Valence State Of Iron Research Articles

Dehydration experiments on natural omphacites: qualitative and quantitative characterization by various spectroscopic methods

A series of natural omphacites from a wide range of P, T occurrences were investigated by electron microprobe (EMP), infrared (IR)-, Mössbauer (MS)- and optical spectroscopy in the UV/VIS spectral range (UV/VIS), secondary ion mass spectrometry (SIMS) and single crystal structure refinement by X-ray diffraction (XRD) to study the influence of hydrogen loss on valence state and site occupancies of iron. In accordance with literature data we found Fe2+ at M1 as well as at M2, and in a first approach assigned Fe3+ to M1, as indicated by MS and XRD results. Hydrogen content of three of our omphacite samples were measured by SIMS. In combination with IR spectroscopy we determined an absorption coefficient: ε i,tot = 65,000 ± 3,000 lmolH2O −1 cm−2. Using this new ε i,tot value, we obtained water concentrations ranging from 60 to 700 ppm H2O (by weight). Hydrogen loss was simulated by stepwise heating the most water rich samples in air up to 800°C. After heat treatment the samples were analyzed again by IR, MS, UV/VIS, and XRD. Depending on the type of the OH defect, the grade of dehydration with increasing temperature is significantly different. In samples relatively poor in Fe3+ (<0.1 Fe3+ pfu), hydrogen associated with vacancies at M2 (OH bands around 3,450 cm−1) starts to leave the structure at about 550°C and is completely gone at 780°C. Hydrogen associated with Al3+ at the tetrahedral site (OH bands around 3,525 cm−1, Koch-Müller et al., Am Mineral, 89:921–931, 2004) remains completely unaffected by heat treatment up to 700°C. But all hydrogen vanished at about 775°C. However, this is different for a more Fe3+-rich sample (0.2 Fe3+ pfu). Its IR spectrum is characterized by a very intense OH band at 3,515 cm−1 plus shoulder at 3,450 cm−1. We assign this intense high-energy band to vibrations of an OH dipole associated with Fe3+ at M1 and a vacancy either at M1 or M2. OH release during heating is positively correlated with decrease in Fe2+ and combined with increase in Fe3+. That dehydration is correlated with oxidation of Fe2+ is indirectly confirmed by annealing of one sample in a gas mixing furnace at 700°C under reducing conditions keeping almost constant OH− content and giving no indication of Fe2+-oxidation. Obtained data indicate that in samples with a relatively high concentration of Fe2+ at M2 and low-water concentrations, i.e., at a ratio of Fe2+ M2/H > 10 dehydration occurs by iron oxidation of Fe2+ exclusively at the M2 site following the reaction: \( {\left[ {{\text{Fe}}^{{{\text{2 + [ M2]}}}}{\text{OH}}^{ - } } \right]} = {\left[ {{\text{Fe}}^{{{\text{3 + [ M2]}}}} {\text{O}}^{{{\text{2}} - }} } \right]} + {\text{1/2}}\;{\text{H}}_{{\text{2}}} \uparrow . \) In samples having relatively low concentration of Fe2+ at M2 but high-water concentrations, i.e., ratio of Fe2+ M2/H < 5.0 dehydration occurs through oxidation of Fe2+ at M1.

FERRATE(VI) FOR WASTE WATER TREATMENT : OXIDATION OF CYANIDE IN AQUEOUS MEDIUM

The higher valence state of iron i.e., Fe(VI) was employed for the oxidation of one of an important toxic ion, cyanide in the aqueous medium. Cyanide was oxidized into cyanate, which is 1,000 times less toxic to cyanide and often accepted for its ultimate disposal. It was to be noted that Fe(VI) is a very powerful oxidizing agent and can oxidize most of the cyanide within few minutes i.e., ca 5 mins of contact. The data was obtained by the UV-Visible measurements for the Fe(VI) decomposition. The UV-Visible data was used to evaluate the overall rate constant for second order redox reaction between ferrate(VI) and cyanide. Also the pseudo first order rate constant was calculated as keeping the cyanide concentration in excess.

