IRON CHEMICAL STATE AND STRUCTURAL PROPERTIES OF Fe-Zn MANGANITES AND THEIR CARBONATE PRECURSORS

Preparation and characterization of both iron-zinc manganites at different iron content (x = Fe/(Fe + Zn) = 0, 0.01, 0.05, 0.10) and their carbonate precursors are reported. In particular, structural properties and the iron chemical state were investigated. Precursors, obtained by coprecipitation at constant pH, were in all cases monophasic and made by a rhodochrosite-like, FexZnyMn(1-x-y)CO3, phase. Mössbauer spectroscopy proved that, surprisingly, Fe3+ was 95% of total iron. A hypothesis about the charge compensation mechanism in the rhodochrosite-like structure is given. The carbonate precursors were decomposed in air at 723 and 973 K, leading at both temperatures to monophasic iron-zinc manganites characterized by a distorted spinel-like structure with tetragonal symmetry. In such a structure, trivalent Mn3+ ions and bivalent Zn2+ ions are perfectly ordered in the octahedral and tetrahedral sites of the crystal lattice, respectively, as expected for a ‘normal’ spinel. Mössbauer investigation revealed that iron is present as trivalent ions in the octahedral sites and that no iron extra-phases were formed. Moreover, three doublets corresponding to octahedral Fe3+ ions were detected, likely related to different distributions of Fe3+/Mn3+ cations in the next-nearest coordination sphere.