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Effect of Co Substitution on the Crystal and Magnetic Structure of SrFeO2.75−δ: Stabilization of the “314-Type” Oxygen Vacancy Ordered Structure without A‑Site Ordering Sourav Marik,* Madhu Chennabasappa,† Javier Fernández-Sanjulián, Emmanuel Petit, and Olivier Toulemonde CNRS, Université de Bordeaux, ICMCB, 87 avenue du Dr. A. Schweitzer, Pessac F-33608, France. S Supporting Information *

ABSTRACT: A study of the structure−composition−properties correlation is reported for the oxygen-deficient SrFe1−xCoxO2.75−δ (x = 0.1−0.85) materials. The introduction of Co in the parent SrFeO2.75 (Sr4Fe4O11) structure revealed an interesting structural transformation. At room temperature (RT), an orthorhombic (space group Cmmm, 2√2ap × 2ap × √2ap type, ap = lattice parameter of the cubic perovskite) → tetragonal (space group P4/mmm, ap × ap × 2ap type) → tetragonal (space group I4/mmm, 2ap × 2ap × 4ap type) structural transformation is observed in parallel with increasing Co content and decreasing oxygen content in the structure. At the same time, a rich variation in the magnetic properties is explored. The samples with x = 0.25, 0.3 show temperature-induced magnetization reversal. With increasing Co content in the structure, magnetic interactions start to weaken due to the random distribution of Fe and Co in the structure; the x = 0.5 sample shows frustration in the magnetic behavior with much smaller magnetization value. With a further increase in the Co content in the structure, RT ferrimagnetic-type behavior is observed for the sample with x = 0.85. The nuclear and magnetic structure refinements using RT and low-temperature neutron powder diffraction (NPD, 10 K) patterns confirm the formation of a “314type” novel oxygen vacancy ordered phase for the sample with x = 0.85, which is the first case of “314-type” novel oxygen vacancy ordering without A-site (ABO3−δ type perovskite) ordering. The magnetic structure is G-type antiferromagnetic starting at room temperature. Further, the stabilization of the “314-type” complex superstructure is related to the ordering of oxygen vacancies in the oxygen-deficient Co−O layers, and the same assists in building a network of Co ions with different coordination environments, each with different spin states, and forms the spin-state ordering. high Fe4+ oxidation state. The magnetic ordering is of the helical-antiferromagnetic (AFM) type. The oxygen-deficient SrFeO2.518,19 exhibits Fe3+ cations on both tetrahedral and octahedral coordination and is known to adopt an orthorhombic brownmillerite type structure (√2ap × 4ap × √2ap supercell, ap = lattice parameter of the cubic perovskite subcell), in which vacancies order to form alternating layers of octahedra and tetrahedra. Between these two phases, two other oxygenvacancy ordered phases, tetragonal SrFeO2.87 (Sr8Fe8O23)2 and orthorhombic SrFeO2.75 (Sr4Fe4O11),2,3 are known to exist. The interesting oxygen-vacancy ordered intermediate member SrFeO2.75 (Sr4Fe4O11) phase contains an equal number of tetravalent and trivalent irons (Fe) and shows a variety of interesting physical properties, which includes unusual magnetic behavior, exchange-bias like properties, and anomalous thermoelectric power.5,6 From a structural point of view, this compound shows an orthorhombic 2√2ap × 2ap × √2ap

1. INTRODUCTION The ability of perovskite-related oxides to form stable crystal structures with a rich structural variety, especially for a range of oxygen stoichiometry, is one of the main reasons for the plentiful chemical and physical properties observed in these materials. Among them, the celebrated high-temperature superconducting cuprates (for instance YBa2Cu3Oy1) give a few examples of properties depending on the variation of oxygen content. In this article, we focus on the oxygen-deficient strontium-based ferrites−cobaltates. Oxygen-vacancy (OV) ordered perovskite-related oxygendeficient ferrites are the aim of great number of studies in recent years in connection with their rich structural variety,2−9 and cobaltates10−15 show a variety of interesting chemical and physical properties relevant for potential applications such as in oxygen storage devices, thermoelectric properties, solid oxide fuel cells, etc. The variation of structures depends on different factors, including the degree of anion deficiency and the composition, and the ordering of oxygen vacancies yields different superstructures. The oxygen-replete end member SrFeO316,17 is a cubic perovskite and possesses an unusually © 2016 American Chemical Society

