Studies of iron in deep-sea sediments by Mössbauer spectroscopy

The distribution of the forms of Fe in the solid phases in core samples of sediments from the Peru Basin has been investigated by Mössbauer spectroscopy with special attention to the cause of the sharp color transition between an upper green colored and a lower tan colored part. An important part of sample handling includes strict exclusion of oxygen during preparation of absorbers and measurements at cryogenic temperatures. The measurement strategy includes measurements between 77 K and 300 mK in zero external magnetic field, supplemented by measurements in external magnetic fields at 4.2 and 300 mK (up to 6.2 and 1 T, respectively). The temperature scans allow detection, identification and quantification of superparamagnetic iron oxides (goethite and hematite). The oxides are only present in samples from the upper tan-colored part of the core. The major part of the Fe(II) and Fe(III) (>80%) is present in a magnetic structure similar to that of layer silicates. The relative Fe(II) content of the layer silicates is practically identical to that determined from the paramagnetic components measured at liquid nitrogen temperature. This shows that the color transition in the sediment coincides with a change in the relative Fe(II) content in layer silicates from 11 to 37%. The color change can thus be explained by an increase in occurrence of Fe(II)–Fe(III) pairs exhibiting absorption bands due to intervalence electron transfer.     

57Fe Mössbauer study of a deposit in an industrial cooling circuit supplied with raw river water

In this work, the nature of the deposit found inside an industrial cooling circuit (which consists of a mixture of different iron containing phases) has been characterized in detail by ⁵⁷Fe Mössbauer spectroscopy. Electron Paramagnetic Resonance spectroscopy was also used to check for the presence of other metals, mainly manganese and copper, detected by the Inductive Coupled Plasma method. We conclude that the deposit contains a large amount of Fe(III), probably consisiting of ferrihydrite nanoparticles and of goethite, either bulk or as large particles. It also contains traces of an Fe(II) species (about 3%), probably adsorbed on the iron oxides. Mn(II) and traces of Cu(II) are also present.     

Investigation of near-source basaltic glasses using 57Fe Mössbauer spectroscopy

Two basaltic glass samples have been studied, one from the Masaya volcano in Nicaragua and one from the Villarrica volcano in Chile using ⁵⁷Fe Mössbauer spectroscopy. The iron(II) to iron(III) ratio has been determined for each sample. The 80 K data shows some separation of the sites and is fitted allowing for discrete doublets.     

Coordination and Oxidation States of Iron Incorporated in Mesoporous MCM41

Mesoporous Fe–MCM41 samples (Si/Fe=25) were synthesized and characterized under evacuation and reducing/oxidizing treatments by in situ FTIR and Mössbauer spectroscopies. Both Fe(II) and Fe(III) located in low coordination states in top layers of pore walls exhibit Lewis acidity and may participate in Fe(III) Fe(II) processes at low temperatures (570 K). Furthermore, Fe(III) Fe(II) cycles can be achieved and repeated with participation of the full amount of iron at higher temperatures (670 K). The accompanying formation of oxygen vacancies and restoration of the structure in the reverse process does not result in extended damages; the MCM41 structure retains its stability under the conditions applied.     

Electric quadrupole interaction of 100Rh in antimony, hafnium and rhenium

The effects of the Mg(II)/Fe(III) ratio on the structure and Fe microenvironments in MgFe LDH substances were investigated. The LDHs were prepared by the co-precipitation method with Mg(II)/Fe(III) ratios from 2:1 to 6:1. The materials were characterized by ⁵⁷Fe Mössbauer spectroscopy and powder X-ray diffractometry. The ⁵⁷Fe Mössbauer spectra exhibited asymmetric doublet corresponding to high-spin Fe(III) microenvironments in all LDH structure. It was found that the quadrupole splitting decreased with increasing Mg(II)/Fe(III)ratio reflecting change in the electric field gradient due to the incorporation of different amounts of iron into the Mg-containing layers.     

Mössbauer spectroscopic study of frozen solutions of57Co using a specially designed cryostat for time differential coincidence emission Mössbauer spectroscopy of237Np

Experiments on the⁵⁷Fe Mössbauer effect in frozen solutions were carried out in order to demonstrate the capabilities of a cryostat which was developed for the emission Mössbauer spectroscopy of²³⁷Np. We confirmed the existence of an aliovalent state of⁵⁷Fe in the temperature range from 5.5 to 35 K. However, the relative amounts of Fe(II) and Fe(III) in the frozen matrix were almost constant for the temperature range from 5.5 to 150 K. The design, operation and capabilities of the cryostat are also described.     

