Electron Delocalization in a New Mixed-Valence σ-Bridged Biferrocenium Cation

The physical properties of mixed-valence 1,3-diferrocenyl-propan-1-one (1) are reported. The Mössbauer spectrum of mixed-valence cation 1 at 300 K shows two doublets, one for a Fe¹¹ metallocene (ΔE_Q = 2.192 mm s^?1) and the other for a Fe^III metallocene (ΔE_O = 0.179 mm s^?1) Thus, the thermal electron-transfer rate in 1 is less than the time scale of the Mössbauer technique (～10⁷ s^?1). Furthermore, the NMR, electrochemical IR, and near-IR data conclusively indicate that there is no significant interaction between the two ferrocenyl moieties.     

Mössbauer Spectroscopic Investigation of FeII and FeIII 3,4,5‐Trihydroxybenzoates (Gallates) – Proposed Model Compounds for Iron‐Gall Inks

Iron gallates with iron in the oxidation states Fe²⁺ and Fe³⁺ were prepared and studied by Mössbauer spectroscopy, X‐ray diffraction, and IR spectroscopy. Fe^III 3,4,5‐trihydroxybenzoate (gallate) Fe(C₇O₅H₄) · 2H₂O, whose structure was first determined by Wunderlich, was obtained by the reaction of gallic acid and metallic iron or by oxidation of the Fe^II gallate, which was obtained by the reaction of ferrous sulfate with 3,4,5‐trihydroxybezoic acid (gallic acid) under anoxic conditions. Trials to reproduce the hydrothermal preparation method of Feller and Cheetham show that the result depends crucially on the free gas volume in the reaction vessel. If there is no free volume one obtains the same Fe^III gallate as in the other preparation methods. With a large free volume another compound was found to form whose composition and structure could not be determined. It could be specified only by Mössbauer spectroscopy. Fe^III gallate, the Fe^II gallate, and the new phase show magnetic ordering at liquid helium temperature.     

Hydrothermal Synthesis,Crystal Structure Determination,and Magnetic Properties of a New Calcium Iron Iridium Hydrogarnet Ca3(Ir2–xFex)(FeO4)2–x(O4H4)1+x with 0 ≤ x ≤ 1

The new calcium iron iridium hydrogarnet Ca₃(Ir_2–xFe_x)(FeO₄)_2–x(H₄O₄)_1+x (0 ≤ x ≤ 1) was obtained by hydrothermal synthesis under strongly oxidizing alkaline conditions. The compound adopts a garnet‐like crystal structure and crystallizes in the acentric cubic space group I4 3d (no. 220) with a = 12.5396(6) Å determined at T = 100 K for a crystal with a refined composition Ca₃(Ir_1.4Fe_0.6)(FeO₄)_1.4(O₄H₄)_1.6. Iridium and iron statistically occupy the octahedrally coordinated metal position, the two crystallographically independent tetrahedral sites are partially occupied by iron. Hydroxide groups are found to cluster as hydrogarnet defects, i.e. partially substituting oxide anions around the empty tetrahedral metal sites. The presence of hydroxide ions was confirmed by infrared spectroscopy and the hydrogen content was quantified by carrier gas hot extraction; the overall composition was verified by energy dispersive X‐ray spectroscopy. The structure model is supported by ⁵⁷Fe‐Mössbauer spectroscopic data evidencing different Fe sites and a magnetic ordering of the octahedral iron sublattice at room temperature. The thermal decomposition proceeds via three steps of water loss and results in Ca₂Fe₂O₅, Fe₂O₃ and Ir. Mössbauer and magnetization data suggest magnetic order at ambient temperature with complex magnetic interactions.     

A Redox‐Induced Spin‐State Cascade in a Mixed‐Valent Fe3(μ3‐O) Triangle

One‐electron reduction of a pyrazolate‐bridged triangular Fe₃(μ₃‐O) core induces a cascade wherein all three metal centers switch from high‐spin Fe³⁺ to low‐spin Fe^2.66+. This hypothesis is supported by spectroscopic data (¹H‐NMR, UV‐vis‐NIR, infra‐red, ⁵⁷Fe‐Mössbauer, EPR), X‐ray crystallographic characterization of the cluster in both oxidation states and also density functional theory. The reduction induces substantial contraction in all bond lengths around the metal centers, along with diagnostic shifts in the spectroscopic parameters. This is, to the best of our knowledge, the first example of a one‐electron redox event causing concerted change in multiple iron centers.     