Received: July 5, 2016 Published: September 13, 2016 9778

DOI: 10.1021/acs.inorgchem.6b01554 Inorg. Chem. 2016, 55, 9778−9789

Article

Inorganic Chemistry

Figure 1. Room-temperature X-ray powder diffraction (RT XRD) patterns for SrFe1−xCoxO2.75−δ (x = 0.1−0.85) samples. From x = 0.1 to x = 0.3, the XRD patterns are consistent with the parent SrFeO2.75 (orthorhombic, SG Cmmm; the + sign indicates the SrCO3 secondary phase) phase. With increasing Co content in the structure, from x = 0.4 the characteristic peaks for the parent orthorhombic structure (111 peak at 2θ = 21.7°, enlarged part i) and the splitting (at 2θ ≈ 47, 79°) disappear and display extra reflections (enlarged parts ii and iii) with respect to the oxygen-vacancy disordered cubic cell (SG Pm3̅m). Cubic perovskite peaks are indicated as P*, and extra peaks are shown by red triangles (also in enlarged parts ii and iii).

oxidation states in the structure, which can trigger complex magnetic properties, provide a fascinating opportunity to explore the magnetic properties of the SrFe1−xCoxO2.75−δ system for a wide range of Co doping. In this paper, we present and discuss a systematic and detailed composition−structure−properties relation for the SrFe1−xCoxO2.75−δ (x = 0.1−0.85) samples. A detailed structural and magnetic phase transformation with variation of Co and oxygen content is shown here.

type structure and can be visualized as an oxygen-vacancy ordered perovskite like superstructure. The SrCoO3−δ systems mimic structurally similar phases; for instance, SrCoO2.520,21 exists in the brownmillerite structure and SrCoO2.8021 with a tetragonal unit cell with a = b = 2√2ap and c = 2ap. In addition, the recently discovered perovskiterelated cobaltate, the so-called “314 phase” with the idealized chemical composition Sr3YCo4O10.5 (Sr0.75Y0.25CoO2.625)14,22,23 is known to form in a tetragonal structure (space group (SG) I4/mmm) having unit cells a = b = 2ap and c = 4ap with a Gtype long-range antiferromagnetic (AFM) ordering below 340 K. The “314” and related phases are reported to be layered structures consisting of alternating oxygen-replete and oxygendeficient layers in close similarity to the brownmillerite structure. The difference between the brownmillerite and the “314” phases is the existence of tetragonal pyramids in the oxygen-deficient layer due to the oxygen intake for the latter phase instead of the tetrahedra observed in brownmillerite phases. The evolution of the “314-type” structure and the novel OV ordered superstructure is suggested to be related to the Asite (related to ABO3−δ perovskite) ordering.24,25 James et al.26 highlighted a tetragonal P4/mmm symmetry with a = b = ap and c = 2ap type superstructure in the absence of A-site ordering. The latter superstructure was also suggested for the La1−xSrxCoO3−δ materials by electron diffraction techniques.27 The aim of the present work is 2-fold: (i) a systematic and detailed investigation of the effect of Co substitution on the structure (nuclear and magnetic) and magnetic properties of SrFeO2.75 (Sr4Fe4O11) phases and (ii) a detailed investigation to elucidate the driving force for stabilization in the “314-type” oxygen-vacancy ordered phase. With this in mind, a series of samples with chemical composition SrFe1−xCoxO2.75−δ have been analyzed by means of X-ray diffraction (XRD) and neutron powder diffraction (NPD) techniques. At the same time, the availability of different kinds of magnetic cations (Fe3+, Fe4+, Co3+, and Co4+), their different possible spin states and the possibility of heterogeneous distribution of Co and Fe

