ARTICLE pubs.acs.org/cm
Topotactic Synthesis and Crystal Structure of a Highly Fluorinated RuddlesdenPopper-Type Iron Oxide, Sr3Fe2O5+xF2x (x ≈ 0.44) Yoshihiro Tsujimoto,*,† Kazunari Yamaura,‡,§ Naoaki Hayashi,*,^ Katsuaki Kodama,& Naoki Igawa,& Yoshitaka Matsushita,# Yoshio Katsuya,@ Yuichi Shirako,O Masaki Akaogi,O and Eiji Takayama-Muromachi‡,§,|| †
)
International Center for Young Scientists (ICYS) and International Center for Materials Nanoarchitechtonics (MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan ‡ Superconducting Materials Center, NIMS, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan § JST, Transformative Research-Project on Iron Pnictides (TRIP), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan ^ Graduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8501, Japan & Quantum Beam Science Directorate, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan # NIMS Beamline Station at SPring-8, NIMS, Kouto 1-1-1, Sayo-cho, Hyogo 679-5148, Japan @ SPring-8 Service Co., Ltd., Kouto 1-1-1, Sayo-cho, Hyogo 679-5148, Japan O Department of Chemistry, Faculty of Science, Gakushuin University, Mejiro, Toshima-ku, Tokyo 171-8588, Japan MANA, NIMS, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan
bS Supporting Information ABSTRACT: Topotactic reaction of the n = 2 RuddlesdenPopper phase Sr3Fe2O7δ (δ ≈ 0.18) with polytetrafluoroethylene (PTFE) yields a highly fluorinated phase Sr3Fe2O5+xF2x (x ≈ 0.44), compared with Sr3Fe2O6F0.87 prepared by the reaction of Sr3Fe2O6 and F2 gas. Structure analyses based on powder neutron diffraction, synchrotron powder diffraction, and 57 Fe M€ossbauer spectroscopy measurements demonstrate that the new oxyfluoride perovskite has no anion deficiencies and adopts the tetragonal structure (space group I4/mmm) with the lattice constants a = 3.87264(6) Å and c = 21.3465(6) Å at room temperature. The fluoride ions preferentially occupy the terminal apical anion sites with oxide ions in a disordered manner, which results in square pyramidal coordination around iron. The present compound also shows an antiferromagnetic order with a Neel temperature (TN) of 390 K, in sharply contrast to Sr3Fe2O6F0.87, which has a TN value that is lower than room temperature. KEYWORDS: low-temperature fluorination, topotactic reaction, iron oxyfluoride, Sr3Fe2O5.44F1.56, RuddlesdenPopper phase
1. INTRODUCTION The development of transition-metal oxides with perovskitebased structures has stimulated the search for mixed-anion systems, because the incorporation of two different anions in one structure provides further opportunities to effectively control and enhance the chemical and physical properties in the pure oxides. For example, the fluorine anion has almost the same ionic radius as the oxygen anion but different charge and electronegativity; this allows us to modify the electronic configuration of the transition-metal cation and the superexchange interactions mediated by the anion through substitution of F for O.1 However, the oxyfluoride phase is basically difficult to synthesize by a simple high-temperature reaction, because of the high chemical stability of the simple fluoride starting materials, although a few examples such as BaScO2F,2 BaFeO3xFy,3 and Sr2FeO3F4 have been prepared. This problem can be overcome using a highpressure synthesis or a low-temperature reaction method. The r 2011 American Chemical Society
former is a powerful technique for stabilizing oxyfluoride phases, as exemplified by the syntheses of PbMO2F (M = Sc, Fe)5,6 and Sr2CoO3F.7 However, the number of materials that may be prepared via such a method is also quite limited, because the reaction technique requires specialized equipment. In contrast, the latter reaction method is straightforward and contributes greatly to the development of oxyfluoride chemistry. To date, various fluorinating agents have been reported, including F2 gas,1,8,9 XeF2,10 NH4F,11,12 AF2 (A = Ni, Cu, Zn, Ag),12,13 poly(vinylidene)fluoride (PVDF),1416 and polytetrafluoroethylene (PTFE).17 Each fluorinating agent exhibits a distinct fluorinating power and reaction pathway. The reaction efficiency and the fluorine contents in the resultant material are dependent on the choice of not only the fluorinating agent, but also the Received: April 14, 2011 Revised: June 16, 2011 Published: August 01, 2011 3652
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Figure 1. Schematic crystal structures of (a) Sr3Fe2O7 and (b) Sr3Fe2O5+xF2x. White, black, small and large gray spheres represent Sr, Fe, O, and O/F, respectively. (c) The magnetic structure of Sr 3Fe2 O5+x F 2x . For clarity, only Fe moments are displayed. The solid line denotes the chemical unit cell in panels a and b and the magnetic unit cell in panel c.
