Topotactic Synthesis and Crystal Structure of a Highly Fluorinated Ruddlesden–Popper-Type Iron Oxide, Sr₃Fe₂O5+xF2–x (x ≈ 0.44)

Topotactic reaction of the n = 2 Ruddlesden–Popper phase Sr 3 Fe 2 O 7−δ (δ ≈ 0.18) with polytetrafluoroethylene (PTFE) yields a highly fluorinated phase Sr 3 Fe 2 O 5+ x F 2– x ( x ≈ 0.44), compared with Sr 3 Fe 2 O 6 F 0.87 prepared by the reaction of Sr 3 Fe 2 O 6 and F 2 gas. Structure analyses based on powder neutron diffraction, synchrotron powder diffraction, and 57 Fe Mössbauer spectroscopy measurements demonstrate that the new oxyfluoride perovskite has no anion deficiencies and adopts the tetragonal structure (space group I 4/ 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 Sr 3 Fe 2 O 6 F 0.87, which has a T N value that is lower than room temperature.