Characterization of Natural Chromite Samples from Ophiolite Complexes in the Philippines by 57Fe Mössbauer Spectroscopy

Mössbauer spectroscopy was applied to natural chromite samples from ophiolite complexes in the Philippines. Chemical and structural characterization of the chromite samples was also carried out using X-ray absorption fine structure (XAFS), X-ray diffraction (XRD) and X-ray fluorescence spectrometry (XRF). The Mössbauer spectra of the samples consisted of quadrupole doublets ascribable to Fe³⁺ in octahedral site, Fe³⁺ in tetrahedral site, and Fe²⁺ in tetrahedral site. The relative percentage of Fe³⁺ and Fe²⁺ ions suggested that these Philippine samples were formed under relatively high oxygen fugacity.     

Quantitative multielemental analysis of titaniferous magnetites

Two representative titaniferous magnetite samples procured from Moulabhanj, Orissa, India have been studied by PIXE, EDXRF, Mössbauer spectroscopy, and XRD techniques. Major iron-bearing phases identified in the samples by Mössbauer spectroscopy and XRD are magnetite, hematite, ferrous ilmenite and ferric ilmenite. The Fe²⁺/Fe³⁺ ratio and the relative percentages of different minerals were determined from the resonance areas of Mössbauer spectra. Quantitative multielemental analysis was carried out by energy dispersive X-ray fluorescence (EDXRF) and proton induced X-ray emission (PIXE). Nineteen minor and trace elements have been quantified by EDXRF whereas by PIXE eighteen elements have been analyzed quantitatively. Concentrations of trace elements determined by EDXRF and PIXE were used in interpreting the physico-chemical condition of the depositional basin.     

Characterization of silica supported Rh-Fe catalysts by Mössbauer technique

Silica supported Rh–Fe catalysts were characterized by means of in situ⁵⁷Fe Mössbauer spectroscopy. The Mössbauer spectra indicated that iron on the silica support existed either as Fe/O/ in the Rh–Fe alloy or as Fe³⁺ in Rh–Fe metal cluster compounds. The (Fe³⁺/Fe/O/) ratio and Mössbauer parameters were found to depend on the (Fe/Rh) atomic ratio in the catalysts. Such dependence corresponded to the change of catalytic properties of the supported Rh–Fe catalyst with the varying (Fe/Rh) ratio.     

Mössbauer Spectroscopy and Preparation of Hg-1223, Tl-1223 and Tl-1212 Superconductors

Introduction of the Mössbauer nuclei ⁵⁷Fe into Hg-1223 phase and that of ⁵⁷Fe and ¹⁵¹Eu into Tl-1212 and Tl-1223 superconductors were investigated. Samples of high phase purity were obtained. Scanning electron microscopy and optical microscopy in normal and polarized light were employed to study the microstructure of the specimens. Energy dispersive X-ray analysis showed that ⁵⁷Fe in the Hg-based samples and ⁵⁷Fe as well as ¹⁵¹Eu in the Tl-based compounds, entered superconducting phases. Incorporation of Eu³⁺ into the superconducting phase favored the formation of the Tl-1212 phase. Mössbauer spectroscopy showed that Eu³⁺ entered the Ca-site. Two doublets found in the ⁵⁷Fe Mössbauer spectra in both the Hg-1223 and the Tl-1223 phase referred to two different micro-environments of Fe³⁺. The assignment of the ⁵⁷Fe Mössbauer spectra was made under the assumption that Fe favored the 1223 phase. Fe³⁺ may replace Cu in both the square pyramidal, five-fold oxygen coordinated Cu sites between the Ca and Ba-(Sr)-O layers, and in the square planar, four-fold oxygen coordinated Cu sites the Ca layers in the superconducting phases in both the Hg- and the Tl-based materials. From the relative areas of the two doublets, we concluded that the Fe³⁺ preferred the square planar Cu site.     

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.     

Oxide Transformation During Preparation of Black Pottery in Hungary

Traditional black pottery produced in Nádudvar, E-Hungary, was studied by ⁵⁷Fe Mössbauer spectroscopy, X-ray diffractometry and microscopy. Quartz, feldspar, clay minerals (kaolinite, smeetite, illite) and calcite were identified in the basic clay material by X-ray diffractometry (XRD). Mössbauer spectroscopy (MS) of the original clay revealed that about 35% of iron compounds were present in goethite while the rest in clay minerals (illite and smectite). After firing the clay in air using an electric furnace (red pottery is prepared in the same way), the Mössbauer spectra showed hematite as the only iron oxide or hydroxide phase, being in good agreement with X-ray diffractometry. In the black product itself, fired in the traditional open-flame furnace, the Mössbauer spectra reflected the presence of iron in magnetite and in sheet silicates with approximately the same relative ratio of oxides and silicates as in the starting material. This can be interpreted as a result of the transformation of goethite to hematite in the first step of firing (in air), and as a reduction of hematite to magnetite in the second step of firing (closed from air). A significant difference was found in the distribution of iron at the Fe²⁺ and Fe³⁺ cation sites in the black surface (more Fe²⁺) and at the dark gray bulk of the fired pottery (less Fe²⁺), showing that the reduction of Fe³⁺ occurs in the silicates instead of further reduction of the magnetite (e.g., to wüstite).     

