Synthesis and characterization of non-stoichiometric nickel-copper manganites

Non-stoichiometric nickel-copper manganites Ni(x)Cu(y)Mn(3-x-y)square(3 delta/4)O(4+delta) were synthesized by thermal decomposition of mixed Nix/3Cuy/3Mn(3-x-y)/3C2O4, nH(2)O oxalates in air at low temperature (623-673 K). X-ray diffraction showed that, for a nickel content x(Ni) greater than or equal to 0.1, the oxalates precipitated presented a mixed crystal structure up to a limit value of copper extent, whereas the oxalates obtained with x(Ni) < 0.1 were not mixed. This could be explained by the intermediate structure of nickel oxalate (beta orthorhombic form) between those of copper and manganese (alpha monoclinic form) oxalates. The structure (alpha or beta) Of the mixed oxalates obtained was also investigated and their lattice parameters are given. The Ni(x)Cu(y)Mn(3-x-y)square(3 delta/4)O(4+delta) oxides crystallize in the spinel structure in a wide range of composition and a stabilizing effect of copper was evidenced. They are highly divided (Sw > 100 m(2) g(-1)) however Sw tends to decrease with increasing y(Cu). The non-stoichiometry delta 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 Mn5O8 acid nickel manganites synthesized in the same conditions, showed that delta 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. (C) 1999 Elsevier Science B.V. All rights reserved.