Control of\nCrystallization Pathways in the BiFeO<sub>3</sub>–Bi<sub>2</sub>Fe<sub>4</sub>O<sub>9</sub> System

Bismuth ferrites, specifically perovskite-type BiFeO<sub>3</sub> and mullite-type Bi<sub>2</sub>Fe<sub>4</sub>O<sub>9</sub>, hold\nsignificant technological promise as catalysts, photovoltaics, and\nroom-temperature multiferroics. However, challenges arise due to their\nfrequent cocrystallization, particularly in the nanoregime, hindering\nthe production of phase-pure materials. This study unveils a controlled\nsol–gel crystallization approach, elucidating the phase formation\ncomplexities in the bismuth ferrite oxide system by coupling thermochemical\nanalysis and total scattering with pair distribution function analysis.\nWe tune the crystallization pathways in the BiFeO<sub>3</sub>–Bi<sub>2</sub>Fe<sub>4</sub>O<sub>9</sub> system by adjusting the metal\nto complexing agent ratio and pH during precursor preparation with\na fixed Bi/Fe ratio of 1:2. Although all precursors undergo an amorphization\nprocess during heating, our results demonstrate a consistent correlation\nbetween the crystallization pathway and the initial structural entities\nformed during gel formation. Pair distribution function analysis reveals\nstructural differences in the intermediate amorphous structures, which\npreferentially crystallize into either BiFeO<sub>3</sub> or Bi<sub>2</sub>Fe<sub>4</sub>O<sub>9</sub>. This study offers mechanistic\ninsights into the formation processes in the system and synthetic\nguidance for the controlled synthesis of pure Bi<sub>2</sub>Fe<sub>4</sub>O<sub>9</sub> and mixed BiFeO<sub>3</sub>/Bi<sub>2</sub>Fe<sub>4</sub>O<sub>9</sub> nanomaterials. Additionally, it elucidates the\nunusual growth behavior and structural size dependence of Bi<sub>2</sub>Fe<sub>4</sub>O<sub>9</sub>, particularly highlighting significant\ndistortions in the local structure likely induced by the proximity\nof Bi’s stereoactive lone electron pairs at small sizes.