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Cite This: Inorg. Chem. 2017, 56, 15241−15250

Combining Multiscale Approaches for the Structure Determination of an Iron Layered Oxysulfate: Sr4Fe2.5O7.25(SO4)0.5 Bruno Gonano, Yohann Bréard,* Denis Pelloquin, Vincent Caignaert, Olivier Perez, Alain Pautrat, and Philippe Boullay Laboratoire CRISMAT, UMR 6508 CNRS ENSICAEN, 6 bd du Maréchal Juin, 14050 Caen Cedex 4, France

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Philippe Bazin Laboratoire LCS, UMR 6506 CNRS ENSICAEN, 6 bd du Maréchal Juin, 14050 Caen Cedex 4, France

Jean-Marie Le Breton Groupe de Physique des Matériaux, UMR 6634 CNRS-Université de Rouen, Site universitaire du Madrillet, avenue de l’Université BP 12, 76801 Saint Etienne du Rouvray Cedex, France S Supporting Information *

ABSTRACT: The new iron layered oxysulfate Sr4Fe2.5O7.25(SO4)0.5 has been prepared by a solid-state reaction in closed ampules into the form of ceramics and single crystals. Its atomic structure has been solved by means of spectroscopy, diffraction techniques, and high-resolution electron microscopy. Sr4Fe2.5O7.25(SO4)0.5 is a layered structure that derives from the Ruddelsden−Popper (RP) phases with the layer stacking sequence SrO/SrFeO2.5/SrFe0.5(SO4)0.5O1.25/SrFeO2.5. Within the mixed Fe3+/SO42− layer, the sulfur atoms are slightly shifted from the B site of the perovskite and each sulfate group shares two corners with iron pyramids in the basal plan without any order phenomenon. The electronic conductivity is thermally activated, while no ionic conductivity is detected.

1. INTRODUCTION Iron is a very attractive transition metal due to its capability of exhibiting several oxidation and spin states in different environments which often induce exotic properties. Up to now, iron oxide systems have generated considerable experimental and theoretical studies, since they offer a wide variety of chemical and physical properties such as multiferroicity (BiFeO3),1 magnetoresistivity (SrFeO3),2 thermoelectricity (CuFe1−xNixO2 (0 ≤ x ≤ 0.05)),3 and ionic conductivity (SrFeO3−x),4 to name but a few. In the same way, complex structures involving iron and oxyanions (carbonate, phosphate, sulfate, borate) have resulted in just as many applicative properties such as SOFCs or batteries (LiFePO4,5 Li2Fe(SO4)2,6 Na2Fe(SO4)26). More fundamental issues are also being studied such as the unusual environment (coordination 12) for iron in CsFe(NO3)47 or complex magnetism in LaFe3(BO3)48 (La = Sm,9 Dy,10 Nd,8 Tb8)). In contrast, there are fewer iron compounds involving both oxygen atoms and oxyanions sharing the same anionic framework. Fe2O(BO3),11 Fe3O2(BO3),12 and SrFe3O(PO4)313 show respectively L-type ferrimagnetism, charge ordering, and a ferromagnetic-like second-order transition. Also, it has been © 2017 American Chemical Society

shown in the perovskite ABO3 that it is actually possible to substitute the B cation for an oxyanion groupthe central atom of the group is thus lying on the B site. This has been amply demonstrated in cuprate superconductors ((Sr2CuO2(CO3),14 (La1.85Sr0.15Cu1−x(SO4)xO4)15) or in cobaltites (SrCo1−x(SO4)xO3−y).16 Concerning iron compounds, the substitution SrFe1−x(SiO4)xO3−y17 is known for x < 0.15; such a substitution was shown to introduce oxygen vacancy disorder which stabilizes the cubic structure whatever the temperature conditions, making these compounds good candidates for SOFC cathodes. As far as we know, this is the only example of oxyanion substitution in iron perovskite. However, note that in the Ruddelsden− Popper phase Sr4Fe3O10,18 having a three-octahedra-thick perovskite block (RP3), the whole FeO2 central layer can be replaced by carbonate groups, leading to the compound Sr4Fe2O6(CO3),19,20 which leads to a high substitution rate in this derivative perovskite compound. In this article, we explore this substitution strategy but with a larger and more electronegative oxyanion group: the sulfate SO42−. After many experiments, Received: October 10, 2017 Published: December 7, 2017 15241

DOI: 10.1021/acs.inorgchem.7b02572 Inorg. Chem. 2017, 56, 15241−15250

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Inorganic Chemistry

Figure 2. Infrared spectrum of Sr4Fe2.5O7.25(SO4)0.5 registered at room temperature.

