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Determination of solid solution limits based on the thermal behaviour of aluninium substituted iron hydroxides and oxides

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Abstract

The formation of substitutional solid solutions of the isostructural oxyhydroxides α-FeOOH-α-AlOOH (goethite-diaspore and γ-FeOOH-γ-AlOOH (lepidocrocite-boehmite) was investigated by X-ray powder diffraction technique and by thermal analysis.

The unit cell parameters of both orthorhombic structures of α-Fe1−x-AlxOOH (Pbnm) and γ-Fe1−xAlxOOH (Cmcm) decrease clearly with the increase of aluminium content up tox=0.10.

Thermal analysis reveals the sensitivity of DTA and DTG effects to the composition of solid solutions. The temperatures of topotactic formation of rhombohedral α-(Fe, Al)2O3 and of the defect-spinel γ-(Fe, Al)2O3 during the dehydroxylation of α-(Fe, Al)OOH and γ-(Fe, Al)OOH respectively increase as the aluminium concentration rises up to 10 mol%. The dehydration curves of samples with greater aluminium content exhibits the same maximumΔm/ΔT as for the 10 mol% Al-preparation, indicating indirectly the end-members of the solid solution ranges.

Zusammenfassung

Mittels Röntgendiffraktion und Thermoanalyse wurde die Bildung von Substitutions-Mischkristallen der isostrukturellen Oxyhydroxide α-FeOOH-α-AlOOH (Goethit-Diaspor) und γ-FeOOH-γ-AlOOH (Lepidokrokit-Böhmit) untersucht. Die Elementarzellenparameter beider rhombischer Strukturen von α-Fe1−xAlxOOH (Pbnm) und γ-Fe1−xAlxOOH (Cmcm) werden mit zunehmenden Aluminiumgehalt bis zux=0.10 eindeutig kleiner.

Thermoanalyse zeigt die Empfindlichkeit von DTA- und DTG-Effekten gegenüber der Zusammensetzung der Mischkristalle. Steigt die Aluminiumkonzentration bis 10 mol% an, so erhöhen sich auch die Temperaturen für die topotaktische Bildung von rhomboedrischem α-(Fe,Al)2O3 und des Defekt-Spinells γ-(Fe,Al)2O3 bei der Dehydroxylierung von α-(Fe,Al)OOH und γ-(Fe,Al)OOH. Die Dehydratationskurven von Proben mit einem größeren Aluminiumgehalt zeigen dasselbe Δm/ΔT-Maximum wie für die Proben mit 10 mol%, was indirekt auf die Grenze des Mischkristallbereiches deutet.

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Authors and Affiliations

  1. Department of Magnetochemistry, A. Mickiewicz University, Poznan UL. Grunwaldzka 6, PL-60780, Poznan, Poland

    E. Wolska, W. Szajda & P. Piszora

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  1. E. Wolska
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  2. W. Szajda
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  3. P. Piszora
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Wolska, E., Szajda, W. & Piszora, P. Determination of solid solution limits based on the thermal behaviour of aluninium substituted iron hydroxides and oxides. Journal of Thermal Analysis 38, 2115–2122 (1992). https://doi.org/10.1007/BF01979624

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