Mössbauer and XRD study of Al-Sn linished steel bimetal alloy

Aluminium alloy free CS1 type steel (0.06 wt% C, 0.45 wt% Mn) and samples of cold roll bonded steel bimetal alloys (MAS15 and MAS16) were fabricated and investigated by X-ray diffraction (XRD), ⁵⁷Fe conversion electron Mössbauer spectroscopy (CEMS) at room temperature. XRD has revealed only the existence of the alpha iron solid solution (steel) phase in the steel only sample, while identified steel and metallic Al and Sn constituent phases in the bimetallic alloys. ⁵⁷Fe Mössbauer spectroscopy revealed the presence of 4 % secondary iron-bearing phase attributed mainly to iron oxide/ oxyhydroxides (ferrihydrite) besides the steel matrix on the surface of the steel sample. A significant difference between the occurrences of the secondary phase of differently prepared bimetal alloys found in their ⁵⁷Fe CEM spectra allowed to identify the main phase of debris as different iron oxide/ oxyhydroxides.     

Mössbauer emission spectroscopy of metastable electronic states

In⁵⁷Fe Mössbauer emission spectroscopy, the parent nucleus⁵⁷Co is incorporated in the matrix by diffusion, implantation or co-precipitation. Mössbauer emisssion spectra often differ from the corresponding absorption spectra in several aspects. The “anomalous” features (denoted as “after-effects” of nuclear transformation) comprise mainly (a) anomalous charge states, (b) anomalous spin states, and (c) metastable electronic populations within these charge states. The first two classes of effects were extensively studied during the sixties and seventies. More recently, several investigations have shown that, under certain circumstances, the electronic systems of Fe(II) and Fe(III) have not reached the thermal equilibrium within the Mössbauer time window. These effects have been evidenced (i) within the crystal field ground-state manifolds, but also (ii) as spectral contributions from excited crystal field states. In the present paper, an overview of these experiments will be given, together with their theoretical interpretation and in comparison to results obtained with other methods (e.g. optical spectroscopy). Special emphasis will be given to the systems⁵⁷Co/LiNbO₃ and⁵⁷Co/MgO, where nonequilibrium populations within the⁶A₁ ground manifold of Fe(III) and Γ_5g ground manifold of Fe(II) have been observed in an external magnetic field.     

Characterisation by Mössbauer Spectroscopy of the Forms of Iron in Sulfide-Rich Fulvic Acid Solutions

Variations in the nature of the bonding between iron and fulvic acid as a function of pH in sulfide-rich solutions have been investigated by ⁵⁷Fe Mössbauer spectroscopy. In all solutions iron was enriched to 95% in the ⁵⁷Fe isotope and ratios of Fe:fulvic acid were maintained at 1:100 on a weight basis for all measurements. When the pH was decreased below 5.0, there was a progressive change in the composition of the spectra, which contained three distinct components that were similar to those seen in the absence of sulfide; i.e., a sextet from magnetically-dilute Fe(III) and doublets from Fe(II) and Fe(III). There was, however, a higher proportion of the iron as Fe(II) in the sulfide-containing solutions and this represented the only form of iron at very low pH. Establishment of equilibria was slow as evidenced by considerable hysteresis between the compositions of the solutions with decreasing and increasing pHs. On increasing the pH, sulfide prevented the formation of magnetically dilute Fe(III) species and instead, in the range 3.5–6.0, compounds were generated with parameters consistent with sulfur being coordinated to the iron. One had parameters similar to those of pyrite and hence probably contains disulfide (S₂ ^2–) units, whilst a second, with poorly defined magnetic hyperfine splitting, may contain monosulfide (S^2–) units, with structures related to either pyrrhotite (Fe_1–x S) or greigite (Fe₃S₄). A third component probably corresponds to a mononuclear low spin Fe(II) complex, involving sulfur and fulvic acid in the iron coordination sphere. These species were oxygen-sensitive and decomposed to yield magnetically dilute Fe(III) complexes and Fe(III) oxyhydroxides on aeration of the solutions.     

Time-dependent57Fe-species in57Co-labeled cobalt(II) iodate studied by means of coincidence Mössbauer spectroscopy

A lifetime of unstable⁵⁷Fe(II)-species formed through EC-decay of⁵⁷Co-labeled Co(IO₃)₂ was estimated using a coincidence technique. KX-ray-gated and time-resolved emission Mössbauer spectra were measured at room temperature. No remarkable difference was found ont he relative intensity ratio of⁵⁷Fe(II)/⁵⁷Fe(III) between the X-ray-gated and time-integral emission spectra. A mean lifetime of the unstable⁵⁷Fe(II)-species was estimated to be 43±5 ns from the time-resolved emission Mössbauer spectroscopy.     

