Anion Order-to-Disorder Transition in Layered Iron Oxyfluoride

Article pubs.acs.org/crystal

Anion Order-to-Disorder Transition in Layered Iron Oxyfluoride Sr2FeO3F Single Crystals Yoshihiro Tsujimoto,*,† Yoshitaka Matsushita,‡ Naoaki Hayashi,§ Kazunari Yamaura,∥ and Tetsuo Uchikoshi† †

Materials Processing Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Materials Analysis Station, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan § Institute for Integrated Cell-Material Sciences, Kyoto University, Yoshida-Ushinomiya, Sakyo, Kyoto 606-8501, Japan ∥ Superconducting Materials Unit, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡

S Supporting Information *

ABSTRACT: Controlling the distribution of mixed anions around a metal center is a long-standing subject in solid state chemistry. We successfully obtained single crystals of an ironbased layered perovskite compound, Sr2FeO3F, by utilizing a high-pressure and high-temperature technique. The phase prepared at 1300 °C and 3 GPa crystallized in tetragonal space group P4/nmm with O/F atoms at the apical sites being ordered. However, a temperature of 1800 °C and a pressure of 6 GPa resulted in partial O/F site disordering. The degree of anion disordering, which was 5%, showed that the anionordered arrangement was quite robust, in sharp contrast to that of Sr2BO3F (B = Co or Ni) with the fully disordered state. 57 Fe Mössbauer spectroscopy measurements revealed no large difference in Néel temperatures between the two phases, but the phase prepared under the latter condition exhibited a quasicontinuous distribution of hyperfine fields caused by O/F site disordering. We discuss the mechanism of the anion order-todisorder transition observed in related oxyfluoride perovskites.

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INTRODUCTION Transition metal oxides with perovskite-related structures show important functional properties for a wide range of applications such as photocatalysts, electric power cables, and ferroelectric or thermoelectric devices.1−4 The unique structural and physical properties of metal oxides originate from the metal− ligand interactions associated with the crystal field splitting of the metal centers, superexchange interactions, and chargetransfer energy.5 Most of the studies aimed at changing the properties of metal oxides have focused on cation substitution involving valence changes and chemical pressure. In contrast, controlling the properties by anion substitution in oxide sublattices has been investigated to a lesser extent. Each anion possesses specific characteristics, including electronegativity, polarizability, ionic radius, and valence state. Thus, the electronic states of metal oxides can be modified more effectively by anion substitution than by cation substitution, through changes in the coordination environments around the metal centers. The main reason for the limited number of mixed anion compounds is the high stability of the metal−oxygen bond compared with the stabilities of other anions. Nonetheless, the development of synthesis methods allows for the incorporation of heteroanions in the oxide sublattices. Hayward et al. reported © 2014 American Chemical Society

the novel cobalt oxyhydride LaSrCoO3H0.7, which was obtained through a topotactic route using a binary metal hydride as a reducing agent.6 Greaves et al. successfully synthesized the Fdoped superconducting copper oxide Sr2CuO2F2+δ by the treatment of the oxide precursor in F2 gas.7 In addition, highpressure synthesis was reported to be a useful approach to the synthesis of oxyfluoride and oxynitride compounds.8−10 Their pioneering work has stimulated the search for new mixed anion systems. Recently, we demonstrated the high-pressure synthesis of a new layered oxyfluoride Sr2BO3F (B = Co or Ni).11,12 These phases belong to the Ruddlesden−Popper (RP) series expressed as An+1BnO3n+1 (A is an alkali or alkaline earth metal, B is a transition metal, and n is the number of perovskite block layers), where the oxygen sites are partially replaced with fluorine anions. Figure 1 shows three representative structural features observed in the n = 1 RP-type A2BO3F phase. Across this series, one fluoride ion occupies the apical anion sites, equally with an oxide ion, but the relationship between the coordination of the B site cation and the anion site ordering is Received: December 11, 2013 Revised: May 8, 2014 Published: July 28, 2014 4278

dx.doi.org/10.1021/cg500484e | Cryst. Growth Des. 2014, 14, 4278−4284

Crystal Growth & Design

Article

temperature (T > 1500 °C) conditions required for the preparation of Sr2BO3F (B = Co or Ni).11,12 The main goal of this work was to understand the origin of O/F site disordering. We attempted to make the anion sites ordered for Sr2BO3F (B = Co or Ni), which has not yet been successfully accomplished. Instead, the anion order-to-disorder transition was investigated by synthesizing Sr2FeO3F (SFOF) under the high-pressure and high-temperature conditions, and its single crystals with ordered or disordered anions, which allowed for a detailed structural analysis, were found to grow under certain reaction conditions.

