The mechanism of the topochemical conversion from the Aurivillius to the perovskite structure has been proposed by Poterala et al. (Poterala et al., 2010). The Aurivillius phase converts directly to the perovskite phase as in NaNbO3 or via an intermediate phase, which is formed by multiple nucleations on the Aurivillius phase, as in BaTiO3 and PbTiO3. The conversion occurs in two sequential stages. The first stage is the multiple topotactic nucleation of the perovskite phase either directly on the Aurivillius phase or on the intermediate phase. The nuclei grow to small crystallites. They are slightly misaligned from the Aurivillius parent structure. At the end of the first stage, the Aurivillius single-crystal particle changes to an aggregate of aligned perovskite crystallites with a slight misorientation. In the second stage, the aligned crystallites in the aggregate grow to form a dense platelike particle. When the growth of the aligned crystallites is complete, the perovskite particles are single-crystals with almost the same shape and size as those of the Aurivillius particles. When the growth is incomplete, the perovskite particles are polycrystalline in nature. The change from single-crystalline, platelike Bi4Ti3O12 particles to single-crystalline, platelike Bi0.5Na0.5TiO3 particles via poly-crystalline, platelike aggregates is also observed in the stage of the template particle formation in Bi0.5Na0.5TiO3 textured by the reactive-templated grain growth process (Motohashi & Kimura, 2008).