Unveiling the Fluorination Pathway of Ruddlesden–Popper Oxyfluorides: A Comprehensive in Situ X-ray and Neutron Diffraction Study

Abstract

Ruddlesden–Popper oxyfluorides exhibit unique properties, but their synthesis is often hindered by low thermodynamic stability. To overcome this challenge, understanding the formation mechanism of these materials is crucial for optimizing the reaction conditions and accessing new products. This study presents an in-depth investigation of the fluorination reaction of La₂NiO₄ with poly(vinylidene fluoride) (PVDF), targeting the oxyfluorides La₂NiO₃F₂ and La₂NiO_2.5F₃, which exhibit distinct structural distortions. In situ X-ray diffraction experiments, performed on a laboratory diffractometer, revealed the presence of four distinct reaction intermediates. The crystal structures of these intermediates were further elucidated through X-ray and neutron powder diffraction experiments, complemented by in situ neutron powder diffraction data obtained using a setup featuring a low-background cell made from single-crystalline sapphire. ¹⁹F MAS NMR spectroscopy was employed to localize the fluoride ions and to track the consumption of PVDF. By systematically optimizing reaction conditions, we successfully obtained both oxyfluorides and quantified the phase evolution of all intermediates through extensive Rietveld refinements, yielding the following reaction steps: La₂NiO₄ (I4/mmm) → Inter#1 (Fmmm) → Inter#2 (Fmmm, with increased orthorhombic distortion) → Inter#3 (C2/c) → La₂NiO₃F₂ (Cccm). In the presence of 50% excess PVDF, La₂NiO₃F₂ is not obtained from Inter#3 and the reaction instead progresses via Inter#4 (P4₂/nnm) to La₂NiO_2.5F₃ (P4₂/nnm, with a larger unit cell). This study demonstrates the power of laboratory in situ XRD experiments in elucidating complex fluorination reaction mechanisms, enabling the synthesis of new oxyfluorides with interesting physical properties. The in situ approach represents a significant advancement over traditional trial-and-error methods, which are still prevalent in solid-state synthesis.