Fe Valence State Research Articles

Investigating nanoscale mineral compositions: IronL3-edge spectroscopic evaluation of iron oxide and oxy-hydroxide coordination

The iron (Fe) L 2,3 -edge X-ray absorption near-edge structure (XANES) spectrum is sensitive to the local coordination environment around the Fe metal center, making it a useful probe for understanding Fe mineral speciation. The two dominant spectral peaks in the Fe L 3 -edge are parameterized according to the difference in the energy position (ΔeV), and the quotient (intensity ratio) of the two peaks’ maxima. Variations in the ΔeV value are strongly linked to factors that impact on the strength of the ligand field (e.g., Fe valence state, coordination number, and the nature of ligand bonding). The intensity ratio is affected by the strength of the ligand field and by the composition of the resultant molecular orbitals. The Fe valence state also strongly affects the intensity ratio, and an intensity ratio equal to one can be used to distinguish between Fe 2+ and Fe 3+ minerals. The effects of polyhedral distortion on the magnitudes of ΔeV and intensity ratio values were tested by considering the Fe oxide and -oxy-hydroxide mineral system, in which ligand effects are limited to the differences between the oxygen and hydroxyl ligands. In this system, the distribution of Fe oxide and -oxy-hydroxide minerals on a ΔeV vs. intensity ratio two-parameter plot could be explained by considering the Fe valence state, the ligand chemistry and the site symmetry of the Fe polyhedra. Furthermore, the ΔeV and intensity ratio values were found to be anti-correlated with respect to the various distortion measures considered in this study (e.g., polyhedral volume distortion percentage). This two-parameter plot is thus presented as a standard-less phase-specific identification tool for use in Fe speciation studies, applicable to both natural systems (e.g., aerosols, aquatic colloids) as well as to engineered systems (e.g., nanoparticle synthesis). A major advantage of this technique is that it is applicable to both crystalline and poorly crystalline phases, thus enhancing our ability to study amorphous and nano-crystalline phases that are typically difficult to characterize using X-ray diffraction techniques.

ChemInform Abstract: Tuning the Mn and Fe Valence States into New Ca0.7Mn2‐xFexO4 (0 < x ≤ 0.60) Solid Solution During Reversible Redox Processes.

AbstractCa0.7Mn2‐xFexO4 (x = 0—0.6) with Mn4+ amounts varying from 33% to 43% are prepared by an aqueous self‐combustion route involving stoichiometric mixtures of Ca, Mn, and Fe nitrates with powdered glycine in a 1:2 molar ratio (hot plate in stainless steel beaker, 220 °C) followed by firing the resultant ashes in air (850 °C, 12 h).

Tuning the Mn and Fe Valence States into New Ca0.7Mn2–xFexO4 (0 < x ≤ 0.60) Solid Solution during Reversible Redox Processes

New Mn4+-rich Ca0.7Mn2–xFexO4 complex oxides (0 < x ≤ 0.6) with Mn4+ amount varying from 33% to 43% have been prepared by aqueous self-combustion routes and characterized by powder X-ray Diffraction (XRD) and Mössbauer spectroscopy. These oxides crystallize with a monoclinic symmetry (CaMn3O6-type, space group P21/a) for low Fe content and with an orthorhombic symmetry (CaFe2O4-type, space group Pnma) for x > 0.14, thus stabilizing the highest Ca defects content (30%) for this series. A regular decrease of the Jahn–Teller Mn3+ site distortion is observed versus Fe content. For low iron content (x < 0.33), Fe3+ ions are preferentially but not exclusively located at the Mn4+-rich site which exhibits lower octahedral distortion, whereas for higher iron doping rates (x ≥ 0.33), Fe3+ ions are almost equally distributed in the two more or less distorted transition metal sites. The same distribution of Fe3+ in both of these atomic positions contributes to the stabilization within these phases of a high content of Mn4+ even at higher temperatures (T > 1000 °C), whereas the unsubstituted Ca0.66Mn2O4 phase is only stable up to 850 °C. Furthermore, these phases can be reduced at T = 550 °C under Ar/5% H2 into a Mn2+ and Fe2+/Fe3+ phase based on the rock salt structure and then reoxidized at T = 700 °C under air to get pure phases with the same orthorhombic unit cell and a Fe3+ distribution in the two atomic positions similar to the initial phase. Despite the additional stabilization of metal iron Fe0 during the reduction at T > 550 °C, a pure Mn4+/Mn3+/Fe3+-based oxide with CaFe2O4-type structure has been recovered after oxidation at T > 700 °C under air. Mössbauer spectroscopy of the reduced phase has revealed the presence of Fe2+ ions at two distinct octahedral sites corresponding to Fe3+ within the oxidized phase. The reversibility of this redox system could be explained by the occurrence of two rutile double chains in the CaFe2O4-type network, which is closely related to the rock salt structure with Ca2+ ions in the vicinity of Mn and Fe atoms. During the steps of oxygen release and storage, this network, derived from the rock salt framework, seems to preserve the Fe and Mn organization at the local scale.