In situ — High Temperature Mössbauer Spectroscopy of Iron Nitrides and Nitridoferrates

The stoichiometric iron nitrides γ′‐Fe₄N, ε‐Fe₃N and ζ‐Fe₂N were characterized by Mössbauer spectroscopy. The thermal decomposition of ε‐Fe₃N was studied in‐situ by means of a specially developed Mössbauer furnace. We found ε‐Fe₃N to γ′‐Fe₄N and ε‐Fe₃N_x (x ≥ 1.3) as decomposition products and determined the border of γ′/ε transformation at T ? 930 K. Mössbauer spectroscopy was applied to study in‐situ the thermal decomposition of the nitridometalate Li₃[Fe^IIIN₂] and the formation of Li₂[(Li_1‐xFe^I_x)N], the compound with the largest local magnetic field ever observed in an iron containing material. The kinetics of formation and the stability of Li₂[(Li_1‐xFe^I_x)N] was of particular interest in the present study.     

Properties of the perovskites, SrMn1−xFexO3−δ (x=1/3, 1/2, 2/3)

The SrMn_1−xFe_xO_3−δ (x=1/3, 1/2, 2/3) phases have been prepared and are shown by powder X-ray and neutron (for x=1/2) diffraction to adopt an ideal cubic perovskite structure with a disordered distribution of transition-metal cations over the six-coordinate B-site. Due to synthesis in air, the phases are oxygen deficient and formally contain both Fe³⁺ and Fe⁴⁺. Magnetic susceptibility data show an antiferromagnetic transition at 180 and 140 K for x=1/3 and 1/2, respectively and a spin-glass transition at 5, 25, 45 K for x=1/3, 1/2 and 2/3, respectively. The magnetic properties are explained in terms of super-exchange interactions between Mn⁴⁺, Fe^(4+δ)+ and Fe⁽³⁺⁾⁺. The XAS results for the Mn-sites in these compounds indicate small Mn-valence changes, however, the Mn-pre-edge spectra indicate increased localization of the Mn-e_g orbitals with Fe substitution. The Mössbauer results show the distinct two-site Fe⁽³⁺⁾+/Fe^(4+δ)+ disproportionation in the Mn- substituted materials with strong covalency effects at both sites. This disproportionation is a very concrete reflection of a localization of the Fe-d states due to the Mn-substitution.     

Mößbauer- und EPR-spektroskopische Untersuchungen hydrothermal behandelter A-Zeolithe

Studies on Hydrothermal Treatment of A-Zeolites by Mössbauer and ESR Spectroscopy The influence of hydrothermal treatment on the structure of primary pores of A-zeolites was systematically investigated by ESR and Mössbauer spectroscopy. The structure of primary pores of these zeolites was modified by Mg²⁺, Li⁺ and paramagnetic Fe³⁺ ions incorporated into the initial zeolite. Coordinations and positions of the iron ions inside of the large cavity were investigated by ESR; magnetically ordered and not magnetically ordered phases of iron on the surface of the zeolites were identified by Mössbauer spectroscopy. The hydrothermal stability of A-zeolites is essentially lowered by Mg²⁺ cations at high degrees of iron exchange. On the other hand the hydrothermal treatment only changes positions of the iron ions and the mass ratios and particle sizes of the iron phases on the surface of the crystallites. The hydrothermal stability of the structure of primary pores is not influenced thereby.     

Mixed valence states of Cr and Fe in La1−xSrxFe0.8Cr0.2O3−y

The La_1?xSr_xFe_0.8Cr_0.2O_3?y (x = 0.2, 0.4, 0.6 and 0.8) phases were studied by X-ray photoelectron spectroscopy at room temperature and ⁵⁷Fe Mössbauer spectroscopy at different temperatures. Mixed valence states were observed both for chromium and iron ions, justifying the complex magnetic behaviour exhibited by these compounds. The Mössbauer results indicate the simultaneous presence of Fe³⁺, Fe⁴⁺ and Fe⁵⁺ at 4.2 K and the co-existence of Fe³⁺ and Fe⁽³⁺ⁿ⁾⁺ at T = 293 K, with the latter fraction increasing with increasing strontium content. The presence of Cr^3+/4+ is interpreted as being mainly responsible for the incomplete charge disproportionation reaction of iron at low temperature, as deduced from the Mössbauer results.     

