Mechanistic Interpretation of the Aurivillius to Perovskite Topochemical Microcrystal Conversion Process

The kinetics of topochemical and morphological conversion of platelet-shaped Na 3.5 Bi 2.5 Nb 5 O 18, PbBi 4 Ti 4 O 15, and BaBi 4 Ti 4 O 15 Aurivillius phases to NaNbO 3, PbTiO 3, and BaTiO 3 perovskites were studied. Reaction of the ⟨001⟩ oriented Aurivillius phases with excess Na 2 CO 3, Pb 3 (CO 3 ) 2 (OH) 2, and BaCO 3, respectively, yielded high-aspect-ratio perovskite microcrystals with ⟨001⟩ orientation. Only the Na 3.5 Bi 2.5 Nb 5 O 18 to NaNbO 3 conversion occurred directly, whereas TEM analysis of reacted BaBi 4 Ti 4 O 15 and PbBi 4 Ti 4 O 15 revealed previously unknown Aurivillius-type intermediate phases with Bi 2 O 2 2+ layers ∼86 and ∼78 Å apart, respectively. Observations from TEM and field emission SEM show that perovskite crystallites grow from multiple nucleation sites, but become slightly misaligned during growth. This misalignment is caused by a loss of epitaxy with the parent Aurivillius phase and subsequent exfoliation of the particles, likely caused by the expulsion of byproduct Bi 2 O 3 liquid on phase boundaries. This conversion process results in substantial microstructure damage, which is healed with an annealing step between 950 and 1050 °C. The pathway for formation of ⟨001⟩ oriented, polycrystalline or single-crystal perovskite platelets is illustrated in a general model for topochemical conversion of Aurivillius phases.