On the Valence State of Iron in Sr0.52+Ca0.52+Fe0.54+ Me0.54+O32−

Powder samples of Sr_0.5Ca_0.5Fe_0.5Me_0.5O₃ (Me = Co, Zr or Mn) and Sr_0.3La_0.7FeO₃ are investigated by X-ray diffraction and Mössbauer effect spectroscopy. Analysis of the completely ordered spectra suggested three kinds of iron ions coexist in general where the resolution into the different valence state is clearly seen. The Mössbauer effect parameters values are found to be close to those expected for Fe³⁺, Fe⁴⁺ and Fe⁵⁺ indicating that some of the tetravalent iron ions in its high spin state disproportionate into Fe³⁺ and Fe⁵⁺ ions passing through temperature dependent intermediate valence states.     

Influences of La3+ substitution on the structure and magnetic properties of M-type strontium ferrites

M-type strontium ferrites with substitution of Sr²⁺ by rare-earth La³⁺, according to the formula Sr_1−xLa_xFe₁₂O₁₉, are prepared by the ceramic process. Influences of the substituted amount of La³⁺ on structure and magnetic properties of Sr_1−xLa_xFe₁₂O₁₉ compounds have systematically been investigated by XRD, VSM and Mössbauer spectrum. When the substituted amount x is below 0.30, X-ray diffraction shows that the samples are single M-type hexagonal ferrites. It is found that the suitable amount of La³⁺ substitution may remarkably increase saturation magnetization σ_s and intrinsic coercivity H_cJ. With the La³⁺ addition for the same sintering temperature, σ_s and H_cJ increase at first, then decrease gradually. However, the substituted amount x at the maximum value of H_cJ is bigger than that of σ_s. Mössbauer spectroscopy of ⁵⁷Fe in Sr_1−xLa_xFe₁₂O₁₉ has shown that the substitution of Sr²⁺ by La³⁺ is associated with a valence change of Fe³⁺ to Fe²⁺ at 2a or 4f₂ site. The magnetic properties are reflected in the Mössbauer spectra which indicate that the magnetic hyperfine field (H_hf) is detected with the highest value at x=0.20. The different exchange paths between the iron sublattices are discussed according to the increased hyperfine fields of the 12k- and 2b-site. The Curie temperature of Sr_1−xLa_xFe₁₂O₁₉ decreases linearly with increasing La³⁺ substitution.     

Study of phase transition on nanocrystalline (La, Sr)(Mn, Fe)O system using high-pressure Mössbauer spectroscopy and high-pressure X-ray diffraction

Pressure-induced structural changes on nano-crystalline La_0.8Sr_0.2Mn_0.8Fe_0.2O₃ were studied using high-pressure Mössbauer spectroscopy and high-pressure X-ray diffraction. Mössbauer measurements up to 10 GPa showed first order transition at 0.52 GPa indicating transformation of Fe^4?+? to high spin Fe^3?+?, followed by another subtle transition at 3.7 GPa due to the convergence of two different configurations of Fe into one. High-pressure X-ray diffraction measurements carried up to 4.3 GPa showed similar results at 0.6 GPa as well as 3.6 GPa. Attempts were made to explain the changes at 0.6 GPa by reorientation of grain/grain boundaries due to uniaxial stress generated on the application of pressure. Similarly variation at 3.6 GPa can be explained by orthorhombic to monoclinic transition.     

X-ray photoelectron and mössbauer spectroscopy studies of the valence state of transition metal ions in Co1–x Fe x Cr2O4 (x = 0.1, 0.2, 0.5) ceramics

The valence state of transition metal ions in the Co_1–x Fe _x Cr₂O₄ (x = 0.1, 0.2, 0.5) system has been investigated using X-ray photoelectron and Mössbauer spectroscopy. It has been shown that, in this system, there are Fe²⁺ and Fe³⁺ ions. The relative Fe³⁺/Fe²⁺ contents have been determined by fitting the experimental Fe 2p photoelectron spectra by a superposition of theoretical spectra of the Fe²⁺ and Fe³⁺ ions, as well as using Mössbauer spectroscopy.     

