Atomic insights into topochemical fluorination and strong octahedral tilt in La₂CoO₄

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 La₂CoO₄ and quantified the lattice distortion of three ordered structures: La₂CoO_3.5F, La₂CoO₃F₂, and La₂CoO_2.5F₃ 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 CoO₄F₂ 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.