STRUCTURE, STOICHIOMETRY, AND PHASE PURITY OF CALCIUM SUBSTITUTED LANTHANUM MANGANITE POWDERS

Calcium-doped lanthanum manganite La1-xCaxMnO3, synthesized by the gly cine/nitrate method, was characterized by high resolution synchrotron X-ray powder diffraction, electron diffraction, and infrared spectrosc opy, A strong correlation was observed between the cooling rate from t he calcination temperature and the powder quality, indicating the impo rtance of a homogeneous oxygen stoichiometry, The structure refinement reveals that La1-xCaxMnO3 with x = 0.2, 0.3, 0.4, and 0.6 has orthorh ombic symmetry with space group Pnma. The MnO6 octahedra are fairly sy mmetrical, but the octahedra are tilted about 20 degrees relative to t he ideal perovskite structure. Infrared spectroscopy revealed that onl y the O-Mn-O bending mode is significantly influenced by the substitut ion 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 Pr ess.