THE CENTER SHIFT IN MOSSBAUER SPECTRA OF MAGHEMITE AND ALUMINUM MAGHEMITES

Abstract--Synthetic, relatively well-crystallized aluminum-substituted maghemite samples, y-(Aly. Fe~_y)203, with y = 0, 0.032, 0.058, 0.084, 0.106 and 0.151 have been studied by X-ray diffraction and zero-field Mrssbauer 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 (6) of the tetrahedral (A) and octahedral (B) ferric ions. From the applied-field measurements i is concluded that there is a constant difference of 0.12 _+ 0.01 mm/s between 6B and 6A, regardless of the A1 content. For the Al-free sample the center shifts are found as: 64 = 0.370 mm/s and 6a = 0.491 mm/s at 4.2 K and 6n = 0.233 mm/s and fib = 0.357 mm/s at 275 K (relative to metallic iron), with an estimated error of 0.005 mm/s. Both 6A and fib are observed to decrease with increasing AI concentration. The effective hyperfine fields for the non-substituted maghemite sample are: Hefr.A = 575 kOe and Hofr.B = 471 kOe at 4.2 K and He~,A = 562 kOe and H~er.B = 449 kOe at 275 K, with an error of 1 kOe. The B-site hyperfine field remains approximately constant with A1 substitution, while for the A site a slight decrease with increasing A1 content was observed. Key Words--A1-Maghemite, Center shift, Maghemite, Mrssbauer effect