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

Crisanto García-Ramos,María Retuerto,José Alonso

doi:10.3390/ma9120970

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.

Highlights

The ideal ABX3 perovskite structure is usually described in a cubic unit cell (a0 ≈ 4 Å) with the B cations at the corners, the X anions at the midpoint of the edges and the larger A cations at the center of the cube
We present complementary data; we determined the effects of the fluorination on the crystallographic and magnetic structures, studied by Neutron Powder Diffraction (NPD), and on the magnetic properties of these appealing materials, in conjunction with a Mössbauer study
One important result distilled from the Mössbauer data is that the determined TN ’s are substantially higher that those reported for specimens of comparable compositions SrFeO2 F (710 K, [6]), Sr0.5 Ba0.5 FeO2 F (670 K, [14]), as commented above. These results indicate better superexchange interactions in the present samples that could arise from a stricter Sr1–x Bax FeO2 F stoichiometry and Fe3+ contents, originated by the original synthesis procedure starting from citrate precursors for the formation of the perovskite oxide networks

Summary

Introduction

The ideal ABX3 perovskite structure is usually described in a cubic unit cell (a0 ≈ 4 Å) with the B cations at the corners, the X anions at the midpoint of the edges and the larger A cations at the center of the cube. Regarding the perovskite fluorides KMF3 , most of them with M = Mg, V, Mn, Fe, Co, Ni, Zn, they all exhibit a cubic structure [2]. Such as Cu2+ with d9 configuration in the cubic crystal field, the perovskite KCuF3 is a Jahn–Teller (JT) active system, which lowers the crystal symmetry from the Materials 2016, 9, 970; doi:10.3390/ma9120970 www.mdpi.com/journal/materials. This compound has been treated as a prototype system in which the orbital ordering is induced by the superexchange interaction alone in the Kugel–Khomskii (KK) model [3]. The orbital ordering from this model results in a CuF6 octahedral-site distortion that consists of four long and two short Cu-F bonds

Methods

Results

Discussion

Conclusion