Low-Temperature Mössbauer Study of a Aluminum-Substituted Maghemite (γ-Fe2-xAlxO3).

Maghemite (γ-Fe₂O₃) is a ferrimagnetic Fe oxide commonly found in tropical and subtropical soils, especially in the topsoil where it is usually a product of burning. Isomorphic substitution (IS) of the Fe in maghemite by different metals (mainly Al³⁺) can modify its mineralogical and chemical attributes, and these modifications may be important to understanding the formation and properties of this mineral in soils and sediments. Therefore, the objective of this work was characterized synthetic maghemites with different degree of isomorphic substitution of Fe by Al (0.0, 0.9, 2.2, 5.5 and 7.4 mol% Al) by Mössbauer spectroscopy at low temperature (4.2 K). Mossbauer spectra adjusted of all maghemite samples and hyperfine parameters corresponding to sextets. The best adjustment was observed using two sextets for each sample. The Bhf had different behavior with temperature decreasing. At 225 K the A site Bhf decreased with IS and B site Bhf increased. At 225 K the A site presented higher spectral area in samples with 0.00 and 0.89 mol % Al, on the other hand, the samples with 2.21, 5.53 and 7.35 mol % Al the Bhf spectral area were similar. At 4.2 K the Bhf decreased for both sites, but always the site B showed greater spectral area than the site B. At 77 k the data does not show any specific behavior, but the Bhf spectral area was generally smaller for A site than the B site.