Tuning the Mn and Fe valence states into new Ca0.7Mn2-xFexO4 (0 < x £ 0.60) solid solution during reversible redox processes

Stéphanie Lesturgez (1) , Graziella Goglio (1) , Mathieu Duttine (1) , Alain Wattiaux (1) , Etienne Durand (1) , Julien Hernandez (2) , Jérôme Majimel (1) , Alain Demourgues (1)

1 ICMCB - Institut de Chimie de la Matière Condensée de Bordeaux
2 Solvay (France)

New Mn4+-rich Ca0.7Mn2–xFexO4 complex oxides (0 < x ≤ 0.6) with Mn4+ amount varying from 33% to 43% have been prepared by aqueous self-combustion routes and characterized by powder X-ray Diffraction (XRD) and Mössbauer spectroscopy. These oxides crystallize with a monoclinic symmetry (CaMn3O6-type, space group P21/a) for low Fe content and with an orthorhombic symmetry (CaFe2O4-type, space group Pnma) for x > 0.14, thus stabilizing the highest Ca defects content (30%) for this series. A regular decrease of the Jahn–Teller Mn3+ site distortion is observed versus Fe content. For low iron content (x < 0.33), Fe3+ ions are preferentially but not exclusively located at the Mn4+-rich site which exhibits lower octahedral distortion, whereas for higher iron doping rates (x ≥ 0.33), Fe3+ ions are almost equally distributed in the two more or less distorted transition metal sites. The same distribution of Fe3+ in both of these atomic positions contributes to the stabilization within these phases of a high content of Mn4+ even at higher temperatures (T > 1000 °C), whereas the unsubstituted Ca0.66Mn2O4 phase is only stable up to 850 °C. Furthermore, these phases can be reduced at T = 550 °C under Ar/5% H2 into a Mn2+ and Fe2+/Fe3+ phase based on the rock salt structure and then reoxidized at T = 700 °C under air to get pure phases with the same orthorhombic unit cell and a Fe3+ distribution in the two atomic positions similar to the initial phase. Despite the additional stabilization of metal iron Fe0 during the reduction at T > 550 °C, a pure Mn4+/Mn3+/Fe3+-based oxide with CaFe2O4-type structure has been recovered after oxidation at T > 700 °C under air. Mössbauer spectroscopy of the reduced phase has revealed the presence of Fe2+ ions at two distinct octahedral sites corresponding to Fe3+ within the oxidized phase. The reversibility of this redox system could be explained by the occurrence of two rutile double chains in the CaFe2O4-type network, which is closely related to the rock salt structure with Ca2+ ions in the vicinity of Mn and Fe atoms. During the steps of oxygen release and storage, this network, derived from the rock salt framework, seems to preserve the Fe and Mn organization at the local scale.

Domaines: Matériaux; Chimie inorganique

Stéphanie Lesturgez, Graziella Goglio, Mathieu Duttine, Alain Wattiaux, Etienne Durand, et al.. Tuning the Mn and Fe valence states into new Ca0.7Mn2-xFexO4 (0 < x £ 0.60) solid solution during reversible redox processes. Chemistry of Materials, 2016, 28 (14), pp.4935-4944. ⟨10.1021/acs.chemmater.6b01135⟩. ⟨hal-01360634⟩