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 H2O2 to form a peroxo-niobium complex, which was reacted with citric acid at pH similar to 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 citrate 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-Nb2O5 and T-Nb2O5. Consequently, nanocrystalline metastable TT-Nb2O5, with a particle size of similar to 25 nm, crystallizes from the peroxo-citrato-niobium complex at 580 degrees C. On further heating, TT-Nb2O5 transforms to T-Nb2O5 at 800 degrees C, M-Nb2O5 at 1000 degrees C, and H-Nb2O5 at 1200 degrees C. The nanometric particle size is retained after heating to 1200 degrees C and H-Nb2O5 has an average crystallite size of 53 nm.