Topochemical Fluorination Yields Long-Range Superlattice in Epitaxial La₂NiO₄ Thin Films

Layered nickelates host a variety of correlated electronic phenomena that can be tuned through doping, strain, and dimensionality. Here, we explore anion engineering as an alternative tuning knob to modify the properties of layered nickelate thin films. First, we synthesize epitaxial thin films of the n = 1 Ruddlesden-Popper nickelate, La₂NiO₄. We then achieve transformation to crystalline La₂NiO₃F₂ thin films through redox-neutral, topochemical fluorination. X-ray diffraction and electron microscopy confirm the atomic structure and crystallinity of La₂NiO₄ and La₂NiO₃F₂. X-ray absorption spectroscopy further confirms a NiO₄F₂ coordination environment and Ni²⁺ oxidation state following fluorination, while electronic transport measurements reveal semiconducting behavior across a range of compressive strain states (ϵ = -1.9% to -5.8%). High dynamic range reciprocal space mapping reveals nanoscale periodicity that emerges upon fluorination, and computational analysis indicates that La₂NiO₃F₂ is susceptible to transverse structural distortions. Overall, we illustrate topochemical fluorination and anion engineering as a tuning knob to modify the chemical and electronic properties of complex oxide thin films.