Pressure effect on the electronic structure of iron in (Mg,Fe)(Si,Al)O3 perovskite: a combined synchrotron Mössbauer and X-ray emission spectroscopy study up to 100 GPa

We investigated the valence state and spin state of iron in an Al-bearing ferromagnesian silicate perovskite sample with the composition (Mg0.88Fe0.09) (Si0.94Al0.10)O3 between 1 bar and 100 GPa and at 300 K, using diamond cells and synchrotron Moss- bauer spectroscopy techniques. At pressures below 12 GPa, our Mossbauer spectra can be sufficiently fitted by a ''two-doublet'' model, which assumes one ferrous Fe 2+ -like site and one ferric Fe 3+ -like site with distinct hyperfine parameters. The simplest interpre- tation that is consistent with both the Mossbauer data and previous X-ray emission data on the same sample is that the Fe 2+ -like site is high-spin Fe 2+ , and the Fe 3+ -like site is high-spin Fe 3+ . At 12 GPa and higher pressures, a ''three-doublet'' model is necessary and sufficient to fit the Mossbauer spectra. This model assumes two Fe 2+ -like sites and one Fe 3+ -like site distinguished by their hyperfine parameters. Between

Valence state and spin transitions of iron in Earth's mantle silicates

Using ab initio simulations, we investigated the valence and spin states of iron impurities in the perovskite ( Pv) and post-perovskite ( PPv) polymorphs of MgSiO 3. In agreement with the previous experimental work, we find a valence disproportionation reaction: 3Fe 2+ → 2Fe 3+ Fe 0 metal. This exothermic reaction results in the predominance of Fe 3+ impurities in lower mantle silicates and produces free metallic iron. It occurs both in Pv and PPv, Al-free and Al-rich, at all lower mantle pressures. This reaction provides a possible mechanism for the growth of the Earth's core and core-mantle chemical equilibration. In the presence of Al 3+, iron forms Fe 3+–Al 3+ coupled substitutions in Pv, but separate Fe 3+-Fe 3+ and Al 3+-Al 3+ substitutions in PPv. Only the high-spin state is found for Fe 2+ impurities at all mantle pressures, while Fe 3+ impurities on the Si-site are low-spin at all pressures in both phases. Fe 3+ impurities on the Mg-site are in the high-spin state in PPv at all mantle pressures, but in Pv we predict a high-spin–low-spin transition. The pressure at which this transition occurs strongly depends on the Al 3+ content and according to our calculations increases from 76 GPa for Al-free to 134 GPa for aluminous Pv; this reconciles many of the previous experimental results. Our findings have implications for the chemical evolution of the Earth and for the radiative conductivity and dynamics of the D” layer.

Valence and spin states of iron in (Mg,Fe)SiO3perovskite and post-perovskite at pressures of the Earth's lower mantle

Valence and spin states of iron in (Mg,Fe)SiO₃perovskite and post-perovskite at pressures of the Earth's lower mantle

Probing electronic and structural properties of the reduced [2Fe−2S] cluster by orientation-selective1H ENDOR spectroscopy: Adrenodoxin versus rieske iron-sulfur protein