A tin-119 Mössbaer spectroscopic study of tin-doped niobium titanium phosphate

¹¹⁹Sn Mössbauer spectroscopy shows that the reaction of metallic tin with niobium titanium phosphate, NbTiP₃O₁₂, results in the accommodation of Sn²⁺ in two types of sites within channels in the NbTiP₃O₁₂ structure. The Sn²⁺ absorptions are characterised by highly positive chemical isomer shifts which decrease as increasing concentrations of Sn²⁺ are incorporated within the NbTiP₃O₁₂ structure. At very high tin concentrations the Mössbauer spectra show the presence of some unreacted elemental tin. The¹¹⁹Sn Mössbauer data are endorsed by X-ray powder diffraction data and are discussed in terms of the incorporation of highly ionic Sn²⁺ species.     

Mössbauer effect in LaFe12O19

Mössbauer spectra of ⁵⁷Fe in LaFe₁₂O₁₉ show that the substitution of La³⁺ for Ba²⁺ or Sr²⁺ in XFe₁₂O₁₉ is associated with a valency change of Fe³⁺ to Fe²⁺ at the 2a or 4f₂ site. Temperature dependences of the hyperfine fields at the various sublattices are given.     

Sol-gel synthesis and dilute magnetism of nano MgO powder doped with Fe

Mg oxides doped with 1 % ⁵⁷Fe were prepared by a sol-gel method, and annealed at various temperatures. Nano-size Mg oxides were characterized by Mössbauer spectrometry, magnetization and XRD measurements. The crystalline size of MgO increases with increase of annealing temperature. Samples annealed at 600 °C and 800 °C gave only doublet peaks of paramagnetic Fe³⁺ in Mössbauer spectra although Fe³⁺ doping into MgO induced a distorted structure and showed weak ferromagnetism. It is considered that the magnetic property is due to defect induced magnetism by doping Fe³⁺ into MgO. For a sample heated at 1000 °C, it is found from low temperature Mössbauer spectra that Fe³⁺ species are located at the core and shell of fine MgFe₂O₄ grains and diluted in MgO matrix.     

M?ssbauer study of carbon coated iron magnetic nanoparticles produced by simultaneous reduction/pyrolysis

Magnetic iron nanoparticles immersed in a carbon matrix were produced by a combined process of controlled dispersion of Fe^3?+? ions in sucrose, thermal decomposition with simultaneous reduction of iron cores and the formation of the porous carbonaceous matrix. The materials were prepared with iron contents of 1, 4 and 8 in %wt in sucrose and heated at 400, 600 and 800°. The samples were analyzed by XRD, Mössbauer spectroscopy, magnetization measurements, TG, SEM and TEM. The materials prepared at 400° are composed essentially of Fe₃O₄ particles and carbon, while treatments at higher temperatures, e.g. 600 and 800° produced as main phases Fe⁰ and Fe₃C. The Mössbauer spectra of samples heated at 400° showed two sextets characteristic of a magnetite phase and other contributions compatible with Fe^3?+? and Fe^2?+? phases in a carbonaceous matrix. Samples treated at temperatures above 600° showed the presence of metallic iron with concentrations between 16?C43%. The samples heated at 800° produced higher amounts of Fe₃C (between 20% and 58%). SEM showed for the iron 8% sample treated at 600?C800°C particle sizes smaller than 50 nm. Due to the presence of Fe⁰ particles in the carbonaceous porous matrix the materials have great potential for application as magnetic adsorbents.     

