Synthesis, structural and magnetic characterisation of the fully fluorinated compound 6H–BaFeO2F

Ruddlesden-Popper Oxyfluorides La2Ni1-xCuxO3F2 (0 ≤ x ≤ 1): Impact of the Ni/Cu Ratio on the Structure

Ruddlesden-Popper oxyfluorides La2Ni1-xCuxO3F2 (0 ≤ x ≤ 1) were obtained by topochemical reaction of oxide precursors La2Ni1-xCuxO4, prepared by citrate-based soft chemistry synthesis, with polyvinylidene fluoride (PVDF) as the fluorine source. Systematic changes of the crystal structure in the oxide as well as the oxyfluoride substitution series were investigated. For 0.2 ≤ x ≤ 0.9, the oxyfluorides adopt the monoclinic (C2/c) structural distortion previously solved for the x = 0.8 compound based on neutron powder diffraction data, whereas the sample with a lower Cu content of x = 0.1 crystallizes in the orthorhombic (Cccm) structure variant of La2NiO3F2. The orthorhombic-to-monoclinic structural transition was found to be the result of an additional tilt component of the Jahn-Teller elongated CuO4F2 octahedra. The structural transitions were additionally studied by DFT calculations, confirming the monoclinic space group symmetry. The "channel-like" anionic ordering of the endmembers La2NiO3F2 and La2CuO3F2 was checked by 19F MAS NMR experiments and was found to persist throughout the entire substitution series.

Cuprate Oxyfluorides La2Cu0.8Ni0.2O3F2 and La2CuO3F2 with "Channel-like" Anion Ordering

Highly fluorinated cuprate Ruddlesden-Popper oxyfluorides La₂Cu_0.8Ni_0.2O₃F₂ and La₂CuO₃F₂ were obtained by topochemical reaction between poly(vinylidene fluoride) (PVDF) and the corresponding oxides La₂Cu_0.8Ni_0.2O₄ and La₂CuO₄ prepared by citrate-based soft chemistry synthesis. The crystal structures of both oxyfluorides were investigated by powder diffraction techniques. The structure of La₂Cu_0.8Ni_0.2O₃F₂ was solved based on combined neutron and X-ray powder diffraction. It crystallizes in a new monoclinic distorted version [C2/c a = 13.1880(3) Å, b = 5.7244(1) Å, c = 5.6007(1) Å, and β = 90.85(1)°] of the anionic ordered structure lately reported for La₂NiO₃F₂. For La₂CuO₃F₂, an even less symmetrical triclinic structure was derived from X-ray powder diffraction data [P1̅ a = 5.6180(5) Å, b = 5.7316(6) Å, c = 7.1978(9) Å, α = 113.32(1)°, β = 90.89(9)°, and γ = 90.16(11)°]. For both compounds, an additional tilt component of the partially Jahn-Teller elongated (Cu,Ni)O₄F₂ octahedra was found as the origin for the lowered symmetry. The formation reaction of La₂CuO₃F₂ was studied by in situ XRD measurements. In these investigations, two new reaction intermediates were identified. The magnetic properties of both oxyfluorides La₂Cu_0.8Ni_0.2O₃F₂ and La₂CuO₃F₂ were characterized by field- and temperature-dependent measurements. An antiferromagnetic ordering with T_N = 240 K was found for La₂Cu_0.8Ni_0.2O₃F₂. In La₂CuO₃F₂, additional weak ferrimagnetism was observed, resulting in a pronounced hysteresis but a weak saturation moment, which was attributed to result from a canted antiferromagnetic spin arrangement.