The1H electron-nuclear double resonance (ENDOR) spectra in frozen buffer solutions of the reduced [2Fe−2S] clusters in adrenodoxin (Adx) and in the “Rieske” iron-sulfur protein (ISP) from the bovine mitochondrial bc1 complex were measured at low temperatures (5–20 K) and analyzed by spectra reconstruction. A single paramagnetic species with iron valence states (II) and (III) connected uniquely to the cluster irons was found in both proteins. For Adx, the experimental spectra from 23 field positions across the nearly axial (g max=2.0241,g int=1.9347, andg min=1.9331) electron paramagnetic resonance (EPR) spectrum were analyzed. Four larger hyperfine couplings were assigned to the cysteine β-protons near the Fe(III) ion. Transfer into the crystal structure showed that the Fe(III) ion was coordinated to the residues Cys55 and Cys92. The spin density was estimated as +1.60 for the Fe(III) and −0.6 for the Fe(II) ion, respectively. Theg-tensor direction with respect to the cluster showed strong similarities with the earlier assignment inArthospira platensis ferredoxin (Canne C., Ebelshauser M., Gay E., Shergill J.K., Cammack R., Kappl R., Huttermann J.: J. Biol. Inorg. Chem. 5, 514, 2000). An Adx mutant (T54A) exhibiting a change (70 mV) in redox potential showed no significant influence at the [2Fe−2S] cluster. The Rieske ISP was subjected to the same analysis. The ENDOR spectra from 35 field positions across the rhombic (g max=2.028,g int=1.891, andg min=1.757) EPR spectrum were simulated. Three major proton contributions were identified from the orientation behavior. Two were assigned to cysteine β-protons and one to a β-proton of His141. In contrast to Adx, the direction of theg max-component was found to lie roughly in the FeS-core plane and the largest proton coupling occurred alongg int. The spin population was estimated as about +1.6 for the oxidized and −0.55 for the reduced iron.

Thermal decomposition of iron(VI) oxides, K 2FeO 4 and BaFeO 4, in an inert atmosphere

The thermal decomposition of solid samples of iron(VI) oxides, K 2FeO 4·0.088 H 2O ( 1) and BaFeO 4·0.25H 2O ( 2) in inert atmosphere has been examined using simultaneous thermogravimetry and differential thermal analysis (TG/DTA), in combination with in situ analysis of the evolved gases by online coupled mass spectrometer (EGA–MS). The final decomposition products were characterized by 57Fe Mössbauer spectroscopy. Water molecules were released first, followed by a distinct decomposition step with endothermic DTA peak of 1 and 2 at 273 and 248 °C, respectively, corresponding to the evolution of molecular oxygen as confirmed by EGA–MS. The released amounts of O 2 were determined as 0.42 and 0.52 mol pro formula of 1 and 2, respectively. The decomposition product of K 2FeO 4 at 250 °C was determined as Fe(III) species in the form of KFeO 2. Formation of an amorphous mixture of superoxide, peroxide, and oxide of potassium may be other products of the thermal conversion of iron(VI) oxide 1 to account for less than expected released oxygen. The thermogravimetric and Mössbauer data suggest that barium iron perovskite with the intermediate valence state of iron (between III and IV) was the product of thermal decomposition of iron(VI) oxide 2.

Energy-loss near-edge fine structures of iron nanoparticles

Body centered cubic (bcc) Fe nanoparticles were fabricated by in situ decomposition of iron fluoride films in a transmission electron microscope. Electron energy-loss near edge structure (ELNES) was used to characterize this exposure process. In particular, the L 3/L 2 white-line intensity ratio (WLR) was used to monitor the iron valence state during exposure, and as an indicator of other properties of the iron nanoparticles. Iron nanoparticles with sizes between 2 and 20 nm exhibit a constant WLR, whose value is same as that for a continuous bcc iron film, suggesting little or no dependence of the local magnetic moment or structure on the particle size. A broad but prominent peak which occurs 40 eV after the L 3-ionization threshold in the iron fluoride, is absent for a metallic iron film but reappears when the iron is converted to an oxide. Long-range ferromagnetic coupling was observed in samples densely populated with iron nanoparticles. Because there is little interaction between particles and the supporting carbon substrate, these samples provide an ideal model system for studying the influence of particle size and interparticle distance on magnetic properties.

The division of iron's core and valence states under high pressures via first-principles calculation

We performed first-principles calculation for the body-centered cubic iron based on density-function theory, employing the pseudopotentionals and plane-wave method. We set the computational precision of the energy of one atom to 0.01 eV and make the spin-polarized total-energy calculation. The calculated results show that the 3p state should be treated as valence state when pressure is higher than 140 GPa, while the contribution to the total energy due to the dispersion of 3s state can be ignored for the whole range of earth's core condition.

