Topochemical Synthesis of LiCoF 3 with a High-Temperature LiNbO 3 -Type Structure.

A novel perovskite fluoride, Li _x CoF ₃ , which has an exceptionally low tolerance factor (0.81), has been synthesized via low-temperature lithium intercalation into a distorted ReO ₃ -type fluoride CoF ₃ using organolithium reagents. Interestingly, this reaction is completed within 15 min at room temperature. Synchrotron X-ray diffractometry and optical second harmonic generation at room temperature have revealed that this compound shows a high-temperature LiNbO ₃ -type structure (space group: R 3̅ c ) involving Li-Co antisite defects and A -site splitting along the c direction. A -site splitting is consistent with the prediction based on hybrid Hartree-Fock density functional theory calculations. Co- L _2,3 edge X-ray absorption spectroscopy, as well as bond valence sum analysis, has verified the divalent oxidation state of Co ions in the lithiated phase, suggesting that its composition is close to LiCoF ₃ ( x ≈ 1). This compound exhibits a paramagnetic-to-antiferromagnetic transition at 36 K on cooling, accompanied by weak ferromagnetic ordering. The synthetic route based on low-temperature lithiation of metal fluorides host paves the way for obtaining a new LiNbO ₃ -type fluoride family.