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.     

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.     

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.     

Valence states of 57Fe decayed from 57Mn implanted into KMnO4

In-beam Mössbauer spectra of ⁵⁷Fe, decayed from short-lived ⁵⁷Mn (T_1/2 = 1.45 min) implanted into potassium permanganate, KMnO₄, were measured at temperatures between 11 K and 130 K. This is the first application of a secondary RI beam to the study of valence states after nuclear transformation. The in-beam Mössbauer spectra obtained below 90 K could be analyzed with two components, a doublet and a singlet. From the calculations of the molecular orbital wave functions, the singlet is suggested to be substitutional ⁵⁷Fe atoms for Mn-sites in tetrahedral [MnO₄]^– with an unusually high valence state of Fe⁸⁺.     

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.     

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.     

Cation distribution and magnetic properties in chromium-substituted nickel ferrites prepared using aerosol route

Cation distribution have been investigated using X-ray diffraction, magnetic and Mössbauer spectral studies in chromium-substituted nickel ferrites prepared by aerosol route. Cation distribution indicates that the chromium atom occupy octahedral site upto x=0.8, and then also enters into tetrahedral site. The saturation magnetization decreases linearly with the increase of chromium concentration due to the diamagnetic nature of the Cr³⁺. However, interesting behaviour is observed in the coercivity. Initially it increases slowly with the chromium concentration but when x>0.8 a very large increase has been observed. This was attributed to the specific cation distribution of Cr³⁺ which results an unquenched orbital angular momentum and a large anisotropy. Room temperature Mössbauer spectra of as obtained samples exhibited a broad doublet resolved into two doublets corresponding to the surface and internal region atoms. The samples annealed at 1200 °C show broad sextets, which were fitted with different sextets, indicating different local environment of both tetrahedral and octahedral coordinated iron cation.     

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.     

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).     

57Fe- and119Sn-Mössbauer effect of oxide glasses

Coordination number of network-former (NWF) and formation of nonbridging oxygen (NBO) at the site neighboring to NWF can be estimated from Mössbauer measurements, since small amounts of Fe³⁺ and Sn⁴⁺ substitute NWF in several oxide glasses. Gamma-ray or thermal neutron irradiation of oxide glasses causes electron or charge transfer from oxygen to the Mössbauer ions, and the probability depends on the fraction of NBO. On the contrary, -ray irradiation of phosphate glasses results in oxidation of Fe²⁺ to Fe³⁺ since iron plays a role of network modifier (NWM) at interstitial sites. Debye temperature _D obtained from low-temperature Mössbauer measurements reflects the site occupation of Mössbauer ions in glasses. A linear relationship between glass transition temperature (T _g ) and quadrupole splitting () of Fe³⁺,T _g — rule, is also effective for determining the site occupation of Mössbauer ions.     

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 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.     

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.     

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 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.     

Preferential Spin Orientation of Fe2+ Towards the c-Axis in FexNi1-xCl2 System (0.034, 0.068 and 0.079)

Fe _x Ni_1-x Cl₂ is a mixed metal chloride and a random mixture with competing orthogonal spin anisotropies. In pure systems, Fe²⁺ shows a preferential spin inclination to the c-axis, while Ni²⁺ shows a weak spin anisotropy in the c-plane. Mössbauer spectra of the Fe _x Ni_1-x Cl₂ system (0.034<x<0.079) observed below the Néel temperature (T _N ) are composed of two types of sub-spectra: spectra I and II. We report the Mössbauer study of the magnetic behavior of different types of Fe²⁺ spins.     

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, 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).     

Iron(III) complexes of a tetradentate tripodal ligand N,N′,N″-tris (2-benzimidazolyl-methyl)amine

Summary Fe^III complexes of a tetradentate ligand with pendant benzimidazolyl groups have been synthesized and characterized. Room temperature Mössbauer spectra depict a quadruple split doublet in the case of NO _inf3 ^p– as co-ligand, while a nearly symmetrical one line spectrum is obtained for complexes with Cl^–as co-ligand. The isomer shift values are towards the lower end of the range found for other high spin Fe^III complexes. ¹H-n.m.r. spectra of the complexes reveal relatively broad linewidths with large isotropic shifts. Paramagnetically shifted resonances are observed in the range of –10.0–+70.0 p.p.m.Author to whom all correspondence should be directed.     

57Fe Mössbauer Investigation of the Substitutional Effect of Praseodymium in 1-2-3 Compounds

⁵⁷Fe Mössbauer spectroscopy measurements were performed on the perovskite compounds Eu_0.7Pr_0.3Ba₂(Cu_0.99 ⁵⁷Fe_0.01)₃O_7-, EuBa_1.5Pr_0.5(Cu_0.99 ⁵⁷Fe_0.01)₃O_7- and EuBa_1.3Pr_0.7(Cu_0.99 ⁵⁷Fe_0.01)₃O_7-. The observed ⁵⁷Fe Mössbauer spectra provided an evidence for the correct site assignment of subspectra originating from ⁵⁷Fe in different microenvironments. Apart from a minor component which was assigned to the ⁵⁷Fe in the Cu(2) site of the copper oxide plane, all the subspectra could be attributed to the ⁵⁷Fe in the Cu(1) copper oxide chain site with a fourfold (doublet D1), fivefold (doublet D2) or sixfold (doublet D3) oxygen coordination. In contrast, in the compound EuBa₂(Cu_0.99 ⁵⁷Fe_0.01)₃O_7- the 6-coordinated (D3) species has not been observed. The substitution of Pr for Eu or for Ba resulted in an increased occupancy of the O(5) antichain oxygen sites, which was explained by the charge neutrality criterion. Especially, the replacement of Ba²⁺ with Pr³⁺ led to an unusually high degree of occupancy of O(5) sites. In the ⁵⁷Fe Mössbauer spectra the relative area of the 6-coordinated species (D3) increased, and that of the 4-coordinated one (D1) vanished completely in the case when Pr was substituted for Ba. Furthermore, the proportion of the 6-coordinated (D3) species increased at the expense of the 5-coordinated (D2) one with an increasing concentration of Pr at the Ba site. These experimental results are consistent with the variety of Mössbauer results reported so far.