Figure 1. Experimental (crosses) and calculated and difference (green and blue solid lines, respectively) X-ray powder diffraction patterns of Sr4Fe2.5O7.25(SO4)0.5 at the end of the pattern matching refinement. The vertical bars are the Bragg positions for the phase. Top right insert: enlargement of the X-ray pattern showing the lack of splitting of the (110) peak. Top left insert: statistic of iron content realized by means of EDS on single-crystal and polycrystalline samples.

it seems that the limiting composition that can be synthesized is Sr4Fe2.5O7.25(SO4)0.5. The reason lies in its atomic structure, which will be presented as well as its transport properties. In this study, to better define the atomic structure of this new complex intergrowth, we used various characterization techniques (precession electron diffraction tomography (PEDT), single-crystal X-ray diffraction (SCXRD), powder X-ray diffraction (PXRD), and high-resolution imaging) on samples of different sizes using both single crystals and polycrystalline samples. The relevance of the use of PEDT and “dynamic refinements”21 for the study of a complex layered compound presenting static disorder phenomena is also discussed, with a reliance on SCXRD data for validation.

Figure 3. TGA curve of Sr4Fe2.5O7.25(SO4)0.5 registered under Ar/H2 (10%) atmosphere. Insert: infrared spectra of Sr4Fe2.5O7.25(SO4)0.5 registered at various temperatures (from room temperature to 873 K) under Ar/H2 (10%) atmosphere.

2. EXPERIMENTAL SECTION 2.1. Synthesis. Polycrystalline ceramic and crystal samples with the Sr4Fe2.5(SO4)0.5O7.25 composition have been synthesized by a direct solid-state reaction using an adequate mixture of SrO, SrSO4, and Fe2O3. Due to the reactivity of SrO in air, all of these precursors were thoroughly mixed in a glovebox (filled with nitrogen gas). The resulting powder was directly introduced in alumina crucibles (for single-crystal growth) or pressed into the form of bars (for ceramic samples) before being introduced in the alumina crucibles, themselves settled in silica tubes later closed under vacuum. The polycrystalline sample tubes were heated over 6 h to 1100 °C for 24 h and cooling to room temperature was performed over 24 h while the crystal sample tubes were heated to 1200 °C over 24 h followed by a slow cooling over 48 h. The polycrystalline sample results in a homogeneous brown powder, while the crystals appear into the form of very thin orange sheets with a maximum length of 160 μm (see Figure S1 in the Supporting Information). Note that any attempts to introduce more than 0.5 SO4 per formula failed (SrSO4 as secondary phase). 2.2. Infrared and Mö ssbauer Spectroscopy. Infrared spectra were recorded with a Thermo Scientific Nicolet 6700 spectrometer, equipped with a DTGS detector (64 scans per spectrum with a spectral resolution of 4 cm−1), and treated with the help of the OMNIC software. Each IR experiment was performed on a 100 mg KBr pressed disk (diameter 1.6 cm) containing 2% of the sample accurately dispersed.

Figure 4. Mössbauer spectrum of Sr4Fe2.5O7.25(SO4)0.5 registered at room temperature: (black crosses) experimental spectrum; (black solid line) calculated spectrum at the end of the fit; (red, blue, and green solid lines) components 1−3, respectively. The 57Fe Mössbauer spectrum at room temperature was recorded in transmission mode, using a conventional spectrometer operating in the constant acceleration mode from −2.5 to 2.5 mm s−1. 57Co/Rh was used as the γ-ray source. 2.3. Thermogravimetric Analysis. The thermogravimetric analysis was performed on a Netzsch TGA apparatus (STA 449 F3 Jupiter), from 25 to 1200 °C with a heating rate of 3 °C min−1 with a reducing Ar/H2 (10%) atmosphere. 15242


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Figure 5. Single-crystal X-ray diffraction patterns of the compound Sr4Fe2.5O7.25(SO4)0.5 oriented to (a) [010], (b) [110], and (c) [001] and precession electron diffraction tomography patterns of the compound Sr4Fe2.5O7.25(SO4)0.5 oriented according to (d) [010], (e) [110], and (f) [001].

