High-spin Fe3 Research Articles

Mössbauer study of atmospheric aerosols of Shanghai.

Three iron-containing atmospheric aerosol samples were collected, one at a tunnel, one near a viaduct, and one at a park in Shanghai City. These samples were studied by Mössbauer spectroscopy. The experimental results show that ultrafine particles alpha-Fe2O3 (about 13 nm) are the main component of the iron-containing atmospheric aerosols in the tunnel, and high-spin Fe3+ in sulfate containing a small amount of alpha-FeOOH is the main component of iron-containing particles in the aerosols from the other two places. The concentrations of iron in these locations, compared with those of some foreign cities, are much higher. To explore the chemical compositions of iron compounds in aerosols, SPM (scanning proton microprobe) has also been used. From these results, a possible chemical process by which the iron oxides converted to iron sulfate in air was discussed.

Temperature dependence of the crystal and magnetic structures of the antiferromagnetic oxides Pb4Fe3O8X, X = Cl and Br

The crystal and magnetic structures of the antiferromagnetic mineral hematophanite, Pb4Fe3O8Cl, and its bromide analogue Pb4Fe3O8Br have been studied over the temperature range 10–650 K using neutron powder diffraction. The materials consist of truncated Pb4Fe3O8 triple perovskite blocks separated by a CsCl-type Pb2X (X = Cl or Br) layer. The basal oxygen of the central FeO6 octahedra exhibits disorder consistent with alternate clockwise/anticlockwise rotations around the c-axis; the degree of rotation increasing upon cooling for both materials, i.e. from 10.3° at 650 K to 12.8° at 10 K for Pb4Fe3O8Br. The order of the Fe moments evolving as a function of temperature has allowed the TNéel (TN) of Pb4Fe3O8Br to be determined for the first time as 600(5) K, a value within experimental error of the TN for Pb4Fe3O8Cl. The moments are coupled antiferromagnetically along all three crystallographic directions, resulting in a magnetic structure related to that of the nuclear structure by amag = √2anuc and cmag = 2cnuc. The magnetic structure is refined with spins perpendicular to the c-axis, giving ordered moments of 3.94(3) μB for Pb4Fe3O8Cl and 4.10(3) μB for Pb4Fe3O8Br at 10 K consistent with the presence of high spin Fe3+.

Evidence for Antisymmetric Exchange in Cuboidal [3Fe−4S]+Clusters

Iron−sulfur clusters with [3Fe−4S] cores are widely distributed in biological systems. In the oxidized state, designated [3Fe−4S]+, these electron-transfer agents have an electronic ground state with S = 1/2, and they exhibit EPR signals centered at g = 2.01. It has been established by Mössbauer spectroscopy that the three iron sites of the cluster are high-spin Fe3+, and the general properties of the S = 1/2 ground state have been described with the exchange Hamiltonian Hexch = J12S1·S2 + J23S2·S3 + J13S1·S3. Some [3Fe−4S]+ clusters (type 1) have their g-values confined to the range between g = 2.03 and 2.00 while others (type 2) exhibit a continuous distribution of g-values down to g ≈ 1.85. Despite considerable efforts in various laboratories no model has emerged that explains the g-values of type 2 clusters. The 4.2 K spectra of all [3Fe−4S]+ clusters have broad features which have been simulated in the past by using 57Fe magnetic hyperfine tensors with anisotropies that are unusually large for high-spin ferric sites. It is proposed here that antisymmetric exchange, HAS = d·(S1 × S2 + S2 × S3 + S3 × S1), is the cause of the g-value shifts in type 2 clusters. We have been able to fit the EPR and Mössbauer spectra of the 3Fe clusters of beef heart aconitase and Desulfovibrio gigas ferredoxin II by using antisymmetric exchange in combination with distributed exchange coupling constants J12, J13, and J23 (J-strain). While antisymmetric exchange is negligible for aconitase (which has a type 1 cluster), fits of the ferredoxin II spectra require |d| ≈ 0.4 cm-1. Our studies show that the data of both proteins can be fit using the same isotropic 57Fe magnetic hyperfine coupling constant for the three cluster sites, namely a = −18.0 MHz for aconitase and a = −18.5 MHz for the D. gigas ferredoxin. The effects of antisymmetric exchange and J-strain on the Mössbauer and EPR spectra are discussed.