2. EXPERIMENTAL SECTION Polycrystalline samples of nominal compositions SrFe1−xCoxO2.75−δ (x = 0.1−0.85) were prepared by standard solid-state reaction methods, starting from stoichiometric amounts of SrCO3 (99.9%), Fe2O3 (99.99%), and Co3O4 (99.6%). The starting materials were repeatedly ground and heat-treated several times before final sintering of the pellet at 1473 K for 40 h; this step was repeated twice. All the samples were initially characterized by X-ray powder diffraction (XRD) at room temperature (RT), performed in a Philips X’Celerator diffractometer (Cu Kα radiation, λ = 1.5406 Å). Further neutron powder diffraction (NPD) data allowed us to determine more precisely the oxygen position, oxygen occupancies, and magnetic structures of the materials. Long scans of NPD at room temperature (RT) and at low temperatures (LT at 10, 50, and 225 K) were recorded for the samples SrFe1−xCoxO2.75−δ with x = 0.25, 0.5, 0.75, 0.85 at the Paul Scherrer Institut, Villigen, Switzerland. A wavelength of λ = 1.494 Å was used for the RT-NPD data collection, and a wavelength of λ = 1.8857 Å was used for the LT-NPD data collection. The high-resolution diffraction patterns were refined (nuclear and magnetic) with the Rietveld28 procedure. The oxygen contents for all the samples were also determined by Mohr salt titration. Mohr salt titration was carried out by titrating a solution containing a mixture of sample in diluted acid and Mohr salt against potassium dichromate. Barium diphenylaminesulfonate along with a few drops of phosphoric acid was used as an indicator. Temperature-dependent direct current (dc) magnetic measurements were performed over the temperature range 5−300 K, using a Squid Quantum Design XL-MPMS magnetometer. dc magnetic measurements were performed under zero field cooling (ZFC) and field cooling (FC) conditions. 9779


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Figure 2. (a) Final observed, calculated and difference profile (orthorhombic, Cmmm, 2√2ap × 2ap × √2ap type unit cell) of the RT NPD pattern for the SrFe0.75Co0.25O2.75 ± 0.02 sample (x = 0.25). 2θ = 37−37.7° is excluded in the refinement due to the appearance of an unindexed peak in the RT NPD pattern. (b) Crystal structure of the same material. The oxygen atoms are shown as red spheres.

3. RESULTS 3.1. Crystal Structures at Room Temperature. Figure 1 shows the RT-XRD patterns for the SrFe1−xCoxO2.75−δ samples with x = 0.1−0.85. This unambiguously shows a change in the XRD patterns with Co substitution. From x = 0.1 to x = 0.3, the XRD patterns are consistent with the parent SrFeO2.75 (orthorhombic, SG Cmmm) phase. However, with increasing Co content in the structure, from x = 0.4 the characteristic peaks for the parent orthorhombic structure (111 peak at 2θ = 21.7°, enlarged part i in Figure 1) and the splitting (at 2θ ≈ 47, 79°) disappear and display extra reflections (enlarged parts ii and iii in Figure 1) with respect to the oxygen-vacancy disordered cubic cell (SG Pm3̅m). The RT-NPD patterns (Figure S1 in the Supporting Information) show a similar trend as well. The detailed structural analyses and their evolution with Co substitution are as follows. 3.1.1. Room-Temperature Crystal Structure for SrFe1−xCoxO2.75−δ Samples with x = 0.1−0.3. Preliminary Rietveld refinement of XRD patterns indicates that all of these compounds (x = 0.1, 0.15, 0.25, 0.3) crystallize in orthorhombic symmetry, SG Cmmm, with a unit cell of ∼2√2ap × 2ap × √2ap type, isostructural with SrFeO2.75 (Sr4Fe4O11). However, a small peak of the secondary phase SrCO3 (main peak at 2θ = 25.25°, indicated by the + sign in Figure 1) is observed for x = 0.1, 0.25 samples. We will use the data of the RT-NPD Rietveld refinement for the x = 0.25 sample to describe the average crystal structure of these four materials (x = 0.1, 0.15, 0.25, 0.3). Figure 2a shows the final plot of the Rietveld refinement using the RT-NPD pattern for the x = 0.25 sample. The obtained lattice parameters, agreement factors, and selected bond lengths and angles resulting from the refinement of the RT NPD data are given in Tables 1 and 2. The oxygenoccupancy refinements for the three oxygen positions (O1, O2, and O3, see Table 1) show no deviation from the full occupancies and are therefore consistent with a total oxygen content of 2.75. The site occupancy refinement indicates that the chemical substitution of the Co ions for the Fe ions occur in both Fe sites. Similarly to the isostructural SrFeO2.75 (Sr4Fe4O11),2−5,29 this compound exhibits a vacancy ordered structure having chains of octahedral Fe2/Co2, which are separated by dimeric units of square-pyramidal Fe1/Co1. Each Fe1/Co1 pyramid shares four base corners with four Fe2/Co2