Magnetic properties of the mixed spinel Mg1+xMnxFe2-2xO4

The structural and magnetic properties of the mixed spinel Mg_1+xMn_xFe_2-2xO₄ system for 0.1<= x <= 0.9 have been studied by means of X-ray diffraction, magnetization, a.c. susceptibility and Mössbauer spectroscopy measurements. X-ray intensity calculations indicate that Mn⁴⁺ ions occupy only octahedral (B) sites replacing Fe³⁺ ions and the added Mg²⁺ ions substitute for A-site Fe³⁺ ions. All samples are magnetic at 12 K displaying Mössbauer spectra that have magnetic sextets coexisting with a central doublet that increases in population with increasing Mn concentration, indicating the presence of short range ordering (clustering). The Mössbauer intensity data show that Mn possesses a preference for the B-site of the spinel over the whole range of concentration. As expected, the hyperfine field and Curie temperature determined from a.c.susceptibility data decrease with increasing Mn content. Magnetization results indicate that on increasing dilution x, the collinear ferrimagnetic phase breaks down at x = 0.3 before reaching the ferrimagnetic percolation limit (x=0.6), as a result of the presence of competing exchange interactions, which is well supported by Mössbauer results. From all the above results, it is proposed that with increasing Mn content from x=0.6 to 0.9, the frustration and disorder increase in the system suppressing the ferrimagnetic ordering, and the system approaches to a cluster spin glass type of ordering at x=0.8 as reflected in the a.c.susceptibility and Mössbauer spectrum.     

Mössbauer study of illitic clays containing iron-rich impurities

The iron mineralogy of nineteen illitic clays from eastern Bavaria was studied by Mössbauer spectroscopy and X-ray powder diffraction. Mössbauer spectra of the <2 μm fraction were taken at RT, 120 K and 4.2 K. The clays contain both paramagnetic Fe³⁺ and Fe²⁺. Superparamagnetic oxides are frequently present. The Fe²⁺ quadrupole splitting and the ratio of Fe³⁺ at 4.2 K to Fe²⁺ at 120 K are correlated and define two groups distinguished by their mineral content. The samples were heated systematically for 48 h up to 1250°C in steps of 50°C. One clay which is rich in chlorite and Fe(II) was studied in detail after firing in air and following a reduction for 3 h at 800°C with charcoal. The transformations of the mineral phases with temperature as shown by X-ray diffraction are also evident in the Mössbauer spectra.     

Magnetic behavior of the oxide spinels: Li0.5Fe2.5−2x Al x Cr x O4

In order to study the effect of substitution of Fe³⁺ by Al³⁺ and Cr³⁺ in Li_0.5Fe_2.5O₄ on its structural and magnetic properties, the spinel system Li_0.5Al _x Cr _x Fe_2.5?2x O₄ (x=0.0, 0.2, 0.4, 0.5, 0.6 and 0.8) has been characterized by X-ray diffraction, high field magnetization, low field ac susceptibility and ⁵⁷Fe Mossbauer spectroscopy. Contrary to the earlier reports, about 50% of Al³⁺ is found to occupy the tetrahedral sites. The system exhibits canted spin structure and a central paramagnetic doublet was found superimposed on magnetic sextet in the Mössbauer spectra (x>0.5).     

Electronic and magnetic properties of the iron borate Fe2BO4

Polycrystalline Fe₂BO₄ was prepared by solid state reactions and its electronic and magnetic properties were investigated by Mössbauer spectroscopy and magnetization measurements. The Mössbauer spectra of Fe₂BO₄ below 270 K indicate the presence of Fe²⁺ and Fe³⁺ sites in the structure, in a ratio 1 : 1. Above this temperature electron delocalization sets in between the divalent and trivalent iron ions and Fe^2.5+ states are observed. The temperature dependence of the Mössbauer spectra and magnetization measurements clearly show the onset of magnetic order below 155 K.     

An iron-57 Mössbauer spectroscopy and EXAFS study of the hydrogen pretreatment of titania-supported iron-ruthenium catalysts

The changes in composition and structure which are induced in a titania-supported iron-ruthernium catalyst following treatment in hydrogen have been investigated in situ by⁵⁷Fe Mössbauer spectroscopy and by EXAFS. The results show that ruthenium dioxide is readily reduced at temperatures below ca. 500°C to ca. 20 Å clusters of metallic ruthenium whilst α-Fe₂O₃ is partially reduced at 130°C to Fe²⁺ and Fe⁰. The Fe³⁺ which is formed by the reoxidation of Fe²⁺ under the reducing conditions at 500°C segregates to the interface of the bimetallic phase and the titania support. It is suggested that continued treatment at 700°C produces a high dispersion of iron which is coordinated to oxygen atoms of the support. The ca. 20 Å clusters of metallic ruthenium may be envisaged as being anchored to the support via iron-ruthenium bonds     

