/personalized medicine, clinical trial enrollment, insurance pricing, portfolio management, and sensor optimization.

Investigation of Nonequilibrium Electronic Dynamics of Warm Dense Copper with Femtosecond X-Ray Absorption Spectroscopy.

Ultrafast optical excitation of matter leads to highly excited states that are far from equilibrium. In this study, femtosecond x-ray absorption spectroscopy was used to visualize the ultrafast dynamics in photoexcited warm dense Cu. The rich dynamical features related to d vacancies are observed on femtosecond timescales. Despite the success in explaining x-ray absorption data in the picosecond regime, the new femtosecond data are poorly understood through the traditional two-temperature model based on the fast thermalization concept and the static electronic structure for high-temperature metals. An improved understanding can be achieved by including the recombination dynamics of nonthermal electrons and changes in the screening of the excited d block. The population balance between the 4sp and 3d bands is mainly determined by the recombination rate of nonthermal electrons, and the underpopulated 3d block is initially strongly downshifted and recovered in several hundreds of femtoseconds.

Structural, magnetic and electric properties of Sr0.95Y0.05Fe12-xZrxO19 (x = 0–2) M−type hexaferrites

The ceramic technique has been utilized to fabricate Sr0.95Y0.05Fe12-xZrxO19 (x = 0–2) samples. Structural and Raman-scattering analyses have revealed that the samples with x = 0 and 0.5 possess the M−type hexagonal-ferrite phase, while the others have the additional Zr-related phase. All samples exhibit the hard-magnetic behavior and the magnetic parameters tend to decrease with increasing Zr-doping content. The study on the electric polarization has indicated that the sample with x = 0 shows the coexistence of paraelectric and weak ferroelectric behaviors, while the other samples (x = 0.5–2) show conductive behaviour with nearly circular loops. The changes in the magnetic and electrical properties mainly related to the co-presence of Fe2+ and Fe3+ ions, as evidenced from analyzing X-ray absorption data.

In Situ X-ray Absorption Spectroscopy and Droplet-Based Microfluidics: An Analysis of Calcium Carbonate Precipitation.

Droplet-based microfluidic systems are ideally suited for the investigation of nucleation and crystallization processes. To best leverage the features of such platforms (including exquisite time resolution and high-throughput operation), sensitive and in situ detection schemes are needed to extract real-time chemical information about all species of interest. In this regard, the extension of conventional (UV, visible, and infrared) optical detection schemes to the X-ray region of the electromagnetic spectrum is of high current interest, as techniques such as X-ray absorption spectroscopy (XAS) provide for the element-specific investigation of the local chemical environment. Accordingly, herein, we report for the first time the integration of millisecond droplet-based microfluidics with XAS. Such a platform allows for the sensitive acquisition of X-ray absorption data from picoliter-volume droplets moving at high linear velocities. Significantly, the high-temporal resolution of the droplet-based microfluidic platform enables unprecedented access to the early stages of the reaction. Using such an approach, we demonstrate in situ monitoring of calcium carbonate precipitation by extracting XAS spectra at the early time points of the reaction with a dead time as low as 10 ms. We obtain insights into the kinetics of the formation of amorphous calcium carbonate (ACC) as a first species during the crystallization process by monitoring the proportion of calcium ions converted into ACC. Within the confined and homogeneous environment of picoliter-volume droplets, the ACC content reaches 60% over the first 130 ms. More generally, the presented method offers new opportunities for the real-time monitoring of fast chemical and biological processes.

Coexistence of Zn and Fe ions influenced magnetic and microwave shielding properties of Zn-doped SrFe12O19 ferrites

We present the structural characterization, valence change of Fe, Raman spectra, and magnetic and microwave shielding properties of polycrystalline SrFe12-xZnxO19 (x = 0–1) samples. X-ray diffraction and Raman analyses indicated the samples having the M-type hexaferrite structure. The Zn doping caused a remarkable change in the lattice constants. Investigating magnetic hysteresis loops, we found an enhanced saturation magnetization (Ms) as increasing x from 0 to 0.1, and a reduced Ms as x > 0.1. Nearly the same tendency was also observed for the coercive force. Such changes have been thought to the Zn substitution and the valence change of Fe, as affirmed from analyzing the X-ray absorption data. Considering the shielding properties, we observed the samples with a thickness of 1.8 mm showing a low reflection loss. Among the investigated samples, the sample with x = 0.4 had the lowest reflection loss of about −20 dB at frequency of 11.7 GHz, corresponding to ~99% microwave energy being absorbed. We have considered that both dielectric and magnetic losses contribute to the microwave-absorption properties of SrFe12-xZnxO19.