Figure 6. (a) Structure of Sr4Fe2O6. (b) Difference Fourier maps between SCXRD and PEDT observations and calculations considering the structure (a) (c) Difference Fourier maps between SCXRD and PEDT observations and calculations considering the structure Sr4Fe2.5O7.25(SO4)0.5 with S and Fe(2) at the B site, highlighting negative residue (blue) at the B site surrounded by positive residue (yellow). (d) Illustration of ADPs ellipsoids for O(3) and O(4) at the end of SCXRD refinement. (e) Illustration of the iron pyramid and sulfate tetrahedron sharing the same O(4) with the splitting of O(3) atoms. large φ and ω scans of the reciprocal spacea total of 2000 frames were collected. Precession electron diffraction tomography (PEDT) were performed with a JEOL 2010 electron microscope (200 kV, LaB6 cathode) equipped with a nanomegas DigiStar precession module and an side entry mounted Gatan Orius 200D CCD camera. PEDT data sets of nonoriented patterns were recorded at room temperature on several different crystals. The precession angle was set to 1.4° with a goniometer tilt step below 1°. Data were analyzed using the software PETS and JANA2006 following a classic procedure described elsewhere.21,22

2.4. Diffraction Data and High-Resolution Electron Microscopy. The powder X-ray diffraction (PXRD) data were collected over the angular range 5−100°, using a Bruker D8 advance Vario1 diffractometer equipped with a primary germanium (111) monochromator (λ(Kα1) = 1.5406 Å) and a LynxEye detector. Single-crystal X-ray diffraction (SCXRD) measurements were performed using Mo Kα radiation produced with a microfocus Incoatec IμS sealed X-ray tube on a CCD (Bruker-Nonius) four-circle diffractometer equipped with an Apex2 CCD (charge-coupled device) detector. A single crystal with approximate dimensions 0.2 × 0.2 × 0.01 mm was used for X-ray crystallographic analysis. The data collection consists of 15243


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check the phase purity and to determine the cell parameters. All of the diffraction peaks can be well indexed in a unique tetragonal cell (space group I4/mmm) with parameters a = 3.904(1) Å and c = 29.14(1) Å (Figure 1). Careful attention was paid to possible distortions indicating a lowering of the symmetry as was reported for RP1 (intergrowth between oneoctahedron-thick perovskite blocks and “rock salt” type layers) La1.85Sr0.15CuOy(SO4)x.15 No evidence of such phenomena was observed in the metrics of the cell or in a splitting of peaks as for instance (hh0) (insert of Figure 1). The stacking parameter is larger than that expected for RP3 Sr4Fe3O10 (28 Å) or observed in Sr4Fe2CO3O6 (27.98 Å), which may indicate the successful presence of (SO4)2− groups in our compound. This point was confirmed by infrared spectroscopy realized at room temperature in the mid-infrared region. In the spectrum shown as Figure 2, the peaks clearly occurring at 690 (ν4), 990 (ν1), and 1123 (ν3) cm−1 are characteristic of SO42− and therefore confirm its presence within our material. The relationship between the sulfate symmetry and their spectra is well established. In our case, the weakness of the nondegenerate symmetric stretching ν1 band indicates that the sulfate tetrahedra are very distorted and the splitting of the triply degenerate asymmetric stretching ν3 band underlines that the sulfate groups adopt a nonfree configuration but rather a bidentate form.23 The ν4 band is masked by the absorption peak at 577 cm−1 which is due to the elongation and deformation vibrations of the metal−oxygen bonds. The average ratios Sr/Fe deduced from the EDS analysis are the expected ones (1.58 for crystals and 1.59 for polycrystalline ceramic) with a standard deviation of ±0.15 indicating a moderate homogeneity of the materials (insert in Figure 1). The presence of sulfur atoms is also confirmed, but this element is too light to be rigorously quantified. Therefore, thermal gravimetric analysis has been done in the temperature range room temperature to 1500 K under an Ar/H2 (10%) reducing atmosphere (Figure 3). The decomposition results in a two-step loss. The first loss occurs between 600 and 900 K and corresponds to a loss of 5.2% in mass. Infrared spectroscopy has been done with the exact same conditions as for TGA (insert of Figure 3), and one can see that the ν3 band characteristic of (SO4)2− decreases in this temperature range (insert in Figure 3) while at the same time the M−O band remains unchanged (see Figure S2 in the Supporting Information), revealing that the departure concerns the sulfate groups. The second loss between 900 and 1500 K (9.0%) corresponds to the departure of oxygen atoms. The resulting powder has been analyzed by X-ray diffraction: the identification of the different phases (Fe, SrS, SrO, and Sr(OH)2(H2O) due to exposure of SrO to air during the X-ray acquisition) indicates that the first loss is due to departure of oxygen atoms of the (SO4)2− groups (and not SO2), which leads to a content of 0.54 (SO4)2− per formula unit: i.e., the nominal composition. The quantification of the SrO and Sr(OH)2(H2O) phases has been not reliable enough to allow the calculation of the oxygen stoichiometry (see Figure S3 in the Supporting Information). The oxygen stoichiometry was indirectly determined by means of Mossbauer spectroscopy. The spectrum presented Figure 4 was recorded at room temperature and is composed of three sextets, indicating that the compound is magnetically ordered at that temperature. The three components (1 in red, 2 in blue, 3 in green) showing respectively the proportions 80%, 10%, and 10% have isomeric shifts (δ) typically characteristic of trivalent iron (δ1 = 0.31 mm s−1; δ2 = 0.19 mm s−1; δ3 = 0.16 mm s−1).

