Topochemical Reduction of La₂NiO₃F₂: The First Ni-Based Ruddlesden–Popper n = 1 T′-Type Structure and the Impact of Reduction on Magnetic Ordering

Within this study, we show that a broad range of reduced phases La 2 NiO 3 F 2−Δ can be derived from Ruddlesden–Popper-type La 2 NiO 3 F 2 using a reductive topochemical defluorination method based on reactions between the oxyfluoride and sodium hydride. The selective extraction of fluoride results in strong structural changes, and the nuclear and magnetic structures of the obtained phases have been determined. Due to the high anion content in La 2 NiO 3 F 2, the formation of Ni + -containing compounds has been observed under reduction. For the first time, this has led to the formation of the highest reduced end member with a T′-type structure with an approximate composition of La 2 NiO 3 F in the n = 1 Ruddlesden–Popper-type series containing only Ni + as the B-cation. This compound can be described as stacking of alternating (La/O 2 /La) fluorite-type layers with La n –1 (NiO 2 ) n infinite layer structural blocks. The underlying reaction mechanism has been investigated by means of X-ray and neutron diffraction, elemental analysis, and magnetic measurements. Furthermore, to gain a deeper understanding of structural distortions, density functional theory-based calculations were performed, providing information about structural rearrangements upon defluorination and magnetic interactions. It is highlighted that the presence of even small amounts of d 9 -configured Ni + cations leads to strong structural changes in La 2 NiO 3 F 1.93, introducing long-range antiferromagnetic ordering between the adjacent perovskite building blocks by increasing the Ni–X–X–Ni super-superexchange interactions as compared to stoichiometric La 2 NiO 3 F 2 .