Perovskite microplatelets of the composition 0.4(Na1/2Bi1/2) TiO3–0.6PbTiO3 (0.4NBT–0.6PT) were synthesized by topochemical conversion of the Aurivillius phase PbBi4Ti4O15 in a NaCl/Bi2O3/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  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 (Na1/2Bi1/2)TiO3 substitution is kinetically favored as it reduces A-site diffusion during the topochemical conversion process.