Fe K-edge EXAFS Research Articles

Coke formation during high-temperature CO2 electrolysis over AFeO3 (A = La/Sr) cathode: Effect of A-site metal segregation

In this study, strontium-doped lanthanum ferrite perovskite oxides (LSF) with different A-site occupancies were used as cathode catalysts for CO2 electrolysis at 800 °C. XRD, EXAFS, XPS, FTIR, and temperature-programmed desorption were used to characterize the properties of the material. Fe K-edge EXAFS indicated that oxygen vacancy concentration and oxidation states of Fe atoms increased with concomitant improvement of electronic and ionic conductivities with a decrease in the A-site occupancy. XPS spectra revealed that Sr atoms segregated on the surface of A-site excess LSF92 to a greater degree than that on the A-site deficient LSF72. These segregated A-site metal oxides, especially SrO, are alkaline in nature and hence strongly bind to acidic CO2 forming stable SrCO3, which consequently facilitates carbon formation. This study highlights the effect of A-site stoichiometric modulation on carbon formation during electrolysis of CO2, which is crucial for improving the long- term performance of an SOEC.

Electronic characterization of redox (non)-innocent Fe2S2 reference systems: a multi K-edge X-ray spectroscopic study.

Di-iron dithiolate hydrogenase model complexes are promising systems for electrocatalytic production of dihydrogen and have therefore been spectroscopically and theoretically investigated in this study. The direct effect of ligand substitution on the redox activity of the complex is examined. In order to understand and eventually optimize such systems, we characterised both metal and ligand in detail, using element specific X-ray absorption Fe- and S-K edge XAS. The (electronic) structure of three different [Fe2S2] hydrogenase systems in their non-reduced state was investigated. The effect of one- and two-electron reduction on the (electronic) structure was subsequently investigated. The S K-edge XAS spectra proved to be sensitive to delocalization of the electron density into the aromatic ring. The earlier postulated charge and spin localization in these complexes could now be measured directly using XANES. Moreover, the electron density (from S K-edge XANES) could be directly correlated to the Fe–CO bond length (from Fe K-edge EXAFS), which are in turn both related to the reported catalytic activity of these complexes. The delocalization of the electron density into the conjugated π-system of the aromatic moieties lowers the basicity of the diiron core and since protonation occurs at the diiron (as a rate determining step), lowering the basicity decreases the extent of protonation and consequently the catalytic activity.

Preference of Co over Al for substitution of Fe in goethite (α-FeOOH) structure: Mechanism revealed from EXAFS, XPS, DFT and linear free energy correlation model

Goethite is the most stable iron hydroxide mineral in geological environments, and it is usually crystallized with more than one type of substituent. However, the synergetic or antagonistic effects of coexisting cations on substitution for lattice Fe in gothite are unclear. In this study, a series of Co-, Al-, and Co + Al co-substituted goethite samples were synthesized at room temperature and then investigated by powder XRD, X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure spectroscopy (XAFS), density function theory (DFT) calculations, and acid dissolution experiments. All the obtained samples were pure goethite, except the one with a high total content of Co + Al that contained a portion of 2-line ferrihydrite. Lattice parameter a of Co + Al co-substituted goethite was linearly decreased with the final Co content, lattice parameter b and calculated crystal density had the best negatively linear relationships with Al content while lattice parameter c, cell volume, average edge-sharing Fe − Me (Me = Fe, Al, and Co) distances along c⁎ and b⁎ axis (E′ and E) and average corner-sharing Fe − Me (DC) distances had the best linear correlations with the total contents of Co + Al. Aluminum and Co were found to be evenly distributed in goethite structure, and both reduced the mineral particle size. Co coexisted in a mixed valance of +2 and +3, and the proportion of Co3+ increased with the increase of Co content in Co-substituted goethite owing to relatively high electric potential energy and decreased with the increase of coexisting Al3+ content in Co + Al co-substituted goethites because of relatively low alkalinity in local environments around Co2+. Incorporation of Co greatly promoted the mineral dissolution in 2 M HCl solution at 298 K. Fe K-edge EXAFS analysis indicated almost constant local order around Fe in these samples. Coexisting Al had almost no effect on Co incorporation into goethite structure but Co suppressed Al substitution for lattice Fe. This preference of Co over Al to substitute for Fe was explained by the predicted partition coefficients (Kd) of common trivalent substituents in goethite by using the linear free energy relationship. It highlights the necessity to take into consideration not only the substituent physical properties (size and charge) but also their chemical properties (chemical bonding energy and chemical potential). These results enhanced our understanding of the incorporation of exotic cations into the iron oxide structures and the mutual effects of coexisting cations on substitution for lattice Fe.

