Synthesis of the Double Infinite-Layer Ni(I) Phase La ₃ Ni ₂ O ₅ F via Sequential Topochemical Reactions

Fluorination of the <i>n</i> = 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 <i>Pbcm</i> in which the fluoride ions in the half-filled interstitial layer are arranged in chains parallel to the <i>y</i>-axis, and the NiO₅F octahedra adopt an <i>a</i>^-<i>a</i>^-<i>c</i>⁺<i>/-(a</i>^-<i>a</i>^-<i>)c</i>⁺ 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 <i>T</i>_N = 225 K, while magnetization data from La₃Ni₂O₅F exhibit a broad maximum centered at 75 K, suggestive of antiferromagnetic order.