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Reduction of dinitrogen by molybdenum nitrogenase relies on complex metalloclusters: the [8Fe:7S] P-cluster and the [7Fe:9S:Mo:C:homocitrate] FeMo-cofactor. Although both clusters bear topological similarities and require the reductive fusion of [4Fe:4S] sub-clusters to achieve their respective assemblies, P-clusters are assembled directly on the NifD(2)K(2) polypeptide prior to the insertion of FeMo-co, which is fully assembled separately from NifD(2)K(2). P-cluster maturation involves the iron protein NifH(2) as well as several accessory proteins, whose role has not been elucidated. In the present work, two NifD(2)K(2) species bearing immature P-clusters were isolated from an Azotobacter vinelandii strain in which the genes encoding NifH and the accessory protein NifZ were deleted, and characterized by X-ray absorption spectroscopy and EPR. These analyses showed that both NifD(2)K(2) complexes harbor clusters that are electronically and structurally similar, with each NifDK unit containing two [4Fe:4S](2+/+) clusters. Binding of the accessory protein NifW parallels a decrease in the distance between these clusters, as well as a subtle change in their coordination. These results support a conformational role for NifW in P-cluster biosynthesis, bringing the two [4Fe:4S] precursors closer prior to their fusion, which may be crucial in challenging cellular contexts. Max-Planck Society; DFG SPP 1927 "Iron-Sulfur for Life" [DE 1877/1-1]; Peter und Traudl Engelhorn Stiftung; U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-AC02-76SF00515]; Bill and Melinda Gates Foundation grant BNF Cereals Phase II [OPP1143172]; Bill and Melinda Gates Foundation grant BNF Cereals Phase III [INV-005889]; U.S. Department of Energy, Office of Science, Basic Energy Science [DE-SC0010834, DE-SC0010867]; Bill & Melinda Gates Foundation [OPP1143172, INV-005889] Published version C. V. S., S. D., and L. D. would like to thank the Max-Planck Society for funding. S. D. and C. V. S. acknowledge the DFG SPP 1927 "Iron-Sulfur for Life" (project DE 1877/1-1) for funding. L. D. thanks the Peter und Traudl Engelhorn Sti.ung for funding. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. C. V. S., S. D., and L. D. gratefully acknowledge Matthew Latimer for his technical assistance during XAS measurements at beamline 9-3. George E. Cutsail III, Justin H. Henthorn, and Patricia Rodr ' iguez Mac ' ia are also thanked for their assistance in XAS data collection. E. J.-V. and A. P. G. were supported by Bill and Melinda Gates Foundation grants BNF Cereals Phase II (OPP1143172) and BNF Cereals Phase III (INV-005889). Work in the laboratories of L. C. S and D. R. D. is supported by grants from the U.S. Department of Energy, Office of Science, Basic Energy Science, DE-SC0010834 and DE-SC0010867, respectively. This work was supported, in whole or in part, by the Bill & Melinda Gates Foundation (OPP1143172, INV-005889). Under the grant conditions of the Foundation, a Creative Commons Attribution 4.0 Generic License has already been assigned to the Author Accepted Manuscript version that might arise from this submission.

The intrinsic stability of the 5 V LiCoPO4-LiCo2P3O10 thin-film (carbon-free) cathode material coated with MoO3 thin layer is studied using a comprehensive synchrotron electron spectroscopy in situ approach combined with firstprinciple calculations. The atomic-molecular level study demonstrates fully reversible electronic properties of the cathode after the first electrochemical cycle. The polyanionic oxide is not involved in chemical reactions with the tluoroethylene-containing liquid electrolyte even when charged to 5.1 V vs Li+/Li. The high stability of the cathode is explained on the basis of the developed energy level model. In contrast, the chemical composition of the cathode-electrolyte interface evolves continuously by involving MoO3 in the decomposition reaction with consequent leaching of oxide from the surface. The proposed mechanisms of chemical reactions are attributed to external electrolyte oxidation via charge transfer from the relevant electron level to the MoO3 valence band state and internal electrolyte oxidation via proton transfer to the solvents. This study provides a deeper insight into the development of both a doping strategy to enhance the electronic conductivity of high-voltage cathode materials and an efficient surface coating against unfavorable interfacial chemical reactions.

Oxycodone is a strong opioid frequently used as an analgesic. Although proven efficacious in the management of moderate to severe acute pain and cancer pain, use of oxycodone imposes a risk of adverse effects such as addiction, overdose, and death. Fast and accurate determination of oxycodone blood concentration would enable personalized dosing and monitoring of the analgesic as well as quick diagnostics of possible overdose in emergency care. However, in addition to the parent drug, several metabolites are always present in the blood after a dose of oxycodone, and to date, there is no electrochemical data available on any of these metabolites. In this paper, a single-walled carbon nanotube (SWCNT) electrode and a Nafion-coated SWCNT electrode were used, for the first time, to study the electrochemical behavior of oxycodone and its two main metabolites, noroxycodone and oxymorphone. Both electrode types could selectively detect oxycodone in the presence of noroxycodone and oxymorphone. However, we have previously shown that addition of a Nafion coating on top of the SWCNT electrode is essential for direct measurements in complex biological matrices. Thus, the Nafion/SWCNT electrode was further characterized and used for measuring clinically relevant concentrations of oxycodone in buffer solution. The limit of detection for oxycodone with the Nafion/SWCNT sensor was 85 nM, and the linear range was 0.5-10 mu M in buffer solution. This study shows that the fabricated Nafion/SWCNT sensor has potential to be applied in clinical concentration measurements.

Selenium (Se) is a toxic contaminant with multiple anthropogenic sources, including 79Se from nuclear fission. Se mobility in the geosphere is generally governed by its oxidation state, therefore understanding Se speciation under variable redox conditions is important for the safe management of Se contaminated sites. Here, we investigate Se behavior in sediment groundwater column systems. Experiments were conducted with environmentally relevant Se concentrations, using a range of groundwater compositions, and the impact of electron-donor (i.e., biostimulation) and groundwater sulfate addition was examined over a period of 170 days. X-Ray Absorption Spectroscopy and standard geochemical techniques were used to track changes in sediment associated Se concentration and speciation. Electron-donor amended systems with and without added sulfate retained up to 90% of added Se(VI)(aq), with sediment associated Se speciation dominated by trigonal Se(0) and possibly trace Se(-II); no Se colloid formation was observed. The remobilization potential of the sediment associated Se species was then tested in reoxidation and seawater intrusion perturbation experiments. In all treatments, sediment associated Se (i.e., trigonal Se(0)) was largely resistant to remobilization over the timescale of the experiments (170 days). However, in the perturbation experiments, less Se was remobilized from sulfidic sediments, suggesting that previous sulfate-reducing conditions may buffer Se against remobilization and migration.