Table 1. Refined Structural Parameters and Agreement Factors (from RT-NPD Refinement) for the SrFe1−xCoxO2.75−δ Material with x = 0.25 a (Å) b (Å) c (Å) cell volume V (Å3) Sr1 (0.5, 0, 0.5) Biso Sr1 (0, 0, 0.5) Biso Sr1 (x, 0, 0) x Biso Fe1/Co1 (0.5, y, 0) y occ Biso Fe2/Co2 (0.25, 0.25, 0.5) occ Biso O1 (0.5, 0, 0) Biso O2 (x, 0, 0.5) x Biso O3 (x, y, z) x y z Biso Rp Rwp RBragg χ2

10.9464(3) 7.6962(3) 5.4691(2) 460.75(3) 0.96(2) 1.07(3) 0.256(1) 0.48(2) 0.2464(5) 0.76(2)/0.24(2) 0.37(3) 0.72(4)/0.28(4) 0.54(2) 0.40 (4) 0.2622(5) 0.64(4) 0.376(1) 0.267(1) 0.246(1) 1.4(1) 5.7 7.45 4.63 5.9

octahedra in such a way that there are no base corners sharing between the Fe1/Co1 pyramids. Therefore, the dimeric units of the Fe1/Co1 are isolated from each other. 3.1.2. Room-Temperature Crystal Structure for SrFe1−xCoxO2.75−δ Samples with x = 0.4−0.6. In view of the extra reflections with respect to the oxygen-vacancy disordered cubic cell (SG Pm3̅m), different superstructure models for 9780


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Inorganic Chemistry Table 2. Selected Bond Lengths (Å) and Angles (deg) (from RT-NPD Refinement) for the SrFe1−xCoxO2.75−δ material with x = 0.25. (Fe1/Co1)−O1 (Fe1/Co1)−O3 (Fe2/Co2)−O2 (Fe2/Co2)−O3 O1−(Fe1/Co1)−O3 (Fe1/Co1)−O1−(Fe1/Co1) (Fe2/Co2)−O2−(Fe2/Co2)

Table 3. Refined Structural Parameters and Agreement Factors (using RT-NPD) for the SrFe1−xCoxO2.75−δ Material with x = 0.5a

1.8963(7) 1.9212(6) × 4 1.9286(7) × 2 1.9588(6) × 4 95.06(2) 180.00 172.05(1)

atom

x

y

z

Biso (Å2)

occ

Sr1 Co1/Fe1 Co2/Fe2 O1 O2 O3

0.5 0 0 0 0 0.5

0.5 0 0 0.054(3) 0.5 0.048(3)

0.2409(3) 0 0.5 0.2605(6) 0 0.5

0.82(2) 0.17(8) 0.45(8) 1.4(1) 0.83(5) 1.4(2)

1.00 0.48(2)/0.51(2) 0.49(2)/0.51(2) 1.00 0.96(2) 0.72(2)

a = b = 3.8679 (1) Å, c = 7.7417 (2) Å, χ2 = 6.4, Rp = 4.58, Rwp = 6.16, Rf = 3.7.

a

oxides were tested. The XRD patterns of these samples (x = 0.4−0.6) can be indexed with the P4/mmm tetragonal symmetry with the ap × ap × 2ap unit cell. The NPD pattern collected for the sample with x = 0.5 at RT shows superstructure peaks, which can be successfully indexed with a tetragonal P4/mmm space group (ap × ap × 2ap), similarly to the RT-XRD pattern. We will use the data of the RT-NPD Rietveld refinement for the x = 0.5 sample to describe the crystal structure of samples with x = 0.4−0.6. In the tetragonal model we used here, there is a doubling of the c axis and this result in a superstructure, ordered along c. In this tetragonal model Sr atoms are placed at 2h sites, Co and Fe are distributed over two Fe/Co sites, 1a (labeled as Fe1/Co1) and 1b (labeled as Fe2/Co2), and three oxygen atoms are distributed over three sites, O1 at 2g, O2 at 2f, and O3 at 2e. Figure 3a shows the final plot of the Rietveld refinement using the RT-NPD for the x = 0.5 sample. The obtained lattice parameters, agreement factors, and selected bond lengths, resulting from the refinement of the RT-NPD data, are given in Tables 3 and 4. The crystal structure is shown in Figure 3b. We have found a random and equal distribution of Co in both Fe sites (Fe1/Co1 and Co2/Fe2). The refinement of oxygen occupancies shows almost full occupancies for O1 and O2, whereas for O3 a significant oxygen deficiency is observed; this indeed suggests that only the Fe2/Co2 sites (at 1b positions) are surrounded by oxygen vacancies (Table 3 and Figure 3b). O1 and O3 have been refined using a displacement from their initial position (O1 from [0,0,z] to [0,y,z] and O3 from [0.5,0,0.5] to

