SYNTHESIS, DECOMPOSITION AND CRYSTALLIZATION CHARACTERISTICS OF PEROXO-CITRATO-NIOBIUM - AN AQUEOUS NIOBIUM PRECURSOR

The synthesis of peraxo-citrato-niobium, a novel aqueous precursor of niobium, is reported. Niobium oxalate was decomposed with acidified H2 O2 to form a peroxo-niobium complex, which was reacted with citric aci d at pH similar to 7.5 to form the stable peroxo-citrato-niobium compl ex. IR and Raman spectroscopy indicate that the peroxo-citrato-niobium complex consists of tridentate bridging citrate and bidentate peroxo ligands. The aqueous complex is binuclear with peroxo groups chelated to niobium and the precipitated complex is polynuclear with bridging t ype peroxo groups. Due to kinetic limitations during crystallization, the formation of seven-coordinated niobium in peroxo-citrato-niobium e nergetically favors the formation of niobium oxides with seven-coordin ated niobium, viz. TT-Nb2O5 and T-Nb2O5. Consequently, nanocrystalline metastable TT-Nb2O5, with a particle size of similar to 25 nm, crysta llizes from the peroxo-citrato-niobium complex at 580 degrees C. On fu rther heating, TT-Nb2O5 transforms to T-Nb2O5 at 800 degrees C, M-Nb2O 5 at 1000 degrees C, and H-Nb2O5 at 1200 degrees C. The nanometric par ticle size is retained after heating to 1200 degrees C and H-Nb2O5 has an average crystallite size of 53 nm.