Chemical composition of interfaces between physical-vapor-deposited TiN and SiO2 as affected by introduction of a thin (0.5–3 nm) alumina interlayer was studied using photoelectron spectroscopy wit...

ispartof: ADVANCED SUSTAINABLE SYSTEMS vol:6 issue:8 status: published

Physical chemistry, chemical physics 22(36), 20311 - 20330 (1-20) (2020). doi:10.1039/D0CP04033A, Permanganate aqueous solutions, MnO$_4$$^-$(aq.), were studied using liquid-micro-jet-based soft X-ray non-resonant and resonant photoelectron spectroscopy to determine valence and core-level binding energies. To identify possible differences in the energetics between the aqueous bulk and the solution-gas interface, non-resonant spectra were recorded at two different probing depths. Similar experiments were performed with different counter ions, Na$^+$ and K$^+$, with the two solutions yielding indistinguishable anion electron binding energies. Our resonant photoelectron spectroscopy measurements, performed near the Mn L$_{II,III}$- and O K-edges, selectively probed valence charge distributions between the Mn metal center, O ligands, and first solvation shell in the aqueous bulk. Associated resonantly-enhanced solute ionisation signals revealed hybridisation of the solute constituents’ atomic orbitals, including the inner valence Mn 3p and O 2s. We identified intermolecular Coulombic decay relaxation processes following resonant X-ray excitation of the solute that highlight valence MnO$_4$$^-$$_{(aq.)}$-H$_2$O$_{(l)}$ electronic couplings. Furthermore, our results allowed us to infer oxidative reorganisation energies of MnO$_4$$^•$$_{(aq.)}$ and adiabatic valence ionisation energies of MnO$_4$$^-$$_{(aq.)}$, revealing the Gibbs free energy of oxidation and permitting estimation of the vertical electron affinity of MnO$_4$$^•$$_{(aq.)}$. Finally, the Gibbs free energy of hydration of isolated MnO$_4$$^-$ was determined. Our results and analysis allowed a near-complete binding-energy-scaled MnO$_4$$^-$$_{(aq.)}$ molecular orbital and a valence energy level diagram to be produced for the MnO$_4$$^-$$_{(aq.)}$ / MnO$_4$$^•$$_{(aq.)}$ system. Cumulatively, our mapping of the aqueous-phase electronic structure of MnO$_4$$^-$ is expected to contribute to a deeper understanding of the exceptional redox properties of this widely applied aqueous transition-metal complex ion., Published by RSC Publ., Cambridge

Simulations based on density functional theory (DFT) were used to investigate the behaviour of substitutional iron in both tetragonal and monoclinic ZrO2. Brouwer diagrams of predicted defect concentrations, as a function of oxygen partial pressure, suggest that iron behaves as a p-type dopant in monoclinic ZrO2 while it binds strongly to oxygen vacancies in tetragonal ZrO2. Analysis of defect relaxation volumes suggest that these results should hold true in thermally grown oxides on zirconium, which is under compressive stresses. X-ray absorption near edge structure (XANES) measurements, performed to determine the oxidation state of iron in Zircaloy-4 oxide samples, revealed that 3 + is the favourable oxidation state but with between a third and half of the iron, still in the metallic Fe0 state. The DFT calculations on bulk zirconia agree with the preferred oxidation state of iron if it is a substitutional species but do not predict the presence of metallic iron in the oxide. The implications of these results with respect to the corrosion and hydrogen pick-up of zirconium cladding are discussed.

All-solid-state batteries promise significant safety and energy density advantages over liquid-electrolyte batteries. The interface between the cathode and the solid electrolyte is an important contributor to charge transfer resistance. Strong bonding of solid oxide electrolytes and cathodes requires sintering at elevated temperatures. Knowledge of the temperature dependence of the composition and charge transfer properties of this interface is important for determining the ideal sintering conditions. To understand the interfacial decomposition processes and their onset temperatures, model systems of LiCoO2 (LCO) thin films deposited on cubic Al-doped Li7La3Zr2O12 (LLZO) pellets were studied as a function of temperature using interface-sensitive techniques. X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), and energy-dispersive X-ray spectroscopy (EDS) data indicated significant cation interdiffusion and structural changes starting at temperatures as low as 300°C. La2Zr2O7 and Li2CO3 were identified as decomposition products after annealing at 500°C by synchrotron X-ray diffraction (XRD). X-ray absorption spectroscopy (XAS) results indicate the presence of also LaCoO3, in addition to La2Zr2O7 and Li2CO3. Based on electrochemical impedance spectroscopy, and depth profiling of the Li distribution upon potentiostatic hold experiments on symmetric LCO|LLZO|LCO cells, the interfaces exhibited significantly increased impedance, up to 8 times that of the as-deposited samples after annealing at 500°C. Our results indicate that lower-temperature processing conditions, shorter annealing time scales, and CO2-free environments are desirable for obtaining ceramic cathode-electrolyte interfaces that enable fast Li transfer and high capacity.

We present results of combined experimental photoelectron spectroscopy and theoretical modeling studies of solvated dicarboxylate species (-O2C(CH2)2CO2-) in complex with Na+ and K+ metal cations. These ternary clusters serve as simple models for the investigation of aqueous ion/solute specific effects that play an important role in biological systems. The experimental characterization of these systems was performed in the presence of up to six solvating waters. In both Na+ and K+ cases, we observe the presence of one major broad band that gradually shifts to higher electron binding energy (EBE) with an increasing number of waters. In the Na+ case further detailed analysis of experimental spectra was performed using ab initio calculations. In particular, we have identified the structures of the lowest energy clusters whose EBE values match well the major band in the experimental spectra. Our results show that evolution of an aqueous solvation shell emphasizes the coordination of the negatively charged carboxylate groups accompanied by simultaneous interaction with metal cations. Calculations also indicate that in the solvation range investigated experimentally (up to 6 waters), Na+ retains direct contact with the dicarboxylate species, i.e. a contact ion-pair (CIP) complex. Preliminary modeling studies show evidence of an alternative solvent separated ion-pair complex once the solvation range approaches 8 waters, however its energy still remains above that of (∼7-8 kcal/ mol-1) the CIP complex. At a higher number of waters (n = 3 for Na+ and n = 5 for K+), the experimental spectra also show the development of a weak low energy band. Its origin cannot be precisely identified. Our calculations in the Na+ case point out the existence of a quaternary complex consisting of Na+, H2O, OH- and a singly protonated dicarboxylate anion (HO2C(CH)2CO2-). Such a complex appears to be stabilized in the solvation range corresponding to the appearance of the low EBE band and does match its peak, even though the energy of such a complex is fairly high compared to the ternary structure. ispartof: PHYSICAL CHEMISTRY CHEMICAL PHYSICS vol:20 issue:46 pages:29051-29060 ispartof: location:England status: published

The exceptional thermal and chemical stability of the UiO-66, -67 and -68 classes of isostructural MOFs [J. Am. Chem. Soc., 2008, 130, 13850] makes them ideal materials for functionalization purposes aimed at introducing active centres for potential application in heterogeneous catalysis. We previously demonstrated that a small fraction (up to 10%) of the linkers in the UiO-67 MOF can be replaced by bipyridine-dicarboxylate (bpydc) moieties exhibiting metal-chelating ability and enabling the grafting of Pt(ii) and Pt(iv) ions in the MOF framework [Chem. Mater., 2015, 27, 1042] upon interaction with PtCl2 or PtCl4 precursors. Herein we extend this functionalization approach in two directions. First, we show that by controlling the activation of the UiO-67-Pt we can move from a material hosting isolated Pt(ii) sites anchored to the MOF framework with Pt(ii) exhibiting two coordination vacancies (potentially interesting for C–H bond activation) to the formation of very small Pt nanoparticles hosted inside the MOF cavities (potentially interesting for hydrogenation reactions). The second direction consists of the extension of the approach to the insertion of Cu(ii), obtained via interaction with CuCl2, and exhibiting interesting redox properties. All materials have been characterized by in situ X-ray absorption spectroscopy at the Pt L3- and Cu K-edges.