Effect of water treatment and Fe doping on Pt sintering and the propane oxidation activity of Pt/Al2O3

The effect of water treatment and Fe doping on Pt sintering at high temperature was studied for 1wt%Pt/Al2O3 catalysts. After aging for 5h at 700 and 800°C, the water-treated catalysts had higher Pt dispersions than the corresponding untreated catalysts, indicating that Pt sintering was suppressed by water treatment. The degree of improvement depended on Fe content. X-ray diffraction and Pt LIII and Fe K edge X-ray absorption fine structure measurements indicated that Pt was not alloyed with Fe species; that metallic Pt particles and trivalent Fe species existed in all of the aged catalysts; and that both water treatment and Fe content did not affect the Pt and Fe valence states. Thus, the effective suppression of Pt sintering in catalysts with low Fe content (molar ratio of Fe/Pt≈1) at 700–800°C is probably due to iron oxides on the alumina surface functioning as pinning sites for Pt nanoparticles. In contrast, high Fe contents (molar ratio of Fe/Pt≳5) promoted Pt sintering, but lowered the propane oxidation temperature. This is probably due to the promotion of dissociative adsorption of the propane CH bond during the oxidation reaction.

Swift heavy ion irradiation induced effects in Fe/MgO/Fe/Co multilayer

Swift heavy ion irradiation induced effects on magnetic multilayer were investigated on Fe/MgO/Fe/Co multilayer stack grown on Si substrate using e-beam evaporation. This stack was irradiated perpendicularly by 120MeV Ag15+ with different fluences ranging from 1×1011 to 1×1013ions/cm2. X-ray diffraction and transmission electron microscopy infer atomic transportation during irradiation. Magnetic measurements carried out on pristine and irradiated stacks show significant changes in the shape of perpendicular magnetic hysteresis, which envisages the re-orientation of magnetic moment along the direction of ion path. X-ray absorption spectroscopic measurements show the onset of changes in the Fe valence state and O (2p)-metal (3d) hybridization after ion irradiation. The change of valence state predicts the oxide formation at interface, which causes the enhancement of in-plane squareness.

Covalency, hybridization and valence state effects in nano- and micro-sized ZnFe2O4

In the present work, Fe valence state, covalency effects, and metal–oxygen hybridization are discussed for ZnFe2O4 using X-ray absorption spectroscopy. A few sets of nano-sized and micro-sized zinc ferrite were synthesized using the nitrate method. Nanoparticles of ZnFe2O4 were synthesized by heating precursor at 300, 400, 500, 800, 1000, and 1200 °C for 1 h. To synthesize micro-sized ZnFe2O4, the obtained nanoparticles were annealed at 1200 °C for 12 h (bulk treatment). X-ray diffraction shows the presence of cubic spinel phase in nano-sized as well as micro-sized ZnFe2O4. Scanning electron microscopy measurements show that particle size ranges are 40–80 nm and 1–2 μm for nano-sized and micro-sized ZnFe2O4, respectively. Fe L-edge spectra of these materials envisage the presence of spectral features corresponding to t2g and eg symmetry states created due to Fe(2p3/2)-Fe(3d) and Fe(2p1/2)-Fe(3d) in octahedral crystal field. This reflects the presence of Fe3+ states in nano-sized and micro-sized ZnFe2O4. eg states dominate in micro-sized ZnFe2O4. O K-edge spectra for these materials can be distinguished by pre-edge and post-edge regions. Pre-edge and post-edge regions are associated with O(2p)–Fe(3d) and O(2p)–Fe(4s,4p) hybridized states. The extent of hybridization estimated from the intensity ratio of O(2p)–Fe(3d) and O(2p)–Fe(4s,4p) hybridized states is higher in nano-sized ZnFe2O4.