Mössbauer studies of styrene-acrylonitrile copolymers containing ferric chloride

Acrylonitrile-styrene /AN-St/ copolymers of different compositions were prepared, with and without ferric chloride by free radical polymerization. It was found using Mössbauer spectroscopy that reduction of Fe³⁺ to Fe²⁺ takes place during the polymerization. The addition of ferric chloride and the reduction of Fe³⁺ was found to influence the thermal stability of the copolymers.     

A new analytical method for 32P: Liquid scintillation counting with solvent extraction

⁵⁷Fe Mössbauer spectroscopy has been applied to the ultramafic rocks collected from the Jinchuan nickel deposit in China to elucidate their mineralization process. Their Mössbauer spectra consisted of two sextets ascribable to magnetite, two doublets ascribable to Fe²⁺ and Fe³⁺ in chromite, and one doublet ascribable to olivine. The closest sample to the ore body did not contain chromite and contained a doublet ascribable to pyrite and a sextet ascribable to pyrrhotite. The valence and site distribution of iron species suggested low oxygen fugacity for the formation of the Jinchuan nickel deposit.     

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.     

Mössbauer spectroscopic characterization of some Hungarian zeolites utilized as catalysts

Some Hungarian zeolites have been studied by Mössbauer spectroscopy. It has been found that Fe³⁺ ions substitute for Si⁴⁺ in tetrahedral position, while Fe²⁺ ions are chargecompensating cations in octahedral sites.     

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.     

Structural studies on some iron 8-oxi-quinolinates using Mössbauer spectroscopy

Using Mössbauer spectroscopy the quinolinates of iron/II/ and iron/III/ have been studied. In iron/II/ quinolinate three sublattices were evidenced, two of them being attributed to Fe²⁺ ions and the third to Fe³⁺ impurities. In the iron/III/ quinolinate five structural sublattices were found, two of them containing Fe³⁺ ions, the other two Fe²⁺ ions and the fifth may be attributed to the interstitial Fe³⁺ ions.     

Mössbauer, vibrational spectroscopic and solution X-ray diffraction studies of the structure of iron(III) complexes formed with indole-3-alkanoic acids in acidic aqueous solutions

The chemical reactions between iron(III) and indole-3-acetic (IAA), -propionic (IPA), and -butyric (IBA) acids were studied in acidic aqueous solutions. The motivation of this work was that IAA is one of the most powerful natural plant-growth-regulating substances (phytohormones of the auxin series). Mössbauer spectra of the frozen aqueous solutions of iron(III) with indole-3-alkanoic acids as ligands (L), showed parallel reactions between Fe³⁺ and the ligands. Partly, it resulted in a complex formation which precipitated in aqueous solution and partly, in a redox process with iron(II) and the oxidised indole-3-alkanoic acids as products. The Mössbauer parameters of the Fe²⁺ species suggested a hexaaquo coordination environment. The chemical composition and coordination structure of the precipitated complexes were investigated using elemental analysis, Mössbauer spectroscopy, Fourier transform infrared (FTIR) and Raman spectroscopic techniques. The complexes were soluble in some organic solvents. So, Mössbauer, FTIR and solution X-ray diffraction measurements were carried out on the solution of complexes in acetone, hexadeutero acetone and methanol, respectively. The data obtained supported the existence of the μ-dihydroxo-bridging structure of the dimer: [L₂Fe<(OH)₂>FeL₂] (where L is indole-3-propionate, -acetate or -butyrate).     

Interplay between Cu and Fe Valences in BaR(Cu0.5Fe0.5)2O5+δ Double Perovskites with R=Lu, Yb, Y, Eu, Sm, Nd, and Pr

We have conducted a systematic ⁵⁷Fe Mössbauer study on BaR(Cu_0.5Fe_0.5)₂O_5+δ double perovskites with various oxygen contents and rare-earth elements (R=Lu, Yb, Y, Eu, Sm, Nd, and Pr). In samples based on R=Lu, Yb, Y, Eu, Sm the oxygen content remained at δ≈0, upon reductive or oxidative heat treatments under normal pressure. The larger rare-earth elements, i.e. Nd or Pr, readily allowed for continuous oxygen content tuning up to δ≈0.3. By employing high-pressure heat treatments higher oxygen contents were achieved for all samples. The Néel temperature of the samples was found to decrease with increasing amounts of oxygen entering the lattice. In high-pressure oxygenated samples the decrease was less severe indicating that despite the incorporation of oxygen a large amount of Fe still remains in the high-spin trivalent state. By using charge-neutrality arguments together with the relative intensities of the Mössbauer spectral components the average valences of Fe and Cu were obtained. Oxygenation under normal pressure led to a corresponding increase of the valence of Fe, while Cu remained divalent. Upon high-pressure heat treatment equal amounts of Fe³⁺ and Cu²⁺ were found to be oxidized to Fe⁵⁺ and Cu³⁺, respectively.     

