New analysis of the Mössbauer spectra of akaganeite
Résumé
The Mössbauer spectra of akaganeite have always been interpreted considering both the tetragonal structure and the chlorine content. However, very recently it has been suggested that the crystallographic structure is not tetragonal but monoclinic, thus another interpretation for the Mössbauer spectra is required. For this purpose, we have prepared and characterized by several techniques synthetic akaganeite. Our results suggest that the two crystallographic sites required by the monoclinic symmetry are not distinguishable in the paramagnetic state as previously assumed, but they are only discernible in the low temperature magnetic region. At room temperature the spectrum is fitted with two doublets whose origin is related to the chlorine content, i.e. one Fe site assigned to Fe3+ ions located close to chloride ions and the other Fe site to those located close to chloride vacancy sites. The low temperature spectra can be adequately fitted with four sextets, whose hyperfine parameters must be subjected to some constraints. The origin of these components is related to the two different crystallographic sites and to the chlorine content. In-field Mössbauer spectrometry at low temperature suggests that the magnetic structure behaves as a system which consists of two asperimagnetic-like structures antiferromagnetically coupled, and not as a collinear antiferromagnet as usually assumed.