X-ray absorption study of the Fe and Mo valence states in Sr2FeMoO6

We studied the Fe and Mo valence states in Sr2FeMoO6 using X-ray absorption spectroscopy. The experimental results were analyzed using atomic multiplet plus crystal field calculations. The analysis indicates that the Fe ions present a fairly ionic Fe3+ (3d5) valence, and that the Mo ions are in a strongly covalent Mo5+ (4d1) state. The presence of Fe ions in a 2+ valence state can be excluded from the Fe-L2,3 spectrum. These results can be understood taking into account the relative energy of the d-levels, the relative strength of the M d–O p mixing, and the exchange stabilization of the Fe3+ ion.

Formation and transformations of Fe-rich serpentines by asteroidal aqueous alteration processes: A nanoscale study of the Murray chondrite

Fe-rich serpentines are an abundant product of the early aqueous alteration events that affected the parent bodies of CM carbonaceous chondrites. Alteration assemblages in these meteorites show a large chemical variability and although water–rock interactions occurred under anoxic conditions, serpentines contain high amounts of ferric iron. To date very few studies have documented Fe valence variations in alteration assemblages of carbonaceous chondrites, limiting the understanding of the oxidation mechanisms. Here, we report results from a nanoscale study of a calcium–aluminum-rich inclusion (CAI) from the Murray chondrite, in which alteration resulted in Fe import and Ca export by the fluid phase and in massive Fe-rich serpentines formation. We combined scanning and transmission electron microscopies and scanning transmission X-ray microscopy for characterizing the crystal chemistry of Fe-serpentines. We used reference minerals with known crystallographic orientations to quantify the Fe valence state in Fe-rich serpentines using X-ray absorption spectroscopy at the Fe L2,3-edges, yielding a robust methodology that would prove valuable for studying oxidation processes in other terrestrial or extra-terrestrial cases of serpentinization. We suggest that aqueous Fe2+ was transported to the initially Fe-depleted CAI, where local changes in pH conditions, and possibly mineral catalysis by spinel promoted the partial oxidation of Fe2+ into Fe3+ by water and the formation of Fe-rich serpentines close to the cronstedtite endmember. Such mechanisms produce H2, which opens interesting perspectives as hydrogen may have reacted with carbon species, or escaped and yield increasingly oxidizing conditions in the parent asteroid. From the results of this nanoscale study, we also propose transformations of the initial cronstedtite, destabilized by later input of Al- and Mg-rich solutions, leading to Fe2+ leaching from serpentines, as well as to random serpentine-chlorite interstratifications. Such transformations towards polysomatic assemblages that are un-equilibrated from the structural, chemical and redox point of views are probably controlled by the various rates of alteration of primary minerals, but also by porosity gradients, as in terrestrial hydrothermal systems. We suggest that the proposed mechanisms may have played a role in the early formation of (Fe2+,Fe3+)-rich serpentines documented in CM chondrites, as well as in their transformation with on-going alteration towards Fe-poorer compositions inferred from previous petrologic, mineralogical and magnetic studies of CM chondrites.

Investigating the firing protocol of Athenian pottery production: A Raman study of replicate and ancient sherds

The black and red coloration of ancient Athenian pottery is due to the different oxidation states of iron oxide: reduced Fe2+ (black) and oxidized Fe3+ (red). It has long been thought that a single, three‐step firing — oxidation–reduction–oxidation — was used to manipulate the Fe valence state and achieve different degrees of vitrification between the slip and the body to produce the high contrast black and red images. However, complex and layered structures, with slips of different color overlaying each other, are sometimes found, raising the question of the adequacy of a single three‐step firing to produce complex architectures. The firing conditions used in the production of an ancient vessel in the collection of the J. Paul Getty Museum, in which a red glossy layer is found between the black gloss and the body, were investigated. The ancient specimen was compared to a set of replicates produced and fired under a series of controlled temperatures and oxygen fugacities. Raman spectroscopy was used to identify the mineralogical phases present in both the ancient and the replicate samples, and the shape and position of the Eg mode in hematite provided a measure of the temperatures at which the minerals were formed. The results suggest the ancient vessel was produced using at least two separate firings: a high temperature firing under oxidizing conditions to create the underlying red glossy layer, followed by a three‐step firing to create the surface black gloss decoration. This finding suggests the firing of ancient Athenian pottery was more complex than previously thought. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Characterization of Bi2Se3:Fe epitaxial films grown by pulsed laser deposition

