Mössbauer Spectra of Synthetic Aluminum Substituted Maghemites (γ-Fe2-xAlxO3).

Mossbäuer spectroscopy has become an increasingly important and applied analytical tool for the characterization of synthetic and natural Fe-containing materials. One of the most rapidly growing application area of this technique is undoubtedly that of soil and sediments. Unlikely for goethite (α-FeOOH) and hematite (α -Fe₂O₃), only a very limited collection of information about the influence of substitutions upon the magnetic properties is available for maghemite (γ-Fe₂O₃). Synthetic, relatively well-crystallized aluminum-substituted maghemite samples, γ-Fe_2-xAl_xO₃, with x = 0, 0.004, 0.011, 0.027, 0.029, 0.063, 0.067, 0.097, 0.124 and 0.145 have been studied by X-ray diffraction and Mossbäuer spectroscopy in 300 K. Mössbauer spectra adjusted of all maghemitas samples obtained the 300 K and the hyperfine parameters corresponding to doublet and sextets subspectra, they are attributable Fe³⁺ high spin (0.2 to 0.7 mm/s). The lack evidence of Fe²⁺ and Fe mixed valency (Fe^3+/2+) it excludes the magnetite occurrence. In all samples were observed sextets corresponding from 100 to 68% of the sample and of the doublet with 32%, was found in the sample with x = 0.124. The doublet observed to room temperature can be characteristically of Fe³⁺, in the structure of paramagnetic minerals or iron oxides in superparamagnetic state, of small particle size. The maghemite magnetic hyperfine interaction values (Bhf) decreased with the isomorphic substitution increasing. However, in the first three points of the adjustment the values of Bhf increase. The increase of the half-width values (Ã) of the mössbauer spectra valleys demonstrated that the increase of the isomorphic substitution promoted increase in Ã, indicating reduction in the of crystallinity degree in the samples.