Synthesis of the Double Infinite-Layer Ni(I) Phase La₃Ni₂O₅F via Sequential Topochemical Reactions

Fluorination of the n = 2 Ruddlesden-Popper oxide, La₃Ni₂O₇, with polyvinylidene fluoride yields La₃Ni₂O₅F₄, a phase in which fluoride ions have been inserted into interstitial sites in the Ruddlesden-Popper framework and also exchanged with the oxide ions residing on apical anion sites. Reaction with LiH at 190 °C reduces La₃Ni₂O₅F₄ by extracting interstitial fluoride ions. The resulting phase, La₃Ni₂O₅F₃, adopts a structure described in space group Pbcm in which the fluoride ions in the half-filled interstitial layer are arranged in chains parallel to the y-axis, and the NiO₅F octahedra adopt an a^-a^-c⁺/-(a^-a^-)c⁺ tilting pattern. Further reduction with LiH at 250 °C converts La₃Ni₂O₅F₃ into La₃Ni₂O₅F, a Ni¹⁺ phase which adopts a T'-structure consisting of double infinite-sheets of apex linked NiO₄ squares, stacked with LaOF fluorite-type layers. Magnetization and neutron diffraction data indicate La₃Ni₂O₅F₃ adopts an antiferromagnetically ordered state below T_N = 225 K, while magnetization data from La₃Ni₂O₅F exhibit a broad maximum centered at 75 K, suggestive of antiferromagnetic order.