A STEM/EELS method for mapping iron valence ratios in oxide minerals

The valence state of iron in minerals has useful applications in the geosciences for estimating redox conditions during mineral formation or re-equilibration. STEM/EELS techniques offer the advantage over other methods of being able to measure Fe valence with very high spatial resolution across mineral grains and grain boundaries. We have modified an EELS method for point analyses of iron valence ratios (Fe 3+/ΣFe) making it possible to generate line scans and maps of Fe valence. We demonstrate this method with measurements at an interface between iron-bearing oxides in a finely intergrown sample of magnetite and ilmenite. The STEM/EELS method is based on a calibrated relationship between Fe 3+/ΣFe and the relative intensities of the Fe L 3 and L 2 white lines in core energy-loss spectra for oxide and silicate minerals. Our method overcomes problems of energy drift in spectrum images by aligning energy-loss edges at a fixed energy position prior to background removal. An automated routine for batch processing of core loss spectra, including additional background removal and calculation of Fe L 3/L 2 intensity ratios, allows for rapid Fe 3+/ΣFe determinations of multiple point analyses or spectrum images and the preparation of Fe valence maps, with an analytical error of ±0.05 to ±0.09 in the Fe 3+/ΣFe measurements.

Structural and Transport Properties of La&lt;sub&gt;1-x&lt;/sub&gt;Sr&lt;sub&gt;x&lt;/sub&gt;Co&lt;sub&gt;1- y-z&lt;/sub&gt;Fe&lt;sub&gt;y&lt;/sub&gt;Ni&lt;sub&gt;z&lt;/sub&gt;O&lt;sub&gt;3-δ&lt;/sub&gt; Perovskites

The La1-xSrxCo0.2Fe0.6Ni0.2O3-d, La1-xSrxCo0.4Fe0.4Ni0.2O3-d and La1-xSrxCo0.2Fe0.4Ni0.4O3-d perovskites were synthesized with Sr contents 0.1 ≤ x ≤ 0.3 using precursors obtained by citric acid method and their structural and transport properties were measured. All obtained samples crystallize in hexagonal R-3c perovskite structure. The thermal stability was found to decrease with increasing Ni and Sr amounts, what leads to appearance of La2NiO4 like secondary phases with spinel structure and even NiO for higher Ni concentration. The DC electrical conductivity and thermoelectric power measurements at low temperatures (77 – 300K) suggest an appearance of variable hopping mechanism in this temperature range. The transport properties were found to improve with both Sr and Ni concentration increases. The moessbauer spectroscopy studies of La1-xSrxCo0.2Fe0.6Ni0.2O3-d samples revealed presence of two valence states of iron (Fe2+, Fe3+). The measured high temperature (870 – 1070K) DC electrical conductivity is relatively high for samples with high Sr and Ni amounts, what makes them good candidates for a possible application as cathode materials for Intermediate Temperature Solid Oxide Fuel Cells and points to further studies of perovskites from the (La, Sr)(Co, Fe, Ni)O3 system.

