Mechanistic Interpretation of the Aurivillius to Perovskite Topochemical Microcrystal Conversion Process

The kinetics of topochemical and morphological conversion of platelet-shaped Na3.5Bi2.5Nb5O18, PbBi4Ti4O15, and BaBi4Ti4O15 Aurivillius phases to NaNbO3, PbTiO3, and BaTiO3 perovskites were studied. Reaction of the ⟨001⟩ oriented Aurivillius phases with excess Na2CO3, Pb3(CO3)2(OH)2, and BaCO3, respectively, yielded high-aspect-ratio perovskite microcrystals with ⟨001⟩ orientation. Only the Na3.5Bi2.5Nb5O18 to NaNbO3 conversion occurred directly, whereas TEM analysis of reacted BaBi4Ti4O15 and PbBi4Ti4O15 revealed previously unknown Aurivillius-type intermediate phases with Bi2O2 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 Bi2O3 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.