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EXPERIMENTAL SECTION

Synthesis. In previous reports, Sr2FeO3F was synthesized in an evacuated silica tube or air at approximately 1000 °C.16−18 The use of these methods always gave the product in powder form. High-pressure synthesis provides a closed reaction environment that suppresses the compositional deviation even at high temperatures. In this work, two sets of starting materials were examined for a high-P and high-T synthesis. One consisted of SrF2 (99.9%, Rare Metallic Co., Ltd.), inhouse prepared SrO2, and Fe (99.99%, Rare Metallic Co., Ltd.); the other consisted of SrF2, SrO (prepared by heating SrCO3), and Fe2O3 (99.99%, Wako Pure Chemical Industries Ltd.). The starting materials were thoroughly mixed in a stoichiometric ratio, put into a Pt capsule, and then set in a high-pressure cell as shown in Figure 2a. Subsequently, the mixture was heated at 3 or 6 GPa at temperatures ranging from 1300 to 1800 °C. After being heated for 1 h, the sample was quenched to room temperature, and the pressure was gradually released. Red-brown single crystals of Sr2FeO3F were obtained at 3 GPa and 1300 °C or at 6 GPa and 1800 °C, from the reaction of SrF2, SrO, and Fe2O3. Herein, the single crystals obtained under the former and latter conditions are abbreviated as LP-SFOF and HP-SFOF, respectively. Crystals of Sr2FeO3F were carefully selected for singlecrystal X-ray diffraction studies. Powder X-ray Diffraction. Powder X-ray diffraction data were collected at room temperature using a PANalytical X’Pert diffractometer equipped with a graphite monochromator and Cu Kα radiation (λ = 1.5418 Å; 45 kV, 40 mA) and compared to patterns in the Inorganic Crystal Structure Database (ICSD).19 The recorded 2θ values ranged from 10° to 70° in 0.018° increments. Single-Crystal X-ray Diffraction. LP-SFOF crystals that were 0.120 mm × 0.081 mm × 0.077 mm in size and HP-SFOF crystals that

Figure 1. Structural relationship between B site-centered coordination and anion-ordered patterns in layered oxyfluoride perovskites expressed as A2BO3F. (a) Square pyramidal coordination with disordered anion sites. (b) Octahedral coordination with disordered anion sites. (c) Square pyramidal coordination with ordered anion sites.

rather complex.13 Sr2BO3F (B = Co or Ni), shown in Figure 1a, forms a square pyramidal coordination around the B metal center with the O/F anions being disordered. In contrast, as shown in Figure 1b, d0 metal-based oxyfluorides such as K2NbO3F14 and Ba2ScO3F15 exhibit O/F site disordered arrangements similar to that of Sr2BO3F (B = Co or Ni); however, the B-centered coordination is not a square pyramid, but an octahedron. Moreover, Sr2FeO3F with a high-spin state for Fe3+ (d5) and Ba2InO3F with the d0 electron configuration15 exhibit the formation of the (Fe/In)O5 square pyramid with the O/F site being ordered (Figure 1c).16 In light of these structural features, it seems that the formation of square pyramidal coordination rather than the electronic configuration is correlated with anion ordering. However, Sr2BO3F (B = Co or Ni) shows the coexistence of square pyramidal coordination and anion disordering. In our previous reports, we had suggested that anion disordering might be attributed to the specific synthetic conditions: dense (P = 6 GPa) and/or high-

Figure 2. (a) Schematic view of cell assembly for high-pressure synthesis. (b) Photograph of a mechanically broken pellet of LP-SFOF prepared at 3 GPa and 1300 °C. Products in the capsule were separated into three regions: single crystals in the two outer regions and concretion in the middle region marked by white lines. 4279



