Electrochemical corrosion behaviour of ion-beam-mixed Fe-Al alloy: A conversion electron Mössbauer spectroscopic study

The electrochemical corrosion behaviour of ion-beam-mixed Fe-Al composites has been studied by using the technique of interface-sensitive conversion electron Mössbauer spectroscopy (CEMS). Polycrystalline aluminium foils deposited with a film 50 Å thick of the ⁵⁷Fe Mössbauer isotope and an overlayer 250 Å thick of natural iron have been bombarded with 100 keV Ar⁺ ions at a dose in the range of about (1–3)×10¹⁶ ions cm^-2 to induce atomic mixing at the interface. A number of as-deposited and ion-beam-mixed composites have been subjected to electrochemical corrosion treatment in a buffer solution by employing the three- sweep potentiokinetic polarization technique. It has been shown that the atomically mixed interface region exhibits corrosion behaviour considerably different from that exhibited by either iron or aluminium foils. The as-deposited as well as the ion-beam-mixed composites have been characterized by ⁵⁷Fe CEMS prior to the corrosive attack and after passivation so as to identify the structural state of the interface and the transformations occuring in the interface as a result of the corrosive reactions. The standard Mössbauer fitting procedure was used in conjunction with a method of obtaining hyperfine field distributions to establish that the corrosive action of the aqueous medium on the ion-beam-mixed Fe-Al composite leads to the elimination of the magnetic spectral components and to the growth of passive non-magnetic phases such as β-γ-FeOOH and FeO.     

The state of iron in natural zeolites: A Mössbauer study

The state of iron in natural zeolites is an important feature for the practical applications of these materials. A Mössbauer study, complemented with other methods, of iron-containing zeolitic rocks from several countries (Mexico, Cuba, Czechoslovakia, and the USSR) was developed. It has been determined that iron is located as high spin Fe³⁺ in framework tetrahedral sites, in extraframework octahedral sites as free Fe(H₂O)₆³⁺, and as high spin Fe²⁺ in octahedral coordination in extraframework sites or in another aluminosilicate associated with the zeolite. Iron is also located in magnetite contained in the zeolite rocks.     

Mössbauer spectroscopy study of the formation of Fe layers in Fe/Cr superlattices

The formation of Fe layers in Fe/Cr superlattices has been studied by Mössbauer spectroscopy. It is established that magnetic clusters, which appear in the initial stage of formation of Fe layers and impart superparamagnetic properties to these nanostructures, involve not only Fe atoms constituting a core of the three-dimensional cluster but also the neighboring Cr atoms exchange-coupled to this core. The thicknesses of Fe layers corresponding to their transition from superparamagnetic to ferromagnetic state are determined.     

Mössbauer study of nanocrystallization in amorphous Fe-P-Si-V alloys

Nanocrystallization of amorphous Fe-P-Si-V alloys during annealing is studied by Mössbauer spectroscopy and x-ray diffraction. The resulting nanostructured materials are shown to contain not only equilibrium phases ( -Fe and Fe₃P) but also a significant amount of the metastable paramagnetic phase Fe₂P and trace levels of the and D phases. Nanocrystallization markedly enhances the strength of the alloys but has an adverse effect on their magnetic properties.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 4, 2005, pp. 427–433.Original Russian Text Copyright © 2005 by Baldokhin, Vavilova, Kovneristyi, Kolotyrkin, Palii, Perfilev.     

Mössbauer study of bacterial ferrihydrite

Mössbauer measurements reveal four inequivalent Fe sites in ferrihydrite produced by Klebsiella oxytoca. The origin of these sites can be understood in terms of two nanosized structural regions in the bacterium and a certain ordering of bilayers and single layers of Fe-occupied octahedra.     

57Fe Mössbauer studies on MgAlxCrxFe2-2xO4 spinel system

Based on ⁵⁷Fe Mössbauer spectroscopy studies conducted at 300 K, the magnetic behaviour of the spinel ferrite system MgAl_xCr_xFe_2-2xO₄ (x=0.0–0.5) has been investigated. The Mössbauer spectra for x=0.0–0.2 exhibit two Zeeman sextets, one due to the Fe³⁺ octahedral ions and the other due to the Fe³⁺ tetrahedral ions, while the compositions x=0.3, 0.4 and 0.5 showed central paramagnetic doublet superimposed on the magnetic sextets. The variation of hyperfine parameters and origin of the central doublet on the magnetic sextets have been discussed.     

Defect formation in Gd3Fe5O12-based garnets: a Mössbauer spectroscopy study

The Mössbauer spectroscopy of Gd_2.2Pr_0.8Fe₅O₁₂ garnet, which exhibits higher electronic conduction with respect to Gd₃Fe₅O₁₂ due to the presence of Pr⁴⁺ cations, showed that praseodymium doping decreases the coordination of Fe³⁺ in octahedral sites. Penta-coordinated Fe³⁺ ions, in combination with small quantities of Fe⁴⁺, are also formed in the lattice of Gd_2.5Ca_0.5Fe₅O₁₂ where the variations of ionic and electronic transport properties indicate charge compensation via generation of oxygen vacancies and electron holes. The mechanisms of garnet lattice disorder, induced by acceptor- and donor-type doping, appear thus quite similar; in all cases, the ionic defect formation requires substantial structural reconstruction, probably associated with direct linking of iron–oxygen tetrahedra. Due to the low concentration of charge carriers and the important role of lattice relaxation in the oxygen ion migration processes, this behavior results in similar activation energies for the ionic conductivity in all Gd₃Fe₅O₁₂-based garnets.     

