On the Sr1−xBaxFeO2F Oxyfluoride Perovskites: Structure and Magnetism from Neutron Diffraction and Mössbauer Spectroscopy

Abstract

Four oxyfluorides of the title series (x = 0.00, 0.25, 0.50, 0.75) have been stabilized by topotactic treatment of perovskite precursors Sr1−xBaxFeO3−δ prepared by soft-chemistry procedures, yielding reactive materials that can easily incorporate a substantial amount of F atoms at moderate temperatures, thus avoiding the stabilization of competitive SrF2 and BaF2 parasitic phases. XRD and Neutron Powder Diffraction (NPD) measurements assess the phase purity and yield distinct features concerning the unit cell parameters’ variation, the Sr and Ba distribution, the stoichiometry of the anionic sublattice and the anisotropic displacement factors for O and F atoms. The four oxyfluorides are confirmed to be cubic in all of the compositional range, the unit cell parameters displaying Vergard’s law. All of the samples are magnetically ordered above room temperature; the magnetic structure is always G-type antiferromagnetic, as shown from NPD data. The ordered magnetic moments are substantially high, around 3.5 μB, even at room temperature (RT). Temperature-dependent Mössbauer data allow identifying Fe3+ in all of the samples, thus confirming the Sr1−xBaxFeO2F stoichiometry. The fit of the magnetic hyperfine field vs. temperature curve yields magnetic ordering TN temperatures between 740 K (x = 0.00) and 683 K (x = 0.75). These temperatures are substantially higher than those reported before for some of the samples, assessing for stronger Fe-Fe superexchange interactions for these specimens prepared by fluorination of citrate precursors in mild conditions.

Existence of Local Polar Domains in Perovskite Oxyfluoride, BaFeO2F

“…While an orthorhombic structure is reported for SrFeO 2 F, we considered that it was cubic in this study because any diffractions indicating the orthorhombic structure were not observed in the SXRD pattern. An inspection of the refinement results reveals that the thermal displacement parameters for the Fe 3+ ion and the anions decrease and increase, respectively, with decreasing x , consistent with the results reported in a previous study of Sr 1– x Ba x FeO 2 F . The variation of the thermal displacement parameter is shown in Figure A.…”

“…As Fe-containing oxyfluoride perovskites are interesting compounds from the perspectives of both fundamental and applied research, numerous compounds, including A FeO 2 F ( A = Pb, Ba, Sr), − PbFe 0.5 Ti 0.5 O 2 F, 3 Sr 1– x BaFeO 2 F, − La 1– x SrFeO 3– x F x , , 6H-BaFeO 2 F,15R-BaFeO 2 F, AgFeOF 2, and La 1– x Sr x Fe 1– y Co y O 3−δ have been synthesized, and their structure, magnetic properties, and photocatalytic activity have been investigated . BaFeO 2 F is particularly attractive because its tolerance factor is greater than unity, which indicates that it is potentially a ferroelectric material.…”

“…Although a unique local structure would be detected as a result of the displacement of Fe in BaFeO 2 F, this local structure has not yet been investigated. In addition, it is reported that the thermal displacement parameter for the Fe site with the substation of Ba with Sr is reduced . The local structure has also not been investigated, although variation of the local structure with the decrease in t is predicted.…”