Author:
He 何 Yuzhou 玉洲,Lin 林 Ting 挺,Wang 王 Shiyu 诗雨,Gao 高 Ang 昂,Meng 孟 Ziang 子昂,Ying 应 Tianping 天平,Liu 刘 Zhiqi 知琪,Gu 谷 Lin 林,Zhang 张 Qinghua 庆华,Ge 葛 Binghui 炳辉
Abstract
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 La2CoO4 and quantified the lattice distortion of three ordered structures: La2CoO3.5F, La2CoO3F2, and La2CoO2.5F3 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.