57Fe Mössbauer Effect Study of Al-Substituted Lepidocrocites

E. De Grave1, ‡, G. M. da Costa1, #x000A7;, L. H. Bowen2, U. Schwertmann3 and R. E. Vandenberghe1
1 Department of Subatomic and Radiation Physics, University of Gent, B-9000 Gent, Belgium
2 Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27965-8204, USA
3 Lehrstuhl für Bodenkunde, Technische Universität München, D-85354 Freising-Weihenstephan, Germany
Contribution No. SSF95-01-06 from the Department of Subatomic and Radiation Physics, University of Gent, Belgium.
Research Director, National Fund for Scientific Research, Belgium.
§ On leave from Departamento de Quimica, Universidade Federal de Ouro Preto, Ouro Preto, MG, Brazil.

Abstract: Seven Al-containing lepidocrocite samples, γ-Fe1−xAlXOOH, prepared from FeCl2/Al(NO3)3 solutions with initial Al/(Al + Fe) mole ratios Ci of 0.0025, 0.01, 0.025, 0.05, 0.075, 0.10 and 0.15 mol/mol, were examined by means of Mössbauer spectroscopy at room temperature (RT) and at various temperatures in the range of 8 to 80 K. The spectra at RT and 80°K consist of broadened quadrupole doublets and were analyzed in terms of a single doublet and of a model-independent quadrupole-splitting distribution, the latter yielding the best fit. The observed variations of the quadrupole-splitting parameters with increasing Ci are inconclusive as to whether the Al cations are substituting into the structure. The temperature at which the onset of magnetic ordering is reflected in the spectra, was measured by the thermoscan method with zero source velocity. A gradual shift from 50 K for Ci = 0.0025 mol/mol to 44 K for Ci = 0.10 mol/mol was observed for that temperature. As compared to earlier studies of Al-free γ-FeOOH samples with similar morphological characteristics, the fractional doublet area in the mixed sextet-doublet spectra at 35 K is significantly higher for the present lepidocrocites. This observation is ascribed to the substitution of Al cations into the lepidocrocite structure. A similar conclusion is inferred from the variation with Ci of the maximum-probability hyperfine field derived from the spectra recorded at 8 K and fitted with a model-independent hyperfine-field distribution. The magnetic results suggest that for the sample corresponding to Ci = 0.15 mol/mol, not all of the initially present Al has been incorporated into the structure.

Key Words: Al Substitution • Lepidocrocite • Mössbauer Spectra

Clays and Clay Minerals; April 1996 v. 44; no. 2; p. 214-219; DOI: 10.1346/CCMN.1996.0440206
© 1996, The Clay Minerals Society
Clay Minerals Society (www.clays.org)