Structure, magnetism and thermal stability of the n = 3 Ruddlesden-Popper oxyfluoride La₄Ni₃O_8.4F_3.5

Oxides of the Ruddlesden-Popper (RP) series A_n+1B_nO_3n+1 are ideal candidates for the incorporation of additional anions like F^-, H^-, or N^3- in the crystal lattice. The resulting mixed anionic compounds usually exhibit clearly different physical properties compared to the precursor oxides due to changes in their atomic and electronic structures. We present the synthesis of the highly fluorinated n = 3 oxyfluoride La₄Ni₃O_8.4F_3.5 by topochemical fluorination of La₄Ni₃O₁₀, with poly(vinylidene difluoride) (PVDF) as the fluoride source. The structure of this compound was solved based on X-ray and neutron powder diffraction data. A monoclinic (P2₁/a, a = 5.4206(3) Å, b = 5.5081(3) Å, c = 29.9823(18) Å, and β = 90.85(4)°) distorted variant of the n = 3 RP structure was found showing a strong elongation perpendicular to the perovskite slabs resulting from full occupation of the interstitial anion positions. The formation reaction and the decomposition reaction were investigated by in situ X-ray diffraction. By this, the presence of one distinct formation intermediate was revealed. The thermal decomposition was found to start at 490 °C, accompanied by the release of oxygen as detected by coupled mass spectrometry. Temperature and field dependent magnetization measurements indicate that the title oxyfluoride is a Curie-Weiss paramagnet, in contrast to the parent oxide, which shows Pauli paramagnetism, highlighting the strong impact of anion substitution on the physical properties of these mixed anionic compounds.