Temperature dependence of EFG at the Fe-site in Co(en)3Fe(CN)6

This paper presents the results of⁵⁷Fe Mössbauer Investigation of Co(en)₃Fe(CN)₆, a cation-anion complex where ‘en’ stands for ethylenediamine, for varlous temperatures from 300-88K. In this compound Iron exists in two oxidation states viz., Fe(III) and the abnormal Fe(IV) state. Quadrupole splitting as a function of temperature has been utilised to understand the crystal field splitting at the Fe-site.     

Iron oxide nanoparticles for plant nutrition? A preliminary Mössbauer study

One of the most important micronutrients for plants is iron. We have prepared iron(III) oxyhydroxide and magnetite nanoparticles with the aim to use them as possible nutrition source for plants. The iron(III)-oxide/oxyhydroxide nanoparticles prepared under our experimental conditions as colloidal suspensions proved to be 6-line ferrihydrite nanoparticles as verified by XRD, TEM/SAED and Mössbauer spectroscopy measurements. ⁵⁷Fe Mössbauer spectra of magnetite nanoparticles prepared under different preparation conditions could be analyzed on the basis of a common model based on the superposition of four sextet components displaying Gaussian-shaped hyperfine magnetic field distributions.     

Mössbauer studies of57Fe atoms isolated in CH4 and CO2 matrices

The Mössbauer absorption spectra of matrix isolated⁵⁷Fe atoms have been measured in the inert matrices CH₄ and CO₂ with matrix temperatures between ～3.3 and ～46 K. The isomer shift of the observed resonances is (?0.79±0.02) mm/s and (?0.76±0.05) mm/s with respect to iron metal at 300 K for⁵⁷Fe in CH₄ and for CO₂ respectively. This is within the experimental errors the same isomer shift as that of rare-gas matrix isolated⁵⁷Fe atoms. All spectra show quadrupole doublets due to the noncubic point symmetry of the lattice site occupied by the⁵⁷Fe atoms. The effective Debye temperatures as obtained from the temperature dependence of the resonance absorption areas are (46±4) K for the CH₄ matrix and (121±6) K for the CO₂ matrix annealed at ～20 K.     

Mössbauer and X-Ray Diffraction Investigations of a Series of B-Doped Ferrihydrites

X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy are used to characterize the influence of borate on two-line ferrihydrite's structure and develop likely models for its attachment. Particle sizes were in the 2–4 nm range, and as borate sorption increased, the ferrihydrite particle size decreased. The d-spacings of two-line ferrihydrite increased with increased borate adsorption. Isomer shift and quadrupole splitting exhibit slight increasing trends as well. Also, the phase transformation temperature of ferrihydrite to hematite is significantly raised due to borate coating of the surface. We suggest borate is sorbed onto the surface by attachment to the oxygen corners of the iron octahedra that are on the surface of the nanoparticles, placing boron in a tetrahedral molecular geometry.     

Morin transition suppression in Polycrystalline 57Hematite (α-Fe2O3) exposed to 56Fe(II)

We used the isotope selectivity of ⁵⁷Fe Mössbauer spectroscopy to investigate changes in the magnetic properties of polycrystalline hematite exposed to ferrous iron (Fe(II)). We found that sorption of ⁵⁶Fe(II), followed by interfacial electron exchange, alters the bulk magnetic properties of ⁵⁷hematite. After reaction with ⁵⁶Fe(II), we observed partial suppression of the Morin transition of ⁵⁷hematite to below 13 K. This is significantly lower than the Morin temperature (T _M) of ～230 K measured for isotopically enriched polycrystalline ⁵⁷hematite, as well as the T _M of 264?±?2 K reported for normal polycrystalline hematite.     

A CEMS investigation of57Fe implanted in Al and Cu at low temperature

A new type of CEM-spectrometer allows in situ measurements on metal foils implanted at low temperatures. It has been used to study defect association and clustering of⁵⁷Fe in Al and Cu. For⁵⁷FeAl, the substitutional fraction (f _s) in samples implanted at 120 K is somewhat smaller than expected for a random impurity distribution but much larger than after room temperature implantation. For⁵⁷FeCu,f _s for samples implanted at 120 K is less than 0.5 of the value expected for a random distribution and it falls to zero after annealing at 600 K, where more extensive Fe clustering occurs. Vacancy trapping in stage III does not contribute significantly to the observed defect sites.     