Near-infrared emitting persistent phosphors have promising applications in the field of in vivo medical imaging. In this paper, we prepared the persistent phosphor LiGa5O8:Cr3+ (LGO:Cr) which exhibits emission in the tissue transparency window and shows afterglow for multiple hours after excitation. Addition of Si or Ge improves the persistent luminescence. X-ray diffraction and electron microscopy, coupled with elemental analysis, revealed that there is a maximum amount of Si or Ge that can be incorporated in the host. Via X-ray absorption spectroscopy and electron paramagnetic resonance experiments, we studied the local environment of chromium in the LGO spinel host. The presence of both Cr3+ and Cr4+ on octahedral sites in LGO was confirmed. Electron paramagnetic resonance showed that Cr3+ resides in a rhombically distorted octahedral lattice site and that the Cr3+ local environment is sensitive to variation in point defects in the surrounding.

We explore the high spectral resolution X-ray data towards the quasar 3C273 to search for signals of hot ($\sim10^{6-7}$ K) X-ray-absorbing gas co-located with two established intergalactic FUV OVI absorbers. We analyze the soft X-ray band grating data of all XMM-Newton and Chandra instruments to search for the hot phase absorption lines at the FUV predicted redshifts. The viability of potential line detections is examined by adopting the constraints of a physically justified absorption model. The WHIM hypothesis is investigated with a complementary 3D galaxy distribution analysis, and by comparison of the measurement results to the WHIM properties in the EAGLE cosmological, hydrodynamical simulation. At FUV redshift z=0.09017, we measured signals of two hot ion species, OVIII and NeIX, with a $3.9\sigma$ combined significance level. Considering the line features in all instruments collectively and assuming collisional equilibrium for absorbing gas, we were able to constrain the temperature ($kT=0.26\pm0.03$ keV) and the column density ($N_H\times{Z_\odot/Z}=1.3_{-0.5}^{+0.6}\times10^{19}$ cm$^{-2}$) of the absorber. Thermal analysis indicates that FUV and X-ray absorption relate to different phases, with estimated temperatures $T_{FUV}\approx3\times10^5$ and $T_{X-ray}\approx3\times10^6$ K, which match the EAGLE predictions for WHIM at the FUV/X-ray measured $N_{ion}$-ranges. We detected a large scale galactic filament crossing the sightline at the redshift of the absorption, linking the absorption to this structure. This study provides insights into co-existing warm and hot gas within a WHIM filament and estimates the ratio of the hot and warm phases. Because the hot phase is thermally distinct from the OVI gas, the estimated baryon content of the absorber is increased, conveying the promise of X-ray follow-up studies of FUV detected WHIM in refining the picture of the missing baryons., Comment: 17 pages, 10 figures, accepted for publication in A&A

Transmission electron microscopy (TEM) is commonly used as an ex-situ technique to determine structural changes by comparing images of catalyst particles before and after a reaction. This requires the use of an alcoholic solvent to disperse the particles on a grid. In this work, we will show that Pd catalysts can be transformed during the procedure, by using EXAFS to determine the structure of Pd catalysts in different environments (as dry or wet samples). Supported palladium nanoparticles exposed to aqueous ethanolic solution (50% w/v) are transformed to a common, reduced, and hydrogen-contaminated state, irrespective of their initial habit or support. Catalysts comprised of nanosize PdO are reduced at ca. 350 K, whereas samples comprised of very small (ca. ≤ 10 atoms) Pd particles react with the solvent at just above room temperature and agglomerating with considerable loss of dispersion. As such any potential benefits to catalysis sought through the synthesis of very highly dispersed metallic Pd supported upon a range of inorganic dispersants will be rapidly erased through the action of such solvents.

The high resolution near edge X-ray absorption fine structure spectrum of nitrogen displays the vibrational structure of the core-excited states. This makes nitrogen well suited for assessing the accuracy of different electronic structure methods for core excitations. We report high resolution experimental measurements performed at the SOLEIL synchrotron facility. These are compared with theoretical spectra calculated using coupled cluster theory and algebraic diagrammatic construction theory. The coupled cluster singles and doubles with perturbative triples model known as CC3 is shown to accurately reproduce the experimental excitation energies as well as the spacing of the vibrational transitions. The computational results are also shown to be systematically improved within the coupled cluster hierarchy, with the coupled cluster singles, doubles, triples, and quadruples method faithfully reproducing the experimental vibrational structure. This is the authors’ accepted and refereed manuscript to the article. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in Journal of Chemical Physics and may be found at https://doi.org/10.1063/1.5011148

Once penetrated into the lungs of exposed people, asbestos induces an in vivo biomineralisation process that leads to the formation of a ferruginous coating embedding the fibres. The ensemble of the fibre and the coating is referred to as asbestos body and is believed to be responsible for the high toxicological outcome of asbestos. Lung tissue of two individuals subjected to prolonged occupational exposure to crocidolite asbestos was investigated using synchrotron radiation micro-probe tools. The distribution of K and of elements heavier than Fe (Zn, Cu, As, and Ba) in the asbestos bodies was observed for the first time. Elemental quantification, also reported for the first time, confirmed that the coating is highly enriched in Fe (~20% w/w), and x-ray absorption spectroscopy indicated that Fe is in the 3+ oxidation state and that it is present in the form of ferritin or hemosiderin. Comparison of the results obtained studying the asbestos bodies upon removing the biological tissue by chemical digestion and those embedded in histological sections, allowed unambiguously distinguishing the composition of the asbestos bodies, and understanding to what extent the digestion procedure altered their chemical composition. A speculative model is proposed to explain the observed distribution of Fe.

In this review we provide an overview of the potential of synchrotron radiation techniques to understand the structural and electronic properties of coordination compounds. Besides the largely employed multi-wavelength anomalous dispersion (MAD) and X-ray absorption spectroscopy (XAS), in both near (XANES) and post (EXAFS) edge regions, we also discuss the contribution arising from more specialized techniques that have become more widely used in the last years, such as the total scattering approach in the XRPD data analysis and X-ray emission spectroscopy (XES). Comparison with the commonly used laboratory techniques (XRD, UV–vis, luminescence, NMR, EPR) is used to underline the added value of synchrotron radiation techniques when applied to already well characterized samples. The fundamental role of DFT calculations in interpreting both diffraction and spectroscopic data to understand structural and electronic properties of coordination complexes is highlighted. A perspective summary is reported at the end of the manuscript.

The cosmological missing baryons at z < 1 most likely hide in the hot (T ≳ 105.5 K) phase of the warm hot intergalactic medium (WHIM). While the hot WHIM is hard to detect due to its high ionisation level, the warm (T ≲ 105.5 K) phase of the WHIM has been very robustly detected in the far-ultraviolet (FUV) band. We adopted the assumption that the hot and warm WHIM phases are co-located and therefore used the FUV-detected warm WHIM as a tracer for the cosmologically interesting hot WHIM. We performed an X-ray follow-up in the sight line of the blazar PKS 2155–304 at the redshifts where previous FUV measurements of O VI, Si IV, and broad Lyman-alpha (BLA) absorption have indicated the existence of the warm WHIM. We looked for the O VII Heα and O VIII Lyα absorption lines, the most likely hot WHIM tracers. Despite the very large exposure time (≈1 Ms), the Reflection Grating Spectrometer unit 1 (RGS1) on-board XMM-Newton data yielded no significant detection which corresponds to upper limits of log N(O VII(cm−2)) ≤ 14.5−15.2 and log N(O VIII(cm−2)) ≤ 14.5−15.2. An analysis of the data obtained with the combination of the Low Energy Transmission Grating (LETG) and the High Resolution Camera (HRC) on-board Chandra yielded consistent results. However, the data obtained with the LETG, combined with the Advanced CCD Imaging Spectrometer (ACIS) lead to the detection of an feature resembling an absorption line at λ ≈ 20 Å at simple one-parameter confidence level of 3.7σ, consistent with several earlier LETG/ACIS reports. Given the high statistical quality of the RGS1 data, the possibility of RGS1 accidentally missing the true line at λ ∼ 20 Å is very low: 0.006%. Neglecting this, the LETG/ACIS detection can be interpreted as Lyα transition of O VIII at one of the redshifts (z ≈ 0.054) of FUV-detected warm WHIM. Given the very convincing X-ray spectral evidence for and against the existence of the λ ∼ 20 Å feature, we cannot conclude whether or not it is a true astrophysical absorption line. Considering cosmological simulations, the probability of the LETG/ACIS λ ∼ 20 Å feature being due to the astrophysical O VIII absorber co-located with the FUV-detected O VI absorber is at the very low level of ≲0.1%. We cannot completely rule out the very unlikely possibility that the LETG/ACIS 20 Å feature is due to a transient event located close to the blazar.