Metallic and nonmetallic double perovskites: A case study ofA2FeReO6(A=Ca, Sr, Ba)

We have investigated the structure and electronic properties of ferrimagnetic double perovskites, A2FeReO6 (A= Ca, Sr, Ba). The A=Ba phase is cubic (Fm3m) and metallic, while the A=Ca phase is monoclinic (P21/n) and nonmetallic. 57Fe Mossbauer spectroscopy shows that iron is present mainly in the high-spin (S=5/2) Fe3+ state in the Ca compound, while it occurs in an intermediate state between high-spin Fe2+ and Fe3+ in the Ba compound. It is argued that a direct Re t2g - Re t2g interaction is the main cause for the metallic character of the Ba compound; the high covalency of Ca-O bonds and the monoclinic distortion (which lifts the degeneracy of t2g states) seem to disrupt the Re-Re interaction in the case of the Ca compound, making it non-metallic for the same electron count.

Magnetoresistance effect in the fluctuating-valence BaSmFe2O5+w system

The occurrence of negative magnetoresistance (MR) in semiconductive BaSmFe2O5+w double-perovskite samples is demonstrated. A peak in the MR value was observed at the Verwey–type transition temperature. The transition signifies the charge separation of the Fe2.5+ fluctuating mixed valence state into high-spin Fe2+ and Fe3+. The samples were ferrimagnetic with a Curie temperature of ∼710 K. Upon oxidizing/reducing the samples the size of the MR peak and the temperature at which the peak occurred varied. The largest MR value observed was 1.4% at 7 T.

Evidence for valence fluctuation of Fe in Sr2FeMoO6−w double perovskite

In this letter evidence for the formation of a valence-fluctuation state of iron, formally denoted as Fe2.5+, is presented. The system under study is the Sr2FeMoO6−w double perovskite, known for exhibiting a very large magnetoresistance. Samples of Sr2FeMoO6−w were synthesized by means of an encapsulation technique utilizing an Fe getter technique and characterized by Fe57 Mössbauer spectroscopy. From 5 K to room temperature the Mössbauer spectrum is dominated by a component with hyperfine parameter values between those expected for high-spin Fe3+ and high-spin Fe2+.

Structural studies of KNbO3 in niobate glass-ceramics

Potassium niobophosphate glasses and glass-ceramics of the family [xNb2O5·(50−x)P2O5·50K2O]:yFe2O3 were studied by X-ray powder diffraction, Mössbauer, infrared and Raman scattering spectroscopy. For the potassium–phosphate samples [50P2O5–50K2O], the iron oxide presents a network forming behavior for 5mol% of Fe2O3 doping. From our Mössbauer analysis, high spin Fe2+ and Fe3+ in a distorted octahedral coordination are present in all samples. One can conclude from the Mössbauer parameters that iron is present in a very broad site distribution, which is characteristic of an amorphous structural neighborhood. The precipitation of crystalline K4Nb6O17 and the ferroelectric KNbO3 crystals was confirmed by X-ray powder diffraction in the samples with x=40 and 50mol%, respectively. The infrared and Raman scattering spectroscopy results suggest that the increase of the ratio Nb2O5/P2O5 leads the niobium to sites of octahedral symmetry and consequently to the formation of ferroelectric KNbO3, as seen by X-ray diffraction analysis.

Local interactions of 57Fe after electron capture of 57Co implanted in -Al2O3 and in -Fe2O3

-Al2O3 and -Fe2O3 were implanted with 57Co at room temperature. -Al2O3 showed complex Mössbauer spectra indicating paramagnetic hyperfine splitting for high-spin Fe3+ and electric quadrupole-split spectra for Fe2+ ions. In canted antiferromagnet -Fe2O3, the implanted Co atoms were positioned substitutionally in Fe2O3 and at higher doses in Fe3O4 and in Fe1-xO. In the different phases, the ions were in Fe2+ and/or in Fe3+ states after the electron capture of 57Co; no after-effects were observed. The results suggest that the phases are formed during fast cooling and crystallization of the oxide from the high-temperature state of the thermal spike.