57Fe Mössbauer spectroscopic characterisation of a ferromanganese nodule from the Central Indian Ocean

The iron bearing phases present in a ferromanganese nodule from the Central Indian Ocean have been determined using⁵⁷Fe Mössbauer spectroscopy. The Mössbauer results have been corroborated by XRD, IR and TG-DTA studies. The Mössbauer spectrum of a ferromanganese nodule shows a broad line width which indicates the presence of more than one iron bearing paramagnetic oxide or oxyhydroxide phases where iron is present as Fe³⁺. -FeOOH has been distinctly characterised as one of the iron bearing phases in the nodule. Other oxyhydroxide and oxide phases of iron in the nodule have been ruled out. A typical paramagnetic doublet persists even at very high temperature which has been proposed to be due to iron(III)phosphate. Formation of solid solution of Mn₂O₃–Fe₂O₃ has been observed in the heat treated nodule at 1073 K, which has been characterised by the Mössbauer technique.     

151Eu and57Fe Mössbauer and X-ray diffraction study of high temperature Tl-containing superconductor

⁵⁷Fe and¹⁵¹Eu Mössbauer spectroscopy and X-ray diffractometry measurements were performed with europium and iron containing Tl₂(Ca_0.8Eu_0.2)Ba₂(Cu_0.98Fe_0.02)₂O₈ and EuBa₂Cu₃O_7-d high temperature superconductor samples in order to study the effect of europium incorporated in a Tl-containing high temperature superconductor. Significant differences have been found between the⁵⁷Fe Mössbauer spectra of Tl-containing samples with and without Eu or between the¹⁵¹Eu Mössbauer spectra of Tl-containing and 1-2-3 type superconductors. The results can be interpreted by assuming that Eu can be localized at the Ca site in the Tl-containing superconductor.     

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.     

Impact of structural features on pigment properties of α-Fe2O3 haematite

Various α-Fe₂O₃ haematite samples were synthesized by precipitation routes (under standard or hydrothermal conditions) followed by thermal treatments under air. The trigonal distortion (C_3v point group) of the Fe³⁺ octahedral sites, which depends on the synthesis route and thermal treatment, was investigated by X-ray diffraction, Mössbauer spectroscopy and visible-near infrared (Vis-NIR) spectroscopy. The correlation between diffuse reflectance spectra and structural features of the haematite samples is reported and discussed herein. The slight increase of the average distortion of the Fe³⁺ octahedral sites, which depends on the annealing temperature of the precipitated sample, directly linked to the crystallite size, contrasts with the larger reduction of the sites distortion for the compound prepared by hydrothermal route due to the occurrence of hydroxyl groups substituted for O²⁻ anions as well as Fe³⁺ cationic vacancies. On a local point of view, as shown by Mössbauer spectroscopy, the Fe³⁺ octahedral sites distortion decreases from the centre towards the surface of the grains. Then the smaller the grain size, the lower the average site distortion. Finally, the reduction of the octahedral distortion was directly correlated to the two FeO charge transfer bands in the visible range and the colour of as-prepared haematites.     

Structure and magnetic properties of Ca2Fe1−xMnxAlO5+δ

Ca₂Fe_1−xMn_xAlO₅ (0?x?1) compounds were prepared by a self-combustion method under air (x=0, 0.1, 0.2 and 0.3) and nitrogen (x=0.5, 0.7 and 1.0). The samples prepared under nitrogen were successfully oxidized after short annealing under air. Both X-ray powder diffraction (XRD) Rietveld analysis and electron diffraction revealed that all compounds adopt the brownmillerite-type structure. All samples present an overall antiferromagnetic behaviour and data from magnetic measurements and Mössbauer spectroscopy allowed to conclude that the transition temperature decreases as Mn content increases for x?0.3 and increases in the case of the x?0.5 compounds. Except for x=1, chemical disorder due to the occupancy of both octahedral and tetrahedral sites by different metals as well as the competition between different moments’ orientation induce a complex magnetic behaviour characterized by magnetic frustration and canted antiferromagnetism. Mössbauer spectroscopy and chemical titrations also allowed to conclude about the preferential oxidation of Mn³⁺ over Fe³⁺, obtained by thermal treatment under air of the x=0.5 and 0.7 compositions.     

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.