Table 4. Selected Bond Lengths (Å) (from RT-NPD Refinement) for the SrFe1−xCoxO2.75−δ Material with x = 0.5 (Fe1/Co1)−O1 (Fe1/Co1)−O2 (Fe2/Co2)−O1 (Fe2/Co2)−O3

2.027(5) 1.934(3) 1.867(5) 1.942(3)

× × × ×

2 4 2 4

[0.5,y,0.5]) to reduce the initially observed high isotropic thermal factors (initial Biso = 2.6(1) and 2.5(1) for O1 and O3, respectively). The structure can be visualized as an alternate stacking of oxygen-replete layers (Fe1/Co1 sites) and oxygendeficient layers (Fe2/Co2 sites) along the c axis to form a layered structure. The total oxygen content as obtained from the refined oxygen occupancies is 2.68 for the sample with x = 0.5. The origin of the tetragonal structure could therefore be the long-range oxygen-vacancy ordering. 3.1.3. Room-Temperature Crystal Structure for SrFe1−xCoxO2.75−δ Samples with x = 0.75−0.85. The RTXRD pattern of the sample with x = 0.75 is similar to the RTXRD patterns of the samples discussed in section 3.1.2. However, the RT-XRD patterns starting from x = 0.8 show the clear appearance of extra peaks (Figure S2 in the Supporting Information) with respect to the tetragonal model discussed in section 3.1.2. In view of the RT-XRD pattern, we first attempted to refine the RT-NPD pattern of the x = 0.75 sample with the P4/mmm tetragonal model. The refinement

Figure 3. (a) Final observed, calculated, and difference profiles (tetragonal, P4/mmm, ap × ap × 2ap type unit cell) of the RT NPD pattern for the SrFe0.5Co0.5O2.68 ± 0.02 sample (x = 0.5). (b) Crystal structure of the same material. Black, red, and green spheres represent O1, O2, and O3 atoms, respectively. The refinement shows that the Fe2/Co2 sites are surrounded by oxygen vacancies, whereas the occupancy of the O2 site is full (see Table 3). 9781


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extra peaks and peak broadening, which cannot be modeled using the P4/mmm tetragonal model. These extra peaks and the peak broadening become more prominent with increasing Co content in the structure (for the x = 0.85 sample, Figure S2) and in fact are characteristic peaks for the “314-type” oxygenvacancy ordered structure. Therefore, we have refined the RTNPD patterns for both samples (Figures 4 and 5a) using the “314-type” structural model14,22,30 with the I4/mmm tetragonal space group. Moreover, the NPD data collected at RT for the sample with x = 0.85 shows the appearance of magnetic reflections (weak peak at Q = 1.39 Å−1, Figure S3 in the Supporting Information). The obtained lattice parameters, agreement factors, and selected bond lengths and angles, resulting from the refinements of RT NPD data for the x = 0.75 and 0.85 samples, are given in Tables 5 and 6. The crystal structure is shown in Figure 5b. The unit cell parameters at room temperature are close for both samples, and the relation c = 2 × a is fulfilled with high accuracy. Similarly to the other Co-substituted compounds (discussed in previous sections), site occupancy refinements indicate that the chemical substitution of the Co ions for the Fe ions occur in both sites (Co1/Fe1 and Co2/Fe2, Table 5 and Figure 5a). As was revealed earlier, the oxygen atoms (O4) residing on the Co2/Fe2 plane show full occupancy. On the other hand, there are two different crystallographic positions where the oxygen atom could be located in the oxygen-deficient layers ((Co2/Fe2)−(O2,O3)) of the crystal structure. We have observed a significant oxygen deficiency in this layer. There is an important difference in the oxygen distribution in oxygendeficient layers of the present investigated materials in comparison to the previously reported “314-type” phases having close oxygen stoichiometry. For the reported “314type” structure the oxygen positions in the oxygen-deficient layers are very close, imposing that only one-fourth of them are occupied, and one needs to split one of the oxygen positions in the oxygen-deficient layer with oxygen uptake.28 Nevertheless, for the present materials, the O3−O3 distances are 2.28(1) Å, long enough to locate more than one-fourth of the atoms. In fact, the occupancy fraction can be increased for both O2 and O3 crystallographic sites with oxygenation. Therefore, in contrast to what was proposed for the “314-type” structure, it

