Numerous studies are always devoted to mixed valence state iron oxides in materials research due to their complex magneto transport properties like the famous Verwey transition [1].  Among these oxides, a special attention is focused on the orthoferrites LnFeO₃ (Ln= rare earth) related to the distorted GdFeO₃-type perovskite structure which can exhibit some possible spin reorientation transitions versus temperature and the nature of Ln [2]. Recently, iron based oxides like LnFe₂O₄ have also focused a large attention due to their ability to exhibit some multiferroic properties [3]. In these systems, both kinds of Fe species (Fe²⁺ and Fe³⁺) localize magnetic moments leading to a ferrimagnetic ordering associated to ferroelectric properties. An exciting challenge is to evidence similar properties in other iron based systems. The Ca-Fe-O system offers several interesting candidates like the CaFe₅O₇ and CaFe₃O₅ phases in regard to the richness of its phase diagram.

CaFe₅O₇ oxide exhibits a complex structure which can be described as an intergrowth between one CaFe₂O₄ unit and n=3 slices of FeO Wustite-type structures [4]. A recent structural study performed by transmission electron microscopy (TEM) observations has revealed a supercell with a lower monoclinic symmetry [5]. From the intensity extraction and hkl conditions deduced from the precession electron diffraction (PED) study, a structural model considering to this supercell and the centrosymmetric P2₁/m setting can be proposed. The fine structural analysis combining Rietveld refinements from neutron and X-ray data evidence six independent iron sites and two specific oxygen environments with coordination 6 and 5+1 respectively. According to the chemical formula CaFe₅O₇, the iron species average state valence is +2.4 and implies the coexistence of Fe⁺³ and Fe²⁺. The magnetic dependence versus temperature has been studied and susceptibility measurements have revealed discontinuity around 360K [6]. The structural evolution of CaFe₅O₇ depending on temperature has been also tuned from diffraction techniques. A clear reversible transition (monoclinic to orthorhombic) has been detected in the same temperature range with the disappearing of the supercell [6]. A complementary STEM-HAADF study has allowed to highlight the impact of this superstructure at atomic scale (Fig.1) : ordered contrasts at the level of calcium rows can be observed. A second ferrite, CaFe₃O₅ related to the n=1 member of the generic (CaFe₂O₄)(FeO)n series, has also been analysed by TEM techniques. Thus a superstructure is revealed but the first STEM-HAADF highlight a complex nanostructural feature related to the coexistence of two polymorphs (Fig.2)

[1] E J W Verwey, Nature 144, 327 (1939)

[2] R. Bozorth & al  Phys. Rev. Lett., 1, 3, (1958)

[3] M. Hervieu & al, Nature Materiels, 13 (2014)

[4] O. Evrard & al,  JSSC 35, 112 (1980)

[5] C. Delacotte & al Key Engineering Materials (2014)

[6] C. Delacotte & al Inorg. Chem. (2014)

Figures:

Figure 1 : [101] oriented HAADF of CaFe5O7 recorded at RT

Figure 2 : [100] oriented HAADF of CaFe3O5 recorded at RT

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EMC Abstracts - https://emc-proceedings.com/abstract/orderdisorder-mechanisms-in-complex-cafe2o4feon-ferrites-n13/