Mechanistic interpretation of the aurivillius to perovskite topochemical microcrystal conversion process

The kinetics of topochemical and morphological conversion of platelet-shaped Na_3.5Bi_2.5Nb₅O₁₈, PbBi₄Ti₄O₁₅, and BaBi₄Ti ₄O₁₅ Aurivillius phases to NaNbO₃, PbTiO ₃, and BaTiO₃ perovskites were studied. Reaction of the (001) oriented Aurivillius phases with excess Na₂CO₃, Pb₃(CO₃)₂(OH)₂, and BaCO ₃, respectively, yielded high-aspect-ratio perovskite microcrystals with (001) orientation. Only the Na_3.5Bi_2.5Nb ₅O₁₈ 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.