Table 1. Experimental Details for SCXRD and PEDT Data Collections and Analysis for Sr4Fe2.5O7.25(SO4)0.5a SCXRD diffractometer temp λ(Mo Kα) distance (crystal−detector) recording mode time and angular scan angular domain (min− max) limiting indices

no. of measd rflns no. of indep rflns (I > 3σ) no. of indep obsd rflns (I > 3σ) Rint (before abs cor/after abs cor) abs cor cor of the secondary extinction

Bruker-Nonius Kappa Apex2Mo source Microfocus Incoatec 298 K 0.71073 Å 35 mm scanning ω 730 s/image, 0.5°/image 5.288 < 2θ < 37.76° −5 ≤ h ≤ 6 −6 ≤ k ≤ 3 −16 ≤ l ≤ 47 1748 392 365 5.63/5.58 Sadabs: empirical method based on redundant reflections B-C type I, Gaussian, isotropic (g = 0.004796) PEDT

microscope temp λ acceleration voltage time and angular scan precession angle (φ) no. of recorded frames angular domain (min−max) limiting indices

completeness (0.625 Å) no. of measd rflns no. of indep rflns (I > 3σ) no. of indep obsd rflns (I > 3σ) max gmax (Å−1); Smax g (matrix); Sg (refine); RSG; Nor no. of refined params a

JEOL 2010 298 K 0.02510 Å 200 kV 0.4 s/image, 1.08°/image 1.4° 99 −50 < 2θ < 50° −5 ≤ h ≤ 5 −5 ≤ k ≤ 5 −43 ≤ l ≤ 43 100% 7987 2318 1588 1.8; 0.01; 0.1; 0.3; 128 119

See ref 22 for parameters related to PEDT dynamic refinements.

High-resolution imagery was performed on a corrected probe JEOL ARM 200F microscope operated at 200 kV and equipped with HAADF (high angle annular dark field). Image simulations have been calculated with JEMS software. Electron dispersion spectroscopy (EDS) was carried out on a JEOL 2010 FEG microscope on 50 crystals (single crystals) and microcrystals (ceramic sample). 2.5. Ionic Conductivity and Impedance. Platinum paste was applied to both sides of the pellets, and electrochemical impedance spectroscopy (FRD 1025 Princeton Applied Research) measurements were carried out in the 300−425 K temperature range, with 100 mV ac test signal amplitude in the frequency range of 0.1 Hz to 1 MHz under an ambient atmosphere. The impedance contributions ascribed to grain boundaries and grain interiors (bulk) were obtained after deconvolution of impedance spectra by fitting to conventional equivalent circuits using ZSimpWin software (version 3.1).