Ferrihydrite (Fh) is an yet enigmatic nano Fe(III)-oxide material, omnipresent in nature that can bind ions in large quantities, regulating bioavailability and ion mobility. Although extensively studied, to date no proper view exists on the surface structure and composition, while it is of vital importance to our understanding of ion complexation in terrestrial and aquatic systems. Here, the surface structure of Fh is elucidated in relation to that of the mineral core, showing a unique surface composition differing from the mineral core. The mineral core is basically defect-free for all Fh particles. Key for understanding Fh is the very large contribution of the surface as an “inter-phase”. The surface of Fh is depleted by two specific types of polyhedra (Fe2,Fe3). Surface depletion (SD) explains the observed particle size dependency of the (a) Pair Distribution Function (PDF) derived from High Energy X-ray total Scattering (HEXS), (b) thermogravimetric water content, and (c) mass density. It also explains the isotopic 57Fe exchange ratio. Due to surface groups, two-line Fh particles are water rich but its mineral core is hydrogen poor. The SD model elucidates the surface structure of crystal faces of idealized Fh such as the 1 −1 0 and 1 −1 1 type of faces that may strongly contribute (e.g. ∼75 ± 10%) to the total surface area. These faces are terminated by protruding Fe1 octahedra, creating the singly-coordinated FeOH(H) groups at the Fh surface. Alternating rows of Fe1 octahedra with singly-coordinated surface groups enable the formation of either double corner (2C) or edge (1E) surface complexes. For Fh, the site densities derived are much higher than for goethite. In combination with the high surface area, it makes Fh to an extremely reactive natural phase.

A series of Li1-xFe1-xVxPO4/C (with 0

International audience; The impact of deep-subsurface pressure conditions on microbial activity is still poorly constrained. In particular it is unknown how pressure of deep environments affects microbial transformations of iron. We investigated the effects of high hydrostatic pressure (HHP) on the rate and the extent of bacterial dissimilatory iron reduction (DIR). We employed a novel experimental setup that enables in situ monitoring of Fe oxidation state and speciation in bacterial cultures in an optimized HHP incubation system using X-ray Absorption Near-Edge Structure (XANES) spectroscopy. The iron-reducing bacterium Shewanella oneidensis MR-1 was incubated at 30 degrees C with Fe(III) citrate and tryptone at pressures between 0.1 and 100 MPa. For pressures up to 70 MPa strain MR-1 (10(8) cells ml(-1)) was able to reduce all 5 mM Fe(III) provided. Above 70 MPa, the final amount of Fe(III) that MR-1 could reduce decreased linearly and DIR was estimated to stop at 109 +/- 7 MPa. The decrease in the reduction yield was correlated with the dramatic decrease in survival (as determined by CFU counts) above 70 MPa. The initial rate of DIR increased with pressure up to 40 MPa, then decreased to reach zero at about 110 MPa. Increased rates of DIR activity and relatively high growth rates for pressures below 40 MPa would potentially ensure the maintenance of MR-1 in most of deep subsurface environments where moderate pressures occur, i.e. deep-sea environments. This study not only provides the first in situ quantitative results for microbial iron metabolism under HHP conditions but also sets the stage for future investigations of deep-sea pressure-adapted iron reducers. Moreover it demonstrates for the first time that XANES at the Fe K-edge is a powerful probe for in vivo monitoring of iron transformations in living microbial cultures. (C) 2012 Elsevier Ltd. All rights reserved.

The core level photoemission and near edge X-ray photoabsorption spectra of guanine in the gas phase have been measured and the results interpreted with the aid of high level ab initio calculations. Tautomers are clearly identified spectroscopically, and their relative free energies and Boltzmann populations at the temperature of the experiment (600 K) have been calculated and compared with the experimental results and with previous calculations. We obtain good agreement between experiment and the Boltzmann weighted theoretical photoemission spectra, which allows a quantitative determination of the ratio of oxo to hydroxy tautomer populations. For the photoabsorption spectra, good agreement is found for the C 1s and O 1s spectra but only fair agreement for the N 1s edge.

Pt(13) clusters with extraordinarily high magnetization are interesting samples in the area of CO chemistry. For the first time paramagnetic platinum carbonyl samples were prepared and characterized by electron paramagnetic resonance (EPR) spectroscopy to improve the understanding of the CO adsorption effect on the structure of small Pt particles with X-ray absorption spectroscopy and Fourier transform infrared (FTIR) methods. In contrast to existing studies on the reduced Pt cluster-CO interaction in the literature, the structure of the paramagnetic Pt sample before CO adsorption is entirely known. The well-defined Pt(13) cluster in NaY zeolite reconstructs into small Pt(2)(CO)(m) aggregates under CO atmosphere at room temperature. X-Band EPR spectroscopy shows that the multiplet spectrum of the Pt(13) cluster at g(iso) = 2.35 disappears, and a new spectrum belonging to the Pt(2)(CO)(m) sample appears at g(iso) = 1.98. Extended X-ray absorption fine structure spectroscopy supports the Pt(13) Cluster destruction upon admission of CO and the formation of a dimeric platinum carbonyl complex with a nearest-neighbor coordination number that decreases from ca. 5.8 to 0.9. FTIR spectroscopy is a very sensitive technique for the C-O vibrational frequency. It reveals that Pt(2)(CO)(m) species have one type of linear and two types of bridge bonded CO groups. Initially, a weak band at 2108 cm(-1) shows that Pt has a small positive charge, which is converting into zero charge upon CO desorption-adsorption cycles. The surprisingly sharp bands in the CO bridge bond region facilitate assignment, in contrast to the bands of CO bonds of other neutral platinum carbonyl samples.

The paper presents a preliminary structural investigation of the 20% Pt-Co (1:1) alloy on Vulcan XC-72 catalyst using X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM) and X-ray diffraction (XRD). XAS results have been obtained ex situ and in situ using a specially optimized for XAS measurement fuel cell (down to 6 keV). The results are compared with those obtained for pure Pt catalyst on the same carbon support under the same working conditions. (C) 2007 Elsevier B.V. All rights reserved.