Soil organo-mineral associations formed by co-precipitation of Fe, Si and Al in presence of organic ligands

Weathering of silicates supplies a range of cations (mainly Si, Al, Fe, but also Ca, Mg, Na, K, Mn) to the soil solution. There, cations can interact with charged functional groups of dissolved soil organic matter (OM). Unlike Al and Fe, Si does not directly bind to natural OM. However, the role of Si in the mechanisms of OM stabilization by coprecipitation with short range order mineral phases (SRO) may have been underestimated. The formation of coprecipitates was tested by titrating a biotite-weathering solution up to pH 5 in presence of 3,4-Dihydroxy-L-phenylalanine (DOPA) with initial (Fe + Al):C ratio ranging from 3 to 0.003. Size, crystallinity, chemical composition and the local structure of the coprecipitates were analyzed by TEM-EDX and Fe K-edge EXAFS. Coprecipitates are amorphous particles whatever the (Fe + Al):C ratio, but their size, composition and local structure were nevertheless seen to progressively vary with increasing C content. In low C samples (high (Fe + Al):C), coprecipitates were 2-40 nm in size and were dominated by Si (30-70%). Fe represented only 20-50% of the mineral phase and was structured in small oligomers of Fe octahedra. Around 20% of the Fe of the coprecipitates were bound to C. Conversely, in high C samples (low (Fe + Al):C), coprecipitates were 10-90 nm in size and Fe was the main component (45-70%). Fe was almost exclusively linked to OM by monomeric Fe-O-C bonds. Si (5-40%) and Al (15-35%) were able to form oligomers occluded in the Fe-OM network. In samples with intermediate C content ((Fe + Al):C = 0.3), the coprecipitates had 5-200 nm size particles. We suggest these coprecipitates are structured in a loose irregular 3D network of amorphous small oligmers of Fe (25-75%), Si (15-50%), and Al (10-35%), forming an amorphous and open-structured mineral skeleton. Within this mineral network, we suggest the organic compounds are linked either by bonds with Fe and Al to the skeleton, by monomeric Fe-O-C in the porosity of the network, or by weak bonds with other OM. This conceptual model provides an alternative to the standard view that SRO-OM is formed by ferrihydrite and amorphous Al(OH)(3). We suggest naming the structure Nanosized Coprecipitates of inorganic oLlgomers with organiCs with nanoCLlCs as acronym. The presence of Si in the inorganic structures may have an impact not only on the amount of OM stabilized by the nanoCLlCs, but in the longer term, on the persistence of the OM stabilization potential by metallic oligomers.

Structure and spectroscopic study of aqueous Fe(III)-As(V) complexes using UV–Vis, XAS and DFT-TDDFT

Aqueous complexes between ferric (Fe(III)) and arsenate (As(V)) are indispensable for understanding the mobility of arsenic (As) in Fe(III)-As(V)-rich systems. In this study, aqueous Fe(III)-As(V) complexes, FeH2AsO42+ and FeHAsO4+, were postulated based on the qualitative analysis of UV–Vis spectra in both Fe(III)-As(V)-HClO4 and Fe(III)-As(V)-H2SO4 systems. Subsequently, monodentate structures were evidenced by Fe K-edge EXAFS and modeled as [FeH2AsO4(H2O)5]2+ and [FeHAsO4(H2O)5]+ by DFT. The feature band at ∼280 nm was verified as electron excitation chiefly from Fe-As-bridged O atoms to d-orbital of Fe in [FeH2AsO4(H2O)5]2+ and [FeHAsO4(H2O)5]+. The structural and spectral information of Fe(III)-As(V) complexes will enable future speciation analysis in Fe(III)-As(V)-rich system.