3. RESULTS AND DISCUSSION 3.1. Physicochemical Characterizations. The polycrystalline compound was characterized by X-ray diffraction to 15244


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Inorganic Chemistry Table 2. Summary of the Crystallographic Parameters Obtained by XRPD, SCXRD, and PEDTa PXRD, SCXRD, PEDT atom

site

x

y

z

n

Sr(1) Sr(2) Fe(1) Fe(2) O(1) O(2) O(3) O(3′) O(4) S(1)

4e 4e 4e 2a 4e 8g 4c 4e 4c 8h

0.0 0.0 0.0 0.0 0.0 0.0 0.5 0.0 0.5 0.09710(4), 0.092(1), 0.13203(5)

0.0 0.0 0.0 0.0 0.0 0.5 0.0 0.0 0.0 0.09710(4), 0.092(1), 0.13203(5) SCXRD, PEDT

0.57855(8), 0.58001(4), 0.578147(7) 0.70293(8), 0.70292(3), 0.702729(6) 0.1467(2), 0.1466(6), 0.14555(8) 0.0 0.21162(4), 0.2127(3), 0.2123(2) 0.13523(4), 0.1367(2), 0.1364(2) 0.0626(1), 0.06795(2), 0.0691(1) 0.04912(2), 0.053468(2), 0.05074(1), 0.0 0.0

1 1 1 0.5 1 1 0.5 0.5 0.625 0.125

atom Sr(1) Sr(2) Fe(1) Fe(2) O(1) O(2) O(3) O(3′) S(1)

Sr(1)−O(2) Sr(1)−O(3) Sr(1)−O(3′) Sr(2)−O(1) S(1)−O(3′) S(1)−O(4)

U11 (Å2) 0.02033(4), 0.00896(3), 0.00671(4), 0.0936(2) 0.02044(2), 0.01523(2),

0.025167(9) 0.01185(8) 0.01148(9) 0.01459(3) 0.00253(2)

0.03503(6)

U22 (Å2)

U33 (Å2)

Ueq (Å2)

=U11, =U11 =U11, =U11 =U11, =U11 =U11, =U11 =U11, =U11 0.0056(2), 0.01129(7)

0.01473(5), 0.0128(1) 0.0106(5), 0.00935(9) 0.0141(6), 0.01647(13) 0.0295(9) 0.0101(3), 0.017233(3) 0.03576(3), 0.03617(3)

0.0183(2), 0.0210(6) 0.0095(2), 0.0110(5) 0.0091(3), 0.0131(6) 0.072(9), 0.0363(3) 0.0170(14), 0.0155(16) 0.0189(14), 0.0167(14) 0.081(12), 0.082(7) 0.081(12), 0.082(7) 0.028(5), 0.0363(3)

=U11 0.0154(1) distance d (Å) SCXRD, PEDT d

X

n

2.555(4), 2.603(3) 2.775(9), 2.794(3) 2.86(2), 2.894(8) 2.767(9), 2.795(6) 1.641(7), 1.656(2) 1.627(8), 1.537(4)

X X X X

4 4 4 4

Fe(1)−O(1) Fe(1)−O(2) Fe(1)−O(3) Fe(2)−O(3)

d

X

n

1.930(9), 1.955(6) 1.978(1), 1.984(7) 2.77(3), 2.72(7) 1.984(3), 2.019(1)

X X X X

1 4 1 2

angle (deg) SCXRD, PEDT O(3′)−S−O(3′) O(3′)−S−O(4)

angle

X

n

143.9(6), 127.3(4) 99.9(4) 100.9(6)

X X

1 4

O(4)−S−O(4)

angle

X

n

115.4(6), 129.5(7)

X

1

a ADPs, distances and angles are the one obtained at the end of SCXRD and PEDT dynamic refinements. XRPD = X-ray powder diffraction: a = 3.8911(5) Å, c = 29.0706(5) Å, RBragg = 5.5%, Rwp = 5.115%, and χ2 = 3.25. SCXRD = single-crystal X-ray diffraction: a = 3.9041(8) Å, c = 29.1372(7) Å, R(obs) = 3.96, Rw(obs) = 7.26, and GOF = 1.32. PEDT = precession electron diffraction tomography: a = 3.925(1) Å, c = 29.2568(7) Å, Robs = 13.61, Rw(obs) = 15.61, and GOF = 7.01.

since it was impossible to localize correctly oxygen and sulfur atoms within this mixed Fe/(SO4) layer. To gain further information on the structure of this mixed layer, refinements based on SCXRD and PEDT data were performed. PEDT data were acquired in a TEM on submicrometer single crystals, while the crystal for SCXRD had a micrometer size. This allowed us to perform a different scale approach for the structural determination. This was found to be particularly relevant to estimate whether the static disorder observed in our compound was size-dependent or not on the basis of accurate single crystal data analysis. PEDT data refined using dynamic diffraction theory has indeed recently been proved to be a valuable tool to locate light atoms.24 First, for both techniques and materials, the systems of Bragg reflections are consistent and confirmed the I4/mmm space group (Figure 5) with a and c parameters close to 3.9 and 29.2 Å, respectively. No additional reflections or diffusion phenomena were found on the diffraction patterns. Thus, structure refinements were performed with

