Control of Crystallization Pathways in the BiFeO ₃ –Bi ₂ Fe ₄ O ₉ System

Bismuth ferrites, specifically perovskite-type BiFeO₃ and mullite-type Bi₂Fe₄O₉, hold significant technological promise as catalysts, photovoltaics, and room-temperature multiferroics. However, challenges arise due to their frequent cocrystallization, particularly in the nanoregime, hindering the production of phase-pure materials. This study unveils a controlled sol-gel crystallization approach, elucidating the phase formation complexities in the bismuth ferrite oxide system by coupling thermochemical analysis and total scattering with pair distribution function analysis. We tune the crystallization pathways in the BiFeO₃-Bi₂Fe₄O₉ system by adjusting the metal to complexing agent ratio and pH during precursor preparation with a fixed Bi/Fe ratio of 1:2. Although all precursors undergo an amorphization process during heating, our results demonstrate a consistent correlation between the crystallization pathway and the initial structural entities formed during gel formation. Pair distribution function analysis reveals structural differences in the intermediate amorphous structures, which preferentially crystallize into either BiFeO₃ or Bi₂Fe₄O₉. This study offers mechanistic insights into the formation processes in the system and synthetic guidance for the controlled synthesis of pure Bi₂Fe₄O₉ and mixed BiFeO₃/Bi₂Fe₄O₉ nanomaterials. Additionally, it elucidates the unusual growth behavior and structural size dependence of Bi₂Fe₄O₉, particularly highlighting significant distortions in the local structure likely induced by the proximity of Bi's stereoactive lone electron pairs at small sizes.