Because of the clear two-step reaction process seen in BCZT (Section 3.1), we assumed that after the formation of CaZrO3, the process would closely follow the reaction route of pure BaTiO3. However, the phase development deviates from the reactions during the solid-state synthesis of BaTiO3 above the phase transition temperature of BaCO3, i.e., above ~900 °C. The deviation of the reaction mechanism is indicated by the appearance of new diffraction peaks in the 2𝜃 range between 27.5° and 30°, as well as at higher angles (Figure ). These reflections can only partly be explained by the formation of Ba2TiO4 and we hypothesize that they come from two novel intermediate oxycarbonate phases with different unit cell volumes and with the 𝑅3 ̅ 𝑚 symmetry (named ABO3-x1(CO3)x1 and ABO3-x2(CO3)x2 in the following; indexed in Figure ). Further research will be conducted to create a structural model of these phases and fully explain their formation. Tracking their phase fractions in the range of 800-1000 °C (Figure , Figure , and supplementary data) revealed that the oxycarbonate phase with a smaller unit cell volume forms first at about 850 °C, while the larger-volume oxycarbonate phase only appears between 875 °C and 900 °C.