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. 2016 Nov 29;9(12):970.
doi: 10.3390/ma9120970.

On the Sr 1-x Ba x FeO₂F Oxyfluoride Perovskites: Structure and Magnetism From Neutron Diffraction and Mössbauer Spectroscopy

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Free PMC article

On the Sr 1-x Ba x FeO₂F Oxyfluoride Perovskites: Structure and Magnetism From Neutron Diffraction and Mössbauer Spectroscopy

Crisanto A García-Ramos et al. Materials (Basel). .
Free PMC article

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.

Keywords: BaFeO2F; Mossbauer spectroscopy; Sr0.5Ba0.5O2F; SrFeO2F; SrFeO3−δ; antiferromagnetic structure; neutron diffraction; oxyfluoride.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
(a) X-ray diffraction (XRD) patterns (Cu Kα) for Sr1−xBaxFeO2F (x = 0.00, 0.25, 0.50, 0.75). They all may be indexed in cubic Pm-3m symmetry; (b) unit cell parameter variation with the Ba content (x); (c) view of the cubic crystal structure, displaying the anisotropic ellipsoids of (O,F) atoms, drawn with a 95% probability; (d) G-type antiferromagnetic structure determined from Neutron Powder Diffraction (NPD) data.
Figure 2
Figure 2
Observed (red crosses), calculated (black line) and difference of (bottom line) NPD Rietveld profiles for Sr1−xBaxFeO2F (x = 0.00, 0.25, 0.50, 0.75) at room temperature (RT), obtained with λ = 1.594 Å. Green lines represent crystallographic Bragg reflections. Pink lines represent magnetic Bragg reflections.
Figure 3
Figure 3
(a) Magnetic susceptibility vs. temperature for Sr1−xBaxFeO2F; (b) magnetization vs. field. ZFC, Zero Field-Cooled.
Figure 4
Figure 4
Mössbauer spectra for Sr1−xBaxFeO2F: (af) x = 0.00 and (gj) x = 0.25.
Figure 5
Figure 5
Dependence of the magnetic hyperfine field with the temperature for: (a) x = 0.00; (b) x = 0.25; (c) x = 0.50; and (d) x = 0.75.
Figure 6
Figure 6
Mössbauer spectra for Sr1−xBaxFeO2F: (ae) x = 0.50 and (fi) x = 0.75.
Figure 7
Figure 7
Variation of the Néel temperature with x across the Sr1−xBaxFeO2F series.

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