Iron doped bismuth selenide (Bi2Se3:Fe, 5–8at.%) films were deposited on Si (100) substrates using pulsed laser deposition at different substrate temperatures. The epitaxial films with high c-axis orientation were grown at 400°C and 450°C. X-ray photoelectron spectroscopy revealed the presence of oxidized surface layer and Fe–Se bonding with the Fe valence state of +3. The valence state was proved by Conversion Electron Mössbauer Spectroscopy. These results, along with quantitative element analysis confirmed the substitution of Bi by Fe ions. The Mössbauer spectroscopy showed paramagnetic behavior of the sample grown at 450°C, whereas the magnetization measurement revealed weak ferromagnetic signal (~103A/m), attributed with the oxidized layer. The growth at elevated temperatures resulted in a heavy oxidation of the samples.

Theoretical calculations of valence states in Fe-Mo compounds

The half-metallic ferromagnetic double perovskite compound Sr2FeMoO6 is considered as an important material for spintronic applications. It appears to be fundamental to understand the role of electronic parameters controlling the half-metallic ground state. Fe-Mo double perovskites usually present some degree of Fe/Mo disorder which generally increases with doping. In this work, we study the valence states of Fe-Mo cations in the off-stoichiometric system Sr2Fe1+xMo1−xO6 (−1 ≤ x ≤ 1/3) with disorder. Our results for Fe and Mo valence states are obtained using the Green functions and the renormalization perturbation expansion method. The model is based on a correlated electron picture with localized Fe-spins and conduction Mo-electrons interacting with the local spins via a double-exchange-type mechanism.

Charge defects and highly enhanced multiferroic properties in Mn and Cu co-doped BiFeO3 thin films

Pure BiFeO3 (BFO) and Mn, Cu co-doped BiFeO3 (BFMCO) thin films were deposited on fluorine doped tin oxide (FTO) substrates by a chemical solution deposition method. Detailed investigations were made on the effects of Mn and Cu co-doping on the crystal structure, the defect chemistry, multiferroic properties of the BFO thin films. With the co-doping of Mn and Cu, a structural transition from the rhombohedral (R3c:H) to the biphasic structure (R3c:H + P1) is confirmed by XRD, Rietveld refinement and Raman analysis. X-ray photoelectron spectroscopy (XPS) analysis shows that the coexistence of Fe2+/Fe3+ and Mn2+/Mn3+ ions in the co-doping films are demonstrated. Meanwhile, the way of the co-doping at B-sits is conducive to suppress Fe valence state of volatility and to decrease oxygen vacancies and leakage current. It's worth noting that the co-doping can induce the superior ferroelectric properties (a huge remanent polarization, 2Pr ∼ 220 μC/cm2 and a relatively low coercive field, 2Ec ∼ 614 kV/cm). The introduction of Mn2+ and Cu2+ ions optimizes the magnetic properties of BFO thin films by the biphasic structure and the destruction of spin cycloid.

Influence of Mn Doping on the Magnetic Properties of CoFe2O4

A series of CoFe2−x Mn x O4 ceramics were synthesized successfully by conventional solid state reaction. The X-ray diffraction analysis proved that all samples were found to have a cubic spinel structure. Diffractograms were used for Rietveld refinement to determine lattice parameters, of which lattice parameter increased with increasing Mn concentration. The microstructure of the samples was studied using scanning electron microscopy. The vibrating sample magnetometer measurements showed that the highest saturated magnetization of 118.11 emu/g and coercivity of 46.89 Oe were observed in CoFe1.85Mn0.15O4 ceramic. Furthermore, X-ray absorption spectra of the samples were recorded to determine the Co, Fe and Mn valence states and their preferentially sites of the spinel structure.