Mössbauer study of the thermal decomposition products of BaFeO4

A pure sample of a hexavalent iron compound, BaFeO₄, was decomposed at temperatures below 1200°C at oxygen pressures from 0.2 to 1500 atm. In addition to the already known BaFeO_x (2.5 ≦ x < 3.0) phases with hexagonal and triclinic symmetry, two new phases were obtained as decomposition products at low temperatures. One of the new phases, with composition BaFeO_{2.61 – 2.71}, has tetragonal symmetry; lattice constants are a₀ = 8.54 Å, c₀ = 7.29 Å. The phase is antiferromagnetic with Néel temperature estimated to be 225 ± 10 K. Two internal fields observed on its Mössbauer spectra correspond to Fe³⁺ and Fe⁴⁺. In the other new phase, with composition BaFeO_2.5, all Fe³⁺ ions had the same hyperfine field; it too is antiferromagnetic with a Néel temperature of 893 ± 10 K. Mössbauer data on the hexagonal phase coincided with earlier results of Gallagher, MacChesney, and Buchanan [J. Chem. Phys.43, 516 (1965)]. In the triclinic-I BaFeO_2.50 phase, internal magnetic fields were observed at room temperature, and it was supposed that there were four kinds of Fe³⁺ sites. The phase diagram of BaFeO_x system was determined as functions of temperature and oxygen pressure.     

Structural Properties,Mössbauer Spectra,and Magnetism of Perovskite‐Type Oxides SrFe1–xTixO3–y

Perovskite‐type phases SrFe_1–xTi_xO_3–y with 0.1 ≤ x ≤ 0.7 have been prepared from the oxides, and, in order to reach high oxygen contents and Fe^IV fractions, annealed at oxygen pressures of 60 MPa. The materials were characterised by powder x‐ray and neutron diffraction, ⁵⁷Fe Mössbauer spectroscopy, and magnetic susceptibility measurements. All samples of the series crystallise in a cubic perovskite structure and reveal considerable oxygen deficiency. The Mössbauer parameters suggest that for x = 0.1, where the Fe^IV fraction is about 90%, the itinerant electronic state of SrFeO₃ is essentially retained. In materials with larger x increasing amounts of Ti^IV and Fe^III ions lead to a stronger localisation of the σ* (Fe 3 d – O 2 p) electrons. There is no evidence for a charge disproportionation of Fe^IV in any of the materials. Magnetic susceptibility measurements show a divergence of zero‐field cooled and field‐cooled data below a temperature T_m and deviations from Curie‐Weiss behaviour above T_m. The data are indicative of spin‐glass behaviour due to disorder and competing exchange interactions.     

Crystal structure and the magnetic properties of tris(2-chloromethyl-4-oxo-4H-pyran-5-olato-κ2O5,O4)iron(III)

The tris(2-chloromethyl-4-oxo-4H-pyran-5-olato-κ²O⁵,O⁴)iron(III), [Fe(kaCl)₃], has been synthesized and characterized by the crystal structure analysis, magnetic susceptibility measurements, Mössbauer, and EPR spectroscopic methods. The X-ray single crystal analysis of [Fe(kaCl)₃] revealed a mer isomer. The magnetic susceptibility measurements indicated the paramagnetic character in the temperature range of 2 K–298 K. The EPR and Mössbauer spectroscopy confirmed the presence of an iron center in a high-spin state. Additionally, the temperature-independent Mössbauer magnetic hyperfine interactions were observed down to 77 K. These interactions may result from spin–spin relaxation due to the interionic Fe³⁺ distances of 7.386 Å.     

Preparation,Characterization, and Reactions of Dinuclear Tris(2,2,2-trichloroethoxy) Iron(III), Fe(OCH2CCl3)3

The preparation, electrical conductivity, magnetic moments, infrared, reflectance, and ⁵⁷Fe Mössbauer spectra of tris(2,2,2-trichloroethoxy) iron(III) and its adducts with some oxygen and nitrogen donor ligands are reported. Cryoscopic data of the parent compound and its complex with ethylacetate suggest these compounds to be dimeric in nitrobenzene and benzene respectively. All the compounds are covalent with Fe^III having distorted octahedral arrangement which is achieved through alkoxy bridging. The magnetic moments are lesser than those required for the spin only value indicating antiferromagnetic interactions in Fe^III atoms. The Mössbauer spectra are explained in terms of two Fe^III high spin sites corresponding to trans- and cis-positions in the structure.     

