Preparation of the Ba2CrNbO6 and NiNb2O6 Oxides by Thermal Decomposition of the Mixture of Oxalate Precursors

Beside their applications in catalysis, photoluminescence, magnetism, gas storage and separation, metal-organic coordination systems have recently been used as molecular precursors for the preparation of the mixed metal oxides by their thermal decomposition. It has been observed that the use of a well-defined heterometallic precursor can produce crystalline oxide materials under conditions that are significantly milder than those applied in traditional solid-state synthesis. Moreover, the single-source precursors provide better control over the stoichiometry of the metal ions in the final products as well as the homogeneity of the materials due to the mixing of the metals at the molecular level. The existence of bridging or chelating ligands in the precursors prevents metal separation during oxide formation. The C2O42− anion easily decomposes to the vapour phases CO2 and CO, by the low- temperature routes, and hence, heterometallic oxalate complexes are very convenient for the preparation of mixed metal oxides [1]. Heterometallic oxalate complexes do not always contain the appropriate stoichiometry for the formation of the desired single phase mixed-metal oxide. So, we have tested whether the multimetallic oxides could be prepared by mixing two or more different oxalate precursor in various ratios prior to thermal decomposition [2]. A highly crystalline materials Ba2CrNbO6 were obtained after thermal decomposition of the mixture of the well-defined and structurally characterized oxalate-based compounds {; ; ; Ba2(H2O)5[NbO(C2O4)3]HC2O4}; ; ; ·H2O [3] and (NH4)3[Cr(C2O4)3]·3H2O taken in equimolar ratio and grinded in agate mortar prior to heating treatment. Following the same strategy, oxide NiNb2O6 was prepared from the mixture of [Ni(bpy)3]2[NbO(C2O4)3]Cl·12H2O [4] and (NH4)3[NbO(C2O4)3]·H2O [3] complexes, taken in a 1 : 3 ratio. The phase formation and structural ordering of oxides obtained by modified molecular precursor-to-material method has been investigated through a combination of thermal analysis (TGA and DTA) and powder X-ray diffraction.