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Clays and Clay Minerals

, Volume 42, Issue 5, pp 628–633 | Cite as

The Center Shift in Mössbauer Spectra of Maghemite and Aluminum Maghemites

  • G. M. Da Costa
  • E. De Grave
  • L. H. Bowen
  • R. E. Vandenberghe
  • P. M. A. De Barker
Article

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 

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Copyright information

© The Clay Minerals Society 1994

Authors and Affiliations

  • G. M. Da Costa
    • 1
    • 2
  • E. De Grave
    • 1
  • L. H. Bowen
    • 3
  • R. E. Vandenberghe
    • 1
  • P. M. A. De Barker
    • 1
  1. 1.Laboratory of Magnetism, Department of Subatomic and Radiation PhysicsUniversity of GentGentBelgium
  2. 2.Departamento de QuímicaUniversidade Federal de Ouro Preto OuroPretoBrazil
  3. 3.Department of ChemistryNorth Carolina State UniversityRaleighUSA

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