Investigation of a Canadian Ilmenite as an Oxygen Carrier for Chemical Looping Combustion

The use of chemical looping combustion (CLC) for power generation is an advanced energy technology that can capture CO2 inherently, which could prove to be the next electricity generation technology in a carbon-constrained future. For commercial-scale application of the CLC process, the availability of cost-effective oxygen carriers (OCs) with stable performance is imperative. Given the composition of ilmenite, there may be potential for its application as a cost-effective alternative OC for CLC. In this study, the performance of a Canadian ilmenite was investigated and showed promising results. Complete reduction of ilmenite takes a relatively long time; therefore, it is not practical for ilmenite to reach its maximum oxygen transport capacity by counter-balance of capital investment in the fuel reactor. Reduction in the first 30 min was selected as the effective reaction for the application of CLC, which gave an oxygen transport capacity of about 5.52%. To understand the phase and Fe valence state (Fe3+...

Realization of high thermoelectric performance in p-type unfilled ternary skutterudites FeSb2+xTe1−x via band structure modification and significant point defect scattering

FeSb2Te, a ternary derivative of binary CoSb3, displays anomalous electrical and thermal transport properties because of considerable modifications in the band structure induced by Fe and significant mixed valence state (namely Fe2+ and Fe3+) scattering of phonons. The substitution of Te for Sb generates more holes without notably affecting the band structure, while markedly improving the electrical conductivity and retaining a high Seebeck coefficient due to the enhanced density of states, thereby leading to dramatically increased power factors. Furthermore, the heat carrying phonons are strongly scattered with increasing x value because of the formation of solid solutions between two end members: □FeSb2Te and □FeSb3 (where □ can be viewed as a vacancy). Consequently, high thermoelectric figures of merit were achieved in the FeSb2+xTe1−x compounds, with the largest ZT value reaching ∼0.65 for the sample with x=0.2. This is the highest value among all p-type unfilled skutterudites and is comparable with some filled compositions. Prospects for further improving the performance of p-type FeSb2Te-based skutterudites are discussed.

Iron valence state of fine-grained material from the Jupiter family comet 81P/Wild 2 – A coordinated TEM/STEM EDS/STXM study

The oxidation state of transition metal elements is an indicator of the environmental conditions during formation and history of extraterrestrial materials. We studied the iron valence state of fine-grained material from a bulbous track extracted from the Stardust cometary collector. It likely originated from primitive material of the comet Wild 2. We used synchrotron-based Scanning Transmission X-ray Microscopy (STXM) to collect Fe L3-XANES spectra at a spatial resolution of about 20nm. Maps of Fe valence state were combined with the elemental maps recorded by energy dispersive X-ray spectroscopy (EDS) with a transmission electron microscope (TEM), on the same areas and with a comparable electron probe size (5–20nm). As for most Stardust fine-grained material, the samples are severely damaged by the hypervelocity impact in the aerogel collector blocks. They show of a wide range of oxidation state at a micrometer scale, from Fe metal to Fe3+. This heterogeneity of oxidation state can be due to the extreme conditions of the collection. Two major parameters can favor changes in redox state. The first is the high temperature regime, known to be highly heterogeneous and to have locally reached extreme values (up to 2000K). The second is the local chemical environment. It may contain elements that could favor a reduction or oxidation reaction within the flash-heated Wild 2 fragments. Comparison of maps by STXM and EDS shows evidence for several correlation trends between element concentrations and the iron valence state. These observations, together with the study of a melted rim of a larger particle, suggest that the redox state was not completely redistributed within the impact melts. These local signatures are compatible with precursors that could have been close to primitive matrix material of chondrites or to chondritic interplanetary dust particles. On average, the fine-grained material from Wild 2 displays a molar fraction (Fe2+oxide+Fe3+oxide)/(total Fe) equal to 0.80±0.10. It appears more oxidized than the average value measured for the comet, when done on larger particles (Westphal et al., 2009). This fine-grained material from Wild 2 does not seem to have sampled reducing environments in the solar nebulae in contrast with the larger particles of Wild 2. This observation confirms the high degree of diversity of materials in Wild 2 and is in good agreement with the dual distribution of high temperature minerals and matrices in carbonaceous chondrites.

