β-BaCO3 facilitates a topochemical reaction route for the high temperature synthesis of perovskite-type (Ba,Ca)(Zr,Ti)O3 via oxycarbonate intermediates

Ferroelectric oxides from the perovskite-type (Ba1−xCax)(ZryTi1−y)O3 system are emerging as one of the upcoming lead-free piezoelectrics for future sensing and actuating applications. Improving their functionality requires a precise adjustment of precursor choice, pre-treatment, as well as reaction and sintering times and temperatures to optimize the defect chemistry and microstructure of the materials. In this respect, understanding the reaction pathways of the precursors has a crucial impact on the final functional properties. In this article, we show that the high temperature modification of BaCO3 precursors can strongly influence the reaction by facilitating a topochemical reaction pathway via the formation of an intermediate oxycarbonate phase of BaTiO3−x(CO3)x (x ≈ 1/3). This results from the diffusion of TiO2 into the material in combination with CO2 outgassing, distinguishing this reaction path from the previously reported diffusion mechanism via an intermediate phase of Ba2TiO4 which happens in parallel. We used high-temperature in-situ X-ray diffraction and automated diffraction tomography to derive a structural model of this novel intermediate oxycarbonate phase, which can be understood as a trigonal distortion variant of the cubic perovskite structure, where the trigonal distortion originates from the planar nature of the carbonate anion. This mechanism could have implications on how the BaCO3 precursor and its decomposition behavior can decisively influence particle morphology and composition obtained prior to sintering of BCZT ferroelectric oxides.