How Hydrogen and Oxygen Vapor Affect the Tribochemistry of Silicon- and Oxygen-Containing Hydrogenated Amorphous Carbon under Low-Friction Conditions: A Study Combining X-ray Absorption Spectromicroscopy and Data Science Methods.

The incorporation of silicon and oxygen into hydrogenated amorphous carbon (a-C:H) is an effective approach to decrease the dependence of the tribological properties of a-C:H on the environment. Here, we evaluate the effect of hydrogen and oxygen partial pressures in vacuum on the tribological response of steel pins sliding against films consisting of silicon- and oxygen-containing a-C:H (a-C:H:Si:O). Experiments are conducted in the low-friction/low-wear regime, where sufficient gas pressure prevents steel from adhering to the a-C:H:Si:O, with the velocity accommodation mode being interfacial sliding between the tribotrack formed in the a-C:H:Si:O film and the carbonaceous tribofilm that is formed on the countersurface. The experiments indicated a decrease (increase) in friction and wear with the hydrogen (oxygen) pressure (hydrogen pressures between 50 and 2000 mbar; oxygen pressures between 10 and 1000 mbar). Characterization by X-ray photoelectron and absorption spectroscopies indicated the occurrence of tribologically induced rehybridization of carbon-carbon bonds from sp3 to sp2. This mechanically induced structural transformation coincided with the dissociative surface reaction between hydrogen (oxygen) gas molecules and sp2 carbon-carbon bonds that are highly strained, which results in the formation of carbon-hydrogen groups (carbonyl or ether groups together with silicon atoms having higher oxidation states). On the basis of variations of the fraction of these surface functional groups with gas pressure, a phenomenological model is proposed for the gas pressure dependence of friction for steel when sliding on a-C:H:Si:O films: while the decrease in friction with hydrogen pressure is induced by an increase in the percentage of carbon-hydrogen groups, the increase in friction with oxygen pressure is caused by a progressive increase in the relative fraction of silicon atoms having higher oxidation states and an increase in surface oxygen concentration.

Bacterial production of vanadium ferrite spinel (Fe,V)3O4 nanoparticles

AbstractBiogenic nanoscale vanadium magnetite is produced by converting V(V)-bearing ferrihydrites through reductive transformation using the metal-reducing bacterium Geobacter sulfurreducens. With increasing vanadium in the ferrihydrite, the amount of V-doped magnetite produced decreased due to V-toxicity which interrupted the reduction pathway ferrihydrite–magnetite, resulting in siderite or goethite formation. Fe L2,3 and V L2,3 X-ray absorption spectra and data from X-ray magnetic circular dichroism analysis revealed the magnetite to contain the V in the Fe(III) Oh site, predominately as V(III) but always with a component of V(VI), present a consistent V(IV)/V(III) ratio in the range 0.28 to 0.33. The bacteriogenic production of V-doped magnetite nanoparticles from V-doped ferrihydrite is confirmed and the work reveals that microbial reduction of contaminant V(V) to V(III)/V(IV) in the environment will occur below the Fe-redox boundary where it will be immobilised in biomagnetite nanoparticles.

Sensing alterations of the local environment of 3d, 4d, and 4f central ions in polyoxopalladates with soft X-ray magnetic dichroisms

Polyoxopalladates hosting 3d, 4d or 4f central ions form a new class of molecular materials with well-defined, tunable morphology and ions coordination to be considered as model systems for basic research. We investigated the isotropic X-ray absorption near-edge structure (XANES), X-ray magnetic circular dichroism (XMCD), and X-ray magnetic linear dichroism (XMLD) to monitor the influence of symmetry changes on electronic and magnetic properties. After a brief introductory review about recent results on 3d and 4d transition metal ions, we (i) show that a change of the coordination symmetry from cubic eightfold (tetrahedral) to sixfold (octahedral) reduces the magnetic moment of Co2+ significantly and (ii) investigate whether axial distortions can modify the 4f states of trivalent lanthanide ions. While for Gd3+ there is no effect visible within experimental uncertainties, changes were observed for Dy3+ and Ho3+. The results are compared to atomic multiplet calculations including the intra-atomic spin-dipole term 〈Tz〉. Furthermore, several experimental pitfalls are discussed that may lead to misinterpretation of X-ray absorption data but can be excluded in the present study.