Existence of Local Polar Domains in Perovskite Oxyfluoride, BaFeO₂F

We prepared high-purity Sr1–xBaxFeO2F by a low-temperature fluorination method, investigated the average structure by synchrotron X-ray diffraction (SXRD), and measured second harmonic generation (SHG) in this material. For BaFeO2F, the average and local structures were investigated by time-of-flight (TOF) neutron diffraction and the X-ray pair distribution function (PDF), respectively. In addition, the magnetic moment under a magnetic field was measured. SHG was observed for Sr1–xBaxFeO2F (x = 1.0, 0.9, and 0.7) and was below the detection limit for x = 0.5. The average crystal structure of BaFeO2F was determined by the refinement of the TOF neutron diffraction data to be a cubic structure in which Fe3+ ions are displaced to (x, x, 1/2) off-site positions with a 1/12 occupancy. The magnetic structure was a G-type antiferromagnetic structure, and the magnetic moments of the Fe3+ ion were 3.84 and 3.50 μB at 3 and 300 K, respectively. According to the X-ray PDF analysis, the local structure was not a cubic structure with the equivalent displacement of the Fe3+ ion to the off-site positions but a noncentrosymmetric structure in which Fe3+ ions are displaced in various directions in distorted BX6 octahedra. These local polar domains induce the SHG. The coexistence of polar domains with different polarization directions results in a cubic average structure. The formation of local polar domains was supported by the canted ferromagnetic components observed in magnetic moment measurements carried out under an applied magnetic field.