Structure, Magnetism and Thermal Stability of the n = 3 Ruddlesden-Popper Oxyfluoride La4Ni3O8.4F3.5

Abstract

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 structure. We present the synthesis of the highly fluorinated n = 3 oxyfluoride La₄Ni₃O_8.4F_3.5by topochemical fluorination of La₄Ni₃O₁₀, with poly(vinylidene difluoride) (PVDF) as fluoride source. The structure of this compound is solved based on X-ray and neutron powder diffraction data. A monoclinic (P2₁/a,a = 5.4206(2) Å, b = 5.5081(2) Å, c = 29.9823(2) Å, and β = 90.85(1)°) 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 as well as the decomposition reaction was 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 490°C and is accompanied by the release of oxygen as detected by coupled mass spectrometry. Temperature and field depended 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.