Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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AgFeOF2: A Fluorine-Rich Perovskite Oxyfluoride Fumitaka Takeiri,† Takafumi Yamamoto,† Naoaki Hayashi,‡ Saburo Hosokawa,§ Kazunari Arai,† Jun Kikkawa,∥ Kazutaka Ikeda,⊥ Takashi Honda,⊥ Toshiya Otomo,⊥ Cédric Tassel,† Koji Kimoto,∥ and Hiroshi Kageyama*,†,# †
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan ‡ Research Institute for Production Development, Sakyo-ku, Kyoto 606-0805, Japan § Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, Nishikyo-ku, Kyoto 615-8245, Japan ∥ National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan ⊥ Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan # CREST, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan S Supporting Information *
ABSTRACT: We synthesized a silver iron oxyfluoride AgFeOF2 by using a high-pressure reaction. Synchrotron X-ray and neutron diffraction, X-ray absorption, and 57Fe Mössbauer spectroscopy indicate that AgFeOF2 crystallizes in the ideal perovskite structure with iron in a trivalent state, although electron microscopy revealed weak super-reflections. A possible partial ordering in the FeO2F4 octahedron is inferred from Mössbauer spectroscopy. The synthesis of the fluorine-rich sample offers an opportunity to study a composition-property relation in AFeIIIO3−nFn (n = 0, 1, and 2). AgFeOF2 exhibits a G-type antiferromagnetic ordering below TN ≈ 480 K, which is much lower than the n = 0 and 1 cases, suggesting a weaker superexchange interaction between Fe moments via F 2p orbitals (vs O 2p orbitals).
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oxides LnFeIIIO3 (Ln = lanthanide) also has the G-type spin order with similar TN values (622−750 K).20 The TN variations, or the magnetic interactions in AFeIIIO2F and LnFeIIIO3, have been discussed from different viewpoints, the former associated with the Fe−O(F) bond length12 and the latter associated with the Fe−O−Fe buckling angle.20 Thus, the effect of fluorine substitution on the magnetic property of the AFeIIIO3−xFx system has remains unclarified. For this reason, the synthesis of iron-based oxyfluorides with a different F/O ratio, such as AFeOF2, would benefit the global understanding of the magnetic properties. So far, a chargedisproportionate TlITlIIIOF221 is the only system with the ABOF2 stoichiometry. In this paper, we report on a silver iron oxyfluoride (AgFeOF2) synthesized via high-pressure techniques. As with AFeIIIO2F, this fluorine-rich compound adopts a pseudo-cubic perovskite structure and a G-type spin structure, but possesses a much lower TN of ∼480 K, suggesting a weaker superexchange interaction via the Fe−F−Fe pathway than via the Fe−O−Fe one. Furthermore, the analysis of Mössbauer spectrum implies a possible deviation from a random F/O distribution, unlike the previous reports on AFeO2F (A = Sr,9 Ba19) that claimed a complete random F/O distribution.
INTRODUCTION Over the years, mixed-anion oxides, where oxide and another anionic species coexist, have attracted much attention, because of their appealing characters.1 Fluorine substitution for oxygen in oxide materials is one of the promising strategy to improve various properties. For example, the enhanced rate capacity is observed in a fluorine-substituted lithium-ion battery cathode Li(Ni1/3Co1/3Mn1/3)(O,F)2,2 while the emission and excitation wavelength is widely tuned in phosphor materials Sr3(Al1−xSix)O4+xF1−x (0 ≤ x < 1).3 These features are partly related to the more ionic character of the F− anion, compared with the oxide anion. The perovskite structure affords the formation of several oxyfluorides with transition metals at the B-site, as found in KTiO2F,4 ANbO2F (A = Na, K),5 PbMO2F (M = Sc,6 Mn7), and AFeO2F (A = Sr,8−14 Pb,15−17 Ba18,19), and they are accessible via conventional solid-state reactions (e.g., ANbO2F), high-pressure reactions (e.g., KTiO2F and PbMO2F), and topochemical fluorination reactions (e.g., AFeO2F). Among those perovskite oxyfluorides, AFeIIIO2F (A = Sr, Pb, Ba) with a (pseudo)cubic symmetry have been extensively studied from the viewpoint of magnetism. All of these compounds undergo an antiferromagnetic (AFM) ordering of the G-type with the transition temperature well beyond room temperature: TN = 710, 655, and 645 K for A = Sr, Pb, and Ba, respectively.12,17,19 The series of orthorhombic perovskite © XXXX American Chemical Society
Received: April 3, 2018
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DOI: 10.1021/acs.inorgchem.8b00500 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
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EXPERIMENTAL SECTION
the stoichiometric anion composition was assumed, the refinement converged reasonably with the agreement indices of Rp = 1.94% and Rwp = 3.58% (Figure 1 and Table 1).
A polycrystalline sample of AgFeOF2 was synthesized under high pressure using a cubic anvil press. Powder specimens of Ag2O (99%, Kojundo Chemical), Fe2O3 (99.99%, Rare Metallic), and anhydrous FeF3 (99%, Rare Metallic) were used as received; they were intimately mixed in a stoichiometric ratio in a nitrogen-filled glovebox (H2O, O2 < 0.1 ppm), pelletized, charged into a platinum capsule, placed into a high-pressure cell made of pyrophyllite, and heated at a temperature of 800−1000 °C for 30 min under a pressure of 2−7 GPa. The obtained samples were examined by powder X-ray diffraction (PXRD), using a D8 AVANCE diffractometer (Bruker AXS) with Cu Kα radiation. Energy-dispersive X-ray spectroscopy (EDX) was collected by an Oxford Inca X-act detector mounted on a Hitachi S3400N scanning electron microscopy (SEM) system. A roomtemperature synchrotron PXRD experiment was performed using the large Debye−Scherrer camera with an imaging plate as a detector, installed at SPring-8 BL02B2. An incident beam from a bending magnet was monochromatized to λ = 0.41867 Å. Sieved powder sample (