anion lattice of the precursors. For example, a highly reduced YBa2Cu3O7δ (δ ≈ 0.89) reacts with XeF2 to incorporate a larger amount of fluorine atoms than a slightly reduced phase (δ ≈ 0.05), which strongly affects the critical temperature of superconductivity induced through fluorination.18 Some RuddlesdenPopper (RP) cupric oxyfluorides also vary greatly in their fluorine contents and structures, depending on the fluorinating agents used.19 For iron oxide, Weller and his collaborators successfully obtained the n = 2 RP-type Sr3Fe2O6F0.87 from Sr3Fe2O6 with oxygen vacancies at the central anion sites, using F2 gas.9 During the reaction, the fluorine atoms selectively occupy the terminal apical anion sites accompanied by oxygen displacement to the central vacant sites. Such a structural rearrangement in the fluorinating process was observed in related compounds such as Sr2CuO2F2+δ1 and Sr2PdO2F2.15 Despite the incorporation of almost one fluorine atom, Sr3Fe2O6F0.87 shows structural and physical features similar to a pure oxide Sr3Fe2O7 (Figure 1). The deviation of the OFeO bond angle in the plane from the ideal value of 180 is only 7.6, much smaller than the 20 observed in Sr2FeO3F (n = 1) with oxygen/fluorine ordering.4 The 57Fe M€ossbauer spectra of Sr3Fe2O6F0.87 are also qualitatively similar to those of Sr3Fe2O7 and provide evidence of mixed-valence states between +3 and +5 of the iron and a magnetic order at 4.2 K.9 In this study, we report the successful synthesis of a morefluorinated phase, Sr3Fe2O5+xF2x (x ≈ 0.44), via a lowtemperature reaction of slightly oxygen-deficient Sr3Fe2O7-δ with PTFE. The structure and magnetic properties, which are significantly different from those in Sr3Fe2O6F0.87, are discussed on the basis of Rietveld refinements using both synchrotron powder X-ray and neutron powder diffraction data, as well as 57Fe M€ossbauer spectroscopy.
2. EXPERIMENTAL SECTION Synthesis. A slightly oxygen-deficient Sr3Fe2O7δ phase was synthesized using a method similar to that reported in the literature.20 Stoichiometric quantities of SrCO3 (99.99%, Rare Metallic Co., Ltd.)
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and Fe2O3 (99.99%, Wako) were ground together and heated in an alumina crucible for 12 h at 1000 C and 24 h at 1200 C in air, with one intermediate grinding. The product was black in color and was confirmed to be a single phase by powder X-ray diffraction (XRD). The lattice parameters calculated by the least-squares method were a = 3.8667(3) Å and c = 20.161(1) Å. The oxygen contents were evaluated by thermogravimetric analysis (TGA), using a PerkinElmer Pyris 1 TGA system. The precursor was heated using a 5% H2/Ar gas up to 1000 C at a rate of 15 C/min and then cooled to room temperature at the same rate, yielding a reduced phase of Sr3Fe2O6.20 Thus, the oxygen deficiency in the precursor was shown to be δ = 0.18 (i.e., the precursor was Sr3Fe2O6.82). The fluorination of Sr3Fe2O6.82 was carried out using PTFE (Aldrich) as a solid-state fluorinating agent. The precursor and PTFE (C2F4 monomer unit) were intimately mixed in several molar ratios (precursor: PTFE = 1:0.501:0.60), and the mixture was pelletized and heated at 270 C for 2 d under flowing argon gas. Characterization. Powder X-ray diffraction data were collected on a PANalytical X’Pert diffractometer equipped with a graphite monochromator and Cu KR radiation (λ = 1.5418 Å). A fluorinated sample was also investigated by powder neutron diffraction (NPD) and synchrotron XRD (SXRD) at room temperature. NPD data were collected on the high-resolution powder diffractometer (HRPD) installed at the JRR-3M reactor at the Japan Atomic Energy Agency (JAEA), Tokai. The incident neutron wavelength was 1.8239 Å. The sample (1.6 g) was placed in a vanadium cylinder. The data were recorded in 0.05 increments in a 2θ range of 10150. The SXRD data were collected using a DebyeScherrer camera installed on BL15XU at SPring-8, with λ = 0.65298 Å.21 The sample was contained in a glass capillary 0.2 mm in diameter. The data were recorded in 0.002 increments in a 2θ range of 250. The NPD and SXRD data were analyzed by the Rietveld method, using the program RIETAN-FP.22 The pseudo-Voigt function was used as a profile function. The weighting R index (Rwp), the Bragg R index (RI), and goodness of fit (S) are defined as the following profile: Rwp = [∑i wi(yioyic)2/∑i wiyio2] and RI = ∑k | IkoIk|/∑k Iko, where yio and yic are the observed and calculated intensities, wi is the weighting factor, and Iko and Ik are the observed and calculated integrated intensities. S = Rwp/Rexp. Rexp = [(N P)/∑i wiyio2]1/2, where N is the total number of yio data when the background is refined, and P is the number of adjusted parameters. 