Structure, stoichiometry, and phase purity of calcium substituted lanthanum manganite powders

Calcium-doped lanthanum manganite La1-xCaxMnO3, synthesized by the glycine/nitrate method, was characterized by high resolution synchrotron X-ray powder diffraction, electron diffraction, and infrared spectroscopy, A strong correlation was observed between the cooling rate from the calcination temperature and the powder quality, indicating the importance of a homogeneous oxygen stoichiometry, The structure refinement reveals that La1-xCaxMnO3 with x = 0.2, 0.3, 0.4, and 0.6 has orthorhombic symmetry with space group Pnma. The MnO6 octahedra are fairly symmetrical, but the octahedra are tilted about 20 degrees relative to the ideal perovskite structure. Infrared spectroscopy revealed that only the O-Mn-O bending mode is significantly influenced by the substitution of La with Ca. In La0.8Ca0.2MnO3 we found diffraction evidence of a superstructure in domains in some of the grains. We propose that the superstructure in La0.8Ca0.2MnO3 is due to ordering of Ca2+ ions on A (La3+) sites in the perovskite ABO(3) structure. (C) 1998 Academic Press.