A comparison of Fe(III) reduction rates between fresh and aged biogenic iron oxides (BIOS) by Shewanella putrefaciens CN32

This study compared the chemical and physical properties of freshwater biogenic iron oxides (BIOS) aged at 4°C for ~5years in the dark to simulate early diagenesis to previously published findings from fresh samples. In addition, the reactivity of aged BIOS in the presence of a well-characterized iron reducing bacterium, Shewanella putrefaciens CN32, was investigated. The composition and proportion of the iron minerals were examined using XRD, Fe k-edge EXAFS, and chemical extractions, and results were similar in both fresh and aged BIOS. Aged BIOS were dominated by 2-line ferrihydrite indicating that the intermixed bacterial cells and their exudates inhibited the transformation of 2-line ferrihydrite into crystalline phases. However, despite the similarity of mineralogy between aged and fresh BIOS, the rates of bacterial iron reduction in aged BIOS were found to be significantly lower than that of their fresh counterparts. Further characterization of aged BIOS by TEM revealed unique morphologies which were not previously observed in fresh BIOS. TEM also showed thickly coated bacterial exudates and presumably those formed as a result of continued accretion or aggregation of iron oxide nanoparticles. Aggregation of the iron oxide particles and subsequent decrease in solubility are likely responsible for the low rates of bacterial iron reduction in aged BIOS. Phase stability and aggregation in BIOS have important implications for the long-term reactivity of BIOS and the preservation of organic matter in the environment.

Retention Mechanisms of Citric Acid in Ternary Kaolinite-Fe(III)-Citrate Acid Systems Using Fe K-edge EXAFS and L3,2-edge XANES Spectroscopy.

Organic carbon (OC) stability in tropical soils is strongly interlinked with multivalent cation interaction and mineral association. Low molecular weight organic acids (LMWOAs) represent the readily biodegradable OC. Therefore, investigating retention mechanisms of LMWOAs in mineral-cation-LMWOAs systems is critical to understanding soil C cycling. Given the general acidic conditions and dominance of kaolinite in tropical soils, we investigated the retention mechanisms of citric acid (CA) in kaolinite-Fe(III)-CA systems with various Fe/CA molar ratios at pH ~3.5 using Fe K-edge EXAFS and L3,2-edge XANES techniques. With Fe/CA molar ratios >2, the formed ferrihydrite mainly contributed to CA retention through adsorption and/or coprecipitation. With Fe/CA molar ratios from 2 to 0.5, ternary complexation of CA to kaolinite via a five-coordinated Fe(III) bridge retained higher CA than ferrihydrite-induced adsorption and/or coprecipitation. With Fe/CA molar ratios ≤0.5, kaolinite-Fe(III)-citrate complexation preferentially occurred, but less CA was retained than via outer-sphere kaolinite-CA complexation. This study highlighted the significant impact of varied Fe/CA molar ratios on CA retention mechanisms in kaolinite-Fe(III)-CA systems under acidic conditions, and clearly showed the important contribution of Fe-bridged ternary complexation on CA retention. These findings will enhance our understanding of the dynamics of CA and other LMWOAs in tropical soils.

Local structures of Ca, Ti and Fe in meteorite fusion crusts

The local structures of meteorite fusion crusts were studied by Ca, Ti and Fe K-edge XANES and EXAFS spectroscopy. The surface of meteorites were melted and volatilized with extreme high temperature and large temperature gradient when meteorites were rushed into atmosphere. This study indicated that meteorite fusion crusts have unique local structures. The local structures of fusion crusts differ from tektites especially in intensity of the shoulder in the rising flank of the edge in Ca XANES spectra. It is consistent with chemical composition change by the volatilization of Si at fusion during atmospheric entry. The high estimated Fe3+/ (Fe2++Fe3+) ratio in meteorite fusion crusts indicates that meteorite fusion crusts are formed into atmospheric oxidation condition. The Ca-O distances in meteorite fusion crusts are 2.612.66 A and are extremely longer than in other natural glasses. The fusion crusts have unique local structure since they experienced extremely high temperature and short quenching time. The XAFS method is effective in distinction of meteorite fusion crusts and classification of natural glass.