57 Fe M€ossbauer spectra on a fluorinated sample were taken over the temperature range of 4410 K in transmission geometry using a 57Co/ Rh γ-ray source. The source velocity was calibrated by R-Fe as a reference material. Morphological characterization was also conducted using a HITACHI Model S-4800 field-emission gun scanning electron microscopy (SEM) system equipped with energy-dispersive X-ray spectroscopy (EDX) analysis.
3. RESULTS AND DISCUSSION Synthesis and Structure Refinements. The sample quality after the fluorinating reactions was assessed by laboratory XRD measurements. After the reaction between the precursor and PTFE in the ratio of 1:0.5, Bragg peak positions in the XRD patterns shifted relative to those of the precursor. Most of the major peaks were readily indexed on a simple body-centered tetragonal structure consistent with the topotactic reaction of Sr3Fe2O7δ. Several weak reflections from the unreacted precursor phase were also detected. Reactions using an additional amount of PTFE, namely, with precursor:PTFE ratios of 1:0.55 or 1:0.60, yielded the product as a single phase. No impurity phases were observed within experimental error. The sample color did not significantly change even after fluorination, but 3653
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Figure 2. SEM images of fluorinated samples taken from different regions. Small graphite particles with an average size of 100 nm were grown over the surface of grains.
remained black. This implies the presence of tetravalent iron in the product. The lattice parameters calculated via the leastsquares method are a = 3.878(4) Å and c = 21.406(2) Å. While the a-axis remained almost unchanged, the c-axis was significantly longer than that of the related oxygen-deficient phases (0 e δ e 1) including the precursor. It should also be noted that the product obtained in this study has a longer c-axis than Sr3Fe2O6F0.87 (20.779(3) Å).9 These results suggest that more fluorine atoms are incorporated in the present compound and preferentially occupy the apical anion sites. Figure 2 shows representative SEM images of the product. We found that small particles with an average size of 100 nm, which were not observed in the precursor alone, were grown on the surface of bigger grains. EDX analyses revealed that the area where the small particles were absent was composed of Sr, Fe, O, and F, and that the atomic ratio of Sr:Fe is 3.17:2.03 in accordance with the expected ratio. In contrast, an additional element—namely, carbon—was detected from the EDX data collected on the small particles. The carbon species should be associated with decomposition of PTFE during the fluorination reaction; this will be discussed later. The sample for structure refinement was synthesized in a precursor:PTFE molar ratio of 1:0.55. Rietveld refinement using the room-temperature SXRD data collected from the fluorinated sample was conducted to determine the chemical crystal structure. A model based on the Sr3Fe2O7 structure with the space group I4/mmm was examined. Several additional peaks were also visible, but these extra reflections could be assigned as carbon graphite (see Figure 3a), which is consistent with the results of SEM-EDX measurements. Contributions from the graphite were ignored during the structure refinement, because the diffraction intensities were considerably small, relative to the main phase. It is impossible to distinguish precisely between oxygen and fluorine, because their neutron
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Figure 3. (a) Synchrotron powder diffraction and (b) powder neutron diffraction patterns collected at room temperature from the product after fluorination. Indices with “n” and “m” denote nuclear and magnetic Bragg reflections, respectively. The nuclear reflections are assigned on the basis of the chemical cell with an√= 3.873 Å and cn = 21.346 Å. The magnetic cell is given as am = bm = 2an and cm = cn.