This was suspected due to the brown color of the samples. Note that components 1 and 2 highlight quadrupole splitting typical of a lightly distorted environment (−0.34 and −0.11 mm s−1) while component 3 (0.79 mm s−1) indicates a high distortion of iron polyhedra. The chemical composition deduced from all these analyses is Sr4Fe2.5(SO4)0.5O7.25. 3.2. Structure Determination. Rietveld refinements from powder X-ray diffraction data have been performed at room temperature, considering first a regular RP3 model. It appears that in the perovskite block there are clearly two sets of Sr−Sr distances parallel to the c axis (Figure 6a). In the outer layers bordering the rock salt layers, the Sr(2)−Sr(1) distance is 3.6 Å, which could correspond to an oxygen-deficient perovskite distance. In the central layer, the Sr(1)−Sr(1) distance (4.6 Å) is definitely too large to be a simple perovskite distance. The starting structural model was thus considered by replacing 50% of the central FeO6 octahedral layer of the perovskite block by sulfate groups. This leads to moderate agreement factor values 15245


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Figure 7. (a) Example of a possible atomic structure (without tilting) of Sr4Fe2.5O7.25(SO4)0.5 oriented along the c axis. (b) Projection on (001) without considering any tilting between polyhedra. (c) Projection on (001) showing some possible tiltings of sulfates and iron polyhedra.

the tetragonal I4/mmm space group. It has to be noted that the space group is not dependent on the single-crystal size. To locate atoms positions within the central layer, Fourier difference maps were calculated. To do so, the required Fc was obtained by considering only the Sr(1) and Fe(1) atoms and their surrounding oxygen atoms (O(1) and O(2)) which are not involved in the examined mixed layer (Figure 6b,c). With both SCXRD and PEDT, Fo − Fc then allows the clear location of only the two atomic positions (0,0,0) and (0,0,0.06) (Figure 6b). No evidence of residue corresponding to the equatorial oxygen atoms was detected, which may indicate a high dispersion of these atoms around an average position. The large density observed in (0,0,0) was attributed to iron Fe(2) and sulfur (S) atoms while the smaller density (0,0,0.06) was ascribed to oxygen atom O(3). The refinements were then pursued by adding these atoms with full occupancy (the (0,0,0) position is considered to be filled by 50% Fe and 50% S) and isotropic atomic displacement parameters (ADPs) (Fe(2)/S were restricted to be identical with that of Fe(1)). Once convergence has been achieved, a second set of Fourier difference maps was calculated. As illustrated Figure 6c, negative residue was observed at the (0,0,0) position site while being surrounded by a doughnut of positive residue visible by both SCXRD and PEDT. To lighten the (0,0,0) site in electrons to the benefit of the doughnut, Fe(2) and S atoms have been separated by moving S in a further refined (0.09,0.09,0)

position (Table 2). This ties in with the usual S−O distances and improves the goodness of the refinement. Once again, no residue corresponding to the equatorial oxygen atoms can be detected. For the final step, positions, occupancies, and anisotropic ADPs values of every atom have been refined, except in PEDT, for which ADPs of Fe(2) and S atoms have been refined with the same isotropic value. For both techniques, the large anisotropic ADPs detected for oxygen O(3) in the form of a prolate ellipsoid along z axis suggest a possible splitting of the atomic position along the stacking axis and we assume that the two positions O(3) (0,0,0.067) and O(3′) (0,0,0.053) should be considered according to the polyhedron (iron or sulfate one) encounteredsuch splitting of the O(3) atom with isotropic ADPs leads also to the best results and corresponds to the usual observed distances Fe−O (1.9 Å) and S−O (1.4 Å). Taking into account the refined occupancy of O(1), O(2), O(3), and O(3′), the oxygen stoichiometry deduced from Mossbauer spectroscopy allows us to estimate the occupation of the nonrefined O(4) site (n = 0.625). Finally, the same structural model has been used for PXRD, SCXRD, and PEDT. The resulting structural parameters are given in Tables 1 and 2. The multiscale approach used to characterize the material allows showing that the disorder between the sulfate and iron polyhedron within the mixed layer is totally independent of the crystal size and is present from the nanometer to the 15246