Local structure and optical absorption characteristic investigation on Fe doped TiO2 nanoparticles**Supported by National Basic Research Program of China (2012CB825801), Science Fund for Creative Research Groups of NSFC (11321503), National Natural Science Foundation of China (11321503, 11179004) and Guangdong Natural Science Foundation (S2011040003985)

The local structures and optical absorption characteristics of Fe doped TiO2 nanoparticles synthesized by the sol-gel method were characterized by X-ray diffraction (XRD), X-ray absorption fine structure spectroscopy (XAFS) and ultraviolet-visible absorption spectroscopy (UV-Vis). XRD patterns show that all Fe-doped TiO2 samples have the characteristic anatase structure. Accurate Fe and Ti K-edge EXAFS analysis further reveal that all Fe atoms replace Ti atoms in the anatase lattice. The analysis of UV-Vis data shows a red shift to the visible range. According to the above results, we claim that substitutional Fe atoms lead to the formation of structural defects and new intermediate energy levels appear, narrowing the band gap and extending the optical absorption edge towards the visible region.

Catalytic defluorination of perfluorinated aromatics under oxidative conditions using N-bridged diiron phthalocyanine.

Carbon-fluorine bonds are the strongest single bonds in organic chemistry, making activation and cleavage usually associated with organometallic and reductive approaches particularly difficult. We describe here an efficient defluorination of poly- and perfluorinated aromatics under oxidative conditions catalyzed by the μ-nitrido diiron phthalocyanine complex [(Pc)Fe(III)(μ-N)Fe(IV)(Pc)] under mild conditions (hydrogen peroxide as the oxidant, near-ambient temperatures). The reaction proceeds via the formation of a high-valent diiron phthalocyanine radical cation complex with fluoride axial ligands, [(Pc)(F)Fe(IV)(μ-N)Fe(IV)(F)(Pc(+•))], which was isolated and characterized by UV-vis, EPR, (19)F NMR, Fe K-edge EXAFS, XANES, and Kβ X-ray emission spectroscopy, ESI-MS, and electrochemical techniques. A wide range of per- and polyfluorinated aromatics (21 examples), including C6F6, C6F5CF3, C6F5CN, and C6F5NO2, were defluorinated with high conversions and high turnover numbers. [(Pc)Fe(III)(μ-N)Fe(IV)(Pc)] immobilized on a carbon support showed increased catalytic activity in heterogeneous defluorination in water, providing up to 4825 C-F cleavages per catalyst molecule. The μ-nitrido diiron structure is essential for the oxidative defluorination. Intramolecular competitive reactions using C6F3Cl3 and C6F3H3 probes indicated preferential transformation of C-F bonds with respect to C-Cl and C-H bonds. On the basis of the available data, mechanistic issues of this unusual reactivity are discussed and a tentative mechanism of defluorination under oxidative conditions is proposed.

Selective behaviour of dilute Fe3 + ions in silicate glasses: an Fe K-edge EXAFS and XANES study

Iron K-edge EXAFS and XANES have been used to study the local structural environment of dilute quantities (0.2mol% Fe2O3) of Fe3+ in seven representative SiO2–R2O–R’O glasses where R=(Li, Na, K), R’=(Mg, Ca, Sr, Ba). Sample preparation was carried out by adding excess CeO2 as an oxidising agent and it was confirmed by optical absorption spectroscopy that iron was fully oxidised to Fe3+. This methodology has made possible, for the first time, an unambiguous EXAFS study of the coordination and local structural environment of dilute levels of Fe3+ in alkali–alkaline earth–silica glasses without the complications presented by the presence of Fe in multiple oxidation states. Coordination of Fe3+ in these glasses is strongly affected by the nature of the alkali and alkaline earth cations, with Fe3+–O CN increasing from 4.0±0.4 to 6.2±0.9 with decreasing alkali/alkaline earth ionic radius ratio (IRR). Respective FeO bond lengths of 1.87±0.01Å to 1.92±0.01Å were also obtained. Dilute Fe3+ in the studied glasses displays selective behaviour, as demonstrated by the opposing effects of alkali and alkaline earth ions on measured Fe3+ CN and Fe3+O bond length.

