CeFex (x = 0, 1, 3, 5, 10, 15 and 20 at.%) mixed oxides synthesized by an adapted Pechini method were characterized by Raman spectroscopy, high-resolution transmission electron microscopy, electron paramagnetic resonance, magnetization and 57Fe Mössbauer spectroscopy (57Fe-MS) measurements in order to evaluate the oxygen vacancies formation and the chemical environment of Fe+3 inserted into the CeO2 crystalline lattice. Fe+3 introduction into the CeO2 structure resulted in an increase of the oxygen vacancies concentration, which indicates that this is the predominant charge compensation mechanism in the formation of CeFex solid solutions by the Pechini method. Fe+3 insertion in CeO2 led to the formation of substitutional solid solutions, in which Fe+3 replaced octahedral Ce+4 sites in the crystalline lattice. Fe+3 could be found in the form of isolated sites with orthorhombic distortion or Fe+3 species in pairs or clusters coupled by strong spin–spin interactions. No evidence of Fe+3 insertion into tetrahedral interstitial sites was found. Isolated Fe+3 species showed a less distorted chemical environment and greater ionic character of the Fe–O bonds than the clusters, being the concentration of both type sites approximately equal for all the Fe+3 doped contents. It was found that pure CeO2 and all the CeFex mixed oxides presented ferromagnetic properties even at room temperature possibly due to their small crystallite size and the presence of oxygen vacancies. At high Fe+3 concentrations (above 10 at.%), probably super-exchange interactions (Fe+3–O−2–Fe+3), with an antiferromagnetic character, also took place, reducing the ferromagnetism of the CeFex mixed oxides.
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The authors thank the Foundation for Research Support of the State of Rio de Janeiro (FAPERJ) and the National Counsel of Technological and Scientific Development (CNPq) for financial support (scholarship). We thank Rosa B. Scorzelli for made available the Mössbauer facility at Centro Brasileiro de Pesquisas Físicas (CBPF)—Brazil. The authors also acknowledge Núcleo de Microscopia da COPPE-UFRJ for the use of the facilities.
Funding was provided by Universidade Federal do Rio de Janeiro.
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