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were 0.057 mm × 0.086 mm × 0.030 mm in size were selected, and each of them was mounted on a glass fiber for the single-crystal X-ray diffraction measurements. All the data were collected at 293 K on a Rigaku Saturn CCD diffractometer with a VariMax confocal optical system for Mo Kα radiation and a high-flux rotating-anode X-ray generator (RA-Micro7, 50 kV, 24 mA). A total of 1440 oscillation images, covering an entire sphere of 6° < 2θ < 55°, were collected using the ω-scan method. The crystal-to-detector distance was set at 45 mm. The data were processed using CrystalClear version 1.3.620 and corrected for Lorentz polarization and absorption effects.21 The structures were resolved by the Patterson method with SHELXS-9722 and were refined on F2 with full-matrix least-squares techniques with SHELXL-9722 using the WinGX package.23 Energy Dispersive X-ray Analysis. Elementary analysis was conducted by using a scanning electron microscope (SEM, HITACHITM3000) equipped with energy dispersive X-ray spectroscopy (EDX) analysis (Oxford Instruments, SwiftED3000). The accelerating voltage was 15 kV. Magnetic Measurements. The magnetic susceptibilities of LPSFOF and HP-SFOF were measured using a SQUID magnetometer (Quantum Design, MPMS-XL). Several milligrams of the crystals were collected from the pellets for the measurements. The samples were measured at an applied magnetic field (H) of 1000 Oe, in the range of 10−400 K, under both zero-field-cooled (ZFC) and field-cooled (FC) conditions. 57 Fe Mö ssbauer Spectroscopy Measurements. 57Fe Mössbauer spectra of LP- and HP-SFOF were recorded at 298 and 4 K in transmission geometry using a 57Co/Rh γ-ray source. Crystals of LPand HP-SFOF were crushed into fine powders with an agate mortar and pestle. The source velocity was calibrated with α-Fe as a reference material.

SrF2 as the main phases, with Sr2FeO3F as a minor phase in powder form. No single crystals were obtained. Figure 2b shows a photograph of a mechanically broken pellet of LP-SFOF, showing that the sample was separated into three regions: two outer regions where single crystals were mainly formed and a middle region composed of some concretion. Elementary analysis with SEM−EDX equipment detected no appreciable difference in the average atomic ratios of strontium, iron, and fluorine among three regions, nearly close to the nominal composition (2:1:1 Sr:Fe:F) (Figure S1 and Table S1 of the Supporting Information). The amount of oxygen could not be accurately assessed because of the low vacuum level in the SEM sample chamber. The cell parameters of the single crystals, which were computed by a preliminary single-crystal XRD analysis, were a = 3.8644 Å and c = 13.1678 Å and in good agreement with parameters for Sr2FeO3F reported previously. 16,18 Figure S2 of the Supporting Information shows the powder XRD patterns collected from finely crushed samples that include all three regions. The XRD data clearly demonstrated that Sr2FeO3F was obtained as the main phase. Some additional peaks assigned to Sr2Fe2O5 were also found. Given the atomic ratio detected by EDX analysis, uncharacterized peaks observed in the XRD patterns may be derived from phases containing fluorine. Thus, these impurity phases likely correspond to the main phases in the middle region of the pellet. On the other hand, the pellet of HP-SFOF was composed almost entirely of single crystals, without the concretion observed in LP-SFOF. Indeed, the XRD patterns revealed a significant reduction in the amount of impurity phases (Figure S2 of the Supporting Information). The difference in the crystal growth between the two samples can be attributed to a temperature gradient inside the capsule; the reaction temperature for HP-SFOF was sufficiently high to melt the products across the capsule. Structural Determination. In general, applying a high pressure on the order of gigapascals to ambient-pressure phases can transform the crystal structures with a higher density. However, the crystal structure of the LP-SFOF obtained by a high-pressure reaction was the same as the phase synthesized by a conventional solid state reaction; LP-SFOF crystallized in space group P4/nmm [a = 3.86440(10) Å, c = 13.1678(7) Å, and V = 196.643(13) Å3], which was consistent with the earlier reports using powder samples.16,18 No significant deviation from the full occupancy for either atom was found. The final refined crystallographic data and atomic coordinates are listed in Tables 1 and 2, respectively. The local coordination environment around the Fe center is depicted in Figure 3a. As previously reported, LP-SFOF exhibits a 1:1 ordered arrangement of O and F anions at the apical sites, leading to the following stacking: -(SrO)2-(FeO2)-(SrF)2-(FeO2)-. The Fe1−O1 and Fe1−O2 bond distances were 1.9624(8) and 1.931(6) Å, respectively. Meanwhile, the Fe1−F1 distance was 2.696(6) Å. Considering the large difference between the two apical bond distances and the corrugated O1−Fe1−O1 linkage [159.9(2)°], the coordination of the iron center can be viewed as a square pyramid with five oxygen atoms. The strontium cations formed two types of 9-fold coordination environments: one with nine oxygen atoms and the other with four oxygen and five fluorine atoms. HP-SFOF also adopted the same space group and basic structure as LP-SFOF, leading to an R1 of 0.0727 and a wR2 of 0.1193. However, a residual electron density of 7.8 e/Å3 was observed at 0.63 Å for the Fe1 site, which allowed the iron site

