Metal citrate polymerized complex thermal decomposition leading to the synthesis of BaTiO3: effects of the precursor structure on the BaTiO3 formation mechanism

Abstract

The study of BaTiO3 crystallization from X-ray amorphous (Ba,Ti) polymeric organic powders has been carried out by comparing samples previously heat-treated in air at 250 and 400°C. From thermal analysis, X-ray diffraction, infrared and Raman spectroscopies, and 13C NMR spectroscopy, it has been found that the BaTiO3 formation strongly depends upon the initial structure of the used precursor. It is concluded that an intermediate oxycarbonate phase was formed prior to the formation of BaTiO3 above 550°C when the precursor used was a (Ba,Ti)-mixed metal organic complex heat-treated at 250°C, and a nanocrystalline BaCO3 intermediate when the metal organic complex had been initially heat-treated at 400°C. Although not well crystallized, thermoanalytical measurements, the unique XRD pattern, and new IR and Raman structural features revealed that such a metastable intermediate oxycarbonate phase has a stoichiometry close to Ba2Ti2O5·CO3, which is characterized by having CO32− groups different to those of pure BaCO3 located, probably, in an open interlayer BaTiO3 metastable structure. Irrespective of the used precursor, thermal decomposition of the (Ba,Ti)-mixed metal organic above 550°C led to the formation of a mixture of tetragonal and hexagonal BaTiO3 polymorphs rather than cubic. The synthesized BaTiO3 powders are characterized by a high surface area of 40 m2 g−1 up to 700°C, and an equivalent particle size smaller than 25 nm. Raman spectra indicated asymmetry inside the TiO6 octahedra of the BaTiO3 structure.