Effect of BaCO₃ impurities on the structure of BaTiO₃ nanocrystals: Implications for multilayer ceramic capacitors

Miniaturization and optimization of high capacitance multilayer ceramic capacitors and other composite optoelectronic devices is driving development of smaller and smaller oxide nanocrystals with high phase purity, uniform and tunable particle size/shape, and other meritorious characteristics. Ferroelectric barium titanate (BaTiO₃) nanoparticles can be synthesized using a wide variety of solution-based methods, many of which include BaCO₃ impurities in the final product. BaCO₃ impurities have been purported to be a discrete phase or surface layer, but either form can be removed by washing the nanocrystals with a dilute acid. Systematic studies of the effect of the acid wash on the atomic-scale structure, morphology, and particle size distributions of the BaTiO₃ nanoparticles have not previously been undertaken. Here, we present results from a series of six sets of solvothermally synthesized BaTiO₃ nanocubes, where half of each batch was washed with dilute acetic acid. Using synchrotron X-ray diffraction and pair distribution functions, it is found that the crystallite size and atomic-scale structures (local and long-range) are unchanged by the washing step and it is concluded that the BaCO₃ impurity phase observed via X-ray diffraction is a discrete phase and not a surface layer of adsorbed CO₂ or CO₃²⁻. Transmission electron microscopy verified that the nanocubes' morphology and size distributions were unaffected by the washing step. This systematic study suggests that structural characterizations of BaTiO₃ samples with different postsynthesis washing treatments can reasonably and credibly be compared.