Atomic insights into topochemical fluorination and strong octahedral tilt in La₂CoO₄

Abstract Topochemical fluorination introduces significant structural distortions and emerging properties in perovskite oxides via substituting oxygen with fluorine. However, the rapid fluorination process and the similarity between F and O render the O/F site occupation and local lattice evolution during fluorination unclear. Here we investigated the atomic-scale O/F exchange in La 2 CoO 4 and quantified the lattice distortion of three ordered structures: La 2 CoO 3.5 F, La 2 CoO 3 F 2 , and La 2 CoO 2.5 F 3 by utilizing aberration-corrected electron microscopy. Atomic-resolved elemental mapping provides direct evidence for the O/F occupancy in interstitial and apical sites. We revealed that apical F ions induce significant octahedral tilting from 178° to 165°, linearly proportional to the occupancy rate; and cause the obvious change in the fine structure O K edge, meanwhile apical O is exchanged into interstitial sites. The strong octahedral tilt leads to the in-plane elongation of the [CoO 4 F 2 ] octahedra. These findings elucidate the atomic-scale mechanisms of the entire fluorination process and highlight the significant role of F in tuning the octahedral tilt of functional oxides.