Investigation into the formation of phases with a Ba2Ti9O20-type structure in the BaO-TiO2 and BaO-SrO-TiO2 systems

The mechanism of formation of barium titanate Ba₂Ti₉O₂₀ in the BaO-TiO₂ and BaO-SrO-TiO₂ systems is investigated using initial mixtures prepared by three methods, namely, mechanical grinding of the initial reactants, coprecipitation from aqueous solutions of salts, and the sol-gel technique. It is established that, irrespective of the preparation procedure, the formation of Ba₂Ti₉O₂₀ proceeds through the formation of the intermediate phases BaTi₄O₉ and BaTi₅O₁₁. The nature of the intermediate phases is determined by the homogeneity and dispersion of the initial mixture, as well as by the stability of the intermediate phase. The most optimum conditions for the synthesis of Ba₂Ti₉O₂₀ are provided by the formation of BaTi₅O₁₁ as an intermediate phase upon heat treatment of the coprecipitation products in the nanocrystalline state. The metastability and structural defects in the BaTi₅O₁₁ intermediate phase encourage a decrease in the temperature of the final heat treatment by 100–150°C in the course of the preparation of Ba₂Ti₉O₂₀ single-phase ceramics.