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
To cite this abstract:
Charléne Delacotte, Laurine Monnier, Yohann Bréard, Sylvie Hébert, Denis Pelloquin; Order/disorder mechanisms in complex (CaFe2O4)(FeO)n ferrites (n=1,3). The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/orderdisorder-mechanisms-in-complex-cafe2o4feon-ferrites-n13/. Accessed: December 4, 2023« Back to The 16th European Microscopy Congress 2016
EMC Abstracts - https://emc-proceedings.com/abstract/orderdisorder-mechanisms-in-complex-cafe2o4feon-ferrites-n13/