Chemical reduction of hematite with starch

Chemical reduction of hematite with starch in air at elevated temperature was investigated using X-ray powder diffraction, FT-IR and ⁵⁷Fe Mössbauer spectroscopies. On heating the starting mixture for 0.5 and 2 hours at 300 °C, magnetite and a small fraction of hematite were identified by XRD. With the heating time prolonged up to 24 hours, magnetite reoxidized and hematite was obtained again. The formation of magnetite was observed even at 580 °C. However, the magnetite formed at this temperature was substoichiometric, as shown by XRD and Mössbauer spectroscopy. Characteristic IR bands of oxide phases were monitored by FT-IR spectroscopy. Chemical reduction of hematite with starch into a Fe⁰ state was not observed in any sample.     

Nanocomposites NiFe2O4/SiO2 and CoFe2O4/SiO2-Preparation by Sol-Gel Method and Physical Properties

This paper aims to characterise the systems NiFe₂O₄/SiO₂ and CoFe₂O₄/SiO₂ prepared by the sol-gel method. After heat treatment, the various samples have been studied by means of X-ray diffraction, Mössbauer spectroscopy, magnetic measurements and transmission electron microscopy (HR TEM).X-ray diffraction and Mössbauer spectra confirmed the presence of the spinel phase. HR TEM observations revealed the nanocrystals with the size in the range of 2–25 nm. Magnetic measurements showed a superparamagnetic behaviour of the samples heated at lower temperature (800°C) and ferrimagnetic character for the samples heated at higher temperature (900, 1000°C).The final phase composition of the heated samples depends on the preparation conditions. The samples, treated up to 300°C in vacuum and then subsequently heated at 800°C or 900°C, do not contain hematite (the most stable phase at higher temperatures). On the contrary, the samples heated at 1000°C or 1250°C display certain content of hematite.     

M?ssbauer study of transformation mechanism of Fe cations in olivine after thermal treatments in air

The transformation mechanism of Fe cations in natural olivine after thermal treatments in air has been studied using mainly⁵⁷Fe Mössbauer spectroscopy. -Fe₂O₃ nanoparticles appear as the primary Fe³⁺ phase in Mössbauer spectra of olivine samples heated at 600-900 °C. These nanoparticles are thermally unstable and they are transformed to -Fe₂O₃ with the increase of heating time. Another transformation mechanism of iron related with the complete decomposition of olivine structure has been observed at temperatures of 1000 °C and higher. The mixed oxide MgFe₂O₄ with the spinel structure and enstatite MgSiO₃ were identified as iron-bearing decomposition products.     

A Mössbauer study of oxygen vacancy and cation distribution in 6H-BaTi1−xFexO3−x/2

We have synthesized samples in the system BaTi_1−xFe_xO_3−x/2 with x=0.1−0.6 at temperatures of 1200-1300°C under reducing conditions of oxygen fugacity. After drop quenching, samples were characterized using the electron microprobe, X-ray diffraction and Mössbauer spectroscopy. All samples were hexagonal with a 6H-BaTiO₃ type structure. Mössbauer spectroscopy showed all iron to be present as Fe³⁺, occurring in octahedral and pentahedral sites. Analysis of area ratios indicates that oxygen vacancies are distributed randomly over O1 sites, and that a random distribution of Fe and Ti cations over M1 and M2 sites is consistent with the data. No evidence for ordering of oxygen vacancies was found. Results are consistent with conductivity results, which show generally increasing ionic conductivity with increasing oxygen vacancy concentration.     