and X-ray scattering factors are very similar. Thus, we assumed that all the anion sites were occupied by oxygen at the initial stage of the refinement. The atomic positions and displacement parameters were allowed to be refined independently. The occupancies of anion sites were fixed to the full fraction. The structure refinements readily converged well (Rwp = 1.59, RI = 5.65). Observed, calculated, and difference plots are displayed in Figure S1 in the Supporting Information. The crystal structure and the crystallographic data, including the atomic coordinates, selected bond distances, and angles, are shown in Figure S2 in the Supporting Information and Table S1 in the Supporting Information, respectively. Neutron diffraction is more sensitive to the positions and occupancies of O/F atoms than XRD. Thus, the NPD data were collected from the fluorinated sample at room temperature. Figure 3 shows a comparison of the NPD patterns in a low 2θ region with the SXRD patterns. In contrast to the SXRD data, the NPD data exhibited the presence of some intense peaks at 2θ = 21.44 and 24.12, which could not be taken into account by the body-centered tetragonal structure. These additional reflections were likely to originate from magnetic scattering, which was confirmed by the M€ossbauer spectroscopy measurements, as shown later. The magnetic peaks could be indexed as 1 0 2 and 1 0 3 reflections on the basis of the magnetic unit cell, given as am = √ bm = 2an, cm = cn, where an and cn are the chemical unit cell constants. Based on the structure refined against the SXRD data, we performed a Rietveld refinement using the NPD data to determine the occupancies and sites of anions, as well as the magnetic structure. At first, various collinear spin models for magnetic structure were examined, and the best agreement between observed and calculated data was obtained by a simple G-type antiferromagnetic model; each nearest neighbor iron moment (μ = 2.2(2) μB) in a double-layered block is aligned antiparallel and confined in the xy plane at a cant angle of θ ≈ 45 with the aaxis, as shown in Figure 1c. This spin arrangement is similar to that of the iron oxychloride analog Sr3Fe2O5X2 (X = Cl, Br).23 3654
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Table 1. Structural Parameters of Sr3Fe2O5+xF2x at Room Temperature Determined by the Neutron Powder Analysis atom
x
site
y
z
Uiso (Å2)
occupancy
Sr1
2b
0
0
0.5
0.0148(11)
Sr2
4e
0
0
0.32333(13)
0.0045(7)
1 1
Fe
4e
0
0
0.08782(11)
0.0026(5)
1
Oca
2a
0
0
0
0.0189(13)
1
Oeq
8g
0
0.5
0.09903(12)
0.0097(5)
1
Ota
16m
0.055(2)
0.055
0.2065(3)
0.037(3)
0.25
I4/mmm; a = 3.87264(6) Å, c = 21.3465(5) Å, V = 320.141(11) Å3 Rwp = 9.14%, RB = 3.70%, S = 1.58 Bond distances: FeOca = 1.875(2) Å, FeOeq = 1.9511(4) Å, FeOta = 2.552(7) Å, Sr1Oca = 2.73837(3) Å, Sr1Oeq = 2.867(2) Å, Sr2Oeq = 2.549(2) Å, Sr2Ota(1) = 2.515(6)3.110(6) Å, Sr2Ota(2) = 2.512(7) Å Bond angles: OcaFeOeq = 96.97(12)
On the other hand, the refinements of atomic coordinates gave an extraordinarily large atomic displacement parameter for the terminal apical anion site (Uiso (Ota) = 0.071(2) Å2). Such a large value is basically attributed to either anion deficiency or site disordering. The site occupancies of all the anions including the equatorial (Oeq) and central apical (Oca), sites did not deviate from the full fraction during the refinements, and thus we examined several models in which the Ota moved out from the ideal 4e (0, 0, z) to more general sites. Among these, 16m (x, x, z) gave the best improvement; the position of the Ota site was slightly shifted to x = 0.055(2), the Uiso significantly reduced to a reasonable value of 0.037(3) Å2, and the value of goodness-of-fit decreased from S = 1.64 to 1.58. The finally obtained crystallographic data, including atomic coordinates, selected bond distances, and angles, are shown