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Figure 8. Experimental [001] HREM image of Sr4Fe2.5O7.25(SO4)0.5 collected with a defocus value of −50 nm and a thickness from 4 to 6 nm. Bottom left insert: corresponding [001] ED pattern. Top right insert: enlargement of the image together with the corresponding atomic structure projection.

micrometer scale. Moreover, it is interesting to note that the single-crystal SCXRD and PEDT approaches both converge rigorously toward the same structural solution, including an equivalent description of the disorder. For PEDT, it is important to use the dynamic scattering theory for structure refinement in order to obtain significant Fourier difference maps. As already highlighted in the case of light elements such as hydrogen,24 Fourier difference maps based on kinematic refinements often lead to spurious residues and, for the present compound, did not allow us to evidence the static disorder for the mixed sulfate and iron atomic positions. 3.3. Structure Description. The structural model is presented Figure 7. Sr4Fe2.5O7.25(SO4)0.5 can be depicted as a derivative compound from the RP3 phase where 50% of the octahedra of the central layer of the perovkite block is substituted for sulfate groups. The sequence SrO/SrFeO2.5/ Sr(Fe0.5(SO4)0.5)O1.25/SrFeO2.5 is then observed. The environment of Fe(1) atoms can be described as a pyramid; the sixth oxygen neighbor O(3) sits farther apart (Fe(1)−O(3) = 2.371(9) Å)the off centering of the iron from the center of the octahedron being direct evidence. Fe(1) is correlated to the major component 1 (80%) of the Mössbauer spectra. Within the mixed layer, no long-range order has been detected between Fe(2) pyramids and the (SO4)2− tetrahedra distribution. Tilting between these polyhedra indeed has to be considered, which makes the localizations of equatorial O(4) atoms impossible due to their high dispersion. In agreement with infrared spectroscopy, most of the sulfate groups share basal corners with Fe(2) pyramids (Figure 7). Components 2 and 3 of the Mössbauer spectra are correlated to Fe(2). Component 2 is similar to that of Fe(1) pyramids, and component 3 has a more distorted environment. This may indicate different degrees of distortion of Fe(2) pyramids depending on whether they are connected to SO4 tetrahedra or Fe(2) pyramids. The (SO4)2− groups have the peculiarity of possessing at least two “free”

Figure 9. (a) Experimental [100] HAADF of Sr4Fe2.5O7.25(SO4)0.5 and corresponding ED pattern (left bottom insert). The white dashed arrow indicates a sulfate row. The blue double arrow underlines the Sr(2)−Sr(1) distance (3.8 Å), while the red double arrow indicates the Sr(1)−Sr(1) distance (4.6 Å). In the white box a calculated image is given to confirm the goodness of our structural model. Top right insert: enlargement of the experimental image to show the offcentering of the Fe(2)/S atoms. (b) Experimental [100] HAADF of Sr4Fe2.5O7.25(SO4)0.5 with defects along the stacking sequence pointed out by two black arrows. The corresponding ED pattern is given in the bottom right insert. At the bottom of the picture are given line profiles corresponding to the Sr−Sr distances in the red and black boxes showing the inhomogeneity if Sr(1)−Sr(1) distances are correlated to the ratio Fe(2)/S. 15247


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Inorganic Chemistry

dots appear brighter (heavier atom) in comparison to those above the black arrow. The contrast is then typical of a RP3 phase, which means that an increase in brightness is correlated with an increase in the ratio Fe(2)/(SO4)2−. This fact is supported by the corresponding profiles shown at the bottom of Figure 9b. The Sr(1)−Sr(1) distances (4.1 Å) parallel to the c axis extracted from the red area are shorter than those extracted from the black area (4.6 Å) and move toward classical distances obtained for a perovskite. This is related to some inhomogeneity in the Fe(2)/(SO4)2− distribution within the mixed layer. 3.4. Electron and Ionic Conductivity. Unfortunately the single crystals are too small and brittle to deposit electrodes on the sample. Consequently electronic transport has been measured on ceramic samples. These are characterized by about 30% porosity (determined by geometric method) (see SEM photos of the bar in Figure S4 in the Supporting Information). This implies that the extrinsic value of resistivity will be overestimated. The temperature dependence of the resistivity is depicted in Figure 10 and corresponds to a “bad” semiconductor behavior (ρ = 106 Ω cm at room temperature). The linear variation of log R vs 1/T (insert in Figure 10) indicates a thermally activated process. The extracted activation energy (0.17 eV) remains constant over the whole temperature range. Figure 11 displays a sequence of characteristic Nyquist plots recorded with constant potential at three different temperatures. The absolute impedance decreases significantly with increasing temperature. Each plot consists of two semicircles, which are attributed to inter- and intragranular fully electronic contributions (see for instance the plotted fit in Figure 11): i.e., no ionic transport has been detected so far.