Reactivity of pyrrhotite (Fe9S10) surfaces: Spectroscopic studies

Synthetic hexagonal pyrrhotite (Fe9S10) etched in hydrochloric acid solution and then dried in air has been studied using ex situ XPS, X-ray fluorescence, Mossbauer, solid-state NMR and EPR spectroscopies. The metal-deficient non-equilibrium, up to several micrometres thick, layer (NL) formed on pyrrhotite under non-oxidative conditions has been found to be composed predominantly of low-spin Fe2+, nearly equal quantities of di- and polysulfide sulphur (probably, chains of 3–5 atoms) and no or low oxygen. When pyrrhotite with the NL is kept in air, singlet ferrous iron converts into high-spin Fe2+ and Fe3+, oxygen is incorporated into the layer and the surface enrichment in sulfur over iron decreases. A Mossbauer signal with an isomer shift of 0.36 mm s−1 and negligible quadruple splitting has been detected for the etched sample, desiccation in air gives rise to a quadruple split of up to 0.65 mm s−1 and a minor decrease in the isomer shift. The application of variable X-ray tube accelerating voltage has made it possible to obtain depth-resolved Fe-Lα,β spectra of the NL and to find several alteration zones which include different forms of iron. Slow oxidative dissolution of the material in 1 M HCl+0.01 M FeCl3 electrolyte produces only a thin NL with mostly O-bonded Fe3+ and polysulfide prevailing over mono- and disulfide species. Subsequent air-drying of this sample results in an increase in the concentrations of oxygen, S-bonded Fe, and mono- and disulfide species, along with S0 formation. No unpaired electron spins have been registered in any of these NL.

Chemical states and lattice dynamics of alpha-diimine Fe2+ complexes intercalated into gamma-zirconium phosphate.

The alpha-diimine Fe2+ complexes, [Fe(phen)3]2+, [Fe(bpy)3]2+, and [Fe(terpy)2]2+, (phen: 1,10-phenanthroline, bpy: 2,2'-bipyridyl, terpy: alpha,alpha',alpha''-tripyridine) were intercalated into zirconium dihydrogenphosphate phosphate dihydrate (gamma-zirconium phosphate, gamma-ZrP), Zr(PO4)(H2PO4).2H2O. The rate of the intercalation, the molar ratio of Fe to Zr, was found to be 3.82-7.76%. Mössbauer spectra indicated that one part of [Fe(phen)3]2+ and [Fe(bpy)3]2+ changed from a low-spin Fe2+ to high-spin Fe2+ state on intercalation, but [Fe(terpy)2]2+ did not change in chemical state. The lattice dynamics of the complexes and the intercalation compounds were investigated in terms of the temperature dependence of the area intensity on the Mössbauer spectra. A linear relationship was established for all the complex salts and the intercalation compounds investigated between the ln[A(T)/A(82)] and absolute temperature, T, where A(T) and A(82) show the intensities of a doublet at T and 82 K of the Mössbauer spectra, respectively. From the slope of the linear relation, the theta2M values, which were derived based on the Debye approximation of lattice vibration, were evaluated for the complex salts and the intercalation compounds. The Fe2+ complexes showed theta2M values of 1.27 to 2.32 x 10(6), whereas the intercalation compounds showed very similar values to each other, ranging from 2.19 to 2.39 x 10(6), irrespective of different alpha-diimine ligands. The results were explained in terms of the characteristic layered structure of zirconium phosphate, and by the tight bond between the alpha-diimine Fe2+ complexes and the host gamma-ZrP.

