Atomistic Insights into Topochemical Reactions in the BiFeO System.

Room-temperature multiferroic BiFeO (BFO) is a promising candidate for next-generation memory and spintronic devices, but its synthesis is hindered by metastability and complex phase evolution pathways. Achieving atomic-scale control over these pathways is critical for unlocking BFO's functional potential. Here, we integrate atomic-resolution scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy and density functional theory to dissect the BFO formation mechanism using annealed Bi/Fe thin-film model systems. We identified the key intermediate phases: Bi-doped α-FeO and a metastable Aurivillius phase, BiFeO. Our investigation reveals three distinct reaction pathways governed by topochemical constraints: (1) Bi substitution into α-FeO, forming BFO via cation exchange while retaining the oxygen sublattice; (2) Fe incorporation into BiO, driving the layered BiFeO growth; and (3) transformation of the existing BFO into BiFeO through Fe out-diffusion and structural rearrangement at the interfaces. These findings provide an atomistic insight into the BFO formation mechanism, demonstrating how topochemical principles and diffusion dynamics dictate the phase evolution.