Formation mechanism of (001) oriented perovskite SrTiO₃ microplatelets synthesized by topochemical microcrystal conversion

To develop a better understanding of the mechanism responsible for topochemical microcrystal conversion (TMC) from Aurivillius SrBi₄Ti₄O₁₅ precursors to perovskite SrTiO₃ microplatelets, compositional/structural evolutions, morphological development, and reaction interface evolution of the (001) oriented SrBi₄Ti₄O₁₅ microplatelets were investigated during the conversion process. The results show that multiple topotactic nucleation events of SrTiO₃ occurred directly on the surfaces of SrBi₄Ti₄O₁₅ above 700°C, while reacting zones of intermediate phase(s) with less Bi³⁺ contents were observed to form in the interior of SrBi₄Ti₄O₁₅. Extensive exfoliation of the precursors occurred generally parallel to the (001) surfaces above 775°C. At 950°C, the original singlecrystal SrBi₄Ti₄O₁₅ platelet was replaced by a polycrystalline aggregate consisting of (001) aligned SrTiO₃ crystallites and poorly crystallized intermediate phase(s). With further increasing the temperature or holding time, the SrTiO₃ phase formed from related intermediate phase(s), and the aligned crystallites were sintered to form dense SrTiO₃ with strong (001) orientation. The obtained SrTiO₃ microplatelets preserved the shape of SrBi₄Ti₄O₁₅ and show high chemical and phase purity. This TMC mechanism has general applicability to a variety of compounds and will be very useful for the design and synthesis of novel anisotropic perovskite crystals with high quality in the future.