Synthesis and Characterization of Diiron(III)-Substituted Silicotungstate, [γ(1,2)-SiW10{Fe(OH2)}2O38]6-

The full details of synthesis and characterization of diiron(III)-substituted γ-Keggin silicotungstate, [γ(1,2)-SiW10{Fe(OH2)}2O38]6- (abbreviated as I) were reported. This complex was synthesized by the reaction of the lacunary [γ-SiW10O36]8- with Fe(NO3)3 in an acidic aqueous solution and isolated as the tetra-n-butylammonium salt (TBA-I). TBA-I has been characterized by infrared, Raman, 183W NMR, UV−visible, Mössbauer, and ESR spectroscopy, TG/DTA, FAB-mass, and magnetic susceptibility measurements, elemental analysis, and acid/base titration. The 183W NMR, infrared, Raman, and UV−visible spectroscopy indicated that I has a γ-Keggin structure with C2v symmetry. It was shown by the magnetic susceptibility measurement and Mössbauer, ESR, and UV−visible spectroscopy that I shows an antiferromagnetic coupling of the two high-spin Fe3+ centers.

The primary and secondary acceptors in bacterial photosynthesis III. Characterization of the quinone radicals Q A − ⋅ and Q B − ⋅ by EPR and ENDOR

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) techniques were used to investigate the electronic structure of the primary (Q A −⋅ ) and secondary (Q B −⋅ ) ubiquinone electron acceptors in reaction centers (RCs) of the photosynthetic bacteriumRhodobacter sphaeroides. To reduce the EPR linewidth, the high-spin Fe2+ present in native RCs was replaced by diamagnetic Zn2+. Experiments were performed both on frozen solutions and single crystals at microwave frequencies of 9, 35 and 94 GHz. Differences in the EPR/ENDOR spectra were observed for Q A −⋅ and Q B −⋅ , which are attributed to different environments of the quinones in the RC. The differences exhibited themselves in: (i) the g-tensors, (ii) the17O and13C hyperfine coupling (hfc) constants of the quinones labeled at the carbonyl group, (iii) the1H-hfcs of the quinone ring and (iv) the exchangeable protons hydrogen bonded to the carbonyl oxygens. From these results and from H/D exchange experiments, the following conclusions were drawn: both Q A −⋅ and Q B −⋅ have at least two hydrogen bonds of different strengths to the carbonyl oxygens. The hydrogen bonds for Q A −⋅ are stronger and more asymmetric than for Q B −⋅ . For Q A −⋅ the stronger bond (to O4) was assigned to His(M219) and the weaker (to O1) to Ala(M260). For Q B −⋅ the stronger bond (to O4) was assigned to His(L190), with several weaker bonds (to O1) to Ser(L223), Ile(224) and Gly(L225). From the temperature dependence of the hfcs of the exchangeable protons some dynamic properties of the RC were deduced. Hfcs with more distant nitrogens were observed by electron spin echo envelope modulation (ESEEM). For Q A −⋅ they were assigned to Nδ of His(M219) and to the peptide backbone nitrogen of Ala(M260) and for Q B −⋅ to Nδ of His(L190). These interactions indicate the extent of the electron wave function, which is important for the understanding of the electron transfer mechanism. Based on the magnetic resonance results, the function of the quinone acceptors in the reaction center is discussed.

A Model of spin catalysis in bacterial photosynthetic reaction centres

A model of influence of non-heme Fe2+ ion on kinetics of electron transfer between quinone anion-radicals Q A − and Q B − in bacterial reaction centres is proposed and investigated. Physical mechanism of the influence is associated with singlet-triplet transitions in Q A − Q B − Pair caused by interactions with paramagnetic Fe2+. The model incorporates exchange couplings between the particles and zero-field splittings in high-spin Fe2+. These interactions are shown to catalyze electron transfer in triplet pairs and alter the reaction yield significantly.

Preparation of LiCoO2 and LiCo1−xFexO2 using hydrothermal reactions

Iron doped/undoped LiCoO 2 powders could be obtained from hydrothermal reactions of either Fe 3+ or Co 3+ containing co-precipitates or a CoCl 2 -NaOH-NaClO 3 (oxidant) mixture with an excess amount of LiOH·H 2 O at 220 °C for 8-48 h. The LiFe x Co 1–x O 2 solid solution maintained a layered rock-salt structure until x=0.25. 57 Fe Mossbauer and Co K-edge XANES spectra and magnetic susceptibility data reveal that iron and cobalt in the solid solution are in high-spin Fe 3+ (S=5/2) and low-spin Co 3+ (S=0) configurations.

