NaI-Mediated Defluorination: A Mild Route to Reduced Ruddlesden–Popper Oxyfluorides Demonstrated on La ₂ CoO ₃ F ₃ as Model System

High Resolution Image Download MS PowerPoint Slide We introduce NaI as a mild reagent for low-temperature topochemical defluorination of Ruddlesden–Popper oxyfluorides. In a topochemical fluorination and subsequent defluorination process La 2 CoO 3 F 3 is obtained from La 2 CoO 4, using poly(vinylidene fluoride) (PVDF) or polytetrafluoroethylene (PTFE) as fluorine sources, and converted to La 2 CoO 3 F 2 by the addition of NaI. Both processes were studied by laboratory in situ X-ray diffraction, which reveals a stepwise fluorine uptake through four crystalline intermediates. Subsequent NaI treatment enables controlled F – removal to form La 2 CoO 3 F 2 . This Co(II) phase is not accessible by direct topochemical fluorination of La 2 CoO 4 and is not observed along the La 2 CoO 3 F 3 formation pathway. X-ray and neutron powder diffraction establish La 2 CoO 3 F 3 as monoclinic ( P 2 1 / c ) with full occupation of the interstitial anion layer, whereas La 2 CoO 3 F 2 is isotypic to La 2 NiO 3 F 2 ( Cccm ) and exhibits a channel-like interstitial anion arrangement. Thermal analysis by in situ XRD is used to scan the temperature ranges over which both oxyfluorides retain their structure, and magnetization measurements indicate the change in cobalt oxidation and spin state upon fluorination/defluorination. This study uses the La 2 CoO 4 → La 2 CoO 3 F 3 → La 2 CoO 3 F 2 reaction sequence as model system to demonstrate that, sequential topochemical fluorination and NaI-mediated defluorination provides access to metastable, anion-ordered RP oxyfluorides under milder conditions than conventional hydride-based reductions, avoiding both reduction of the metal cations to their metallic state and anion substitution reactions due to size effects of the iodide ion.