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 I 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. Mossbauer 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 721, 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. Mossbauer investigation revealed-that iron is present-As trivalent ions in the octahedral sites and that no iron extra-phases Wefd, 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.