Figure 10. Thermal dependence of the resistivity of Sr4Fe2.5O7.25(SO4)0.5. Insert: log σ vs 1/T fitted with the Arrhenius law with Ea = 0.17 eV.

oxygen corners (the apical O(3′)), as was reported for Na3Fe(SO4)325 for instance. The [001] oriented HREM image presented in Figure 8 gives direct evidence of the random distribution of the Fe(2) pyramids and sulfate tetrahedra in the basal plan. In this photo recorded for a defocus of −50 nm, the bright dots are correlated to high electron density zones, i.e. the rows (along the z axis) of cations (Sr and Fe/S), as revealed by bright dots. No long-range ordering phenomenon has been observed, and the distances measured between rows of cations are in great agreement with those calculated. The stacking mode is further illustrated by the [100] oriented STEM-HAADF image presented in Figure 9a. The simulated image obtained with our structural model fits with the experimental image. The rows of gray dots (one is indicated by a dashed white arrow) correspond to the mixed Fe(2)/(SO4)2− layers that alternate with perovskite blocks separated by a rock salt layer, with high regularity. Note that the sets of distances obtained from the images are in remarkably good agreement with those obtained by Rietveld refinements. The off-centering of SO4 from the B site of the perovskite is also highlighted in this image (top right insert). However, some defects in the stacking mode can be detected, as for instance in Figure 9b. Two small black arrows point out a defect for which the contrast could be interpreted as a double Fe/SO42− layer. Moreover, some variations in the dot intensity could be observed in the mixed layers: above the red arrow the

4. CONCLUSION In contrast to carbonate anions which adopts a planar configuration, the substitution of iron octahedra for 3D sulfate groups in Sr4Fe3−x(SO4)xO10−z is limited to x = 0.5. This is due to steric hindrance, to avoid O−O distances between sulfate neighbors that are too short. In RP3 compounds oxygen vacancies occur in the basal plane of the central layer of the perovskite block (LaSr3Fe3O926), which suggests that this layer offers interatomic bonds with lower binding energy in comparison to the others. It is of interest to note that the substitution occurs also within the central layer of the perovskite block and not in the other layers. This may be also the consequence of a better balance of Coulomb repulsion and attraction forces for this framework.

Figure 11. Experimental Nyquist diagrams of impedance spectra of Sr4Fe2.5O7.25(SO4)0.5 recorded at three temperatures. Insert: experimental and calculated Nyquist diagrams of impedance spectra at 425 K. 15248


Article

Inorganic Chemistry

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The lack of equivalent vacant sites is responsible for the absence of ionic conductivity, and as expected for the trivalent iron 2D compounds, the value of electric resistivity is very high. We have shown that it is actually possible to synthesize (using a close ampule method) intergrowth between iron perovskite and Sr(SO4)Fe layers. This opens up a route for new promising layered compounds based on SrSO4 layers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02572. SEM and regular images of single crystals, infrared spectra of Sr4Fe2.5O7.25(SO4)0.5 registered at various temperatures (from room temperature to 873 K) under an Ar/H2 (10%) atmosphere showing the effect on S−O bonds while the metal bonds remain unchanged, X-ray diagram of the resulting powder after the TGA measurement showing the presence of metallic iron, SrS, and SrO which react to form Sr(OH)2(H2O), SEM image of the surface of the ceramic bar of Sr4Fe2.5O7.25(SO4)0.5 showing the porosity (PDF) Accession Codes

CCDC 1579681 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for Y.B.: [email protected]. ORCID

Bruno Gonano: 0000-0002-7331-2131 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are extremely grateful to Dr. S. Gascoin for collection of X-ray diffraction data, X. Larose for technical support on TEM analyses, F. Veillon for the transport measurements, and C. Le Guillouzer for the infrared spectroscopy data collection.

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REFERENCES

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