Raman microscopic investigations of BaTiO3 precursors with core–shell structure

Due to their outstanding dielectric and ferroelectric properties, barium titanate (BaTiO3)-based ceramics have found many applications in electronic devices. To optimise the final quality of such ceramics, a detailed knowledge of the complex processes involved in the formation of BaTiO3 is required. The phase formation process in ordered structures of the BaCO3/TiO2 system was analysed by X-ray diffraction and by Raman spectral imaging (RSI) as a function of the annealing temperature. RSI was used for the first time as a locally resolving method for phase analysis, and proved to be a useful tool in examining the formation process of BaTiO3 starting from spherical, core–shell structured precursors of the type TiO2 core/BaCO3 shell. The Raman spectra of different BaO–TiO2 phases appearing as intermediate phases during the formation of BaTiO3 were recorded for separately-prepared pure substances. Using these spectra as fingerprints, and choosing phase filters by setting wave number windows, “phase landscape pictures” of the samples at different temperatures during the genesis of BaTiO3 could be created with a lateral resolution of up to 200 nm. These pictures confirm shell-like formation of the different barium titanate phases according to the diffusion of barium and oxygen ions from the Ba-rich shell into the TiO2 core. At an intermediate state of the phase formation process, the phase sequence Ba2TiO4, BaTiO3, BaTi2O5, BaTi4O9 and BaTi5O11 to TiO2 was detected from the outer to the inner parts of the core–shell structures.