Effect of Sn on methane decomposition over Fe supported catalysts to produce carbon

In this work, alumina-supported Sn containing Fe catalysts were investigated in CVD reactions (Chemical Vapor Deposition) using methane for carbon production. The catalysts were prepared with 10 wt.% of Fe (as Fe₂O₃) and 3, 6 and 12 wt.% of Sn (as SnO₂) supported on Al₂O₃ named hereon Fe10Sn3A, Fe5Sn6A and Fe10Sn12A, respectively. These catalysts were characterized by SEM, TPCVD, TPR, TG, Raman, XRD and ⁵⁷Fe and ¹¹⁹Sn Mössbauer spectroscopy. Methane reacts with Fe10A catalyst (without Sn) in the temperature range 680?C900°C to produce mainly Fe⁰, Fe₃C and 20 wt.% of carbon deposition. TPR and TPCVD clearly showed that Sn strongly hinders the CH₄ reaction over Fe catalyst. ⁵⁷Fe Mössbauer suggested that in the presence of Sn the reduction of Fe^?+?3 by methane becomes very difficult. ¹¹⁹Sn Mössbauer showed Sn^?+?4 species strongly interact with metallic iron after CVD, producing iron-tin phases such as Fe₃SnC and FeSn₂. This interaction Sn?CFe increases the CVD temperatures and decreases the carbon yield leading to the production of more organized forms of carbon such as carbon nanotubes, nanofibers and graphite.     

Phase transformation of strontium hexagonal ferrite

The phase transformation of strontium hexagonal ferrite (SrFe₁₂O₁₉) to magnetite (Fe₃O₄) as main phase and strontium carbonate (SrCO₃) as secondary phase is reported here. SrFe₁₂O₁₉ powder was obtained by a heat treatment at 250 °C under controlled oxygen flow. It was observed that the phase transformation occurred when the SrFe₁₂O₁₉ ferrite was heated up to 625 °C in confinement conditions. This transformation took place by a combination of three factors: the presence of stresses in the crystal lattice of SrFe₁₂O₁₉ due to a low synthesis temperature, the reduction of Fe³⁺ to Fe²⁺ during the heating up to 625 °C, and the similarity of the coordination spheres of the iron atoms present in the S-block of SrFe₁₂O₁₉ and Fe₃O₄. X-ray diffraction analysis confirmed the existence of strain and crystal deformation in SrFe₁₂O₁₉ and the absence of them in the material after the phase transformation. Dispersive X-ray absorption spectroscopy and Fe⁵⁷ Mössbauer spectroscopy provided evidences of the reduction of Fe³⁺ to Fe²⁺ in the SrFe₁₂O₁₉ crystal.     

Mössbauer spectroscopy on57Fe in highT c superconductor Bi2Sr2Ca1Cu2O y

Mössbauer measurements were performed on polycrystalline⁵⁷Fe: Bi₂Sr₂Ca₁Cu₂O _y , super-conductor in the temperature range of 77–296 K. The samples were obtained in a solid phase synthesis using 0.01, 0.03, 0.1 and 0.5 mol fractions of α-Fe₂O₃ (96% enriched in⁵⁷Fe). A prevailing quadrupole doublet practically independent of temperature and iron concentration characterizes the obtained Mössbauer spectra. The corresponding hyperfine parameters suggest the presence of high spin Fe¹¹¹ ions in a strongly distorted octahedral symmetry which indicates a probable copper substitution by iron in the system.     