Structural, electrical and magnetic properties of low‐dimensional conductors based on unsymmetrical π donor EDT‐TTF and analogous selenium‐substituted molecules

Organic conductors based on the unsymmetrical π donor molecule EDT-TTF (ethylenedithiotetrathiafulvalene) or its selenium-substituted analogs (EDST, EDTS) and tetrahedral anions GaCl 4 – were prepared. The crystal structure determinations and the extended Huckel tight-binding band calculations indicate these systems to be quasi-one-dimensional conductors similar to TMTTF or TMTSF systems (TM systems). Electrical resistivity and magnetic susceptibility measurements and low-temperature X-ray diffraction experiments suggested a spin-Peierls ground state for (EDT-TTF) 2 GaCl 4 . (EDST) 2 GaCl 4 and (EDTS) 2 GaCl 4 exhibit metallic behavior down to ca. 40 K. The electric and magnetic properties of (EDST) 2 GaCl 4 suggested a semimetallic state at low temperature. In spite of the similarity in the crystal and electronic band structures between TM and EDT systems, these two series of quasi-one-dimensional conductors do not share the same ‘generalized phase diagram’. The electron–lattice interaction seems to be important in EDT conductors. The electric and magnetic properties of the isostructural systems with magnetic FeCl 4 – anions were also examined. The magnetic interaction between the high-spin Fe 3+ ions was found to be very weak.

The primary and secondary acceptors in bacterial photosynthesis: II. The structure of the Fe2+-Q− complex

The primary processes of bacterial photosynthesis take place in a membrane-bound protein called the reaction center (RC). The process involves light-induced electron transfer from a primary electron donor to a sequence of electron acceptors. The terminal acceptors, operating sequentially, are two ubiquinones, QA and QB, that couple magnetically to a high-spin (S = 2) Fe2+ forming an Fe2+-Q− complex. We have used a variety of techniques to investigate the electronic as well as some features of the spatial structure of the Fe2+-Q− complex in RCs fromRb. sphaeroides. These include: (a) static magnetization measurements in the temperature range of 0.7 to 190 K and magnetic fields up to 8 kG, (b) EPR spectroscopy at helium temperatures at 1.2, 9, and 35 GHz, (c) Mossbauer spectroscopy and (d) extended X-ray fine structure (EXAFS) determinations. The results of these measurements showed that Fe2+ resides in an asymmetric ligand field environment forming 6 ligands with a combination of oxygens and nitrogens. From the EXAFS results the distances of the Fe2+ to the first and third coordination shells were determined. These spatial features were subsequently corroborated by the X-ray structure of the RC, which showed the environment of the Fe to be a distorted octahedron, the base plane of which is formed by three Nɛ’s of histidines and one carbonyl oxygen; the apex is formed by a fourth Nɛ and a second carbonyl oxygen. The distances from the Fe2+ to the first and third shell were in good agreement with the values obtained from EXAFS. The most detailed information of the electronic structure of the Fe2+-Q− complex was obtained from the EPR spectra using the spin Hamiltonian formalism. The following spin-Hamiltonian parameters were obtained: for the crystalline field parameters,D = 7.60 K,E/D = 0.25; for the electronicg-values, gFe, x = 2.16, gFe, y = 2.27, gFe, z = 2.04 and for the antiferromagnetic exchange interaction,J x = −0.13 K,J y = −0.58 K,J z = −0.58 K. The use of the spin Hamiltonian was validated by comparing the results obtained from it with those obtained from an exact numerical solution of the 25 lowest energy levels of the orbital Hamiltonian. Possible roles that Fe2+ may play in the function of the RC are discussed. They include a structural role in which Fe2+ liganded to four histidine nitrogens imparts a rigid structure to a four α-helix bundle and a role in the electron transfer kinetics, most notably from the intermediate acceptor I− (bacteriopheophytin) to QA. Replacement of Fe2+ by diamagnetic Zn2+ retained all observed native characteristics of the RC.

