The Center Shift in Mössbauer Spectra of Maghemite and Aluminum Maghemites

Abstract

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 it is concluded that there is a constant difference of 0.12 _+ 0.01 mm/s between 6B and 6A, regardless of the A1 content.

Iron-containing phases in metallurgical and coke dusts as well as in bog iron ore

“…Interesting feature of the obtained results is the fact that despite signifi cant discrepancies in A-site and B-site occupations, the hyperfi ne fi eld values of the obtained spectra are closer to magnetite than to maghemite [17,18]. The serious diffi culty in analysing the magnetite Mössbauer spectra is the fact that even above the Verwey transition (charge ordering transition around 120 K), the spectrum consists of three -not two -components because of mixed valency (Fe 2+ /Fe 3+ ) at octahedral B sites [19].…”

“…In general, the higher value of  is determined (smaller ) and the higher oxidation degree of magnetite is observed. In particular, when concentration of vacancies takes a value [17,18]. However, in the case of two investigated metallurgical dusts (MET2-P4 and MET3-P5), the value of  is higher than 1/3.…”

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

“…At 6 K all five samples with different amounts of AI substitution exhibit an asymmetrical sextet which is characteristic for the presence of a magnetic hyperfine-field distribution (MHFD). The problems associated with the numerical analysis of these spectra have been discussed previously by da Costa et al (1994c). These authors concluded that neither the use of two discrete, but constraint Lorentzian-shaped sextets, nor the use of a superposition of two modelindependent M H F D , yielded acceptable results for the various M6ssbauer parameters.…”

“…Two quadrupole doublets with split-Lorentzian line shape (i.e., different width for the low-and high-AEQ side of the absorption line) were considered to fit the MS, however, with two rather severe constraints: the A-site isomer shifts were forced to be 6A = 6B --0.08 mm/s, and the relative area ratios were fixed at their values found from externalfield measurements (da Costa et al 1994a). The idea behind the isomer-shift coupling is that the difference 6B -6A for the M samples was found to be constant at 0.12 mm/s over a wide range of temperatures (da Costa et al 1994c). The presently used value of 0.08 is the average of ten external-field results, all at 4.2 K and referring to poorly crystalline maghemites (de Bakker et al 1991, da Costa et al 1994a).…”

Structural and Magnetic Properties of Iron Oxide Nanoparticles in Shells of Hollow Microcapsules Designed for Biomedical Applications

“…These values are the typical characteristics of the high spin Fe 3+ ions in maghemite. [14][15][16] The area of the doublet increases with increasing temperature. It should be noted that we did not observed remarkable transformations of the spectra shape at temperatures where the Verwey transition occurs in magnetite Fe3O4 (around 120 K).…”