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RESULTS AND DISCUSSION Synthesis and Crystal Growth. The single-crystal growth of related layered oxyhalide perovskites is often achieved using a flux or melt growth method with simple halide starting materials. For example, Ca2−xNaxCuO2Cl2,24 Srn+1BnO2n+1Xn (B = Fe or Co; n = 1 or 2; X = Cl or Br),25,26 and other oxyhalide compounds27 have been obtained as single crystals by using ACl2 (A = Ca or Sr), NaCl, and SrBr2 as a flux. In contrast, it is relatively difficult to prepare single crystals of oxyfluoride perovskites using fluoride starting materials28−31 because of their low reactivities and high melting points compared with those of the corresponding chlorides and bromides. Alternatively, other reaction routes such as a hydrothermal reaction with a HF solution are employed for the crystal growth of oxyfluoride compounds.32,33 A high-pressure method is also a powerful technique for single-crystal growth.24,34−36 Because of the highly packed and dense reaction environment, both the flux method and melt growth method are available, without loss because of the volatilization of the starting materials during the reaction at elevated temperatures. Indeed, a layered copper oxyfluoride, Ba2Ca3Cu4O8(O1−xFx)2,37 was successfully grown as single crystals at high pressures. On the other hand, there was concern about whether our target compound, Sr2FeO3F, could be synthesized using a HP solid state reaction. Thus, the various reaction conditions described in the Experimental Section were examined. Consequently, it was found that the choice of the starting materials for the preparation of Sr2FeO3F under highpressure conditions strongly affected not only the resultant phases but also the single-crystal growth. For example, HPSFOF single crystals were grown using SrO, Fe2O3, and SrF2 as starting materials at 6 GPa and 1800 °C, but the reaction among SrO2, Fe, and SrF2 under similar pressure and temperature conditions resulted in a mixture of Sr2FeO4 and 4280



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Table 1. Crystal Data and Structural Analysis Data for Sr2FeO3F Prepared at 3 GPa and 1300 °C (LP-SFOF) or at 6 GPa and 1800 °C (HP-SFOF) Sr2FeO3F (LP-SFOF) formula weight T (K) space group a (Å) c (Å) V (Å3) Z Dcalc (g/cm3) F000 no. of measured reflections no. of unique reflections no. of observed reflections [F2 > 2σ(F2)] Rint (%) final R1/wR [F2 > 2σ(F2)] (%) goodness of fit maximum/minimum residual peak (e/Å3)

Sr2FeO3F (HP-SFOF)

298.09 296(2) P4/nmm 3.86440(10) 13.1678(7) 196.643(13) 2 5.034 270 4345 423 400

298.09 293(2) P4/nmm 3.869(2) 13.182(8) 197.3(2) 2 5.017 270 5538 583 388

2.6 3.24/8.45 1.269 2.849/−3.634

8.18 3.54/5.66 0.899 3.182/−1.820

Figure 3. Local coordination environments around the Fe center. O/F anions at apical sites are (a) fully ordered and (b) partially disordered, which involves iron site splitting.

