Synthesis and characterization of non-stoichiometric nickel-copper manganites

Non-stoichiometric nickel-copper manganites Ni(x)Cu(y)Mn(3-x-y)square(3 del ta/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 eq ual to 0.1, the oxalates precipitated presented a mixed crystal structure u p to a limit value of copper extent, whereas the oxalates obtained with x(N i) < 0.1 were not mixed. This could be explained by the intermediate struct ure of nickel oxalate (beta orthorhombic form) between those of copper and manganese (alpha monoclinic form) oxalates. The structure (alpha or beta) O f the mixed oxalates obtained was also investigated and their lattice param eters 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 st abilizing 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-st oichiometry delta of such nickel-copper manganites was for the first time d etermined by selective titration (gas chromatography) of the oxygen release d during TPR experiments in argon. The technique is presented and the resul ts, along with those obtained with manganese oxide Mn5O8 acid nickel mangan ites synthesized in the same conditions, showed that delta depended both on the decomposition temperature of the oxalate and on the chemical compositi on of the oxide. Such results should provide interesting data concerning th e cationic distributions of these non-stoichiometric nickel-copper manganit es. (C) 1999 Elsevier Science B.V. All rights reserved.