The Center Shift in Mössbauer Spectra of Maghemite and Aluminum Maghemites

G. M. Da Costa1, 2, E. De Grave1, L. H. Bowen3, R. E. Vandenberghe1 and P. M. A. De Bakker1
1 Laboratory of Magnetism, Department of Subatomic and Radiation Physics University of Gent, B-9000 Gent, Belgium
2 On leave from Departamento de Química, Universidade Federal de Ouro Preto Ouro Preto, MG, Brazil
3 Department of Chemistry, North Carolina State University Raleigh, North Carolina 27695, USA

Abstract: Synthetic, relatively well-crystallized aluminum-substituted maghemite samples, γ-(Aly·Fe1−y)2O3, with y = 0, 0.032, 0.058, 0.084, 0.106 and 0.151 have been studied by X-ray diffraction and zero-field Mössbauer spectroscopy in the range 8 K to 475 K, and also with an external field of 60 kOe at 4.2 K and 275 K. It was found that there are two different converging models for fitting the zero-field spectra of the maghemites with a superposition of two Lorentzian-shaped sextets, both resulting in inconsistent values for the hyperfine fields (Hhf) and/or the center shifts (δ) of the tetrahedral (A) and octahedral (B) ferric ions. From the applied-field measurements it is concluded that there is a constant difference of 0.12 ± 0.01 mm/s between δB and δA, regardless of the Al content. For the Al-free sample the center shifts are found as: δA = 0.370 mm/s and δB = 0.491 mm/s at 4.2 K and δA = 0.233 mm/s and δB = 0.357 mm/s at 275 K (relative to metallic iron), with an estimated error of 0.005 mm/s. Both δA and δB are observed to decrease with increasing Al concentration. The effective hyperfine fields for the non-substituted maghemite sample are: Heff,A = 575 kOe and Heff,B = 471 kOe at 4.2 K and Heff,A = 562 kOe and Heff,B = 449 kOe at 275 K, with an error of 1 kOe. The B-site hyperfine field remains approximately constant with Al substitution, while for the A site a slight decrease with increasing Al content was observed.

Key Words: Al-Maghemite • Center shift • Maghemite • Mössbauer effect.

Clays and Clay Minerals; October 1994 v. 42; no. 5; p. 628-633; DOI: 10.1346/CCMN.1994.0420515
© 1994, The Clay Minerals Society
Clay Minerals Society (www.clays.org)