Structure and magnetic properties of Ca2Fe1-xMnxAlO5+δ

Ca2Fe1-xMnxAlO5 (0≤x≤1) compounds were prepared by a self-combustion method under air (x=0, 0.1, 0.2 and 0.3) and nitrogen (x=0.5, 0.7 and 1.0). The samples prepared under nitrogen were successfully oxidized after short annealing under air. Both X-ray powder diffraction (XRD) Rietveld analysis and electron diffraction revealed that all compounds adopt the brownmillerite-type structure. All samples present an overall antiferromagnetic behaviour and data from magnetic measurements and Moessbauer spectroscopy allowed to conclude that the transition temperature decreases as Mn content increases for x≤0.3 and increases in the case of the x≥0.5 compounds. Except for x=1, chemical disorder due to the occupancy of both octahedral and tetrahedral sites by different metals as well as the competition between different moments' orientation induce a complex magnetic behaviour characterized by magnetic frustration and canted antiferromagnetism. Moessbauer spectroscopy and chemical titrations also allowed to conclude about the preferential oxidation of Mn3+ over Fe3+, obtained by thermal treatment under air of the x=0.5 and 0.7 compositions. - Graphical abstract: Structure of the Ca2Fe1-xMnxAlO5 compound and electron diffraction pattern obtained along the [1-bar 01] zone axis (x=0.2), showing a brownmillerite structure