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Mössbauer spectra of - and -polymorphs of FeOOH and Fe₂O₃: effects of poor crystallinity and of Al-for-Fe substitution

E. DE GRAVE^*, C. A. BARRERO, G. M. DA COSTA, R. E. VANDENBERGHE and E. VAN SAN

Department of Subatomic and Radiation Physics, Division NUMAT, University of Ghent, B-9000 Ghent, Belgium

^* E-mail: eddy.degrave{at}rug.ac.be

(Received 28 August 2001; revised 17 April 2002)

This paper presents various aspects, revealed by Mössbauer spectroscopy, of structural and magnetic properties of Al-substituted small-particle soil-related oxides. For goethite we focus on the relations between the hyperfine fields on the one hand, and crystallinity and Al content on the other. It is argued that these relations only provide a rough estimate of the Al content in natural samples. The ferrimagnetic-like behaviour reflected in the external-field Mössbauer spectra (4.2 K, 60 kOe) of certain Al goethites is presented. The spectra obtained for lepidocrocites are not spectacular, but confirmed that up to ~10 at.% Al can be incorporated in the structure. Three differently-made series of hematites are considered. The Morin transition and spin structures in hematite are very sensitive to crystallinity and Al content, and probably to the presence of structural OH^–. Integral low-energy electron Mössbauer spectroscopy on non-substituted samples indicates that the Morin-transition temperature in the surface layers (2 to 5 nm) is not significantly shifted from the bulk value. Measurements in extremely high magnetic fields (140 kOe) have shown that a spin-flip transition is induced in highly-substituted samples which exhibit no Morin transition in zero field. The use of external fields is crucial for the characterization and precise determination of the hyperfine parameters and site occupancies for maghemites, and for phase analyses of magnetic soils.

KEYWORDS: Mössbauer spectroscopy, goethite, hematite, maghemite, Al substitutions, soils