E. De Grave

Evaluation of ferrous and ferric Mössbauer fractions

The Mössbauer fractions f for various ferrous- and/or ferric-containing oxides and oxyhydroxides, silicates and carbonates were evaluated from the experimental temperature dependence of their center shifts, using the Debye approximation for the second-order Doppler shift. It is concluded that ferrous ions exhibit a lower fraction as compared to ferric ions. Using standard mixtures of α-Fe2O3 with selected Fe2+ or Fe3+ compounds, it is found that the calculated Fe3+f values are somewhat overestimated with respect to those of Fe2+. Possible explanations for this shortcoming are discussed and it is suggested that a different temperature dependence of the intrinsic isomer shift is the most likely reason. This suggestion is corroborated by analyses of hematite and hedenbergite data which are available for temperatures up to 900 K and 800 K respectively.

On the methodology of the analysis of Mössbauer spectra

A review is presented of the direct fitting procedures which are used in the analysis of Mössbauer spectra. Direct lineshape fitting with alternative profiles as well as shape-dependent, shape-independent and quasi shape-independent distribution fitting methods all can easily be incorporated in one computer program scheme yielding a large versatility for modification and/or extension of the programs according to specific spectra.

Some aspects concerning the characterization of iron oxides and hydroxides in soils and clays

A review of the systematic Mössbauer studies on the most encountered iron oxides and hydroxides is given in which the qualitative and quantitative aspects, helpful in the characterization of natural smaples, are emphasized. The present possibilities of Mössbauer spectroscopy in soil characterization are further illustrated from some examples of natural soils.

Mössbauer characterization of iron oxides and (oxy)hydroxides: the present state of the art

Mössbauer spectroscopy is a powerful direct technique for the identification and quantification of iron oxides and (oxy)hydroxides in soils and sediments. However, further characterization with respect to structural properties such as crystallinity, Al substitution, stoichiometry, water content, etc. is rather limited. With some examples of synthetic and natural goethite and hematite sample series it is illustrated that the hyperfine parameters depend on much more structural features than the Al content and crystallinity alone. Neither the Morin transition in hematite nor the Verwey transition in magnetite is directly applicable for analytical purposes in natural samples.

Influence of nonstoichiometry and the presence of maghemite on the Mossbauer spectrum of magnetite

Several samples of large- and small-particle magnetite (Fe3O4), as well as its thermal decomposition products formed at different temperatures and atmospheres, have been studied extensively by Mössbauer spectroscopy (MS), both with and without an applied field of 6T. Synthetic mixtures of magnetite and poorly- or well-crystallized maghemite have also been studied. Large-particle magnetite (MCD > 200 nm), when heated in air for 12 hours at T < 400°C, transforms to a mixture of well-crystallized hematite and magnetite, the latter one remaining stoichiometric, according to the relative area-ratios obtained from MS. Thermal treatment at 1300°C in a controlled O2 partial pressure, produced a mixture of stoichiometric and nonstoichiometric magnetite, but the latter component seems to be composed of particles with different degrees of nonstoichiometry. The Mössbauer spectra of the decomposition products at T < 200°C in air of small-particle magnetite (MCD ~ 80 nm) could be successfully interpreted as a mixture of magnetite and maghemite, rather than nonstoichiometric magnetite. This suggestion is further supported by the experiments with the synthetic mixtures. It is clearly demonstrated that is not possible, even by applying a strong external field, to separate the contribution of the A-site of magnetite from that of maghemite.

Mössbauer studies of magnetite and Al-substituted maghemites

This paper reviews a systematic Mössbauer study of maghemite γ-Fe2O3 and Al-substituted maghemites γ-(Fe1−yAly)2O3. Three series of samples prepared from different methods and having different morphological characteristics and aluminum contents were investigated. It was found that both the cation distribution and the solubility limit depend on the preparation method, and no general conclusion in that respect could be inferred. From the temperature dependence of the hyperfine fields the exchange integrals could be calculated, and were found as: JAB= −25 K, JAA= −18 K, and JBB= −3 K. The hyperfine fields show a crossing in the vicinity of 300 K, for both substituted and unsubstituted samples, as a result of the relatively strong A–A interaction. The Curie temperatures were found to be in the range of 948–730 K, the lower value referring to the sample containing 22 mole% Al. The influence of maghemite on the Mössbauer spectra (MS) of magnetite was explored in some detail. It was demonstrated on the basis of the MS recorded for a variety of reference mixtures, that it is not possible to resolve the ferric A-site components due to maghemite and magnetite, even with the absorber subjected to a strong external field.

Nature of the morin transition in al-substituted hematite

Abstract The Mossbauer spectra of α(Fe1-cAlc)2O3 with c=0.048 indicate the presence of two distinct spin phases below 245 K, one of which gradually disappears with increasing temperature. A careful study of the hyperfine parameters as a function of temperature reveals complex spin reorientation phenomena in both phases.

The effect of crystallinity and Al substitution on the magnetic structure and morin transition in hematite

Abstract An aluminous hematite with 6.5 at% substitution and obtained by thermal decomposition at 500°C of an aluminous goethite, has been annealed at many different temperatures between 550 and 900°C. The resulting compounds have been characterized by X-ray diffraction and electron microscopy. XPS and zeta-potential measurements indicate an enrichment in Al of the surface of the particles at high sintering temperatures. Mossbauer spectra were recorded as a function of temperature and the results compared with those obtained for as-prepared aluminous hematites as a function of the Al substitution. This shows that the amount of Al leaving the hematite lattice at high temperatures is insignificant. A careful evaluation of the Mossbauer data of thermally treated, substituted and non-substituted hematite has allowed to distinguish the effects of crystallinity from those of the Al substitution. It is further suggested that the two low-temperature phases observed in the Mossbauer spectra, coexist within the particles and that the magnetic exchange interaction is on the basis of the canted spin directions.

57Fe Mössbauer effect study of Mn-substituted goethite and hematite

Abstract57Fe Mössbauer spectroscopy is performed on synthetic Mn-substituted goethites, α-(Fe1−cMnc)OOH, and hematites, α-(Fe1−cMnc)2O3, with c up to 0.08. The hyperfine parameters and in particular the magnetic hyperfine field are found to be less influenced by Mn substitution than in the case of Al. On the other hand, Mn substitution suppresses drastically the Morin transition in hematite which results in a weakly-ferromagnetic state at 80K for compositions with c>0.04.

On the Mossbauer parameters in BiFeO3

The Mossbauerspectrum of BiFeO3 is shown to consist of two super-imposed six-line spectra. The main distinction between the two iron sites is made up by the different ΔEQ values, even having opposite sign. A qualitative explanation is proposed.