The diffusion-controlled reaction starts by the diffusion of Ba 2+ and O 2-ions from BaCO3 into TiO2 and the formation of the intermediate Ba2TiO4 phase. The latter then reacts with remaining TiO2 to form the desired perovskite-type phase, while small amounts of polytitanate phases, e.g., Ba6Ti17O40 and Ba4Ti13O30, have been reported as well . The Ba2TiO4 intermediate phase can also exist as a secondary phase in samples with Ba-excess .