Mössbauer study of Fe-Cr-Co magnetic alloys

Mössbauer spectroscopy has been used to investigate the hyperfine fields in an Fe-27.5% Cr-17.5% Co-0.5% Al alloy for different heat treatments. Spectra for the as-cast condition consist of two superimposed 6 line spectra and a central peak and reveal the three distinct iron environments of ₁, ₂, and phases respectively. Following homogenization a broad distribution of hyperfine fields associated with broad isomer shift distribution is indicative of randomized atomic distribution within a single phase. Upon isothermal heat treatment at 620° C the phase spinodally decomposes into 2 ferromagnetic environments: a Fe-Co rich ₁ phase withH _eff 350 kOe and a Cr-rich ₂ phase withH _eff 200 kOe. Evidence of a third ferromagnetic iron rich environment ( ₃ phase withH _eff 325 kOe) is also found as a transition phase which forms prior to subsequent ageing. The progressive reduction in ageing temperature from 600 to 540° C to improve coercivity transforms the ₁ and ₂ modulated structure into a paramagnetic Cr-rich precipitate giving a single line spectrum and a highly ferromagnetic Fe-Co rich matrix givingH _eff 350 kOe. Anisotropy of the Fe-Co matrix introduced by isothermal decomposition under the influence of the external magnetic field is clearly observable after the ageing process through changes to the Mössbauer spectrum.     

Mössbauer study of Li0.5Fe2.5−x Al x O4: cation distribution and noncollinear spin alignment

Based on⁵⁷Fe Mössbauer spectroscopic investigations of Li_0.5Fe_2.5–x Al _x O₄ (x=0.3 and 0.8) conducted at 4.2 K under an external magnetic field of 40 kG, the exact cation distribution has been worked out. Both the compositions show spin canting for the octahedral sites, with the Yaffet Kittle angle _yk20° forx=0.3 and _yk27° forx=0.8. The Al³⁺ ions show some preference for the octahedral sites but the derived cation distribution is quite different from that reported earlier on the basis of magnetization studies. The reported compensation temperature forx=0.8 has not been observed. The origin of the central quadrupole doublet along with the magnetic sextet and the role of canted spin alignment towards the possible observance of compensation point, are discussed.     

Mössbauer study of vacancy ordering in the system SrTi1−xFexO3−y (0.50 ≤ x ≤ 0.70)

The synthesis of the system SrTi_1−xFe_xO_3−y with 0.50 ≤ x ≤ 0.70 has been performed in order to study the nature of the oxygen vacancy ordering. By means of Electron Microscopy and Diffraction and Mössbauer spectroscopy, we have obtained the main features of vacancy ordering in this system. The results are compared with the predictions of Komornicki-Grenier model of anion vacancies distribution in perovskites.     

Mössbauer,EXAFS, and X-ray diffraction study of Fe3+ clusters in MgO:Fe and magnesiowüstite (Mg,Fe)1−xO-evidence for specific cluster geometries

Mössbauer and EXAFS analysis of Fe³⁺-doped MgO rapidly quenched from 1200° C have yielded octahedral/tetrahedral Fe³⁺ site occupancy ratios which are used to place constraints on the possible defect cluster geometries. The data suggest the samples contain a range of defect aggregates consisting of combinations of basic dimer and trimer units (octahedral Fe³⁺ ions coupled to 1 or 2 cation vacancies) and variations about the 4-1 cluster motif. (Four cation vacancies surrounding one interstitial Fe³⁺.) The groupings are consistent with lattice-energy calculations for defect clusters in both FeO and MgO, the observed sample colour, and prior TEM and ED observations of defect ordering in quenched wüstite. Low-temperature annealing of the samples results in cluster growth and changes in the Mössbauer spectrum attributable to production of magnesioferritelike aggregates. The colour of the samples after annealing is indicative of grouping of magnetically interactive octahedral Fe³⁺ ions such as would be present in spinel nuclei. X-ray diffraction structure factor measurements of magnesiowüstite with 70 cation % Fe/(Fe+Mg) yield defect-concentration ratios similar to those observed in MgO and wüstite. This indicates that initial clustering of Fe³⁺ ions and cation vacancies probably results in similar structures throughout the MgO-FeO-Fe₂O₃ system.     

A temperature-dependent Mössbauer study of mechanically activated and non-activated zinc ferrite

The changes of ZnFe₂O₄ caused by mechanical activation as well as the structural evolution of nonactivated and mechanically activated zinc ferrite occurring during heating, have been investigated by in situ Mössbauer spectroscopy. Attention is directed to mechanically induced changes in magnetic properties of zinc ferrite, the variation of nuclear quadrupolar interactions, the thermally induced changes of the Mössbauer shift, and also to the structural response of mechanically activated zinc ferrite to changes in temperature.