On the formation of an oxycarbonate intermediate phase in the synthesis of BaTO3 from (Ba,Ti)-polymeric organic precursors

The study of BaTiO3 crystallization from X-ray amorphous (Ba,Ti) polymeric organic powders has been carried out on a (Ba,Ti)-citrate polymeric resin heat-treated in air at 250 degreesC. From thermal analysis, X-ray diffraction, infrared and Raman spectroscopies, and C-13 NMR spectroscopy, it has been concluded that an intermediate oxycarbonate phase was formed prior to the formation of BaTiO3 above 550 degreesC. Although not well crystallized, thermoanalytical measurements, the unique XRD pattern, and new IR, Raman, and C-13 NMR structural features revealed that such a metastable intermediate oxycarbonate phase has a stoichiometry close to Ba2Ti2O5CO3, which is characterized by having CO23- groups different to those of pure BaCO3 located, probably, in an open interlayer BaTiO3 metastable structure. A tentative structure for the oxycarbonate phase is proposed. From the XRD crystal size measurements and Raman spectra, it was suggested that the thermal decomposition of the (Ba, Ti) citrate polymeric organic precursor above 550 degreesC led to the formation of a mixture of tetragonal and hexagonal BaTiO3 polymorphs rather than cubic. Despite the specific surface of the synthesized BaTiO3 powders up to 700 degreesC, higher than 40 m(2)/g and an equivalent particle size smaller than 25 nm, Raman spectra indicated asymmetry inside the TiO6 octahedra of the BaTiO3 structure. (C) 2002 Published by Elsevier Science Ltd.