Mechanosynthesis of LaMnO₃ from different manganese oxides

We report on the synthesis of lanthanum manganese oxide (LaMnO₃) by solid state reaction using high energy reactive ball milling from manganese oxides (MnO, Mn₂O₃, MnO₂) mixed with lanthanum oxide in stoichiometric ratios. Morphology and particle size of powders were investigated by SEM and laser diffraction at different milling times. A prolonged milling time (> 630 min) produces an important reduction in particle size but also powder agglomeration. Structural characterization as a function of milling time was carried out by XRD as well as by TEM for specific samples. Lanthanum manganites were successfully obtained by the simplest possible process, a single step milling at room temperature showing however different transformation kinetics respective to the manganese oxide precursors. Complete reaction of LaMnCO₃ using Mn₂O₃ and MnO₂ was achieved only after 210 and 270 min of milling, respectively obtaining a mixture of rhombohedral, cubic and orthorhombic phases. From MnO, mainly rhombohedral LaMnO₃ could only be obtained after a calcination process. In order to avoid iron contamination, precursor mixtures were milled in nylamid vials with zirconia balls at 360 min for Mn₂O₃ and MnO₂ as precursors. The crystalline structure of the manganites prepared using zirconia/nylamid milling system was mainly cubic.