Coexistence of 3d‐Ferromagnetism and Superconductivity in [(Li1−xFex)OH](Fe1−yLiy)Se

Superconducting [(Li_1?xFe_x)OH](Fe_1?yLi_y)Se (x≈0.2, y≈0.08) was synthesized by hydrothermal methods and characterized by single‐crystal and powder X‐ray diffraction. The structure contains alternating layers of anti‐PbO type (Fe_1?yLi_y)Se and (Li_1?xFe_x)OH. Electrical resistivity and magnetic susceptibility measurements reveal superconductivity at 43 K. An anomaly in the diamagnetic shielding indicates ferromagnetic ordering near 10 K while superconductivity is retained. The ferromagnetism is from the iron atoms in the (Li_1?xFe_x)OH layer. Isothermal magnetization measurements confirm the superposition of ferromagnetic and superconducting hysteresis. The internal ferromagnetic field is larger than the lower, but smaller than the upper critical field of the superconductor. The formation of a spontaneous vortex phase where both orders coexist is supported by ⁵⁷Fe‐Mössbauer spectra, ⁷Li‐NMR spectra, and μSR experiments.     

Crystal structure and magnetic properties of tris(2-hydroxymethyl-4-oxo-4H-pyran- 5-olato-κ2O5,O4)iron(III)

Tris(2-hydroxymethyl-4-oxo-4H-pyran-5-olato-κ²O⁵,O⁴)iron(III) [Fe(ka)₃], has been characterised by magnetic susceptibility measurements Mössbauer and EPR spectroscopy. The crystal structure of [Fe(ka)₃] has been determined by powder X-ray diffraction analysis. Magnetic susceptibility and EPR measurements indicated a paramagnetic high-spin iron centre. Mössbauer spectra revealed the presence of magnetic hyperfine interactions that are temperature-independent down to 4.2?K. The interionic Fe³⁺ distance of 7.31?Å suggests spin-spin relaxation as the origin of these interactions.     

Zum Aufbau schlecht geordneter Calciumhydrogensilicate. VI. Chemische,ESR- und Mößbauer-spektroskopische Untersuchungen zum Einbau von Fe3+-Ionen in C?S?H(Di,Poly)

On the Structure of Ill-crystallized Calcium Hydrogen Silicates. VI. Chemical, ESR, and Mössbauer-Spectroscopic Investigations on the Incorporation of Fe³⁺ into C? S? H(Di, Poly) C? S? H(Di, Poly), prepared by precipitation reactions from sodium silicate solutions in presence of Fe³⁺ ions, incorporates at least up to 0.09 Fe₂O₃/SiO₂. Thereby the anion composition of the C? S? H(Di, Poly) ist not affected. The results of ESR and Mössbauer spectroscopic measurements show, that Fe is incorporated as Fe³⁺ at octahedral sites. Only less than 1% of the total amount of Fe³⁺ is situated at tetrahedral sites. In good accordance with that is the change of thermal stability of the silicate anions concerning condensation and degradation reactions.     

Mössbauer and thermogravimetric study of ferrous gluconate

Ferrous gluconate Fe(C₆H₁₁O₇)₂·2H₂O was investigated by means of⁵⁷Fe (14.4 keV)-Mössbauer spectroscopy and thermogravimetry. The Mössbauer study was performed in the temperature range 80 to 423 K. It was found that Fe²⁺ occupies two distinctly different Mössbauer sites in the hydrated phase and a single site in the product of the thermal treatment. All samples were contaminated by some amount of Fe³⁺. A significant oxidation occurs during thermal treatment (about 378 K) at least for the samples exposed to the air. No Goldanskii-Karyagin effect has been detected, in contrast to the previous claim. It has to be noted that the ferrous gluconate is often used as the iron containing component of drugs used in the treatment of anaemia.     

The Nitrogenase FeMo‐Cofactor Precursor Formed by NifB Protein: A Diamagnetic Cluster Containing Eight Iron Atoms

The biological activation of N₂ occurs at the FeMo‐cofactor, a 7Fe–9S–Mo–C–homocitrate cluster. FeMo‐cofactor formation involves assembly of a Fe_6–8–S_X–C core precursor, NifB‐co, which occurs on the NifB protein. Characterization of NifB‐co in NifB is complicated by the dynamic nature of the assembly process and the presence of a permanent [4Fe–4S] cluster associated with the radical SAM chemistry for generating the central carbide. We have used the physiological carrier protein, NifX, which has been proposed to bind NifB‐co and deliver it to the NifEN protein, upon which FeMo‐cofactor assembly is ultimately completed. Preparation of NifX in a fully NifB‐co‐loaded form provided an opportunity for Mössbauer analysis of NifB‐co. The results indicate that NifB‐co is a diamagnetic (S=0) 8‐Fe cluster, containing two spectroscopically distinct Fe sites that appear in a 3:1 ratio. DFT analysis of the ⁵⁷Fe electric hyperfine interactions deduced from the Mössbauer analysis suggests that NifB‐co is either a 4Fe²⁺–4Fe³⁺ or 6Fe²⁺–2Fe³⁺ cluster having valence‐delocalized states.     