in Table 1, and the crystal structure and refined NPD patterns are illustrated in Figures 1b and 4. € ssbauer Spectroscopy. The 57Fe M€ Mo ossbauer spectra of the fluorinated sample were collected to study the valence state of the iron associated with the composition ratio of O/F. Figure 5a shows representative M€ossbauer spectra. The spectrum at 293 K revealed the hyperfine field splitting indicative of magnetic order, which is consistent with the NPD measurements. The 4 K spectrum exhibited more clearly defined sextets, but could not be accounted for by a single site of iron, unlike Sr3Fe2O5X2 (X = Cl, Br), which contains only trivalent iron.23 This suggests a complex hyperfine structure of the iron nucleus related to some disorder and differing site occupancies of O/F in the present compound. The fitting of the 4 K data showed the spectrum to be well-resolved into two characteristic sextets with a peak-area ratio of 0.89/0.11. The fitting results including the M€ossbauer parameters are given in Figure 5a and Table 2. The isomer shift (IS) and hyperfine field (HF) are 0.429 mm s1 and 515 kOe, respectively, for the larger peak area, which can be assigned as Fe(III). For the smaller peak area, IS = 0.062 mm s1 and HF = 230 kOe, values that are much smaller than those of the Fe(III) component. These values are very close to those assigned as charge-disproportionated Fe(V) in some iron oxides such as CaFeO3,24 La1xAxFeO3 (A = Ca, Sr),25 Sr3Fe2O7,26 and CaCu3Fe4O12.27 The nominal oxidation state of iron in these oxides is +4 or intermediate values between +3 and +4, but the instability of Fe(IV) results in its charge disproportionation into Fe(III)/Fe(V) at low temperatures. Sr3Fe2O6F0.87 was also
Figure 4. Observed (crosses), calculated (upper solid line), and difference (bottom solid line) plots from the Rietveld structural refinement against the neutron powder diffraction data collected from Sr3Fe2O5+xF2x at room temperature. The nuclear and magnetic Bragg reflections are indicated by the bottom and upper tick marks, respectively.
found to exhibit similar charge disproportionation with a ratio of Fe(III)/Fe(V) = 0.78/0.22. 9 Here, we can estimate the chemical composition including O/F contents as “Sr3Fe2O5.44F1.56” by taking into consideration the full occupation of anion sites and the peak-area ratio of Fe(III)/Fe(V). In terms of anion disorder around an iron metal center, each sextet could be regarded as a superposition of hyperfine sextets with some distribution of M€ossbauer parameters. Thus, we carried out more-detailed M€ossbauer characterization, allowing the distribution of HF as well as IS and quadrupole splitting (QS). During the fitting, the intensity ratio, the interval of HF, and the full width at half maximum (fwhm) of the sextets were fixed at values of 3:2:1:1:2:3, 4 kOe, and 0.35 mm s1, respectively. The obtained M€ossbauer parameters are given in Table 2. The obtained fitting curves, HF distribution, and average HF vs T plots are shown in Figures 5ac. At 4 K, the resultant HF distribution was clearly divided into two parts, with average HF values of 511 and 219 kOe, and the average IS values and peakarea ratios also agreed well with the initial fitting results. As the temperature increased, it was harder to distinguish the Fe(V) component from the hyperfine sextets, because of noticeable spectral broadening, especially above 200 K. Upon further 3655
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Table 2. M€ ossbauer Parameters of Sr3Fe2O5+xF2x temperature,
isomer shift, ISa
hyperfine field, HFa
quadrupole splitting, ΔE
line width
normalized
T (K)
(mm s1)
(kOe)
(or QS)a,b (mm s1)
(mm s1)
peak area
410
0.195
0
1.09
0.35
1
293
0.310
348
0.52
0.35c
1
200
0.370
395
0.37
0.35c
1
4
0.428
511
0.54
0.35c
0.918
0.105
229
0.21
c
0.082
0.429
515
0.54
0.75
0.886
0.062
230
0.19
0.54
0.114
4 (nonav.)
