Synthesis and characterization of non-stoichiometric nickel-copper manganites

Non-stoichiometric nickel–copper manganites Ni Cu Mn h O were synthesized by thermal decomposition of x y 32x2y 3d / 4 41d mixed Ni Cu Mn C O , nH O oxalates in air at low temperature (623–673 K). X-ray diffraction showed that, x / 3 y / 3 (32x2y) / 3 2 4 2 for a nickel content x $ 0.1, the oxalates precipitated presented a mixed crystal structure up to a limit value of copper Ni extent, whereas the oxalates obtained with x , 0.1 were not mixed. This could be explained by the intermediate structure Ni of nickel oxalate (b orthorhombic form) between those of copper and manganese (a monoclinic form) oxalates. The structure (a or b) of the mixed oxalates obtained was also investigated and their lattice parameters are given. The Ni Cu Mn h O oxides crystallize in the spinel structure in a wide range of composition and a stabilizing effect x y 32x2y 3d / 4 41d 2 21 of copper was evidenced. They are highly divided (Sw. 100 m g ) however Sw tends to decrease with increasing y . Cu The non-stoichiometry d of such nickel–copper manganites was for the first time determined by selective titration (gas chromatography) of the oxygen released during TPR experiments in argon. The technique is presented and the results, along with those obtained with manganese oxide Mn O and nickel manganites synthesized in the same conditions, showed that d 5 8 depended both on the decomposition temperature of the oxalate and on the chemical composition of the oxide. Such results should provide interesting data concerning the cationic distributions of these non-stoichiometric nickel–copper manganites.