Preparation and characterization of nanosize nickel-substituted cobalt ferrites (Co1−xNixFe2O4)

Nanosize nickel-substituted cobalt ferrites were prepared using aerosol route and characterized by TEM, XRD, magnetic and Mössbauer spectroscopy. The particle size of as obtained samples was found to be ∼10 nm which increases upto ∼80 nm on annealing at 1200 °C. The unit cell parameter ‘a’ decreases linearly with the nickel concentration due to smaller ionic radius of nickel. The saturation magnetization for all the samples after annealing at 1200 °C lies in the range 47.6-84.5 emu/g. Room temperature Mössbauer spectra of as obtained samples exhibit a broad doublet, suggesting super paramagnetic nature of the sample. The broad doublet is further resolved into two doublets corresponding to the iron atoms residing at the surface and internal regions of the particle. The samples annealed at 1200 °C showed broad sextet, which is resolved into two sextets, corresponding to tetrahedrally and octahedrally coordinated Fe cations. Cation distribution calculated using XRD and Mössbauer data indicates a decrease in Fe³⁺(oct.)/Fe³⁺(tet.) ratio with increasing nickel concentration.     

Mössbauer study of the thermal decomposition of co-precipitated Fe/NH4/2/SO4/2.6H2O and Ni/NH4/2/SO4/2.6H2O

Using Mössbauer spectroscopy, the thermal decomposition products of co-precipitated Fe/NH_4/2/SO_4/2.6H₂O and Ni/NH_4/2/SO_4/2.6H₂O for two hours at various temperatures in open air have been studied and identified. It has been found that NiFe₂O₄, formed at 900 °C and 1100 °C, has been the final product.     

Mössbauer study of hydrogenated Fe-Zr amorphous alloys

The effect of electrochemical hydrogenation was investigated in Fe₉₀Zr₁₀ and Fe₈₉Zr₁₁ amorphous alloys by means of⁵⁷Fe Mössbauer spectroscopy. Significant changes in the Mössbauer spectra as well as in the hyperfine field distribution of hydrogenated samples were found with increasing hydrogen concentration. It was established that the dependence of Curie temperature on hydrogen content had a maximum, and the hydrogen had two mean localization sites. By comparing the Mössbauer spectra of hydrogenated samples as-quenched and annealed before hydrogenation it was shown that low-temperature relaxation processes were going on at aging temperature as low as 150°C in this amorphous alloy and the low-temperature relaxation processes modify the localization of hydrogen. The combination of the hydrogenation and Mössbauer techniques gives a very sensitive method for detecting structural changes.     

Structural and thermal properties of Co-Cu-Fe hydrotalcite-like compounds

The structural properties and thermal decomposition processes of Co-Cu-Fe ternary hydrotalcites (HT) have been studied through X-ray diffraction, thermogravimetric measurements, Fourier-transform infrared and Mössbauer spectroscopies. Due to the strong Jahn-Teller effect, the Cu-Fe layered system is stabilized only in the presence of Co²⁺. At low Co²⁺ contents, additional phases are segregated in the solids. X-ray patterns show the presence of Cu(OH)₂ and CuO. The decomposition process was investigated by in situ X-ray, in situ Mössbauer and FTIR experiments. By increasing the temperature from 25 °C up to 180 °C we observed that the structural disorder increases. This effect has been likely attributed to the Co²⁺→Co³⁺ oxidation since thermal decomposition was carried out under static air atmosphere. Part of the Co³⁺ cations could migrate to the interlayer region, thus forming a metastable compound that still has a layered structure. Collapse of the layered structure was observed at about 200 °C. By further increasing the temperature the system becomes more crystalline and the formation of Co₃O₄ is observed in the X-ray patterns. In Cu-rich HT, some of the carbonate anions are released at temperatures higher than 550 °C and this phenomenon is attributed to the formation of a carbonate-rich phase. The specific surface area data present its highest values in the temperature range where the collapse of the layered structure takes place.     

