Crystal structure analysis of calcium-doped lanthanum manganites prepared by mechanosynthesis

We report results of crystal structure analyses of calcium-doped lanthanum manganites, La_1-xCa_xMnO₃, obtained by mechanosynthesis. The calcium to lanthanum ratio x was varied from 0 to 1 at increments of 0.1, allowing us to study changes in crystal structure with different degrees of calcium substitution, from LaMnO₃ (x = 0) to CaMnO₃ (x = 1). Metallic oxide precursors, Mn₂O ₃, La₂O₃, and CaO, were mixed in stoichiometric proportions. The powder mixture was milled using a shaker mixer/mill. X-ray powder diffraction was used to monitor the phase transformation as a function of the milling time. Rietveld refinement was used to structurally characterize the manganites. The results show that it is possible to obtain calcium-doped lanthanum manganite by mechanosynthesis using a weight ratio of balls to powder of 12:1. After 4.5 h of milling time, the synthesis is completed; the time is independent of the calcium-doping level. However, increase of the Ca ²⁺ content leads to a monotonic decrease of the orthorhombicity factor b/a for calcium to lanthanum ratios between 0.2 and 0.8. When the doped level is increased, a peak displacement is observed, which is associated with a distortion of the crystal structure and variation in the cell parameter. All the manganites crystallized with the same O-type orthorhombic perovskite structure as pure LaMnO₃, with a space group Pnma. The structural distortion in the orthorhombic lattice with the Ca²⁺ content is associated with a partial oxidation of the manganese ion, the increment on vacancies, and the cationic substitution.