Structure\nand Magnetic Properties of the <i>n</i> = 3 Ruddlesden–Popper\nOxyfluoride La<sub>0.5</sub>Sr<sub>3.5</sub>Fe<sub>3</sub>O<sub>7.5</sub>F<sub>2.6</sub>

Ruddlesden–Popper (RP) compounds of the general\nformula\n(AX)(ABX<sub>3</sub>)<sub><i>n</i></sub> with their unique\nsequence of perovskite-like (ABX<sub>3</sub>) and rock-salt-like units\n(AX) promise applications in diverse fields such as catalysis and\nsuperconductivity. Fluorination of RP oxides often leads to dramatic\nchanges in the material properties, caused by differences in the atomic\nand electronic structure. While current research focuses on fluorination\nof <i>n</i> = 1 type RP oxides (A<sub>2</sub>BO<sub>4</sub>), <i>n</i> = 3 RP oxyfluorides have remained elusive.\nWe present the synthesis of the first iron-based <i>n</i> = 3 RP oxyfluoride, La<sub>0.5</sub>Sr<sub>3.5</sub>Fe<sub>3</sub>O<sub>7.5</sub>F<sub>2.6</sub>, which was obtained from an oxide\nprecursor by topochemical fluorination with poly(vinylidene fluoride).\nJoint Rietveld refinements of neutron and powder X-ray diffraction\ndata were used to determine the crystal structure. Best results were\nobtained in the space group <i>Pbca</i> (No. 61) with <i>a</i> = 5.5374(1) Å, <i>b</i> = 5.5441(1) Å,\nand <i>c</i> = 29.2541(2) Å. The effect of the aliovalent\nincorporation of fluoride ions is particularly evident with respect\nto changes in structure and magnetic properties. The magnetic behavior\nwas studied using field- and temperature-dependent magnetization measurements,\nMößbauer spectroscopy, and neutron diffraction. Additional\nmagnetic Bragg reflections observed in the room-temperature neutron\ndata were successfully refined in the space group <i>Pbca</i> (61.1.497 in Opechowski–Guccione notation), indicating a\nG-type antiferromagnetic ordering with a surprisingly high Néel\ntemperature above 300 K. This strong increase of <i>T</i><sub>N</sub> by several hundred Kelvin compared to the parent oxide\nis particularly remarkable.