These include the known intermediate phase (Ba,Ca)2(Zr,Ti)O4, a polytitanate phase, and two novel oxycarbonate phases with a perovskite-related structure. The phase formation process of the latter might be related to Ti 4+ diffusion and incorporation into the 𝑅3 ̅ 𝑚 BaCO3 lattice, resulting in two different types of perovskites containing carbonate ions. In contrast to previous reports on solid-state reactions from oxide and carbonate precursors, this demonstrates the possiblity for perovskite-type phase formation even before complete carbonate removal from the system, a topochemical reaction route starting from the high temperature modification of BaCO3, and the stability of oxycarbonate phases even at ~ 900-1000 °C.