Mössbauer spectroscopic evidence for iron(III) complexation and reduction in acidic aqueous solutions of indole-3-butyric acid

Mössbauer spectroscopic studies were carried out in acidic (pH 2.3) ⁵⁷Fe^III nitrate containing aqueous solutions of indole-3-butyric acid (IBA), rapidly frozen in liquid nitrogen at various periods of time after mixing the reagents. The data obtained show that in solution in the presence of IBA, iron(III) forms a complex with a dimeric structure characterised by a quadrupole doublet, whereas without IBA under similar conditions iron(III) exhibits a broad spectral feature due to a slow paramagnetic spin relaxation which, at liquid nitrogen temperature, results in a large anomalous line broadening (or, at T = 4.2 K, in a hyperfine magnetic splitting). The spectra of ⁵⁷Fe^III+IBA solutions, kept at ambient temperature under aerobic conditions for increasing periods of time before freezing, contained a gradually increasing contribution of a component with a higher quadrupole splitting. The Mössbauer parameters for that component are typical for iron(II) aquo complexes, thus showing that under these conditions gradual reduction of iron(III) occurs, so that the majority (85%) of dissolved iron(III) is reduced within 2 days. The Mössbauer parameters for the iron(III)-IBA complex in aqueous solution and in the solid state (separated from the solution by filtration) were found to be similar, which may indicate that the dissolved and solid complexes have the same composition and/or iron(III) coordination environment. For the solid complex, the data of elemental analysis suggest the following composition of the dimer: [L₂Fe-(OH)₂-FeL₂] (where L is indole-3-butyrate). This structure is also in agreement with the data of infrared spectroscopic study of the complex reported earlier, with the side-chain carboxylic group in indole-3-butyrate as a bidentate ligand. The Mössbauer parameters for the solid ⁵⁷Fe^III-IBA complex at T = 80 K and its acetone solution rapidly frozen in liquid nitrogen were virtually identical, which indicates that the complex retains its structure upon dissolution in acetone.     

In situ Mössbauer spectroscopy studies of the electrochemical reaction of lithium with KFeS2

In situ Mössbauer spectroscopy has been used to study the electrochemical reaction of lithium with KFeS₂. Compositions KLi_xFeS₂ with Δx = 0.25 were obtained by coulometric titration for one complete discharge and recharge. Mössbauer spectra were obtained at each composition. Three new iron sites are identified in addition to Fe³⁺ in KFeS₂. A mechanism to account for the electrochemical and Mössbauer data is proposed. The end product KLiFeS₂ has been synthesized and found to be body-centered tetragonal with a = 3.938(2) Å and c = 13.135(5) Å.     

Magnetic Resonance and Mössbauer Studies of Superparamagnetic γ‐Fe2O3 Nanoparticles Encapsulated into Liquid‐Crystalline Poly(propylene imine) Dendrimers

We present the first results of electron magnetic resonance (EMR) and Mössbauer spectroscopy studies of γ‐Fe₂O₃ nanoparticles (NPs) incorporated into liquid‐crystalline, second‐generation dendrimers. The mean size of NPs formed in the dendrimers was around 2.5 nm. A temperature‐driven transition from superparamagnetic to ferrimagnetic resonance was observed for the sample. Low‐temperature blocking of the NP magnetic moments has been clearly evidenced in the integrated EMR line intensity and the blocking temperature was about 60 K. The physical parameters of magnetic NPs (magnetic moment, effective magnetic anisotropy) have been determined from analyses of the EMR data. The effective magnetic anisotropy constant is enhanced relative to bulk γ‐Fe₂O₃ and this enhanced value is associated with the influence of the surface and shape effects. The angular dependence of the EMR signal position for the field‐freezing sample from liquid‐crystalline phase showed that NPs possessed uniaxial anisotropy, in contrast to bulk γ‐Fe₂O₃. Mössbauer spectroscopy determined that fabricated NPs consisted of an α‐Fe core and a γ‐Fe₂O₃ shell.