Polymerized complex route to barium titanate powders using barium-titanium mixed-metal citric acid complex

Barium titanate (BaTiO³) powders were prepared by a polymerized complex method based on the Pechini-type reaction route, wherein a mixed solution of citric acid (CA), ethylene glycol (EG), and barium and titanium ions, with a molar ratio of CA:EG:Ba:Ti = 10:40:1:1, was polymerized to form a transparent resin, which was used as a precursor for BaTiO₃. Characterization of the initial precursor solution of EG, CA, and barium and titanium ions by Raman scattering and ¹³C-NMR spectroscopy indicated that barium and titanium ions were simultaneously stabilized with CA to form a barium-titanium mixed-metal CA complex with a stoichiometry similar to Ba:Ti:CA = 1:1:3. Raman and ¹³C-NMR spectra of the liquid mixture at various reaction stages indicated that the fundamental coordination structure of the mixed-metal complex remained almost unchanged throughout the polymerization process. X-ray diffractometry (XRD) measurements indicated formation of pseudo-cubic BaTiO₃ free from BaCO₃ and TiO₂ when the barium-titanium polymeric precursor was heat-treated in air at 500°C for 8 h or at 600°C for 2 h. However, the Raman spectra of the same powders indicated the formation of tetragonal (rather than cubic) BaTiO³, with traces of high-temperature hexagonal BaTiO3 stabilized at room temperature. XRD of a pyrolyzed product at 500°C for 2 h revealed a simple mixture of BaTiO₃ and an intermediate phase, Ba₂Ti₂O₅·CO₃. A solid-state reaction between BaCO₃ and TiO₂ was concluded as not being responsible for the BaTiO₃ formation; rather, BaTiO₃ formed directly by thermal decomposition of the intermediate Ba₂Ti₂O₅·CO₃ phase at temperatures >500°C. In addition, by Raman scattering measurements, the intermediate Ba₂Ti₂O₅·CO₃ phase was found to be unstable in ambient air, yielding BaCO₃ as one of the decomposed products.