Phase transitions in the SrSnO3-SrFeO3 solid solutions: X-ray diffraction and Mössbauer studies

SrSn_1−xFe_xO_y (0?x?1) oxides were prepared by conventional solid-state reaction in air using high-purity (?99%) SrCO₃, SnO₂ and Fe(C₂O)₂·2H₂O at elevated temperatures of 1300°C for 24 h and furnace cooled. Samples obtained from 1300°C were annealed at 620°C for 2 days and quenched in liquid nitrogen (LN). Powder XRD analysis by Rietveld refinement and Fe Mössbauer spectroscopy measurements were employed to characterize synthesized perovskites. Samples obtained from furnace cooled and LN quenched undergo two compositionally driven phase transitions, which are supposed to be of second order. The x=0-0.3 members crystallize in orthorhombic parent SrSnO₃ structure (Space group Pbnm), whereas samples x=0.4-0.9 have a simple cubic perovskite cell and end-member SrFeO_2.74 composition crystallize orthorhombic structure (Space group Cmmm). The composition of the first phase transition (x≈0.3) is slightly shifted to higher x with decreasing annealing temperature. Mössbauer data show that the Fe⁴⁺/Fe^tot ratio is depending on composition under constant synthesis conditions. The phase compositions have been discussed in terms of ternary solid solution of compounds SrSnO₃-SrFeO_2.74-SrFeO_2.5 superior to a simple binary solid solution (SrSnO₃-SrFeO₃).     

Structural analysis of mixed tin oxides produced by the sol-gel method

Mixed oxides SiO₂/SnO₂ with 80/20 nominal weight composition have been obtained by the sol-gel method with different precursors. X-ray diffraction and low temperature transmission Mössbauer spectroscopy have been used to follow the structural evolution of the samples after treatments in the temperature range RT-1050 °C. The main results are that changes in the precursor nature and gel preparation affect the Sn(IV) homogeneity in the SiO₂ matrix and that the use of Sn-alkoxides increases the content of residual carbon which promotes reduction of Sn(IV) at intermediate temperatures.     

Study of the crystallization kinetics in amorphous Fe83P17 alloy

The crystallization kinetics of Fe₈₃P₁₇ amorphous alloy has been studied by Mössbauer spectroscopy and X-ray diffractometry. The samples were annealed isothermally at two different temperatures (315 °C and 325 °C). During isothermal annealing of the samples three phases were observed: crystalline Fe₃P phase, crystalline -Fe phase and the amorphous phase. The value of the Avrami exponent was found to be about 2.0 at each annealing temperature. This suggests that the growth rate of the crystals is controlled by volume diffusion and the nucleation rate decreases during crystallization. The activation energy obtained for the overall crystallization process was 193±43 kJ mol^–1.     

LaxSr2−xFeyRu1−yO4±δ: a new family of K2NiF4 type oxides

The phases La_xSr_2−xFe_yRu_1−yO_4±δ (x=0.2-0.8; y=0.6-0.9) have been synthesized by solid-state techniques and yield tetragonal structures with I4/mmm symmetry. The oxygen stoichiometry and high-temperature structures have been examined using diffraction techniques and in situ Mössbauer spectroscopy at temperatures up to ∼600°C. Furthermore, new reduced phases that adopt structures with Immm symmetry have been discovered. Unusual coordination numbers have been determined for the most highly reduced samples with square planar coordination evident for the B site cations. The reduced orthorhombic Immm phases were found to readily reoxidize in air to the tetragonal I4/mmm structure at relatively low temperatures of only ∼500°C.     

Mössbauer spectroscopic studies of thermal decomposition of substituted pentacyanoferrate(II) complexes

The thermal behaviour of substituted pentacyanoferrates(II) of the type Na₃[Fe(CN₅)L]·xH₂O, whereL=n-, sec-, tert- oriso-butylamine,di-iso-butylamine ortri-n-butylamine, was investigated with the aid of Mössbauer spectroscopy, XRD and TG-DTG-DTA. The Mössbauer spectra of these complexes exhibit a quadrupole doublet with E _Q=0.70–0.83 mm s^–1 at room temperature. The isomer shift, =0.00±0.03 mm s^–1 suggests that the iron atom is in the +2 low-spin state. The complexes start to decompose at 50°C, yielding a residual mass of 5.8 –21.3% in the temperature range 900–950°C. The Mössbauer spectra recorded after heating at 150 and 300°C exhibit an asymmetric doublet, suggesting partial decomposition. The Mössbauer spectra at higher temperature are complex. At different stages of the thermal process, the presence of -Fe₂O₃, -Fe₂O₃, -Fe, Fe₃C and Fe₃O₄ was demonstrated.On leave from A. N. College, Anandwan-442 914, IndiaWe are grateful to the Monbusho (Ministry of Education, Science, Sports and Culture) for the award of a fellowship to RBL and for financial assistance for the research work. Thanks are also due to Dr. T. Nakamoto for valuable cooperation.     

