NaI-Mediated Defluorination: A Mild Route to Reduced Ruddlesden-Popper Oxyfluorides Demonstrated on La2CoO3F3 as Model System

We introduce NaI as a mild reagent for low-temperature topochemical defluorination of Ruddlesden–Popper oxyfluorides. In a topochemical fluorination and subsequent defluorination process La2CoO3F3 is obtained from La2CoO4, using poly(vinylidene fluoride) (PVDF) or polytetrafluoroethylene (PTFE) as fluorine sources, and converted to La2CoO3F2 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 La2CoO3F2. This Co(II) phase is not accessible by direct topochemical fluorination of La2CoO4 and is not observed along the La2CoO3F3 formation pathway. X-ray and neutron powder diffraction establish La2CoO3F3 as monoclinic (P21/c) with full occupation of the interstitial anion layer, whereas La2CoO3F2 is isotypic to La2NiO3F2 (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 magnetisation measurements indicate the change in cobalt oxidation/spin state upon fluorination/defluorination. This study uses the La2CoO4 → La2CoO3F3 → La2CoO3F2 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 as well as anion substitution reactions due to size effects of the iodide ion.