Synthesis of high aspect ratio PbBi₄Ti₄O₁₅ and topochemical conversion to PbTiO₃-based microplatelets

Perovskite microplatelets of the composition 0.4(Na_1/2Bi _1/2) TiO₃-0.6PbTiO₃ (0.4NBT-0.6PT) were synthesized by topochemical conversion of the Aurivillius phase PbBi ₄Ti₄O₁₅ in a NaCl/Bi₂O ₃/PbO flux system. To facilitate morphologic control, we investigate the effects of TiO₂ particle size on molten salt growth of the PbBi₄Ti₄O₁₅ phase. We find that the initial nucleation density and [100] thickness of this phase are controlled by the TiO₂ dissolution rate, while the platelet diameter is determined by Ostwald ripening. PbBi₄Ti₄O₁₅ microplatelets produced using these methods can be converted entirely to a tetragonal perovskite phase (c/a=1.051) while retaining the dimensions of the precursor PbBi₄Ti₄O₁₅ phase. We propose that the resulting 0.4NBT-0.6PT composition is favored thermodynamically due to the lower free energy of this composition relative to pure PbTiO₃. In addition, partial (Na_1/2Bi_1/2)TiO₃ substitution is kinetically favored as it reduces A-site diffusion during the topochemical conversion process.