data and atomic coordinates, are listed in Tables 1 and 2, respectively. 57 Fe Mö ssbauer Spectroscopy. To quantitatively evaluate the anion stoichiometry and oxidation state of iron, we conducted the 57Fe Mössbauer spectroscopy measurements of the compounds. Figure 4 shows the spectrum of finely crushed LP-SFOF crystals recorded at 298 K, including three sets of sextets and a paramagnetic doublet. The fitting results are listed in Table 3. The estimated values of the isomer shift (IS) and hyperfine field (HF) for the sextets are in good agreement with reported values for Sr2FeO3F and Sr2Fe2O5: for the former, IS = 0.296 mm s−1 and HF = 310 kOe (Table 3),38,39 and for the latter, IS = 0.380 and 0.180 mm s−1 and HF = 495 and 416 kOe. The doublet peak, which has a typical IS value of trivalent iron, likely comes from the uncharacterized phases detected in the powder XRD pattern. Figure 4b shows the 57Fe Mössbauer spectra of HP-SFOF recorded at 298 and 4 K. As described above, HP-SFOF has two sites of iron involved in anion disordering. Indeed, the observed sextet in the spectrum at 298 K was too broad to be fit by a single site of iron; thus, an attempt was made to fit the spectrum to two sextet lines (and one doublet line for the impurity absorption) (see Figure S3 and Table S2 of the Supporting Information). The fitting curves appeared to

to be split into two in a 0.947(4):0.053(4) Fe1:Fe2 occupancy ratio (see Figure 3b). After the split model had been introduced, R1 and wR2 significantly decreased to 0.0354 and 0.0566, respectively. In the HP-SFOF structure, the O3−Fe1− O3 and O3−Fe2−O3 bond angles in the basal plane were 160.44(17)° and 162.9(5)°, respectively, which indicates that both these iron atoms formed 5-fold coordination. As observed in LP-SFOF, the iron atom moved away from the center position of the FeO5F octahedron to the apical oxygen site. Thus, it is likely that the observed iron site splitting resulted from a random distribution of oxygen and fluorine anions to the apical sites, although it is not possible to distinguish the oxygen and fluorine atoms because of their similar X-ray scattering powers. The F1/O1 and O2/F2 sites were randomly distributed according to the ratio of the iron site occupancies. No deviation from unity was found for any of the anion site occupancies. The final results, including the crystallographic

Table 2. Atomic Coordinates and Equivalent Isotropic Displacement Parameters for Sr2FeO3F (LP-SFOF and HP-SFOF) ga

a

x

Sr1 Sr2 Fe1 F1 O1 O2

1 1 1 1 1 1

0.250 0.2500 0.7500 0.7500 0.2500 0.7500

Sr1 Sr2 Fe1 Fe2 F1/O1 O2/F2 O3

1 1 0.947(4) 0.053(4) 0.947/0.053 0.947/0.053 1

0.750 0.750 0.2500 0.2500 0.2500 0.7500 0.2500

y Sr2FeO3F (LP-SFOF) 0.250 0.2500 0.7500 0.7500 0.7500 0.7500 Sr2FeO3F (HP-SFOF) 0.750 0.750 0.2500 0.2500 0.2500 0.7500 −0.2500

z

U(eq)

0.12190(5) 0.39498(5) 0.27216(9) 0.0674(4) 0.2461(3) 0.4188(4)

0.01002(15) 0.00847(15) 0.00867(19) 0.0223(13) 0.0110(6) 0.0091(9)

0.37822(4) 0.10525(4) 0.22773(8) 0.2751(14) 0.0816(3) 0.5669(3) 0.25302(19)

0.01173(12) 0.01011(11) 0.0081(2) 0.005(3) 0.0139(7) 0.0272(8) 0.0121(5)

Site occupancy factor. 4281



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distribution of HF to vary. In addition, the residual crystal orientation effect was taken into account during the fitting. The Mössbauer parameters and the distribution histogram of HF are listed in Table 3 and shown in Figure 4c, respectively. The IS value for the trivalent iron and the average HF value were slightly smaller than those of LP-SFOF and previously reported samples. On the other hand, the 4 K spectrum of HP-SFOF exhibited a more defined sextet that could be fit to one sextet line. This spectral shape is significantly different from those in Sr2FeO3+xF1−x (x = 0.11 or 0.20) with mixed valence states of Fe(III) and Fe(IV): the spectral broadening persists even at 4.2 K because of the magnetic interactions between the nearest neighbor Fe(III) and Fe(IV).18 Because our phase does not contain any aliovalent iron, the distribution of hyperfine fields is made narrow relative to those in the mixed-valent phases at low temperatures. Magnetism. Figure 5 shows the temperature dependence of magnetic susceptibilities χ (=M/T) of LP- and HP-SFOF.