Solid-State Deuterium NMR of Imidazole Ligands in Cytochrome c Peroxidase

We have measured hyperfine shifted NMR signals associated with deuterated imidazole bound to the high-spin Fe3+ state of cytochrome c peroxidase using deuterium magic angle spinning solid-state NMR. These experiments were performed on a mutant of cytochrome c peroxidase, CcP(H175G), for which replacement of the proximal histidine with glycine produced a cavity that can bind a variety of substituted imidazoles including imidazole or 2-methylimidazole. The mutant with imidazole bound is inactive; specifically its reaction with H2O2 is blocked. We observed deuterium NMR signals from the methyl-d3 group of the perdeuterated 2-MeIm sample bound to H175G CcP. The signal displayed an upfield chemical shift and exhibited non-Curie temperature dependence, indicating the existence of low-lying excited electronic states. Upon introducing a nondeuterated competitive ligand, imidazole, a decrease in the intensity of this signal was detected, consistent with the assignment of the deuterium signal to the bound 2-methyli...

The primary and secondary electron acceptors in bacterial photosynthesis: I. A chronological account of their identification by EPR

A chronological account is presented of the work that led to the identification of the primary and secondary acceptors in photosynthetic reaction centers ofRb. sphaeroides. Both acceptors when reduced give rise to broad EPR signals centered atg = 1.82. When the iron is removed from the reaction center, the EPR line, narrowed by approximately two orders of magnitude, is characteristic of a ubisemiquinone. The broad signal in native reaction centers (RCs) is, therefore, ascribed to an unpaired electron being localized on the ubisemiquinone interacting magnetically with the high spin Fe2+. These findings are consistent with the structure of the RC, as determined a decade later by X-ray diffraction.

Magnetic, Electrical Conductivity, and EPR Investigations of a Low-Spind5System in Fe8V10W16O85

The temperature dependence of the magnetic, electrical conductivity, and electron paramagnetic resonance (EPR) properties of Fe8V10W16O85has been investigated. The magnetic susceptibility measurements revealed an almost Curie–Weiss law behavior above room temperature and an additional magnetic interaction at low temperature, causing a steep rise of magnetization as the liquid helium temperature was approached. The value of the magnetic moment at high temperature,μeff=1.80μB, suggests a predominance of trivalent iron ions in a low-spin state. In the 300–4.2 K temperature range a difference between the zero field cooling (ZFC) and the field cooling (FC) modes was recorded. This irreversible behavior might be related to the presence of weakly coupled clusters. The EPR measurements revealed a broad, temperature-dependent resonance line at high temperature and two weaker lines at low temperature. The two low-temperature lines were attributed to antiferromagnetically coupled high-spin Fe3+ion clusters and to high-spin iron ions placed at sites with low symmetry of the crystal field. The broad line at high temperature was separated into two Lorentzian lines. These component lines were attributed to the two paramagnetic centers connected with the Fe3+ions involved in the magnetic structure of Fe8V10W16O85: dominant low-spin centers and a small admixture (<15%) of the high-spin centers. The line broadening and shift of the resonance field of the two component lines with decreasing temperature were studied and analyzed using a model of the EPR lines of antiferromagnets. The temperature dependence of the electrical conductivity showed a typical semiconducting-type behavior with an activation energy of 0.40 eV. The hopping mechanism of small polarons was proposed to explain the transport properties of the sample.

Spectroscopic Studies of Iron Niobophosphate Glasses

We investigated iron niobophosphate glasses [0.1Nb2O5 · 0.4P2O5 · 0.5Li2O]yFe2O3(y = 0, 0.1, 0.2, 0.3, 0.4, 0.5) prepared in ambient atmosphere using the melt quenching technique. The samples were studied using X-ray powder diffraction, 57Fe Mössbauer and infrared spectroscopy, in order to investigate the effect of the increasing concentration of α-Fe2O3 in the glass matrix. According to Mössbauer results high spin Fe3+ and high spin Fe2+ at distorted octahedral sites are present in all samples. The distortion of the Fe3+ and Fe2+ ions sites increases from low (y = 0.1) to high (y = 0.3) iron content samples. The measurements of the infrared absorption, X-ray powder diffraction and Mössbauer spectroscopy also indicate that LiFePO4 and α-Fe2O3 crystalline phases are present for high iron content samples (y = 0.4, 0.5). The limit of solubility of α-Fe2O3 in these glasses is around 23 mol%.