shows a good matching between the observed and calculated patterns with agreement factors of χ2 = 2.19, Rwp (%) = 3.81, Rp (%) = 4.46, and Rf (%) = 4.42. Nevertheless, a closer inspection of the RT-NPD pattern (Figure 4) reveals the presence of weak

Figure 4. Comparison of Rietveld refinement patterns of roomtemperature neutron powder diffraction (enlarged part, RT NPD) using the tetragonal P4/mmm (ap × ap × 2ap type unit cell) and tetragonal I4/mmm (2ap × 2ap × 4ap type unit cell, “314-type”, see the text) models for the SrFe0.25Co0.75O2.63±0.02 sample (x = 0.75). The extra peaks and the peak broadening which cannot be fitted with the P4/mmm (ap × ap × 2ap type unit cell) model are highlighted by arrows. Rietveld refinement of RT-NPD using the I4/mmm (2ap × 2ap × 4ap type unit cell, “314-type”) model highlights a good agreement between the observed (red) and calculated (black) patterns.

Figure 5. (a) Final observed, calculated, and difference profiles (tetragonal, I4/mmm, 2ap × 2ap × 4ap type unit cell, “314-type”, see text) of the RT NPD pattern for SrFe0.15Co0.85O2.62 ± 0.02 sample (x = 0.85). 2θ = 18−19.5° is excluded in the refinement due to the appearance of a magnetic peak. (b) Crystal structure of the same material. Black, red, green, and dark green spheres represent O1, O2, O3, and O4 atoms, respectively. The refinement shows that the Fe1/Co1 sites are surrounded by oxygen vacancies, whereas the Fe1/Co1 layer is oxygen replete (see Table 5). 9782


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3.2.1. Magnetic Structure for SrFe0.75Co0.25O2.75 (x = 0.25). In comparison with the parent compound (SrFeO2.75), magnetic reflections down to 50 K of the present sample are identical. For the parent compound, two different magnetic structural models were proposed.2−4 In both, Fe3+ and Fe4+ cations order on the square-pyramidal and octahedral sites, but the site preferences are different. However, in any case, as indicated by LT Mossbauer spectroscopy measurements and NPD measurements, only one Fe site orders antiferromagnetically (AFM). Later on, in agreement with the experimental work of Hodges et al.,2 using spin-polarized electronic structure total-energy calculations Vidya et al.4 concluded that SrFeO2.75 can be considered as a phase-separated system, where Fe2 (Fe2/Co2 for the present case, see Figure 2b) sites form a longrange G-type AFM ordering below the AFM transition temperature (TN) and Fe1 (Fe1/Co1 for the present case), on the other hand, with two spin-up Fe2 magnetic moments and two spin-down magnetic moments in the neighbor becomes topologically frustrated. Similar to the magnetic structure of SrFeO2.75, proposed by Hodges et al.,2 we have used the Shubnikov group Cm′m′m to refine the 50 K NPD pattern of the present sample. We have considered that the magnetic moments only at the Fe2/Co2 sites are ordered, whereas those at the Fe1/Co1 site have no contribution. This in fact constrained the Fe2/Co2 site along the [010] direction. The final refinement profile and magnetic structure at 50 K are shown in Figure 6a,b, respectively. Within the accuracy of the experimental and data refinement, the magnetic structure of the present compound is G-type AFM, with a refined magnetic moment at the Fe2/Co2 sites of 1.98(7) μB, at 50 K. This is much lower in comparison with that of the parent compound (for SrFeO2.75 ∼3.5 μB at 50 K).2,3 3.2.2. Magnetic Structure for SrFe1−xCoxO2.75−δ Samples with x = 0.75, 0.85. For the sample with x = 0.85, the appearance of magnetic peaks started at RT, and as the temperature was decreased below RT, the small magnetic peaks observed at RT increased in intensity (Figure S3 in the Supporting Information). However, for the x = 0.75 sample a single and weak magnetic reflection is observed at LT (10 K, Figure S4 in the Supporting Information). Therefore, we will describe the magnetic structure using the LT NPD patterns for the x = 0.85 sample. Since the position of the magnetic peaks (for the x = 0.85) coincide with those for the allowed nuclear reflections, we choose the propagation vector of the magnetic structure to be k = [0,0,0]. The best fit to the observed magnetic reflections in the NPD patterns is obtained using for the magnetic ordering scheme in which magnetic moments of both Co/Fe sites (Co1/ Fe1 and Co2/Fe2) are aligned along the c direction (magnetic space group I4/mm′m). Figure 7a shows the final refinement profile of the NPD pattern collected at 10 K. Importantly, the refinement shows that the magnetic moments on two Co/Fe sites are different. For the 314 phase, Sheptyakov et al.30 recently proposed a similar magnetic ordering scheme. Using the LT NPD patterns, they highlighted that the c components of Co magnetic moments in two sorts of crystallographic sites are in roughly a 2:1 ratio. This important feature of nonequal moment magnitudes for different Co sites has been overlooked and assumed to be equal in all previous publications. In fact, the peak at Q = 1.14 Å−1 proves that Co cations in different sites have different magnetic moments. Similar to the magnetic ordering scheme proposed in ref 30, we have considered that Co atoms in the oxygen-deficient layer (Co1/Fe1 site) possess