The adsorption of Si on goethite (alpha-FeOOH) has been studied in batch experiments that cover the natural range of Si concentrations as found in the environment. The results have been interpreted and quantified with the charge distribution (CD) and multi-site surface complexation (MUSIC) model in combination with an extended Stern (ES) layer model option. This new double layer approach (ES) accounts for ordering of interfacial water molecules leading to stepwise changes in the location of electrolyte ions near the surface [T. Hiemstra, W.H. Van Riemsdijk, J. Colloid Interface Sci. 301 (2006) 1]. The Si adsorption on goethite peaks at a pH of approximately 9 and decreases at lower and higher pH values. Thermodynamically, the pH-dependency of silicic acid adsorption is related to the value of the proton co-adsorption and can also be linked to the Si charge distribution in the interface as is discussed. Based on published EXAFS data, the adsorption of Si on goethite was modeled as the formation of a bidentate surface complex. The ionic charge distribution (CD) of this complex can be calculated from the geometry of this surface complex, optimized with molecular orbital/density functional theory (MO/DFT), and combined with a correction for water dipole orientation. The resulting CD value has been applied successfully in the description of the adsorption data. The use of a theoretical CD value has the practical advantage of a reduction of the number of adjustable parameters with a factor 2. To describe the adsorption at a high Si loading, formation of a Si polymer, e.g. a tetramer, is proposed. Such a species is only contributing to the overall adsorption at solution concentrations above about 10(-4) M, where super saturation with respect to quartz exists. The adsorbed silica polymer hydrolyzes at high pH. The reactive ligand of the polymer is quite acid (logK approximately 6.5-7.1), which is typically for the triple bond SiO(-1) surface groups of polymerized Si, like amorphous SiO(2)(s), and the triple bond SiO(-1) ligand of the aqueous dimer Si(2)O(OH)(5)O(-1)(aq). The applied model correctly predicts the change of particles charge and the shift in IEP due to proton release upon Si adsorption.

One of the limitations to the widespread use of hydrogen as an energy carrier is its storage in a safe and compact form. Herein, recent developments in effective high-capacity hydrogen storage materials are reviewed, with a special emphasis on light compounds, including those based on organic porous structures, boron, nitrogen, and aluminum. These elements and their related compounds hold the promise of high, reversible, and practical hydrogen storage capacity for mobile applications, including vehicles and portable power equipment, but also for the large scale and distributed storage of energy for stationary applications. Current understanding of the fundamental principles that govern the interaction of hydrogen with these light compounds is summarized, as well as basic strategies to meet practical targets of hydrogen uptake and release. The limitation of these strategies and current understanding is also discussed and new directions proposed.

There is an increasing worldwide demand for high energy density batteries. In recent years, rechargeable Li-ion batteries have become important power sources, and their performance gains are driving the adoption of electrical vehicles (EV) as viable alternatives to combustion engines. The exploration of new Li-ion battery materials is an important focus of materials scientists and computational physicists and chemists throughout the world. The practical applications of Li-ion batteries and emerging alternatives may not be limited to portable electronic devices and circumventing hurdles that include range anxiety and safety among others, to their widespread adoption in EV applications in the future requires new electrode materials and a fuller understanding of how the materials and the electrolyte chemistries behave. Since this field is advancing rapidly and attracting an increasing number of researchers, it is crucial to summarise the current progress and the key scientific challenges related to Li-ion batteries from theoretical point of view. Computational prediction of ideal compounds is the focus of several large consortia, and a leading methodology in designing materials and electrolytes optimized for function, including those for Li-ion batteries. In this Perspective, we review the key aspects of Li-ion batteries from theoretical perspectives: the working principles of Li-ion batteries, the cathodes, anodes, and electrolyte solutions that are the current state of the art, and future research directions for advanced Li-ion batteries based on computational materials and electrolyte design.

We used thermogravimetric hydrogen reduction and iodometric titration to determine the bulk valence state of cobalt in Co-perovskites containing surface carbonate hydroxide or hydroxyl groups. It could be shown that thermogravimetric hydrogen reduction experiments are very sensitive to volatile surface groups, but due to their volatility, they can be specified and the bulk valence state of cobalt can still be deduced from these experiments. The iodometric titration is less sensitive to small volatile surface impurities, but precaution has to be taken that oxygen or iodine does not escape from the solution during dissolution of the sample. Best results were obtained if the sample was titrated during dissolution in a closed argon floated titration apparatus. We tested the two methods using LaCoO3 perovskite as a sample with a known valence state. Both methods delivered satisfactory results, and the valence state could be determined with an accuracy of better than 1%.

The energy loss near edge structure (ELNES) of the O-K, Ti-L-23 and Mn-L-23 edges have been recorded in hexagonal Ba(3)Ti(2)NlnO(9) with an energy resolution of 0.10-0.20eV using a monochromator on a commercial transmission electron microscope (TEM) and compared with a tetragonal BaTiO3 reference sample. The formal valency and symmetry of Mn have been determined using atomic multiplets calculations and its effect on the electronic structure of BaTiO3 has been interpreted through a molecular-orbital model. (c) 2005 Elsevier Inc. All rights reserved.

UV-vis spectroscopy was used in a combined in-situ UV-vis/XAFS spectroscopic setup to study the synchrotron radiation effect on aqueous homogeneous copper solutions. Two different systems were studied. In the first study, the focus was on a copper bipyridine-catalyzed oxidation of benzyl alcohol to benzaldehyde with 2,2,6,6- tetramethylpiperidinyl-l-oxy and base as cocatalysts. It was found that when the reaction mixture is exposed to the X-ray beam, the features present in the in-situ UV-vis spectrum develop differently compared to the situation when the reaction mixture is not exposed to the X-ray beam. Besides a temperature effect of the X-ray beam, both the UV-vis analysis and the XAFS analysis showed a reducing influence of the X-ray beam on the sample. To investigate this in more detail, we Studied a series of dilute aqueous copper solutions from different precursor salts, viz., Cu(NO3)(2).3H(2)O, CUSO4.5H(2)O, CuCl2, and CuBr2. It was found that the different aqueous copper solutions have different stabilities under the influence of the X-ray beam. Especially the solution from the CuCl2 precursor salt was found to be unstable and to be subjected to reduction. These examples illustrate the need for a second technique, such as in-situ UV-vis spectroscopy, to evaluate the effect of synchrotron radiation used to measure in-situ XAFS on catalytic systems.

EXAFS spectroscopy, analysed in the frame of the multiple scattering theory, has been able to determine the local structure of [Cu(CO)(2)](+) complexes hosted inside ZSM-5 channels upon contacting the activated zeolite with CO from the gas phase at room temperature. We found that the number of coordinated CO molecules (1.8 +/- 0.3) is in good agreement with the [Cu(CO)(2)](+) stoichiometry suggested by IR. The Cu - C distance obtained for the [Cu(CO)(2)](+) complex is 1.88 +/- 0.02 angstrom, with a C - O distance ( 1.12 +/- 0.03 angstrom). This work complements a previous one [ C. Lamberti, G. Turnes Palomino, S. Bordiga, G. Berlier, F. D'Acapito and A. Zecchina, Angew. Chem. Int. Ed., 2000, 39, 2138], performed at liquid nitrogen temperature, where the structure of [Cu(CO)(3)](+) complexes was identified by combined EXAFS/XANES/IR spectroscopies. An increase of the Cu - C distance of 0.05 angstrom by moving from [Cu(CO)(2)](+) to [Cu(CO)(3)](+) complexes has been observed, which is the local rearrangement needed to accommodate a third CO ligand in the first coordination shell of copper. EXAFS determined that the Cu - C - O bond angle is linear within the error bars ( 170 +/- 10 degrees), while IR and XANES indicate that intrazeolitic [Cu(CO)(2)](+) complexes have C-2v symmetry. The experimentally obtained moieties are in good agreement with the values obtained with advanced quantum mechanical methods.