Redox interactions between Fe and cysteine: Spectroscopic studies and multiplet calculations

The biogeochemical cycle of Fe is intricately linked with that of organic matter. Cysteine represents an organic molecule with functionalities (O, S, N functional groups) and a C backbone that may mimic the functional groups present in organic matter from terrestrial and aquatic environments. In the present study we explore the redox speciation and coordination environment of Fe and the roles of the various ligand atoms of cysteine (C, N, S) in iron-organic redox coupling and transformations. The changes in oxidation state of Fe, C, N, and S in laboratory-synthesized Fe(II)–cysteine (synthesized from ferrous sulfate) and Fe(III)–cysteine (synthesized from ferric nitrate) complexes are monitored as a function of time using synchrotron X-ray absorption spectroscopy (Fe L2,3-edge XANES; C, N and S K-edge XANES; Fe K-edge EXAFS) and theoretical multiplet calculations using the program CTM4XAS (Charge Transfer Multiplet for X-ray Absorption Spectroscopy). CTM4XAS calculations show that 80% of the total Fe in both the Fe(II)–cysteine and the Fe(III)–cysteine complexes is present as Fe2+ initially (t=0), thus indicating preservation of Fe(II) in Fe(II)–cysteine and reduction of Fe(III) in Fe(III)–cysteine at initial conditions, the latter caused by an internal electron transfer reaction from S of –SH on the cysteine molecule. After 12months, however, ∼60% of the total Fe is present as Fe3+ in the Fe(II)–cysteine complex whereas ∼67% of the total Fe is present as Fe2+ in the Fe(III)–cysteine complex. The fact that a larger proportion of the Fe in the Fe(III)–cysteine complex remained reduced after 12months than that in the Fe(II)–cysteine complex suggests that the reduced Fe in Fe(III)–cysteine after 12months is further stabilized via preferential binding with the donor atoms of cysteine. Stabilization via preferential binding is supported by a coordination environment that changed from tetrahedral Fe2+ binding to S at a distance of 2.3Å at t=0 for both Fe(II,III)–cysteine complexes, to Fe3+ in an octahedral coordination with O/N atoms at a distance of 2.05Å (most prevalent in Fe(II)–cysteine) and Fe2+ in tetrahedral coordination with S/O/N atoms at an average distance of 2.15Å (most prevalent in Fe(III)–cysteine) at t=12months. Redox changes in the –NH2 and –SH groups of cysteine accompanied the Fe redox changes thus reflecting the true potential of cysteine as a redox ligand. Our studies of the Fe(II,III)–cysteine complexes add valuable information to the existing literature on the redox chemistry of Fe–cysteine systems by shedding light on the electron exchange pathways that may occur within the complexes and by providing a detailed depiction of the iron-ligand structure and coordination. The presence and persistence of Fe(II) or Fe(III) in complexes with soluble organics have implications for Fe biological availability and Fe mobility and transport in terrestrial as well as in aquatic environments.

Alpha particle damage in biotite characterized by microfocus X-ray diffraction and Fe K-edge X-ray absorption spectroscopy

AbstractCombined microfocus XAS and XRD analysis of α-particle radiation damage haloes around thorium-containing monazite in Fe-rich biotite reveals changes in both short- and long-range order. The total α-particles flux derived from the Th and U in the monazite over 1.8 Ga was 0.022 α particles per atomic component of the monazite and this caused increasing amounts of structural damage as the monazite emitter is approached. Short-range order disruption revealed by Fe K-edge EXAFS is manifest by a high variability in Fe–Fe bond lengths and a marked decrease in coordination number. XANES examination of the Fe K-edge shows a decrease in energy of the main absorption by up to 1 eV, revealing reduction of the Fe3+ components of the biotite by interaction with the 24He2+, the result of low and thermal energy electrons produced by the cascade of electron collisions. Changes in d spacings in the XRD patterns reveal the development of polycrystallinity and new domains of damaged biotite structure with evidence of displaced atoms due to ionization interactions and nuclear collisions. The damage in biotite is considered to have been facilitated by destruction of OH groups by radiolysis and the development of Frenkel pairs causing an increase in the trioctahedral layer distances and contraction within the trioctahedral layers. The large amount of radiation damage close to the monazite can be explained by examining the electronic stopping flux.