Studies on InFeMO4 (M=Mg, Co, Ni, Cu and Zn) compounds: Crystal structure and cation distribution

The crystal structure and the cation distribution in a series of InFeMO₄ compounds (M=Mg, Co, Ni, Cu and Zn) have been studied by means of X-ray powder diffraction and ⁵⁷Fe Mössbauer spectroscopy. The M=Mg, Co and Ni samples were confirmed to crystallize with the cubic spinel structure (space group Fd-3m), whereas the M=Cu and Zn samples adopted a hexagonal structure. For all the phases, the cation stoichiometry was found to deviate from the ideal molecular formula, InFeMO₄. The paramagnetic Mössbauer spectra of the samples were analyzed using a four-component fitting model suggested by a statistical simulation with point-charge calculation. The Mössbauer data confirmed the trivalent state for iron at both cation sites in all samples. The results from the fitting of the Mössbauer spectra were also employed in Rietveld refinement of the X-ray diffraction data for the determination of exact cation distribution. It was seen that the distribution of Fe at the A and B sites follows very closely the 1:2 ratio of the ideal formula AB₂O₄ for all samples, whereas trivalent indium was clearly seen to favor the A site and divalent M cation the B site.     

Mössbauer Study of the Structure of Fe - Zircon System

Iron-doped silicate (zircon), prepared by a ceramic method with the addition of LiF as mineralizer, was analyzed by X-ray powder diffraction (XRD) and ⁵⁷Fe Mössbauer spectroscopy to obtain information on the solid solution formation. The results of X-ray diffraction and Mössbauer spectroscopy have shown that only a small fraction of iron, about 1.5 mol%, is incorporated in the zircon structure as paramagnetic Fe³⁺ species while the remaining Fe³⁺ cations form magnetic -Fe₂O₃ particles which are trapped within the zircon matrix.     

Mössbauer studies in thermally treated iron-samarium mixed oxide system

Mössbauer spectroscopy and X-ray diffractometry have been used to study Sm–Fe mixed oxides (with different SmFe atomic ratios) annealed at 550, 850, 1000 and 1250 °C. The room temperature Mössbauer spectra can be interpreted in terms of one, two or three sextets and in some cases by an additional doublet depending on the composition and the heat treatment. The sextets have been associated with SmFeO₃ perovskite, Sm₃Fe₅O₁₂ garnet and -Fe₂O₃ hematite. These results are in agreement with those of X-ray diffractometric measurements, which give a clear, evidence of the presence of these phases.     

Local structure and chemical durability of FeOOH-fixed sodium silicate glass prepared from water glass

A 12M HCl solution of iron oxyhydroxide (a-FeOOH: goethite) was mixed with water glass (18Na₂O^.36SiO₂ ^.46H₂O) at room temperature. The mixture (sol) changed into a dry gel when dried at 25 °C for 120 hours in air. Glass-ceramic and glass samples were prepared when the dry gel was heated for 1-3 hours in an electric furnace at 800 and 900 °C, respectively. The ⁵⁷Fe Mössbauer spectrum of the dry gel is composed of a magnetic hyperfine structure owing to the formation of g-FeOOH (lepidocrocite). By contrast, the ⁵⁷Fe Mössbauer spectrum of glass-ceramic and glasses is composed of paramagnetic Fe(III) with distorted tetrahedral symmetry. This proves that Fe(III) atoms occupy network-forming Si(IV) sites in the FeOOH-fixed sodium silicate glass. A leaching test of the silicate glass in the acid rain simulant composed of HNO₃ (pH 3.5) and H₂SO₄ (pH 3.5) revealed high chemical durability, indicating that Fe(III) is firmly fixed in the glass matrix.     