Figure 4. (a and b) 57Fe Mössbauer spectra recorded from the finely crushed samples of LP-SFOF and HP-SFOF, respectively, at 298 and 4 K. (c) Hyperfine field distribution of HP-SFOF at 298 K.

Figure 5. Magnetic susceptibility curves for LP-SFOF and HP-SFOF crystals, measured at H = 1000 Oe under ZFC conditions. The filled triangle indicates the magnetic phase transition temperature for HPSFOF.

reproduce the data well, but the peak area ratio of the decomposed sextets was estimated to be 44:45, which contradicts the iron sites determined by single-crystal XRD analysis. This discrepancy can be explained by a random distribution of O/F anions at an atomic scale. It is known that introduction of foreign cations or anions in an iron-centered coordination environment leads to an inhomogeneous distribution of hyperfine fields, as seen in Sr(Fe,Mn)O2,40 Sr2FeO3+xF1−x,18 and Sr3Fe2O5.44F1.56.41 In this context, the fitting of the spectrum was re-examined allowing the

Because no essential difference between the ZFC and FC data was observed, only the ZFC data at H = 1000 Oe are presented. For the LP-SFOF sample, single crystals were carefully collected from the outer regions of the pellet; however, contamination by impurities could not be completely avoided, as detected by powder XRD. Upon cooling from 400 K, the HP-SFOF with anion disordering exhibited the anomaly indicative of antiferromagnetic ordering at around TN = 340 K. This is consistent with the hyperfine splitting observed in the Mössbauer spectrum at 298 K. The magnetic phase transition

Table 3. 57Fe Mössbauer Parameters of Sr2FeO3F (LP-SFOF and HP-SFOF) T (K)

a

LP-SFOF

298

HP-SFOF

298

Sr2FeO3F ref 38

4.2 293 4.2

IS (mm s−1)

HF (kOe)

quadrupole splitting, QS (mm s−1)

line width (mm s−1)

peak area (%)

0.296 0.380 0.180 0.331 0.300a 0.333 0.404 0.290 0.390

310 495 416 0 290a 0 520 311 520

−0.429 −0.694 0.610 0.854 −0.433a 0.890 −0.458 −0.44 −0.47

0.40 0.31 0.31 0.36 − − 0.34

63 11 14 12 89 11 100

Averaged values. 4282



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temperature or pressure are needed to clarify the mechanism of the O/F anion disordering. Unfortunately, however, such studies were not possible because of limitations of experimental techniques and crystal growth conditions: the Pt capsule containing starting materials melted when it was heated above 1300 °C at 3 GPa, while lowering the reaction temperature from 1800 °C at 6 GPa, even by −50 °C, resulted in poor crystalline quality. Nevertheless, we can deduce the driving force of anion disordering from the density of SFOF single crystals. If the high-pressure effect is the dominant factor, the anion-disordered phase would have a density higher than that of the anion-ordered phase, but this is not the case. The density of HP-SFOF (5.017 g/cm3) was lower than that of LP-SFOF (5.034 g/cm3). Finally, the entropic effects associated with the high reaction temperatures seem to play a crucial role in the anion order-to-disorder transition. Notice that only 5% of the apical anions were disordered in HP-SFOF, which was significantly different from the full random disordered arrangement in Sr2BO3F (B = Co or Ni). Further investigation is necessary to understand the cause of the instability of the anion-disordered state in Sr2FeO3F.