Oxidative stress caused by acute and chronic exposition to altitude

In this article, current views on cellular and molecular biology (biochemical) mechanisms are discussed under the aspect of altitude exposition. The Andean, Tibetan, and Ethiopian patterns of adaptation to high-altitude hypoxia are known [Beal et al. (2002) Proc Natl Acad Sci USA 99: 17215-17218]. The phylogenetic tree of the human species suggests that there are genetic differences in adaptation patterns to chronic hypoxic hypoxia. Five defense mechanisms are well established for lowlanders who are exposed to acute hypoxic hypoxia. Consequences of the cellular decrease in ATP are the formation of hypoxanthine and xanthine, which are the substrates for the massive formation of superoxide anion radicals and hydrogen peroxide via the oxidase activity of the xanthine oxidoreductase reaction. Under severe hypoxia, about 51 % of the total inhaled oxygen is used to form superoxide anion radicals in rat liver [Gerber et al. (1989) Adv Exp Med Biol 253B, Plenum Press, New York, 497-504]. The reactivity and selectivity of the superoxide anion radical are modified by specific interactions and electron exchange. It is commonly accepted that the superoxide anion radical in aqueous solutions has a lifetime in the millisecond range. However, electron spin resonance spectroscopy studies in a KO2/H2O/iron ion system revealed for the first time a stabilization of a part of the initially added superoxide anion radicals lasting up to hours at room temperature [Földes-Papp (1992) Gen Physiol Biophys 11: 3-38]. Superoxide anion radicals adsorbed on an oxidic iron hydrate phase in aqueous systems might function as a strong oxidant similar to that species which has been suggested to be a complex between oxygen and different valence states of iron in the initiation of lipid peroxidation by ferrous iron. There were serious doubts about the identity of alkoxy radicals. For the first time, alkoxy radicals were directly demonstrated in solution by electron spin resonance spectroscopy [Földes-Papp et al. (1991) Adv Synth Catal 333: 293-301]. The redox status in mammalian cells is mainly determined by the antioxidant glutathione, which is a key player in maintaining the intracellular redox equilibrium and in the metabolic regulation of the cellular defense against oxidative stress. As reactive oxygen species occupy an essential role in membrane damage, the idea of membrane-bound enzymatic defense mechanisms gets a new dimension [Földes-Papp et al. (1981) Acta Biol Med Ger 40: 1129-1132; Földes-Papp and Maretzki (1982) Acta Biol Med Ger 41: 1003-1008]. The steady-state between antioxidants and pro-oxidants affects the gene expression via hypoxia-induced transcription activities. The transcription factor hypoxia-inducible factor 1 (HIF-1) is a global regulator of oxygen homeostasis. As discussed in this article, hypoxia or 'oxidative stress' is accompanied by appropriate molecular adaptation mechanisms at the enzymatic or epigenetic level (enzymatic and non-enzymatic radical inhibitors, posttranslational modifications) and at the genetic level (transcription, translation).

Effect of oxygen deficiency on the cationic states of iron in a series of oxides with various perovskitelike structures

Compounds representing several series of complex oxides with perovskitelike structures have been studied by Mossbauer spectroscopy. Analysis of the quadrupole splitting-isomer shift diagrams constructed using the results of these measurements shows that increasing oxygen deficiency in the structures of the compounds studied is accompanied by variation of the coordination and valence state of iron.

XAFS studies on vitrified industrial waste

The oxidation state and local environment of Fe atoms in a series of vitrified lead-rich industrial wastes, as a function of the ash content, is studied using EXAFS and NEXAFS measurements at the Fe-K edge. The Fe-K EXAFS results reveal that the Fe–O distance and the coordination number in the 1st nearest neighbour shell decrease with increasing ash content in the vitrified product. Information on the iron's valence state and the role of Pb2+ ions in the glass matrix is drawn from the characteristics of the pre-edge features and the position of the absorption edge in the NEXAFS spectra.

A synchrotron Mössbauer spectroscopy study of (Mg,Fe)SiO3perovskite up to 120 GPa