Determination of the Mössbauer parameters of rare-earth nitroprussides: Evidence for new light-induced magnetic excited state (LIMES) in nitroprussides

Nitroprussides of the rare-earth elements and some mixed rare-earth-sodium nitroprussides are studied by Mössbauer spectroscopy at ambient and lower temperatures. The high precision Mössbauer measurements reveal fine changes in the electronic configurations of the nitroprusside anions. A small increase of the quadrupole splitting reveals charge polarization effects in the nitroprusside anion caused by the oblate or prolate shape of the rare-earth ion and the lanthanide contraction. Despite the very large magnetic moment of holmium a magnetic phase transition is not observed down to 300 mK. The population of the metastable states SI and SII are evidenced in europium and scandium nitroprussides, and most likely they can be populated in all rare-earth nitroprussides. No distinct correlation between the Mössbauer parameters and the decay temperatures T_c of the metastable states are found. In a very thin surface layer strong color change, which remains stable at room temperature, is detected. A quadrupole doublet with Mössbauer parameters typical for Fe(III), low spin S=1/2 state is related to a new colored photoproduct. The photoproduct is called light-induced magnetic excited state (LIMES) and explained with a photochemical redox reaction, which changes the valence, spin, and magnetic state of 4f-3d bimetallic complexes.     

Iron-Iridium Mixed-Metal Carbonyl Clusters: A Mössbauer Study of the Structure and of the Adsorption of [Fe2Ir2(CO)12]2− on MgO

¹⁹³Ir and ⁵⁷Fe Mössbauer spectroscopy was used to investigate the structure of the [Fe₂Ir₂(CO)₁₂]^2- cluster compound and the adsorption of this cluster on hydrated MgO. Supported samples were prepared by impregnation of the magnesia with solutions of [Et₄N]₂[Fe₂Ir₂(CO)₁₂] in acetone. The Mössbauer and FT-IR spectra of the MgO-supported cluster confirm that the bimetallic carbonyl is molecularly physisorbed onto MgO without undergoing any transformation or decomposition. The easy solvent extraction of the intact cluster from the oxide surface excludes ion pairing between the cluster anion and the Mg²⁺ surface sites. Mössbauer spectra are in agreement with the refined structure of the molecular cluster and the temperature dependence of the ⁵⁷Fe Mössbauer spectra above 80 K is consistent with the low degree of interaction of the cluster with the support. This technique, therefore, appears to be promising in order to infer structural information when X-ray determination fails.     

Coexistence of Rutile and Trirutile Phases in a Natural Tapiolite Sample

We report X-ray powder diffraction (XRD), electron probe microanalysis (EPMA), and Mössbauer spectroscopy (MS) measurements performed on a natural tapiolite with composition Fe_0.57Mn_0.37Ti_0.10Ta_1.27Nb_0.67O₆. XRD and MS suggest that besides being partially ordered the as-collected sample is a mixture of trirutile (P4₂/mnm, a=4.7532(9) Å, c=9.228(7) Å) and Nb-rich rutile (P4₂/mnm, a=4.856(2) Å, c=3.098(1) Å) structures. The Mössbauer spectra of the rutile (Fe, Mn, Ta, Nb)O₂ were fitted to Δ=1.72±0.05 mm/s and δ=1.10±0.03 mm/s at 300 K and to Δ=2.10±0.06 mm/s and δ=1.18±0.03 mm/s at 80 K. The present results suggest that cation ordering in compounds of the tapiolite series can be easily assessed by Mössbauer spectroscopy in a way similar to that as previously demonstrated for the columbite series.