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 Sr3Fe2O7-delta (delta approximate to 0.18) with polytetrafluoroethylene (PTFE) yields a highly fluorinated phase Sr3Fe2O5+xF2-x (x approximate to 0.44), compared with Sr3Fe2O6F0.87 prepared by the reaction of Sr3Fe2O6 and F-2 gas. Structure analyses based on powder neutron diffraction, synchrotron powder diffraction and Fe-57 Mossbauer 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) angstrom and c = 21.3465(6) angstrom 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 Neel temperature (T-N) of 390 K, in sharply contrast to Sr3Fe2O6F0.87, which has a T-N value that is lower than room temperature.