0.35
Averaged values, except the 410 K data. b QS = (S2 S1)/2. See Figure 4a. ΔE denotes quadrupole splitting in the paramagnetic state. c The line width of each sextet with an interval of 4 kOe was fixed at 0.35 mm s1 during fitting. a
heating, the average HF rapidly decreased and became zero at TN = 390 K. The spectrum at 410 K showed a paramagnetic quadrupole doublet that could be fitted by one component with the IS value associated with Fe(III). This is in contrast to Sr3Fe2O6F0.87, in which the Fe(V) component persists, even above TN.9 Determination of the Fluorine Sites. It is challenging to determine the fluorine sites in extended transition-metal oxyfluorides, because O and F possess similar X-ray and neutron scattering factors. For iron oxyfluorides with cubic perovskite structures, such as AFeO2F (A = Sr, Pb), the cis- and transarrangements for the fluorine in the octahedron were discussed on the basis of M€ossbauer spectra and bond-valence sum (BVS) calculations.6,16 For layered oxyfluoride perovskite, we can determine the fluorine sites more specifically by comparing the anisotropic local environment around the metal center with that in the oxyhalide analogs. Table 3 shows a comparison of the local environment around the B-site in the n = 2 RuddlesdenPopper oxyhalides described as A3B2O5X2 (A = alkali or alkaline-earth metal, B = transition metal, X = halogen).9,23,2831 The halogen atoms exclusively occupy the terminal apical anion sites; this induces a strong hybridization between the B cation and Oca, which is associated with square-pyramidal coordination. While the BX and BOca bond distances and their ratios (BX/ BOca) vary, depending on both the B- and X-sites, the OcaBOeq bond angles exhibit almost the same values among related oxyhalide compounds (∼97). Therefore, this value can be used as a measure of fluorine occupation and distribution. For example, Sr3Fe2O6F0.87 has a small OcaFe-Oeq bond angle of 93.8, relative to the corresponding measure of value; thus, we can infer that this is a result of mixed occupation of O and F at the terminal apical sites, in addition to small occupancy of F.9 For the fluorinated compounds in this study, the value of the OcaFeOeq bond angle further approaches that expected from an A3B2O5X2 composition, which is consistent with the greater fluorine content in Sr3Fe2O5.44F1.56 than in Sr3Fe2O6F0.87. These structural features also suggest that the fluorine atoms occupy the terminal apical anion sites rather than the equatorial or central apical sites, resulting in square-pyramidal coordination around Fe. Also, the displacement of the terminal apical anion sites and the expansion of the c-axis support this presumption.
Figure 5. (a) M€ossbauer spectra collected from Sr3Fe2O5+xF2x. Red and blue dashed lines represent the contributions from Fe(V) and Fe(III) components, respectively. The solid lines drawn in the spectra at 200 and 293 K represent the total fitting lines consisting of sextets with an interval of 4 kOe. (b) Hyperfine field distribution at 4, 200, and 293 K. (c) Averaged hyperfine field plotted as a function of temperature. 3656
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Table 3. Comparison among Local Environments around the B-Site in n = 2 RuddlesdenPopper-Type Oxyhalides A3B2O5X2, Sr3Fe2O6F0.87, and Sr3Fe2O5.44F1.56 BOca (Å)
BX(Ota) (Å)
OcaBOeq angle ()
BOca/BX(Ota)
reference
Sr3Fe2O6F0.87
1.894(6)
2.26(2)
0.838
93.8(3)
9
Sr3Fe2O5.44F1.56 Ba3In2O5F2
1.875(2) 2.073(2)
2.552(7) 2.597(3)
0.735 0.798
96.97(12) 97.64(9)
this work 28
Sr3Fe2O5Cl2
1.8719(7)
2.9977(2)
0.624
97.65(6)
23
Sr3Co2O5Cl2
1.876(5)
3.053(5)
0.614
98.2(2)
29
Ba3In2O5Cl2
2.049(4)
3.069(7)
0.668
97.9(2)
30
Sr3Fe2O5Br2
1.869(2)
3.217(3)
0.581
97.53(9)
23
Ba3In2O5Br2
2.053(5)
3.242(9)
0.633
97.1(3)
31
