Structure and Magnetic Properties of the n = 3 Ruddlesden-Popper Oxyfluoride La_0.5Sr_3.5Fe₃O_7.5F_2.6

Ruddlesden-Popper (RP) compounds of the general formula (AX)(ABX₃) with their unique sequence of perovskite-like (ABX₃) and rock-salt-like units (AX) promise applications in diverse fields such as catalysis and superconductivity. Fluorination of RP oxides often leads to dramatic changes in the material properties, caused by differences in the atomic and electronic structure. While current research focuses on fluorination of n = 1 type RP oxides (A₂BO₄), n = 3 RP oxyfluorides have remained elusive. We present the synthesis of the first iron-based n = 3 RP oxyfluoride, La_0.5Sr_3.5Fe₃O_7.5F_2.6, which was obtained from an oxide precursor by topochemical fluorination with poly(vinylidene fluoride). Joint Rietveld refinements of neutron and powder X-ray diffraction data were used to determine the crystal structure. Best results were obtained in the space group Pbca (No. 61) with a = 5.5374(1) Å, b = 5.5441(1) Å, and c = 29.2541(2) Å. The effect of the aliovalent incorporation of fluoride ions is particularly evident with respect to changes in structure and magnetic properties. The magnetic behavior was studied using field- and temperature-dependent magnetization measurements, Mößbauer spectroscopy, and neutron diffraction. Additional magnetic Bragg reflections observed in the room-temperature neutron data were successfully refined in the space group Pbca (61.1.497 in Opechowski-Guccione notation), indicating a G-type antiferromagnetic ordering with a surprisingly high Néel temperature above 300 K. This strong increase of T_N by several hundred Kelvin compared to the parent oxide is particularly remarkable.