Enhanced Magnetic Interaction by Face-Shared Hydride Anions in 6H-BaCrO2H

Studies on magnetic oxyhydrides have been almost limited to perovskite-based lattices with corner-sharing octahedra with a M-H-M (M: transition metal) angle of θ ∼ 180°. Using a high-pressure method, we prepared BaCrO₂H with a 6H-type hexagonal perovskite structure with corner- and face-sharing octahedra, offering a unique opportunity to investigate magnetic interactions based on a θ ∼ 90° case. Neutron diffraction for BaCrO₂H revealed an antiferromagnetic (AFM) order at T_N ∼ 375 K, which is higher than ∼240 K in BaCrO_3-xF_x. The relatively high T_N of BaCrO₂H can be explained by the preferred occupancy of H^- at the face-sharing site that provides AFM superexchange in addition to AFM direct exchange interactions. First-principles calculations on BaCrO₂H in comparison with BaCrO₂F and BaMnO₃ further reveal that the direct Cr-Cr interaction is significantly enhanced by shortening the Cr-Cr distance due to the covalent nature of H^-. This study provides a useful strategy for the extensive control of magnetic interactions by exploiting the difference in the covalency of multiple anions.

Structural and Magnetic Properties of BaFeO2.667 Synthesized by Oxidizing BaFeO2.5 Obtained via Nebulized Spray Pyrolysis

A vacancy-ordered perovskite-type compound Ba₃Fe₃O₈ (BaFeO_2.667) was prepared by oxidizing BaFeO_2.5 (P2₁/c) with the latter compound obtained by a spray pyrolysis technique. The structure of Ba₃Fe₃O₈ was found to be isotypic to Ba₃Fe₃O₇F (P2₁/m) and can be written as Ba₃Fe³⁺₂Fe⁴⁺₁O₈. Mössbauer spectroscopy and ab initio calculations were used to confirm mixed iron oxidation states, showing allocation of the tetravalent iron species on the tetrahedral site, and octahedral as well as square pyramidal coordination for the trivalent species within a G-type antiferromagnetic ordering. The uptake and release of oxygen were investigated over a broad temperature range from room temperature to 1100 °C under pure oxygen and ambient atmosphere via a combination of DTA/TG and variable temperature diffraction measurements. The compound exhibited a strong lattice enthalpy driven reduction to monoclinic and cubic BaFeO_2.5 at elevated temperatures.

On the Impact of the Degree of Fluorination on the ORR Limiting Processes within Iron Based Catalysts: A Model Study on Symmetrical Films of Barium Ferrate

In this study, symmetrical films of BaFeO2.67, BaFeO2.33F0.33 and BaFeO2F were synthesized and the oxygen uptake and conduction was investigated by high temperature impedance spectroscopy under an oxygen atmosphere. The data were analyzed on the basis of an impedance model designed for highly porous mixed ionic electronic conducting (MIEC) electrodes. Variable temperature X-ray diffraction experiments were utilized to estimate the stability window of the oxyfluoride compounds, which yielded a degradation temperature for BaFeO2.33F0.33 of 590 °C and a decomposition temperature for BaFeO2F of 710 °C. The impedance study revealed a significant change of the catalytic behavior in dependency of the fluorine content. BaFeO2.67 revealed a bulk-diffusion limited process, while BaFeO2.33F0.33 appeared to exhibit a fast bulk diffusion and a utilization region δ larger than the electrode thickness L (8 μm). In contrast, BaFeO2F showed very area specific resistances due to the lack of oxygen vacancies. The activation energy for the uptake and conduction process of oxygen was found to be 0.07/0.29 eV (temperature range-dependent), 0.33 eV and 0.67 eV for BaFeO2.67, BaFeO2.33F0.33 and BaFeO2F, respectively.

Synthesis and characterisation of fluorinated epitaxial films of BaFeO2F: tailoring magnetic anisotropyviaa lowering of tetragonal distortion

In this article, we report on the synthesis and characterisation of fluorinated epitaxial films of BaFeO₂F via low-temperature fluorination of thin films of BaFeO_2.5+d grown by pulsed laser deposition. Diffraction measurements show that fluoride incorporation only results in a contraction of the film perpendicular to the film surface, where clamping by the substrate is prohibitive for strong in-plane changes. The fluorinated films were found to be homogenous regarding the fluorine content over the whole film thickness, and can be considered as single crystal equivalents to the bulk phase BaFeO₂F. Surprisingly, fluorination resulted in the change of the tetragonal distortion to a nearly cubic symmetry, which results in a lowering of anisotropic orientation of the magnetic moments of the antiferromagnetically ordered compound, confirmed by Mössbauer spectroscopy and magnetic studies.