Preparation of mesoporous ferrisilicate with high content of framework iron by pH-modification method and its catalytic performance

The mesoporous ferrisilicates (MFS) with high iron content were synthesized by pH-modification method, and the iron content could be up to 10.5wt.% (Si/Fe=8). The pH was kept less than 2 at pre-hydrothermal synthesis step and was adjusted to 11 during hydrothermal step. The samples were characterized by XRD, HRTEM, N2-sorption, XRF, FTIR, DRUV–vis, Fe K-edge EXAFS, EPR, and DSC. The results suggested that the MFS materials were ordered 2D hexagonal mesophase of MCM-41, and the iron atoms were tetrahedral coordinated in the silica framework. This material could efficiently catalyze the hydroxylation of phenol in water medium using H2O2 as an oxidant, and the phenol conversion could be up to 52% under the optimal experimental conditions.

(TAML)FeIV═O Complex in Aqueous Solution: Synthesis and Spectroscopic and Computational Characterization

Recently, we reported the characterization of the S = (1)/ 2 complex [Fe (V)(O)B*] (-), where B* belongs to a family of tetraamido macrocyclic ligands (TAMLs) whose iron complexes activate peroxides for environmentally useful applications. The corresponding one-electron reduced species, [Fe (IV)(O)B*] (2-) ( 2), has now been prepared in >95% yield in aqueous solution at pH > 12 by oxidation of [Fe (III)(H 2O)B*] (-) ( 1), with tert-butyl hydroperoxide. At room temperature, the monomeric species 2 is in a reversible, pH-dependent equilibrium with dimeric species [B*Fe (IV)-O-Fe (IV)B*] (2-) ( 3), with a p K a near 10. In zero field, the Mössbauer spectrum of 2 exhibits a quadrupole doublet with Delta E Q = 3.95(3) mm/s and delta = -0.19(2) mm/s, parameters consistent with a S = 1 Fe (IV) state. Studies in applied magnetic fields yielded the zero-field splitting parameter D = 24(3) cm (-1) together with the magnetic hyperfine tensor A/ g nbeta n = (-27, -27, +2) T. Fe K-edge EXAFS analysis of 2 shows a scatterer at 1.69 (2) A, a distance consistent with a Fe (IV)O bond. DFT calculations for [Fe (IV)(O)B*] (2-) reproduce the experimental data quite well. Further significant improvement was achieved by introducing hydrogen bonding of the axial oxygen with two solvent-water molecules. It is shown, using DFT, that the (57)Fe hyperfine parameters of complex 2 give evidence for strong electron donation from B* to iron.

Intercalation of [Fe8(μ3-O)2(μ2-OH)12(tacn)6]8+ single molecule magnets in saponite clay

Abstract We have contacted Na-saponite with aqueous solutions containing a well-defined iron polycation with structuring ligands [Fe 8 ( μ 3 -O) 2 ( μ 2 -OH 2 ) 12 (tacn) 6 ] 8+ , which is known as a promising candidate for molecular magnets. When the resulting suspensions were activated with either microwaves or ultrasound, macroscopic-level characterization of the solid phases obtained after centrifugation (elemental analysis, surface area, XRD) suggested the intercalation of stoichiometrically intact polycations in the interlayer space by cation exchange. Furthermore, 57 Fe Mossbauer spectroscopy and Fe K-edge EXAFS are compatible with a conservation of the structure of the polycations inorganic core: connectivity seems to be maintained, while the polycations must be somewhat flattened by strain due to intercalation. Magnetization curves also appear compatible with a conservation of the polycation nuclearity, although experiments with a magnetically clean clay sample are necessary to confirm this point.

