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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.

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.

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.

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 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.

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.

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.