Nano-gold catalysts on Fe-modified ceria for pure hydrogen production via WGS and PROX: Effect of preparation method and Fe-doping on the structural and catalytic properties

Two series of Fe-doped (5, 10 and 20wt% Fe2O3) ceria oxides were synthesized by mechanochemical mixing (MM) and impregnation (IM) methods. The Au catalysts (3wt%) supported on these mixed oxides were tested in WGSR and PROX. Gold catalysts on supports prepared by IM exhibited WGS activity lower than that of gold on undoped ceria. Significantly better WGS performance was demonstrated over gold catalysts on supports synthesized by MM. The observed differences in the catalytic behavior were explained by the crucial role of the gold dispersion depending on the preparation method. The highest WGS activity of Au5FeCeMM catalyst was attributed to the cooperative catalytic work of Fe-modified ceria and magnetite with the highest extent of non-stoichiometricity. In contrast to WGSR, the catalytic behavior in PROX over gold catalysts on supports prepared by both methods did not differ significantly. Fe likely acted as a ceria modifier because no changes in Fe-phases and Fe valence state were observed upon reaction. The nanosized hematite particles covering the ceria grains in the case of the IM method could diminish the ceria basicity explaining the observed good and stable PROX performance over Au10FeCeIM catalyst in the presence of CO2 and water.

Purplish-red almandine garnets with alexandrite-like effect: causes of colors and color-enhancing treatments

Fine gem-quality, purplish-red garnets from the Tocantins State, Brazil, were investigated for their crystal chemistry and optical properties by several spectroscopic techniques, including electron microprobe analysis, Mossbauer, Raman spectroscopy and optical absorption. Although most garnets are purplish-red, some specimens show color zoning, with deep red color in the core and purple in the outer parts. Electron microprobe analysis showed that these garnets are principally almandine–pyrope solid solution at the rim. However, at the red core, they contain also up to 7 % of spessartine. Mossbauer spectroscopy reveals that the iron content is predominantly Fe2+ (>99 %) in the natural garnets. The optical absorption spectra are dominated by spin-allowed and unusual high-intense spin-forbidden transitions from eightfold coordinated Fe(II) in the near infrared and visible spectral region, respectively. For the red core, in addition, three sharp bands centered in the blue part of the visible spectral range and a broad charge transfer band in the near-UV region are observed. All garnets with purplish colors show also a remarkable color-changing effect from purple in daylight light to red in incandescent light called alexandrite-like effect. Heat treatments in the 700–900 °C temperature range in oxidizing and reducing atmospheres lead to reversible and irreversible color changes which are discussed based on the microscopic changes in the Fe ion coordination and valence states.

Effect of simultaneous chemical substitution of A and B sites on the electronic structure of BiFeO3 films grown on BaTiO3/SiO2/Si substrate

Electrical properties and electronic structure of Bi1−xCaxFe1−yMnyO3−δ grown by pulsed-laser deposition on BaTiO3/SiO2/Si substrate were investigated. Results showed that Ca has drastic effect on symmetry of crystal and electrical poperties of BiFeO3. On the other hand, Mn revealed to have more radical effect on optical properties and energy gap of the compound. XPS results represented that although Ca tend to decrease Fe valence state, Mn tends to stabilize it at 3+ (at least in this concentrations). UV–visible study yielded bandgap of 2.51–2.81 eV (at 300 K) for different concentrations of Ca and Mn. UV–visible spectra also revealed sub-bandgap defect transitions at 2.2 and 2.4 eV. Slave-particle approach has also been applied to elucidate nature of the metal–insulator transition.

Detection of Fe2+ valence states in Fe doped SrTiO3 epitaxial thin films grown by pulsed laser deposition

We present an X-ray absorption spectroscopy study on Fe-doped SrTiO3 thin films grown by pulsed laser deposition. The Fe L2,3 edge spectra are recorded for doping concentrations from 0-5% after several annealing steps at moderate temperatures. The Fe valence state is determined by comparison with an ilmenite reference sample and calculations according to the charge transfer multiplet model. We found clear evidence of Fe(2+) and Fe(3+) oxidation states independently of the doping concentration. The Fe(2+) signal is enhanced at the surface and increases after annealing. The Fe(2+) configuration is in contrast to the mixed Fe(3+)/Fe(4+) valence state in bulk material and must be explained by the specific defect structure of the thin films due to the kinetically limited growth which induces a high concentration of oxygen vacancies.