The electronic environment of the Fe nuclei in two silicate perovskite samples, Fe_(0.05)Mg_(0.95)SiO_3 (Pv05) and Fe_(0.1)Mg_(0.9)SiO_3 (Pv10), have been measured to 120 GPa and 75 GPa, respectively, at room temperature using diamond anvil cells and synchrotron Mossbauer spectroscopy (SMS). Such investigations of extremely small and dilute ^(57)Fe-bearing samples have become possible through the development of SMS. Our results are explained in the framework of the “three-doublet” model, which assumes two Fe2+-like sites and one Fe^(3+)-like site that are well distinguishable by the hyperfine fields at the location of the Fe nuclei. At low pressures, Fe^(3+)/∑Fe is about 0.40 for both samples. Our results show that at pressures extending into the lowermost mantle the fraction of Fe^(3+) remains essentially unchanged, indicating that pressure alone does not alter the valence states of iron in (Mg,Fe)SiO_3 perovskite. The quadrupole splittings of all Fe sites first increase with increasing pressure, which suggests an increasingly distorted (noncubic) local iron environment. Above pressures of 40 GPa for Pv10 and 80 GPa for Pv05, the quadrupole splittings are relatively constant, suggesting an increasing resistance of the lattice against further distortion. Around 70 GPa, a change in the volume dependence of the isomer shift could be indicative of the endpoint of a continuous transition of Fe3+ from a high-spin to a low-spin state.

Structural and magnetic properties of the solid solution series Sr2Fe1–xMxReO6(M = Cr, Zn)

Strong correlations between the electronic, structural and magnetic properties have been found during the study of doped double perovskites Sr2Fe1−xMxReO6 (0 ≤ x ≤ 1, M = Zn, Cr). The interplay between the van Hove singularity and the Fermi level plays a crucial role for the magnetic properties. Cr doping of the parent compound Sr2FeReO6 leads to a non-monotonic behaviour of the saturation magnetization and an enhancement for doping levels up to 10%. The Curie temperatures monotonically increase from 401 to 616 K. In contrast, Zn doping leads to a continuous decrease in the saturation magnetization and the Curie temperatures. Superimposed on the electronic effects is the structural influence which can be explained by size effects modelled by the tolerance factor t. For Cr doping levels higher than 40% we observed a tetragonal–cubic transformation. For Zn doped samples the tetragonal distortion linearly increases with the increasing Zn content. The valence state of iron strongly influencing the structure was determined by Mossbauer spectroscopy to be +2.7.

Structural and magnetic properties of iron-nitride thin films deposited using a filtered cathodic vacuum arc

A filtered cathodic vacuum arc technique has been applied to synthesis iron-nitride thin films on silicon (100) substrate. Two approaches were carried out to introduce the nitrogen gas into the chamber to control the nitrogen content. Films with smooth surface containing α-Fe(N), α′-Fe(N), αʺ-Fe16N2, γ′-Fe4N, ζ-Fe2N, ɛ-Fe3N and γʺ-FeN phases have been obtained, respectively. The magnetic properties of the films show that small amount of nitrogen addition into the iron films increases the saturation magnetization, while excess nitrogen decreases the saturation magnetization. It is explained based on the bond-band-barrier correlation mechanism [C.Q. Sun, Prog. Mater. Sci. 48 (2003) 521] that the electronegative nitrogen changes the valence states of iron into Fe+ or Fe dipoles with higher magnetic momentum compared with an neutral Fe atom in the bulk.

Magnetism of iron-containing MCM-41 spheres

Iron species were loaded into the mesopores of MCM-41 spheres by incipient wetness impregnation procedure with Fe(ACAC) 3 as the precursor. The magnetism of the samples was studied by vibrating sample magnetometer (VSM), diffuse reflectance ultraviolet–visible (DRUV-vis) spectra and Mossbauer spectra. The results show that the magnetic behavior of the iron-loaded MCM-41 spheres depends on the content of iron loading, valence, and coordination state of iron ions, testing temperature and the atmosphere for the pyrolysis of the iron precursor.

