Synthesis, Decomposition and Crystallization Characteristics of Peroxo−Citrato−Niobium:  An Aqueous Niobium Precursor

The synthesis of peroxo−citrato−niobium, a novel aqueous precursor of niobium, is reported. Niobium oxalate was decomposed with acidified H 2 O 2 to form a peroxo−niobium complex, which was reacted with citric acid at pH ∼ 7.5 to form the stable peroxo−citrato−niobium complex. IR and Raman spectroscopy indicate that the peroxo−citrato−niobium complex consists of tridentate bridging citrato and bidentate peroxo ligands. The aqueous complex is binuclear with peroxo groups chelated to niobium and the precipitated complex is polynuclear with bridging type peroxo groups. Due to kinetic limitations during crystallization, the formation of seven-coordinated niobium in peroxo−citrato−niobium energetically favors the formation of niobium oxides with seven-coordinated niobium, viz. TT−Nb 2 O 5 and T−Nb 2 O 5 . Consequently, nanocrystalline metastable TT−Nb 2 O 5, with a particle size of ∼25 nm, crystallizes from the peroxo−citrato−niobium complex at 580 °C. On further heating, TT−Nb 2 O 5 transforms to T−Nb 2 O 5 at 800 °C, M−Nb 2 O 5 at 1000 °C, and H−Nb 2 O 5 at 1200 °C. The nanometric particle size is retained after heating to 1200 °C and H−Nb 2 O 5 has an average crystallite size of 53 nm.