The crystallographic data obtained from the model where F atoms are located in the terminal apical sites are shown in Table S2 in the Supporting Information for NPD data and Table S3 in the Supporting Information for SXRD data. The corresponding Rietveld plots for Sr3Fe2O5.44F1.56 are shown in Figures S3 and S4 in the Supporting Information. These results are essentially the same as the initial refinement results using the NPD data. BVS calculation is useful for confirming the validity of structure refinement, especially the fluorine site. The BVS values obtained are +1.80 for Sr1, +2.10 for Sr2, +3.21 for Fe, 2.08 for Oeq and 1.16 for F; these agree well with those expected from the composition. However, the BVS value for Ota is 0.89, which is significantly larger than the expected value. Similar deviations were observed in Sr3Fe2O6F0.87 (BVS = 1.435 for Ota, 1.16 for F).9 This implies that the actual bond distance between Sr and Ota is longer than the average bond distance between SrOta/F, although the refinement of a model with further splitting on Ota/ F sites did not give any improvement, because of the small Ota content. Reaction Mechanism. Here, we discuss the mechanism of the fluorination reaction in this study. The greater efficiency with which PTFE fluorinates Sr3Fe2O7-δ, compared to fluorine gas, probably originates from the stronger reducing power of carbon than that of F2 gas. Therefore, it is reasonable to assume that the C of the PTFE acts as an oxygen getter and then F atoms from the PTFE are incorporated into the anion lattices. On this assumption, we proposed a possible reaction equation as follows: Sr3 Fe2 O6:82 þ 0:39C2 F4 f Sr3 Fe2 O5:44 F1:46 þ 0:69CO2 þ 0:09C
Note that carbon forms as a byproduct and the amount of C2F4 is smaller than the actual amount used (= 0.55C2F4). In fact, we detected the presence of carbon via synchrotron XRD and SEMEDX experiments, although it is not possible to accurately estimate the amount of carbon impurity. An excess of PTFE is required to compensate for its loss through volatilization. It is interesting to compare fluorination of a regular perovskite SrFeO3 with that of a layered perovskite Sr3Fe2O7. As reported by Berry et al.,16 the reaction of SrFeO3 with PVDF (a derivative of PTFE) gives SrFeO2F containing only Fe(III). In contrast, the oxidation state of iron in Sr3Fe2O5.44F1.56 in this study is, on average, +3.22, despite a greater availability of C for reduction of the precursor. This difference is attributable to the temperature dependence of the reducing power of carbon. Indeed, the fluorination of SrFeO3 is achieved at 400 C, which is much higher than the reaction temperature for Sr3Fe2O5.44F1.56. To obtain Sr3Fe2O5F2 with Fe(III), we reacted Sr3Fe2O7δ with PTFE at higher temperatures; however, this resulted in failure, because the precursor decomposed to an amorphous phase. An
alternative route accessible to such a phase is to use highly reduced phases as starting materials. For example, the reaction of Sr3Fe2O5 (δ = 2)32 with F2 gas or XeF2 may yield Sr3Fe2O5F2 through the insertion of F atoms. A direct synthetic method using high-pressure equipment is a promising route. These experiments are now in progress.
4. CONCLUSION We have successfully synthesized a RuddlesdenPopper iron oxyfluoride Sr3Fe2O5.45F1.56, via the reaction of Sr3Fe2O7δ (δ ≈ 0.25) with polytetrafluoroethylene. This new phase is fluorinated more than Sr3Fe2O6F0.87 that was prepared using fluorine gas. Sr3Fe2O5.44F1.56 exhibits the coexistence of Fe(III) and Fe(V) at low temperatures and a G-type antiferromagnetic order with a Neel temperature of TN = 390 K. ’ ASSOCIATED CONTENT
bS
Supporting Information. The results of the initial structure refinement using the SXRD data and the schematic crystal structure determined. The final results of the structure refinements using the SXRD and NPD data. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mails:
[email protected] (Y.T.), n-hayashi@ saci.kyoto-u.ac.jp (N.H.).
’ ACKNOWLEDGMENT We thank M. Takano and Y. Kobayashi for helpful discussions and Y. Shimakawa for allowing the use of the M€ossbauer spectrometer. This work was supported by the World Premier International Research Center (WPI) initiative on Materials Nanoarchitechtonics (MANA), a Grant-in-Aid for “Transformative Research-project on Iron Pnictides (TRIP)” from JSPS and Grants-in-Aid for Research Activity (22850019 and 21540330) from MEXT of Japan. The SXRD experiments were conducted with the approval of JASRI (2010B4505). The NPD experiments were performed under the NIMS-RIKEN-JAEA Cooperative Research Program on Quantum Beam Science and Technology. ’ REFERENCES (1) Al-Mamouri, M.; Edwards, P. P.; Greaves, C.; Slaski, M. Nature 1994, 369, 382. (2) Needs, R. L.; Weller, M. T. J. Solid State Chem. 1998, 139, 422. 3657
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