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From discrete or polymeric heterometallic complexes to the mixed-metal oxides

Mixed-metal oxides are an important class of advanced materials, due to their stability, low cost, low toxicity, useful photophysical properties and wide range of technological applications. They are mostly utilized as catalysts and structural ceramics, but recently, the use of mixed-metal oxides as sensors, actuators, and smart materials has also been explored. It is known that the effect of crystallinity, particle size, structure and morphology of these materials could highly affect their properties. These can be tuned in part by changing the synthesis methods. The possibility of using coordination polymers through the thermal decomposition process as molecular precursors in the synthesis of nanomaterials with high surface and specific morphology has been considered only recently. This method of obtaining oxide materials, compared with conventional methods, has several advantages: (i) the obtained material is more homogeneous because the metals are mixed at the molecular level ; (ii) the resulting materials have relatively high specific surface areas because the crystalline oxides are formed under significantly milder conditions than those in, for instance, solid-state reaction processes ; (iii ) the existence of bridging or chelating ligands in the precursors prevents metal separation during oxide formation ; (iv) the re is much greater control of the metal stoichiometry in the final oxide. The C2O42− group easily decomposes to gaseous CO2 and CO at low temperatures, and hence, the oxalate-based solids can serve as a convenient source of oxides [1]. Utilizing the preparation of the oxide materials via thermal decomposition, several oxalate-based compounds were tested as molecular precursors. Discrete heterotetranuclear oxo -bridged compound [Cr2(bpy)4(μ-O)4Nb2(C2O4)4]·3H2O (1 ; bpy = 2, 2'-bipyridine) showed to be a good candidate for molecular precursor-to-material conversion, yielding the rutile-type CrNbO4 oxide after heat treatment at 900°C. The thermal processing of heterodimetallic one-dimensional (1D) compound {; ; ; ; ; ; [CaCr2(bpy)2(C2O4)4]·0.83H2O}; ; ; ; ; ; n(2) proved to be a simple, one-step synthesis route for the preparation of the β-CaCr2O4 phase at 1100 °C in nitrogen flow. The r.t. structure of β-CaCr2O4 is isomorphic with calcium ferrite, unlike most chromate structures, which usually crystallize as spinel oxides. A three-dimensional (3D) oxalate-based coordination polymer {; ; ; ; ; ; [Co(bpy)3][Mn2(C2O4)3]·H2O}; ; ; ; ; ; n(3) was used as a single-source precursor for the formation of spinel oxide CoMn2O4 heating at 800 °C. The conversion via thermal decomposition of compounds 1–3 was explored by thermal analysis (TGA and DTA), IR spectroscopy and powder X-ray diffraction (PXRD). References [1] M. Jurić, J. Popović, A. Šantić, K. Molčanov, N. Brničević and P. Planinić, Inorg. Chem. 52 (2013) 1832-1842.

Mixed-Metal Oxides ; Molecular Precursors ; Thermal Decomposition ; X-ray Diffraction

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