Fluorination of epitaxially grown thin films of BaFeO_2.5 to BaFeO₂F results in increased magnetic anisotropy.

Magnetic interactions in cubic-, hexagonal- and trigonal-barium iron oxide fluoride, BaFeO2F

(57)Fe Mössbauer spectra have been recorded from the hexagonal (6H)- and trigonal (15R)- modifications of BaFeO2F and are compared with those previously recorded from the cubic form of BaFeO2F. The spectra, recorded over a temperature range from 15 to 650 K show that all of the iron in all the compounds is in the Fe(3+) state. Spectra from the 6H- and 15R-modifications were successfully fitted with components that were related to the Fe(1) and Fe(2) structural sites in the 6H variant and to the Fe(1), Fe(2) and Fe(3) structural sites in the 15R form. The magnetic ordering temperatures were determined as 597  ±  3 K for 6H-BaFeO2F and 636  ±  3 K for 15R-BaFeO2F. These values are surprisingly close to the value of 645  ±  5 K determined for the cubic form. The magnetic interactions in the three forms are compared with a view to explaining this similarity of magnetic ordering temperature.

The magnetic and crystal structures of Sr1−δFeO2−xFx, a new oxyfluoride

A new quasi-two-dimensional oxyfluoride, Sr_1−δFeO_2−xF_x, has been successfully synthesized by combining topotactic fluoridation and CaH₂ reduction.

Introducing a large polar tetragonal distortion into Ba-doped BiFeO3 by low-temperature fluorination

This article reports on the synthesis and crystallographic and magnetic structure of barium-doped BiFeO3 compounds with approximate composition Bi(1-x)Ba(x)FeO(3-x/2), as well as those of the fluorinated compounds Bi(1-x)Ba(x)FeO(3-x)F(x) (both with x = 0.2, 0.3), prepared by low-temperature fluorination of the oxide precursors using polyvinylidenedifluoride. Whereas the oxide compounds were obtained as cubic (x = 0.2) and slightly tetragonal (x = 0.3, c/a ≈ 1.003) distorted perovskite compounds, a large tetragonal polar distortion was observed for the oxyfluoride compounds (c/a ≈ 1.08 for x = 0.2 and ∼1.05 for x = 0.3), being isostructural to tetragonal PbTiO3. Although described differently in previous reports on Ba-doped BiFeO3, the observed remanent magnetization is found to agree well with the amount of BaFe12O19 only detectable by neutron diffraction and the well-known magnetic properties of BaFe12O19. The oxyfluoride compounds show G-type antiferromagnetic ordering with magnetic moments lying in the a/b plane.

Crystallographic and magnetic structure of the perovskite-type compound BaFeO2.5: unrivaled complexity in oxygen vacancy ordering

We report here on the characterization of the vacancy-ordered perovskite-type structure of BaFeO2.5 by means of combined Rietveld analysis of powder X-ray and neutron diffraction data. The compound crystallizes in the monoclinic space group P2(1)/c [a = 6.9753(1) Å, b = 11.7281(2) Å, c = 23.4507(4) Å, β = 98.813(1)°, and Z = 28] containing seven crystallographically different iron atoms. The coordination scheme is determined to be Ba7(FeO4/2)1(FeO3/2O1/1)3(FeO5/2)2(FeO6/2)1 = Ba7Fe([6])1Fe([5])2Fe([4])4O17.5 and is in agreement with the (57)Fe Mössbauer spectra and density functional theory based calculations. To our knowledge, the structure of BaFeO2.5 is the most complicated perovskite-type superstructure reported so far (largest primitive cell, number of ABX2.5 units per unit cell, and number of different crystallographic sites). The magnetic structure was determined from the powder neutron diffraction data and can be understood in terms of "G-type" antiferromagnetic ordering between connected iron-containing polyhedra, in agreement with field-sweep and zero-field-cooled/field-cooled measurements.