temperature was lower than those of the previously reported samples (TN ∼ 358 K),18 which was consistent with the smaller value of the hyperfine field. On the other hand, the magnitude of χ for LP-SFOF was larger than that for HP-SFOF, and the anomalies at TN were obscure. These observations may be related to the contribution from the residual impurities. Nevertheless, we can infer from the magnitude of hyperfine fields at 298 K that the TN of LP-SFOF is almost the same as that of previously reported samples. It is worth discussing the relationship between the anion disorder and the magnetic interactions before we discuss the decrease in the TN for HP-SFOF. Generally, the interlayer magnetic interactions mediated by the apical anions in the n = 1 RP phases are much weaker than the intralayer interactions, so the magnetic properties are expected to depend little on the degree of anion disorder. In fact, the difference in the magnetic phase transition temperature between anion-disordered Sr2CoO3F (TN = 323 K)42 and anion-ordered Sr2CoO3Cl (TN = 330 K)43 is quite small. However, we recently discovered unusual anion site dependence of the magnetic properties of Sr2NiO3X (X = F or Cl) in a low-spin configuration.12 The anion-disordered Sr2NiO3F undergoes a spin freezing transition in contrast to the anion-ordered Sr2NiO3Cl showing a longrange magnetic order. The orbital symmetry of transition metals and the neighboring ligands is key to understanding the effect of the anion site distribution on the magnetic properties. On the two-dimensional network consisting of corner-sharing BO5X octahedra with the B cation shifted from the O4 basal plane toward the oxide ion at one apical site, the interaction between B dx2−y2 and O 2px,y orbitals; namely, pdσ interaction gives the strongest exchange interaction between B cations, because of the σ−σ bonding. The magnitude of this pdσ interaction along the side directions of a square is not influenced by the anion-ordered pattern in terms of orbital symmetry.44 For Sr2CoO3X, this type of superexchange interaction plays a dominant role in the magnetism;45 thus, the magnetic phase transition temperatures are very similar to each other. On the other hand, Sr2NiO3X (X = F or Cl) with S = 1/2 possesses the half-filled dxy orbital, which creates ddσ (direct exchange) interaction along the diagonal directions of a square and pdπ (superexchange) interaction mediated by O 2px,y orbitals along the side directions. 12 The former interaction, especially, changes the magnitude with anionordered patterns because of off-centering of the metal site toward an apical oxygen and is dominant compared to the pdπ interaction. As a result, the magnetic ground state strongly depends on the kind of halogen anion. Sr2FeO3F takes the high-spin configuration with S = 5/2, and the Fe(dx2−y2)−O(2p)−Fe(dx2−y2) superexchange interaction dominates the magnetic properties. However, Sr2FeO3F also has the half-filled dxy orbital, which is different from Sr2CoO3X with the dxy orbital fully occupied. In this context, the decrease in TN observed in HP-SFOF with respect to previously reported SFOF probably resulted from the bond randomness of the ddσ interactions accompanied by anion disordering. Origin of O/F Anion Disordering. Single-crystal analysis of HP-SFOF demonstrated the splitting of the iron sites along the c axis, accompanied by O/F site disordering, whereas LPSFOF adopted the crystal structure with a full anion-ordered arrangement, similar to that reported previously.16 In our previous studies of Sr2CoO3F,11 we proposed that a highpressure or high-temperature condition would lead to anion disordering. Thus, systematic studies as a function of

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CONCLUSIONS

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ASSOCIATED CONTENT

Single crystals of Sr2FeO3F with ordered and/or disordered apical anions were successfully grown under high-temperature and high-pressure conditions. To the best of our knowledge, this work is the first report of the order-to-disorder transition of O/F anions in which the chemical composition was maintained. However, there are still two questions to be solved. One is the driving force of the anion disordering, and the other is a small degree of anion disordering. In situ high-resolution XRD experiments at high pressures and high temperatures may shed light on this problem.

S Supporting Information *

X-ray crystallographic files in CIF format, SEM−EDX analysis of Sr2FeO3F (LP-SFOF), X-ray powder diffraction data of Sr 2 FeO 3 F (LP- and HP-SFOF), and 57 Fe Mö s sbauer spectroscopic data of HP-SFOF at 298 K. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank K. Fujimaki, T. Ikeda, and T. Taniguchi for their support with the high-pressure synthesis at the National Institute for Materials Science. This research was supported in part by the World Premier International Research Center of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan; by the Japan Society for the Promotion of Science (JSPS) through Grants-in-Aid for Scientific Research (22246083, 25289233, 22340163, and 25287145); and by the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program), Japan. 4283



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