Topotactic Synthesis and Crystal Structure of a Highly Fluorinated Ruddlesden–Popper-Type Iron Oxide, Sr₃Fe₂O_5+xF_2–x (x ≈ 0.44)

Topotactic reaction of the n = 2 Ruddlesden–Popper phase Sr₃Fe₂O_7−δ (δ ≈ 0.18) with polytetrafluoroethylene (PTFE) yields a highly fluorinated phase Sr₃Fe₂O_5+xF_2–x (x ≈ 0.44), compared with Sr₃Fe₂O₆F_0.87 prepared by the reaction of Sr₃Fe₂O₆ and F₂ gas. Structure analyses based on powder neutron diffraction, synchrotron powder diffraction, and ⁵⁷Fe Mössbauer spectroscopy measurements demonstrate that the new oxyfluoride perovskite has no anion deficiencies and adopts the tetragonal structure (space group I4/mmm) with the lattice constants a = 3.87264(6) Å and c = 21.3465(6) Å at room temperature. The fluoride ions preferentially occupy the terminal apical anion sites with oxide ions in a disordered manner, which results in square pyramidal coordination around iron. The present compound also shows an antiferromagnetic order with a Néel temperature (T_N) of 390 K, in sharply contrast to Sr₃Fe₂O₆F_0.87, which has a T_N value that is lower than room temperature.