In-beam Mössbauer study of 57Fe using a secondary 57Mn beam and ion implantation

We have succeeded in obtaining well-resolved M?ssbauer spectra of ⁵⁷Fe arising from short-lived ⁵⁷Mn ( T _1/2 = 1.45 min) in Si and KMnO₄. The M?ssbauer spectra of ⁵⁷Fe in Si are well fitted with a curve consisting of two singlet lines, one being assigned as the interstitial Fe atoms and the other as substitutional ones. The relative intensities of the two lines infer that 60% of ⁵⁷Fe ( ←⁵⁷Mn) atoms land at the interstitial sites and 40% at the substitutional sites at temperatures between 30 K and 296 K. The result for the KMnO₄ sample suggests a presence of an exotic chemical species corresponding to a higher valence state than Fe^{6 +}. Received: 1 May 2001 / Accepted: 4 December 2001     

Mössbauer spectroscopy and X-ray fluorescence studies on sediments from the methanic zone of the Helgoland mud area,North Sea

⁵⁷Fe Mössbauer spectroscopy (MS) and X-ray fluorescence spectroscopy (XRF) were used to determine the identity of iron(III) oxides in surface (top 30 cm ) and subsurface (> 30 cm – 500 cm)sediments from the Helgoland mud area in the German Bight of the North Sea. A 500 cm-long sediment core was cut in 25cm sections while only the top 10 cm of a 30 cm-long sediment core was sampled. Using a MIMOS spectrometer, MS spectra were recorded at 293K (RT) in backscattering geometry. At 80K and 5.5K, MS analysis was carried out in transmission geometry. At RT and 80K only illite was observed, but at 5.5K lepidocrocite was revealed in the MS spectra. The relation between Fe(III) and Fe(II) doublets of illite did not significantly vary with depth, but the relative amount of lepidocrocite increased with depth reaching about 24 % of iron phases, as revealed by MS. XRF measurements showed that the amount of Fe in the sediments varied with depth but was always less than 4 % of total elemental composition. The main component of the sediment was silica and its depth profile alternated with those of other elements, especially aluminium and iron. It was observed that elevated concentrations of dissolved iron in the subsurface sediment of the Helgoland mud area correlated with the depth-wise distribution of distinct microbial populations presumably due to microbial reduction of excess bioavailable iron minerals such as lepidocrocite. These results are thus, important in the context of microbe-mineral interactions in marine sediments as iron oxides are an electron acceptor for microbial anaerobic respiration.     

Thermal evolution of high dose Fe implanted Si(100) at 623 K: A CEMS study

Hyperfine interactions associated to the phases produced by 623 K Fe implantation into Si(100) and by subsequent thermal treatments at temperatures up to 1273 K have been studied by conversion electron Mössbauer spectroscopy. The results suggest that in the as implanted sample the short range order around the ⁵⁷Fe probes is \beta-FeSi₂-like. This phase orders with a thermal treatment at 773 K. A preferential orientation of the electric field gradient was detected from the relative height of the quadrupole interaction components. It was found that the transition to the high temperature phase \alpha-FeSi₂ occurred at least 50 K below the reported value.     

Fe-induced magnetic transition in YBa2(Cu1−x 57Fe x )3O6 by NQR and Mössbauer spectroscopy

Nuclear quadrupole resonance spectroscopy (around 30 MHz) on the chain site Cu(1) nuclei in oxygen deficient YBa₂(Cu_1?x Fe _x )₃O₆ doped with different amounts of⁵⁷Fe (x≤0.01) reveal an onset of magnetic order at low temperatures represented by a symmetrical doublet contribution to the nuclear quadrupole resonance (NQR) spectrum. The onset temperatureT _N2 depends on the concentration of Fe reaching 130 K forx=0.01. The splitting forx=0.01 at 100 K corresponds to a net internal field of 0.09 T with a distribution of ≈0.08 T representing about 70 percent of the Cu(1) nuclei.⁵⁷Fe and⁵⁷Co Mössbauer spectroscopy at 4.2 K with and without an external magnetic field of 5 T revealed that belowx=0.00015 Fe spins are decoupled from the Cu(2) moments in the antiferromagnetic state. Results are interpreted in terms of the magnetic model structure suggested by Kadowaki et al. [1].