Understanding Topochemical Microcrystal Conversion in Molten Salt to Produce High-Aspect-Ratio Perovskites Particles

Highly anisotropic platelet-shaped particles can be used to enhance both structural and functional applications. Anisotropic perovskites are especially in recent focus for their functional properties, including opto- and piezo-electric. Topochemical microcrystal conversion (TMC) in molten salt has been established as a scalable and robust way to fabricate anisotropic perovskite particles from templates. However, TMC often provides a low degree of control on the particle size and aspect ratio. The control over the particle shape is key to the crystallographic texture and mechanical properties of the final material. A comprehensive examination of the full reaction process is still lacking, leaving the underlying reaction mechanisms in large part unresolved. In this work, we employed in situ time-resolved synchrotron X-ray diffraction to monitor the TMC of Bi0.5Na0.5TiO3 (BNT) from Bi4Ti3O12 (BiT) templates under varying TiO2 deficiencies and uncovered new insights into the nucleation and growth mechanisms. We demonstrate that control over nucleation and initial anisotropic crystallite growth continuously dictates the final BNT particles' morphologies. Based on these new insights, we produce BNT platelets with a 1.2- ∼2.1-times higher median aspect ratio than previously reported. In addition, we fabricated thin (170 nm) Bi0.5Na0.5TiO3-BaTiO3 platelets with a median and 90th percentile aspect ratio of up to 17 and 41, 1.4- and 2.9-fold higher than that of BiT templates (∼12 and 15), respectively, further ascertaining our growth model and its applicability to other BNT-based compositions. Our results expand our knowledge of the TMC process and open the way to producing high-aspect-ratio and quality perovskite platelets.