MS-12-P-3011 Evidence of a structural and magnetic transition in CaFe5O7 at 360K

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 phase 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 carried out at room temperature combining transmission electron microscopy (TEM) observations and powder X-ray diffraction data has revealed a supercell with a monoclinic symmetry [5]. From the hkl conditions deduced to the electron diffraction 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+, so magnetic interactions could be expected. To base this peculiar feature, the magnetic dependence versus temperature has been studied and susceptibility measurements have revealed discontinuity around 360K (Fig.1). The structural evolution depending on temperature of CaFe5O7 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 (Fig.2) in agreement with the sensivity of the latter under electron beam in Image mode. However a complementary STEM-HAADF study has allowed to image the superstructure (fig.3), with ordered contrasts at the level of iron rows.

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[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 (in press)