A systematic analysis of the thermodynamic and spectroscopic features of a vast set of data on carbonyl-like complexes present in the literature, combined with some new experimental data here reported, allowed us to attempt to single out the electrostatic U- and ﷿-contributions in the formation of carbonyl bonding for the so-called “nonclassical carbonyls” hosted inside zeolitic nanocavities. In particular, the room-temperature adsorption of CO on well-defined Cu(I) and Ag(I)-ZSM-5 systems (as testified by X-ray absorption experiments) was studied by means of the joint use of IR spectroscopy and adsorption microcalorimetry. The formation in the zeolite pores of heterogeneous [Cu(CO)2] + and [Ag(CO)] + complexes, the stoichiometry of which is in good agreement with the homogeneous nonclassical carbonyls, was monitored as a function of increasing pCO. In the early stage of the interaction, strong and irreversibly bound monocarbonyl species characterized by O ˜ COads > O ˜ COgas are formed on Cu(I) and Ag(I) sites. Conversely, labile adducts (electrostatic in nature) are formed on Na + and K + sites hosted in the same zeolite pores. The zero-coverage enthalpy of CO adsorbed on Cu(I) and Ag(I) sites (-¢adsH 120 and 100 kJ/mol, respectively) is much larger than the -¢adsH values measured for the two alkaline-metal adducts (35 and 28 kJ/mol for Na + and K + , respectively), despite the closeness of the charge/radius ratios of the two sets of metal cations [Na + and Cu(I); K + and Ag(I)]. The high -¢adsH values and the irreversible nature of (a fraction) of the d-block metal carbonyls suggest the onset of a ﷿-back-donation reinforcing the carbonyl bond with respect to a plain U-coordination. A clear deviation from an empirical rule, which linearly correlate ¢O ˜ CO and -¢Hads quantities for a large set of non-d/d 0 /d 10 metal carbonyls, was observed in the case of copper-and silver-carbonyls, confirming the interplay of U- and ﷿-back-donation contributions for such species, otherwise defined nonclassical carbonyls. The ¢O ˜ CO versus -¢adsH empirical rule was found of general validity, in that it allows to infer the enthalpy values from the blue-shift of the C-O stretching frequency (and vice-versa) in the case of non-d/d 0 metal carbonyls, whereas it allows to roughly estimate by the deviation from the line the extent of the ﷿-back-donation in the case of d-block metal carbonyls. Further, the spectroscopic and thermodynamic features of carbonyl species formed on (partially) reduced copper sites have shown that in the absence of strong electrostatic plus U-coordinative components the carbonyl bond is surprisingly weak, despite the presence of ﷿-back-donation.

Intermediate valence is one of the typical phenomena of systems with strong electronic correlation. The Anderson impurity model predicts a scaling of the valence with the reduced temperature T/T(K), which is difficult to observe by traditional surface-sensitive electronic spectroscopies. This paper presents results obtained by resonant inelastic X-ray scattering (RIXS), a bulk-sensitive configuration- and chemical-specific technique. The temperature dependence of the valence of YbInCu(4) and YbAgCu(4) was measured by tuning the incident energy to the resonance of the Yb(2+) spectral component. In the case of YbInCu(4) a sharp valence transition, as known from thermodynamical measurements, has been found. The valence of YbAgCu(4) reveals a smooth dependence consistent with a Kondo temperature T(K) = 70 K. These findings establish RIXS as a powerful tool for measuring bulk electronic properties of solids.

The ratio of the two components of the L 23 M 23 V Auger transition is currently used to determine the stoichiometry of titanium oxides. The reduction of Ti manifests itself by a relative increase (decrease) in intensity of the Auger component lying at high (low) kinetic energy, which is assigned to an intraatomic (interatomic) transition involving Ti 3d (O 2p) valence electrons. The LMV Auger transition is herein revisited via its resonant behaviour at the Ti L 23 absorption edge in TiO 2 . Resonances of the high energy component occurs exclusively at the L 2 edge, while resonances of the other component are seen at the L 3 edge, but also at the L 2 edge. The latter being attributed to a L 2 L 3 V Coster–Kronig transition, the low- and high-lying LMV are identified as L 3 MV (including those following CK) and L 2 MV transitions, respectively, at variance with the previous assignment.

The Cu K- and Ce L-3-edge regions have been measured in the hole doped hybrid structured [Nd-2/3(Ce1-xThx)(1/3)](2)-(Ba2/3Nd1/3)(2)Cu3O9-delta (2-2-3), 0 less than or equal to x less than or equal to 1.0, compounds which are superconducting for x less than or equal to 0.30. From the energy positions of the Cu K-edge features, it is clearly noted that 2-2-3 compounds incorporate the K-edge features seen in the oxygen deficient 1-2-3 and the T'-type 2-1-4 superconductors. On progressive Th substitution in 2-2-3, Ce continues to show mixed valence (3(+), 4(+) predominant), Cu remains mainly in 2 + state and no significant change is seen in the weak feature at Cu1+ position. However, noticeable changes in the split white line features at the Cu K-edge are observed. It is inferred from the analysis of the spectral features that a re-distribution of oxygen on the Cu(1) octahedral (defect) site in 2-2-3 occurs with increasing x, affecting adversely the carrier density. This plausibly causes a steep fall in T-c with Th substitution. (C) 1999 .

The distribution of valence electrons between transition metals (TM) and oxygen has been studied via X-ray absorption near-edge fine structure (XANES) at the TM-L 2, 3 and O-K thresholds for La 2 Li 1/2 Cu 1/2 O 4 , Nd 2 Li 1/2 Ni 1/2 O 4 , and La 2 Li 1/2 Co 1/2 O 4 . Simulations of the TM-L 2, 3 XANES by charge-transfer multiplet calculations result in 3d-hole concentrations that increase from ∼30% 3d 8 in the Cu(III) to ∼57% 3d 7 in the Ni(III) to ∼72% 3d 6 in the Co(III) compound. Concomitantly, the O-K XANES spectra reflect an increase of bond ionicity in the same sequence. The results show that the Li-doped holes in Nd 2 Li 1/2 Ni 1/2 O 4 are distributed between Ni-3d and O-2p orbitals.

EXAFS measurements on all the cations in Nd2CuO4, Nd2-xCexCuOy (NCCO) and Nd(1.82-z)A(z)Ce(0.18)CuO(y) (NACCO), A=Sr, Ca or Ba, x = 0.06 or 0.18, have been made using synchrotron radiation. Bond lengths (R), coordination numbers (N) and Debye-Waller factors (sigma(2)) have been obtained from the EXAFS analysis. The curved wave theory in the single-scattering approximation and the small-atom multiple-scattering theory have been applied to experimental EXAFS data and it is found that the latter gives a better fit. The CuO2 plane contains a strong multiple-scattering contribution. Further, all the dopants (Ce, Sr, Ca and Ba) occupy the Nd site without affecting the basic T' structure. Doping affects the local structure around the substituents as reflected by the changes in R and sigma(2) values. The observed CuO2 layer distortion (or large Cu-O-Cu angle deviation from linearity, 180 degrees) is found to have an adverse effect an T-c as observed in Ca or Ba doped NACCO samples. Further, it is realized that critical levels of charge carriers, oxygen vacancies and CuO2 planar effects have a strong influence on the SC behaviour of the T'-type electron doped cuprates.