The role of oxidative stress, impaired glycolysis and mitochondrial respiratory redox failure in the cytotoxic effects of 6-hydroxydopamine in vitro

The neurotoxin, 6-hydroxydopamine (6-OHDA) has been implicated in the neurodegenerative process of Parkinson's disease. The current study was designed to elucidate the toxicological effects of 6-OHDA on energy metabolism in neuroblastoma (N-2A) cells. The toxicity of 6-OHDA corresponds to the total collapse of anaerobic/aerobic cell function, unlike other mitochondrial toxins such as MPP+ that target specific loss of aerobic metabolism. The toxicity of 6-OHDA paralleled the loss of mitochondrial oxygen (O 2) consumption (MOC), glycolytic activity, ATP, H + ion gradients, membrane potential and accumulation of the autoxidative product, hydrogen peroxide (H 2O 2). Removing H 2O 2 with nonenzymatic stoichiometric scavengers, such as carboxylic acids, glutathione and catalase yielded partial protection. The rapid removal of H 2O 2 with pyruvate or catalase restored only anaerobic glycolysis, but did not reverse the loss of MOC, indicating mitochondrial impairment is independent of H 2O 2. The H 2O 2 generated by 6-OHDA contributed toward the loss of anaerobic glycolysis through lipid peroxidation and lactic acid dehydrogenase inhibition. The ability of 6-OHDA to maintain oxidized cytochrome c (CYT-C-OX) in its reduced form (CYT-C-RED), appears to play a role in mitohondrial impairment. The reduction of CYT-C by 6-OHDA, was extensive, occurred within minutes, preceded formation of H 2O 2 and was unaffected by catalase or superoxide dismutase. At similar concentrations, 6-OHDA readily altered the valence state of iron [Fe(III)] to Fe(II), which would also theoretically sustain CYT-C in its reduced form. In isolated mitochondria, 6-OHDA had negligible effects on complex I, inhibited complex II and interfered with complex III by maintaining the substrate, CYT-C in a reduced state. 6-OHDA caused a transient and potent surge in isolated cytochrome oxidase (complex IV) activity, with rapid recovery as a result of 6-OHDA recycling CYT-C-OX to CYT-C-RED. Typical mitochondrial toxins such as MPP+, azide and antimycin appeared to inhibit the catalytic activity of ETC enzymes. In contrast, 6-OHDA alters the redox of the cytochromes, resulting in loss of substrate availability and obstruction of oxidation–reduction events. Complete cytoprotection against 6-OHDA toxicity and restored MOC was achieved by combining catalase with CYT-C (horse heart). In summary, CYT-C reducing properties are unique to catecholamine neurotransmitters, and may play a significant role in selective vulnerability of dopaminergic neurons to mitochondrial insults.

The mixed-valence iron compound Na 0.1Fe 7(PO 4) 6: crystal structure and 57Fe Mössbauer spectroscopy between 80 and 295 K

The crystal structure of the synthetic iron phosphate Na 0.10(1)Fe 6.99(1)(P 1.00(1)O 4) 6 has been refined at 270 and 100 K from single-crystal X-ray diffraction data. The compound is triclinic, P−1, Z=1, lattice parameters: a=6.3944(9) Å, b=7.956(1) Å, c=9.364(1) Å, α=105.13(1)°, β=108.35(1)°, γ=101.64(1)° at 270 K and adopts the well-known howardevansite structure type. Iron, being both in the divalent and the trivalent valence state, is ordered on the four symmetry non-equivalent iron positions [Fe 2+ on Fe(1) and Fe(3), Fe 3+ on Fe(2) and Fe(4)]. Three of the four iron positions show octahedral oxygen atom coordination, the fourth one, which is occupied by Fe 2+, is five-fold coordinated. The structure consists of crankshafts (buckled chains) of edge sharing Fe-oxygen polyhedra, passing through the unit cell in [101] direction. Structural investigation at 100 K shows no change of symmetry. The valence state and distribution of iron was determined by 57Fe Mössbauer spectroscopy. The compound shows 4 subspectra in agreement with the four different Fe sites. The assignment of the Fe 2+ doublets to the Fe(1) and Fe(3) sites is trivial due to the 2:1 stoichiometry, also found in the Mössbauer spectra. For the Fe 3+ sites, the temperature-dependent variation of structural distortion parameters and the quadrupole splitting led to a clear doublet assignment.