THE CENTER SHIFT IN MOSSBAUER-SPECTRA OF MAGHEMITE AND ALUMINUM MAGHEMITES

Synthetic, relatively well-crystallized aluminum-substituted maghemite samples, gamma-(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 Mossbauer 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 (H(hf) and/or the center shifts (delta) 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 delta(B) and delta(A), regardless of the Al content. For the Al-free sample the center shifts are found as: delta(A) = 0.370 mm/s and delta(B) = 0.491 mm/s at 4.2 K and delta(A) = 0.233 mm/s and delta(B) = 0.357 mm/s at 275 K (relative to metallic iron), with an estimated error of 0.005 mm/s. Both delta(A) and delta(B) are observed to decrease with increasing Al concentration. The effective hyperfine fields for the non-substituted maghemite sample are: H(eff,A) = 575 kOe and H(eff,B) = 471 kOe at 4.2 K and H(eff,A) = 562 kOe and H(eff,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.