The superbulky deca-aryleuropocene [Eu(Cp(BIG))2], Cp(BIG) = (4-nBu-C6H4)5-cyclopentadienyl, was prepared by reaction of [Eu(dmat)2(thf)2], DMAT = 2-Me2N-α-Me3Si-benzyl, with two equivalents of Cp(BIG)H. Recrystallizyation from cold hexane gave the product with a surprisingly bright and efficient orange emission (45% quantum yield). The crystal structure is isomorphic to those of [M(Cp(BIG))2] (M = Sm, Yb, Ca, Ba) and shows the typical distortions that arise from Cp(BIG)⋅⋅⋅Cp(BIG) attraction as well as excessively large displacement parameter for the heavy Eu atom (U(eq) = 0.075). In order to gain information on the true oxidation state of the central metal in superbulky metallocenes [M(Cp(BIG))2] (M = Sm, Eu, Yb), several physical analyses have been applied. Temperature-dependent magnetic susceptibility data of [Yb(Cp(BIG))2] show diamagnetism, indicating stable divalent ytterbium. Temperature-dependent (151)Eu Mössbauer effect spectroscopic examination of [Eu(Cp(BIG))2] was examined over the temperature range 93-215 K and the hyperfine and dynamical properties of the Eu(II) species are discussed in detail. The mean square amplitude of vibration of the Eu atom as a function of temperature was determined and compared to the value extracted from the single-crystal X-ray data at 203 K. The large difference in these two values was ascribed to the presence of static disorder and/or the presence of low-frequency torsional and librational modes in [Eu(Cp(BIG))2]. X-ray absorbance near edge spectroscopy (XANES) showed that all three [Ln(Cp(BIG))2] (Ln = Sm, Eu, Yb) compounds are divalent. The XANES white-line spectra are at 8.3, 7.3, and 7.8 eV, for Sm, Eu, and Yb, respectively, lower than the Ln2O3 standards. No XANES temperature dependence was found from room temperature to 100 K. XANES also showed that the [Ln(Cp(BIG))2] complexes had less trivalent impurity than a [EuI2(thf)x] standard. The complex [Eu(Cp(BIG))2] shows already at room temperature strong orange photoluminescence (quantum yield: 45 %): excitation at 412 nm (24,270 cm(-1)) gives a symmetrical single band in the emission spectrum at 606 nm (νmax =16495 cm(-1), FWHM: 2090 cm(-1), Stokes-shift: 2140 cm(-1)), which is assigned to a 4f(6)5d(1) → 4f(7) transition of Eu(II). These remarkable values compare well to those for Eu(II)-doped ionic host lattices and are likely caused by the rigidity of the [Eu(Cp(BIG))2] complex. Sharp emission signals, typical for Eu(III), are not visible.

The electronic structure of the inorganic misfit-layer compounds (SbS)(1.15)(TiS2)(n) (n = 1,2) has been investigated using x-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) and x-ray absorption spectroscopy (XAS). These compounds are built of alternating modulated double-layers of SbS with the NaCl-type structure and sandwiches TiS2 (for n = 1) or paired sandwiches TiS2 (for n = 2) with 1T-TiS2 structure. The XPS spectra of the Sb 3d core levels show a splitting of 1.1 eV, indicating two states for Sb, corresponding to the two types of Sb atoms present in the crystal structure. The XPS of 2s and 2p of sulphur atoms are a superposition of the spectra of S in SbS and in TiS2. The valence band spectrum is mainly from the S 3p bands; a small peak just below the Fermi energy is attributed to electrons in the a 3d t(2g) conduction band. The Ti 2p x-ray absorption spectrum shows structure, due to multiplet and crystal field interactions. The electronic structures of the misfit-layer compounds can be considered to be a superposition of the electronic structures of the two subsystems SbS and TiS2, but with energy bands broadened due to the incommensurate modulation.

International audience; A large enhancement of the x-ray magnetic circular dichroism is observed at the iron K absorption preedge of magnetite. This is achieved by performing resonant inelastic x-ray scattering (RIXS) experiments with a 2p hole in the final state of the second-order optical process. We measured and calculated the full 1s2p RIXS planes for opposite helicities of the incoming circularly polarized x rays. The crystal field multiplet calculations show that the enhancement arises from 2p-3d Coulomb repulsions and 2p and 3d spin-orbit coupling. The observed magnitude of the RIXS magnetic circular dichroism effect is ∼16%. This opens up new opportunities for a broad range of research fields allowing for truly bulk-sensitive, element-, and site-selective measurements of 3d transition metal magnetic moments and their ordering using hard x-ray photons.

Molecular {Cr7Ni} rings have shown several ideal features for the observation of quantum phenomena and they appear suitable candidates for qubits encoding at low temperatures. We have exploited different functionalization pathways to graft molecular {Cr7Ni} rings onto a Au(111) surface from the liquid phase and here we report a comparative analysis of the results obtained by STM, XPS, XAS and XMCD experimental techniques.

Site-specific radiation damage and anisotropy of anomalous scattering can induce intensity differences in symmetry-related reflections. If the data are kept unmerged, these symmetry-breaking effects can become a source of phase information., The space-group symmetry of a crystal structure imposes a point-group symmetry on its diffraction pattern, giving rise to so-called symmetry-equivalent reflections. Instances in macromolecular crystallography are discussed in which the sym­metry in reciprocal space is broken, i.e. where symmetry-related reflections are no longer equivalent. Such a situation occurs when the sample suffers from site-specific radiation damage during the X-ray measurements. Another example of broken symmetry arises from the polarization anisotropy of anomalous scattering. In these cases, the genuine intensity differences between symmetry-related reflections can be exploited to yield phase information in the structure-solution process. In this approach, the usual separation of the data merging and phasing steps is abandoned. The data are kept unmerged down to the Harker construction, where the symmetry-breaking effects are explicitly modelled and refined and become a source of supplementary phase information.

This paper describes the structure of Er3+-doped SiO2-HfO2 waveguides containing nanocrystals of HfO2. Pure and 1 mol% Er3+-doped 70SiO(2)-30HfO(2) films were deposited by the sol-gel method on amorphous SiO2 substrates using the dip-coating technique. Each waveguide has experienced a single thermal treatment at temperatures ranging from 900 to 1200 degrees C, for either short (30 min) or long (24 h) durations. Crystallization and microstructure were studied by x-ray diffraction (XRD). The local environments of hafnium and erbium ions were determined, respectively, from Hf and Er L-3-edges extended x-ray absorption fine structure (EXAFS) experiments. Both XRD and EXAFS results demonstrate the substitution of Hf4+ by Er3+ ions in the crystalline structure. XRD shows the nucleation of tetragonal HfO2 nanocrystals after heat treatment at 1000 degrees C for 30 min in the pure waveguide, and at 900 degrees C for 24 h in the waveguide doped with Er3+. In both series, partial transformation from tetragonal to monoclinic HfO2 nanocrystals starts after heat treatment at 1100 degrees C for 24 h. The average crystallite size and size distribution can be controlled by thermal annealing temperature and duration, respectively, with brief treatment yielding a more homogeneous nanocrystal size.

Sulfur K-edge XANES has been measured for three sulfur model compounds, dibenzothiophene, dibenzothiophene sulfone, and aliphatic sulfur (DL-methionine). The spectra have been simulated with Density Functional Theory (DFT) by using a number of methods, including the half-core-hole approximation. Dipole transition elements were calculated and the transitions were convoluted with linearly increasing Gaussian functions in the first 20 eV of the near-edge region. In the case of dibenzothiophene, relaxation of the first excited states in the presence of the core-hole gave a further improvement. The theoretical results reproduce well the features of the spectra and give insight in the relation between geometric structure and molecular orbitals. Though DL-methionine and dibenzothiophene show a similar sharp rise of the white line, their molecular levels are quite different., pointing out the difficulties in finding useful "fingerprints" in the spectra for specific compounds.

