Topotactic Synthesis and Crystal Structure of a Highly Fluorinated Ruddlesden-Popper-Type Iron Oxide, Sr3Fe_2O_(5+x)F_(2-x) (x ≈ 0.44)

摘要

Topotactic reaction of the n = 2 Ruddlesden-Popper phase Sr3Fe_2O_(7-σ) (σ ≈ 0.18) with polytetrafluoroethylene (PTFE) yields a highly fluorinated phase Sr3Fe2O_(5+x)F_(2-x) (x ≈ 0.44), compared with Sr3Fe2O6F_(0.87) prepared by the reaction of Sr3Fe_2O6 and F2 gas. Structure analyses based on powder neutron diffraction, synchrotron powder diffraction, and Fe Mossbauer spectroscopy measurements demonstrate that the new oxyfluoride perovskite has no anion deficiencies and adopts the tetragonal structure (space group 14/mmm) with the lattice constants a = 3.87264(6) A and c = 21.3465(6) A 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 Sr3Fe_2O_6F_(0.87) which has a T_N value that is lower than room temperature.