Control of crystallization pathways in the BiFeO3 - Bi2Fe4O9 system

Bismuth ferrates, 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 co-crystallization, particularly in the nano-regime, hindering the production of phase pure materials. This study unveils a controlled sol-gel crystallization approach, elucidating the phase formation complexities in the bismuth ferrate oxide system by coupling thermochemical analysis and total scattering with pair distribution function analysis. We tune the crystallization pathways by adjusting the metal to complexing agent ratio and pH during precursor preparation resulting in preferential crystallization of either BiFeO3 or Bi2Fe4O9. 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 process and synthetic guidance for controlled synthesis of BiFeO₃ and 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.