Raman microscopic investigations of BaTiO₃ precursors with core–shell structure

Abstract

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.