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Variable-Temperature Mössbauer Spectroscopy of Nano-Sized Maghemite and Al-Substituted Maghemites

Published online by Cambridge University Press:  28 February 2024

G. M. Da Costa ,
E. De Grave ,
L. H. Bowen ,
P. M. A. De Barker  and
R. E. Vandenberghe
G. M. Da Costa
Affiliation:
Laboratory of Magnetism, Department of Subatomic and Radiation Physics University of Gent, B-9000 Gent, Belgium On leave from Departamento de Química, Universidade Federal de Ouro Preto, Ouro Preto, MG, Brazil
E. De Grave*
Affiliation:
Laboratory of Magnetism, Department of Subatomic and Radiation Physics University of Gent, B-9000 Gent, Belgium
L. H. Bowen
Affiliation:
Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204
P. M. A. De Barker
Affiliation:
Laboratory of Magnetism, Department of Subatomic and Radiation Physics University of Gent, B-9000 Gent, Belgium
R. E. Vandenberghe
Affiliation:
Laboratory of Magnetism, Department of Subatomic and Radiation Physics University of Gent, B-9000 Gent, Belgium
*
*Research Director, National Fund for Scientific Research, Belgium.
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Abstract

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Synthetic aluminum-substituted maghemite samples, γ-(Fe1-xAlx)2O3, have been prepared by thermal decomposition of Al-lepidocrocite (γ-Fe1-xAlxOOH), with × = 0, 0.04, 0.06, 0.14 and 0.18. The particles are needle-shaped and the mean crystallite diameter along the [311] crystallographic direction was found to be between 2.0 and 5.0 nm. Mössbauer spectra were collected at 6 K and from 80 K up to 475 K at steps of 25 K. In a wide range of temperatures the spectra of the non-substituted sample consist of a superposition of a broad sextet and a superparamagnetic doublet, whereas for the Al-maghemites this range is much smaller. From the temperature variation of the fractional doublet area two different parameters were defined: the temperature corresponding to a 50/50 doublet-sextet spectrum (T1/2), and the temperature below which the doublet ceases to exist (T0). These two parameters (T1/2 and T0) decrease from 390 K and 92 K (Al-free sample), to 118 K and 64 K (4 mole % Al) and to 100 K and 48 K (18 mole % Al), respectively. The average hyperfine fields at 6 K undergo a steep drop in going from the Al-free sample (Hhf = 506 kOe) to the sample with 4 mole % Al (Hhf = 498 kOe), but for higher substitutions the effect is much smaller. The A- and B-site quadrupole splittings, obtained from the data between 220 K and 475 K, were found as: ΔEQ,A = 0.86 ± 0.04 mm/s and ΔEQ,B = 0.65 ± 0.04 mm/s for the 4 mole % Al sample. The characteristic Mössbauer temperature, determined from the temperature dependence of the average isomer shift, was found to be in the range of 500–600 K.

Keywords

Al substitutionMaghemiteMössbauer effect
Type
Research Article
Information
Clays and Clay Minerals , Volume 43 , Issue 5 , October 1995 , pp. 562 - 568
Copyright
Copyright © 1995, The Clay Minerals Society

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