New Fe–Ta and Co–Ta oxalate complexes: structural characterization and thermal behaviour – formation of mixed-metal oxides

Abstract

Two novel heterometallic oxalate-based compounds, [M(phen)₃][Ta(OC₂H₅)(C₂O₄)₃]·H₂O [M = Fe (1), Co (2); phen = 1,10-phenanthroline] were synthesized by using an (oxalato)tantalate(V) aqueous solution as a source of complex anions. As a result of the presence of ethanol in the reaction mixture, the ethoxy group coordinated to tantalum(V) and the [Ta(OC₂H₅)(C₂O₄)₃]²⁻ anion was isolated in the crystalline products. The obtained compounds were characterized by means of IR spectroscopy, X-ray single-crystal diffraction and thermal analysis. Both compounds were investigated as possible molecular precursors for the mixed-metal oxides. Thermal decomposition residues of the compounds at different heating temperatures (800, 1000, 1200 °C) were analyzed by powder X-ray diffraction. When 1 was heated up to 800–1200 °C the mixed-metal oxide FeTaO₄, of different crystallite sizes, formed. The thermal decomposition residue of 2, heat-treated up to 1000 °C, contained an up to now non-investigated mixed-metal oxide, Co₄Ta₂O₉, along with the better known mixed-metal oxide CoTa₂O₆. Co₄Ta₂O₉ was for the first time structurally characterized: the structure is composed of (i) layers containing hexagonally arranged CoO₆ octahedra and (ii) layers where both CoO₆ and TaO₆ octahedra form a hexagonal assembly.