Characterization of CeO₂–Fe₂O₃ Mixed Oxides: Influence of the Dopant on the Structure

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.