Table 5. Refined Structural Parameters and Agreement Factors (using RT-NPD) for the SrFe1−xCoxO2.75−δ Materials with x = 0.75, 0.85 x = 0.75 a = b (Å) c (Å) cell volume V (Å3) Sr1 (0, 0, z) z Biso Sr2 (0, 0, z) z Biso Sr3 (0, 0.5, z) z Biso Co1/Fe1 (x, y, 0) x=y occ Biso Co2/Fe2 (0.25, 0.25, 0.25) occ Biso O1 (x, y, z) x=y z occ Biso O2 (x, 0, 0) x occ Biso O3 (x, 0.5, 0) x occ Biso O4 (0, y, z) y z occ Biso total oxygen content Rp Rwp RF χ2

x = 0.85

7.7237(1) 15.4717(2) 922.98(1)

7.7245(1) 15.4775(3) 923.50(1)

0.1301(8) 1.1(1)

0.1274(7) 1.0(1)

0.631(1) 0.7(1)

0.6363(6) 1.2(1)

0.1304(4) 0.6(1)

0.1319(4) 0.9(1)

0.2529(9) 0.72(2)/0.28(2) 0.63(9)

0.2482(3) 0.84(2)/0.16(2) 0.11 (7)

0.76(2)/0.24(2) 0.25(9)

0.84(2)/0.16(2) 0.27(7)

0.254(1) 0.1186(2) 0.88(2) 2.3(1)

0.2599(7) 0.1181(2) 0.94 (2) 1.9 (1)

0.281(1) 0.42(2) 2.0(1)

0.286(1) 0.44(2) 2.5(1)

0.745(1) 0.34(1) 1.9(1)

0.738(3) 0.24(1) 2.8(1)

0.2509(5) 0.2497(6) 1.00 0.68(3) 2.64(2) 2.88 3.79 4.6 3.06

0.2508(6) 0.2455(4) 1.00 0.73(4) 2.62(2) 3.31 4.30 4.18 2.83

is not necessary to split one or the other oxygen position in order to locate all the oxygen atoms for the present materials. The total oxygen content as obtained from the refined oxygen occupancies are 2.64(2) and 2.62(2) for the samples with x = 0.75 and 0.85, respectively. 3.2. Magnetic Structures. Upon cooling below room temperature new reflections of magnetic origin appear on the NPD patterns for the samples with x = 0.25, 0.85. As mentioned in section 3.1, the NPD data for the sample with x = 0.85 show the appearance magnetic peak at RT. The NPD patterns of the sample with x = 0.5 do not show any magnetic peak down to 10 K. For the sample with x = 0.75, the existence of a weak magnetic reflection (single peak at Q = 1.39 Å−1, Figure S4 in the Supporting Information) is observed at 10 K. 9783