Vanadium oxide (1 wt %) supported on gamma-Al(2)O(3) was used to investigate the interface between the catalytically active species and the support oxide. Raman, UV-vis-NIR DRS, ESR, XANES, and EXAFS were used to characterize the sample in great detail. All techniques showed that an isolated VO(4) species was present at the catalyst surface, which implies that no V-O-V moiety is present. Surprisingly, a Raman band was present at 900 cm(-1), which is commonly assigned to a V-O-V vibration. This observation contradicts the current literature assignment. To further elucidate on potential other Raman assignments, the exact molecular structure of the VO(4) entity (1 V=O bond of 1.58 A and 3 V-O bonds of 1.72 A) together with its position relative to the support O anions and Al cation of the Al(2)O(3) support has been investigated with EXAFS. In combination with a structural model of the alumina surface, the arrangement of the support atoms in the proximity of the VO(4) entity could be clarified, leading to a new molecular structure of the interface between VO(4) and Al(2)O(3). It was found that VO(4) is anchored to the support oxide surface, with only one V-O support bond instead of three, which is commonly accepted in the literature. The structural model suggested in this paper leaves three possible assignments for the 900 cm(-1) band: a V-O-Al vibration, a V-O-H vibration, and a V-(O-O) vibration. The pros and cons of these different options will be discussed.

A comparative study of Li1-xMn2-xO4 (0

Nanocrystalline LiFePO4 and doped LiTi0.01Fe0.99PO4 powders were synthesized via a sol-gel preparation route. High-resolution tunneling electron microscopy observation and energy dispersive spectroscopy, mapping show the homogeneous distribution of dopant Ti cations in the crystals. Fe and O K-edge X-ray absorption near-edge structure (XANES) measurements show that Ti4+ doping induces an increased unoccupied d-state in LiFePO4, resulting in an enhanced p-type semiconductivity. In situ Fe K-edge XANES measurements of Ti-doped and undoped LiFePO4 electrodes have been performed to determine the change of Fe valence during the lithium intercalation and de-intercalation processes. Both LiFePO4 and doped LiTi0.01Fe0.99PO4 cathodes demonstrate good electrochemical performance.

The "spin ladder" is a reduced-dimensional analogue of the high temperature superconductors that was predicted to exhibit both superconductivity and an electronic charge density wave or "hole crystal" (HC). Both phenomena have been observed in the doped spin ladder system Sr14-xCaxCu24O41 (SCCO), which at x=0 exhibits a HC which is commensurate at all temperatures. To investigate the effects of discommensuration we used resonant soft x-ray scattering (RSXS) to study SCCO as a function of doped hole density, d. The HC forms only with the commensurate wave vectors L_L = 1/5 and L_L = 1/3 and exhibits a simple temperature scaling T_(1/3) / T_(1/5) = 5/3. For irrational wave vectors the HC "melts", perhaps through the motion of topological defects carrying fractional charge., Comment: 4 pages, 4 figures

The near edge XAS spectra of the mononuclear copper complex [Cu(TMPA)(OH(2))](ClO(4))(2) (1) have been simulated using the multiple scattering edge simulation package MXAN (or Minuit XANes). These simulations, which employ the muffin-tin (MT) approximation, have been compared to simulations generated using the finite-difference method (FDM) to evaluate the effect of MT corrections. The sensitivity of the MXAN method was tested using structural models that included several different variations on the bond angles and bond distances for the first-shell atoms of 1. The sensitivity to small structural changes was also evaluated by comparing MXAN simulations of 1 and of structurally modified [Cu(TMPA)(L)](n)(+) complexes [where L = -O-(F(8)TPP)Fe(III), -F, -OPO(2)(O-p-nitrophenyl)Zn(II)(TMPA), and -NCMe] to the experimental data. The accuracy of the bond distances obtained from the MXAN simulations was then examined by comparison to the metrics of the crystal structures. The results show that MXAN can successfully extract geometric information from the edge structure of an XAS spectrum. The systematic application of MXAN to 1 indicates that this approach is sensitive to small structural changes in the molecule that are manifested in the XAS edge spectrum. These results represent the first step toward the application of this methodology to bioinorganic and biological systems.

The electronic properties of the isostructural series of heterotrinuclear thiophenolate-bridged complexes of the general formula [LFeMFeL](n+) With M = Cr, Co and Fe where L represents the trianionic form of the ligand 1,4,7-tris(4-tertbutyl-2-mercaptobenzyl)-1,4,7-triazacyclononane, synthesized and investigated by a number of experimental techniques in the previous work(1), are subjected now to a theoretical analysis. The low-lying electronic excitations in these compounds are described within a minimal model supported by experiment and quantum chemistry calculations. It was found indeed that various experimental data concerning the magnetism and electron delocalization in the lowest states of all seven compounds are completely reproduced within a model which includes the electron transfer between magnetic orbitals at different metal centers and the electron repulsion in these orbitals (the Hubbard model). Moreover, due to the trigonal symmetry of the complexes, only the electron transfer between nondegenerate orbital, a(1), originating from the t(2g) shell of each metal ion in a pseudo-octahedral coordination, is relevant for the lowest states. An essential feature resulting from quantum chemistry calculations, allowing to explain the unusual magnetic properties of these compounds, is the surprisingly large value and, especially, the negative sign of the electron transfer between terminal iron ions, beta'. According to their electronic properties the series of complexes can be divided as follows: (1) The complexes [LFeFeFeL](3+) and [LFeCrFeL](3+) show localized valences in the ground electronic configuration. The strong antiferromagnetic exchange interaction and the resulting spin 1/2 of the ground-state arise from large values of the transfer parameters. (2) In the complex [LFeCrFeL](+), due to a higher energy of the magnetic orbital on the central Cr ion than on the terminal Fe ones, the spin 3/2 and the single unpaired a(1) electron are almost localized at the chromium center in the ground state. (3) The complex [LFeCoFeL](3+) has one ground electronic configuration in which two unpaired electrons are localized at terminal iron ions. The ground-state spin S = 1 arises from a kinetic mechanism involving the electron transfer between terminal iron ions as one of the steps. Such a mechanism, leading to a strong ferromagnetic interaction between distant spins, apparently has not been discussed before. (4) The complex [LFeFeFeL](2+) is characterized by both spin and charge degrees of freedom in the ground manifold. The stabilization of the total spin zero or one of the itinerant electrons depends on beta', i.e., corresponds to the observed S = 1 for its negative sign. This behavior does not fit into the double exchange model. (5) In [LFeCrFeL](2+) the delocalization of two itinerant holes in a(1) orbitals takes place over the magnetic core of chromium ion. Although the origin of the ground-state spin S = 2 is the spin dependent delocalization, the spectrum of the low-lying electronic states is again not of a double exchange type. (6) Finally, the complex [LFeCoFeL](2+) has the ground configuration corresponding to the electron delocalization between terminal iron atoms. The estimated magnitude of the corresponding electron transfer is smaller than the relaxation energy of the nuclear distortions induced by the electron localization at one of the centers, leading to vibronic valence trapping observed in this compound. ispartof: Journal of the American Chemical Society vol:125 issue:41 pages:12615-12630 ispartof: location:United States status: published