Synthesis and characterisation of transition metal substituted barium hollandite ceramics

ABSTRACTA range of transition metal bearing hollandite phases, formulated Ba1.2B1.2Ti6.8O16 (B2+ = Mg, Co, Ni, Zn, Mn) and Ba1.2B2.4Ti5.6O16 (B3+ = Al, Cr, Fe) were prepared using an alkoxide - nitrate route. X-ray powder diffraction demonstrated the synthesis of single phase materials for all compositions except B = Mn. The processing conditions required to produce > 95 % dense ceramics were determined for all compositions, except B = Mg for which the maximum density obtained was > 93 %. Analysis of transition metal K-edge XANES data confirmed the presence of the targeted transition metal oxidation state for all compositions except B = Mn, where the overall oxidation state was found to be Mn3+. The K-edge EXAFS data of Ba1.2B1.2Ti6.8O16 (B = Ni and Co) were successfully analysed using a crystallographic model of the hollandite structure, with six oxygen atoms present in the first co-ordination shell at a distance of ca. 2.02Å. Analysis of Fe K-edge EXAFS data of Ba1.2B2.4Ti5.4O16 revealed a reduced co-ordination shell of five oxygens at ca. 1.99Å.

EXAFS study of ion beam mixed Fe/Al multilayers

Composition and structure of Fe/Al multilayer coatings after ion beam mixing at different temperatures are studied. Samples were prepared by consecutive sputter deposition of Fe and Al on well-polished silicon substrate. The samples were ion beam mixed with 330 keV Ar ions at the dose of 3×10 15 ions/cm 2 in the temperature range between room temperature and 400 °C. AES depth profiles showed that mixing between Fe and Al layers started at 100 °C and was complete at 400 °C. Fe K-edge EXAFS, however, reveals that Fe bcc metallic phase is still present at all temperatures. The amount of an additional Fe–Al phase is about 20% up to 300 °C, increasing to 50% at 400 °C.

Hydrolysis of Iron(II) Chloride under Anoxic Conditions and Influence of SiO4Ligands

This study describes the evolution of the nanoscale aqueous ferrous species as a function of the hydrolysis ratio and the influence of SiO4 ligands. Samples were synthesized under anoxic conditions and studied by EXAFS at the Fe K-edge. At low hydrolysis ratio (R = [OH-]added/[Fe(II)]initial = 0.1), the data are consistent with the formation of a planar Fe cluster with five Fe(II) octahedra. Although Si neighbors cannot be detected unequivocally by Fe K-edge EXAFS, they affect the condensation of ferrous species by hindering the linkages between Fe(II) octahedra. Indeed, at low hydrolysis ratio (R = 0.1) and Si/Fe = 1, only small sized Fe clusters such as planar Fe(II) trimers are formed. Moreover, this study clearly indicates that the polymerization of Fe(II) is more affected by the presence of SiO4 than in the case of Fe(III).

Dynamics of wild-type HiPIPs: a Cys77Ser mutant and a partially unfolded HiPIP.

The temperature dependence of the mean square displacement of the (57)Fe nuclei due to motion faster than 100 ns are measured by temperature-dependent Mössbauer spectroscopy for oxidized and reduced HiPIPs from Ectothiorhodospira halophila, Chromatium vinosum WT and a Cys77Ser mutant. The behaviour is interpretable in the frame of the general model of protein dynamics distinguishing two temperature intervals. The character of harmonic and quasi-diffusional modes in HiPIPs is discussed. Dynamic information obtained from Mössbauer spectroscopy and Fe K-edge EXAFS are compared. Structure dynamics of the iron-sulfur cluster in the partially unfolded reduced HiPIP from C. vinosum was investigated by Mössbauer spectroscopy and EXAFS, indicating an intact metal centre and a protein backbone with a largely collapsed secondary structure. The role of the cofactor during protein folding is discussed. Differences in the dynamics between the native protein and the molten globule are found at physiological temperatures only. The structure and dynamic behaviour of the [Fe(4)S(4)]Cys(3)Ser cluster in the Cys77Ser mutant of the HiPIP from C. vinosum are analysed. The temperature dependence of electron relaxation in oxidized HiPIPs is investigated by Mössbauer spectroscopy and analysed theoretically, considering spin-spin and spin-lattice relaxation. The latter consists of contributions from direct phonon bottleneck and Orbach mechanisms. The data agree with former pulsed EPR results. Orbach relaxation is interpreted as due to transitions between electronic isomers of oxidized HiPIPs. With this interpretation, the energetic difference between both isomers equals the energy gap estimated from the temperature dependence of the Orbach relaxation.