The performance of iron-containing catalysts prepared at low temperature for carbon monoxide hydrogenation

The performance for carbon monoxide hydrogenation of amorphous- and crystalline-unsupported iron oxides following low temperature pretreatment in nitrogen, carbon monoxide and hydrogen has been examined. The phase compositions of the catalysts before and after catalytic evaluation have been determined by⁵⁷Fe Mössbauer spectroscopy. Pretreatment of amorphous non-potassium doped precipitates gave the formation of metallic iron catalysts which were catalytically active at low temperatures and which were shown by Mössbauer spectroscopy to be converted during evaluation to iron carbide and the iron oxide Fe₃O₄. Catalysts which were not pretreated were reduced during catalytic evaluation to Fe₃O₄. Pretreated potassium-doped catalysts composed of either iron carbide or a mixture of iron carbide and metallic iron gave hydrocarbon product distributions which showed a higher Schulz-Flory alpha value and a lower selectivity towards methane when the catalyst reached steady state as a result of an increase in carbon monoxide adsorption and/or a decrease in hydrogen adsorption. The used catalysts were shown by Mössbauer spectroscopy to contain iron carbide together with various proportions of metallic iron and the iron oxide Fe₃O₄. The activities of the pretreated amorphous and crystalline catalysts were comparable and may be related to the disintegration of the crystalline catalysts during pretreatment in carbon monoxide which induces the formation of particles with surface areas similar to those observed in the amorphous catalysts.     

Reduction properties of perovskite-related rare earth orthoferrites

Perovskite-related materials of composition LaFe_1?x Co _x O₃ have been prepared by conventional calcination methods and by mechanical milling methods. The reduction properties in a flowing mixture of hydrogen and nitrogen have been monitored by temperature programmed reduction techniques and the nature of the reduced phases characterised by X-ray powder diffraction, X-ray absorption spectroscopy and⁵⁷Fe Mössbauer spectroscopy. The smaller particle materials made by mechanical milling methods are more susceptible to reduction than their counterparts made by conventional solid state methods. The presence of cobalt enhances the reducibility of iron in the perovskite-related structures. In iron-rich systems more limited reduction of iron and cobalt leads to the segregation of discrete metallic phases without destruction of the perovskite-related structure at temperatures up to 1200°C whereas in cobalt-rich systems the reduction of Fe³⁺ and Co³⁺ leads to significant segregation of alloy and metallic phases and is accompanied by destruction of the perovskite-related structure.     

Specific features of magnetic states of impurity iron ions in the perovskite La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>Co<Subscript>0.98</Subscript> <Superscript>57</Superscript>Fe<Subscript>0.02</Subscript>O<Subscript>3</Subscript>

Single-phase polycrystalline La_0.75Sr_0.25Co_0.98⁵⁷Fe_0.02O₃ samples have been prepared by solidstate ceramic technology. The samples have the rhombohedral structure (space group \(R\bar 3c\)). The studies of perovskite La_0.75Sr_0.25Co_0.98⁵⁷Fe_0.02O₃ by Mössbauer spectroscopy on impurity ⁵⁷Fe nuclei in the temperature range of 5–293 K have revealed the existence of a superparamagnetic relaxation in the temperature range of 100–210 K. The parameters of hyperfine interactions (hyperfine magnetic fields, line shifts, and quadrupole shifts) and the anisotropy energy have been measured, and the frequencies of magnetic moment relaxation of iron ions have been estimated.     

Effect of FeCl3 and acetone on the structure of Na–montmorillonite studied by Mössbauer and XRD measurements

⁵⁷Fe Mössbauer spectroscopy, X-ray diffraction, X-ray fluorescence spectroscopy and infrared spectroscopy were used to study the effect of FeCl₃ and acetone on the structure of a Na–bentonite. XRD indicated the incorporation of Fe³⁺ ions into the interlayer space since the basal lattice spacing of montmorillonite increased to 1.6 from 1.24 nm after treatment with FeCl₃ dissolved in acetone. Interlayer Na⁺ ions could be exchanged to Fe³⁺. Magnetically split Mössbauer subspectra with internal magnetic fields 41 and 46 T at 74 K, were associated with two main Fe³⁺ microenvironments within the interlayer regions. The resultant Fe–montmorillonite was successfully applied as a catalyst in the preparation of 1,1-diacetates from aromatic aldehydes and acetic acid anhydride.