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Table 6. Selected Bond Lengths (Å) and Angles (deg) (from RT-NPD Refinement) for the SrFe1−xCoxO2.75−δ Materials with x = 0.75, 0.85 bond length

x = 0.75

x = 0.85

bond angle

x = 0.75

x = 0.85

Sr1−O1 Sr1−O2 Sr1−O4 Sr2−O1 Sr2−O3 Sr2−O4 Sr3−O1 Sr3−O2 Sr3−O3 Sr3−O4 Co1/Fe1−O1 Co1/Fe1−O2 Co1/Fe1−O3 Co2/Fe2−O1 Co2/Fe2−O4

2.78(1) 2.96(1) 2.679(4) 2.69(1) 2.78(1) 2.67(1) 2.74(1) 2.64(1) 2.826(4) 2.67(1) 1.836(3) 1.96(1) 1.91(1) 2.03(1) 1.93(1)

2.84(1) 2.96(1) 2.66(1) 2.64(1) 2.79(1) 2.67(1) 2.74(1) 2.63(1) 2.80(1) 2.66(1) 1.83(1) 1.93(1) 1.95(1) 2.04(1) 1.93(1)

O4−Co2/Fe2−O4 O4−Co2/Fe2−O1 O1- Co1/Fe1−O1 O2−Co1/Fe1−O2 O3−Co1/Fe1−O3 O1−Co1/Fe1−O2 O1−Co1/Fe1−O3 O2−Co1/Fe1−O3 Co2/Fe2−O1−Co1/Fe1

180 89.7(1) 180 102.5(2) 89.2(2) 90.29(6) 89.7(2) 84.2(2) 178.2(1)

180 89.8 1) 180 107.2(1) 83.7(2) 92.4(2) 87.2(2) 94.5(2) 172.9(1)

Figure 6. (a) Final observed, calculated, and difference profiles of the NPD pattern collected at 50 K for the SrFe0.75Co0.25O2.75±0.02 sample (x = 0.25). The solid green line shows the magnetic contribution. (b) Crystal and magnetic structure of the same material. The magnetic moments only at the Fe2/Co2 sites are ordered, whereas those at the Fe1/Co1 site have no contribution.

magnetic structure determination carried out using LT-NPD data for the x = 0.85 sample unambiguously shows that, similar to the 314 phase, the magnetic structure of the present sample is AFM of the G type, with magnetic moments aligned along the c direction of the unit cell having nonequal moment magnitudes for different Co/Fe sites. As mentioned earlier, a single magnetic peak (Figure S4 in the Supporting Information) is observed at 10 K for the x = 0.75 sample. Nevertheless, the peak is in the same position with the most intense magnetic peak observed for the x = 0.85 sample. This strongly suggests that the local chemical structure for the x = 0.75 sample is 314-like (I4/mmm-like), in accordance with the structural analysis presented in section 3.1. 3.3. Magnetic Properties. The temperature variations of the FC and ZFC magnetization (M−T) curves measured at H = 100 Oe for all of the samples are shown in Figure 8. A wide range of interesting magnetic properties are observed with Co substitution. The samples with x = 0.15, 0.25, 0.3, having the same crystal structure (SG Cmmm) show AFM ordering at TN = 255 ± 2 K and then a large difference between the FC (MFC) and ZFC (MZFC) magnetization. The FC magnetizations for the samples with x = 0.25, 0.3 exhibit an interesting temperature-induced magnetization reversal (MR) phenomenon. The compensation

Figure 7. (a) Final observed, calculated, and difference profiles of the NPD pattern collected at 10 K for the SrFe0.15Co0.85O2.62±0.02 sample (x = 0.85). The solid green line is solely the magnetic contribution of the structure (“314-type”). (b) Magnetic structure of the same material. A G-type AFM structure with magnetic moments aligned along the c direction of the unit cell having nonequal moment magnitudes for different Co/Fe sites is highlighted here (see the text).

higher magnetic moments. Within the accuracy of the experimental and data refinement, the magnetic structure of the present compound is G-type AFM, with refined magnetic moments at the Co1/Fe1 and Co2/Fe2 sites of 2.13(7) μB/Co and 1.66(7) μB/Co cation, respectively, at 10 K. Therefore, our 9784


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Figure 8. Temperature-dependent FC and ZFC magnetization curves for SrFe1−xCoxO2.75−δ samples with (a) x = 0.15, (b) x = 0.25, (c) x = 0.3, (d) x = 0.5, (e) x = 0.75, and (f) x = 0.85, measured at 100 Oe.

Figure 9. Variation of lattice parameters (normalized, a, b, and c) and oxygen content with doped Co concentration for SrFe1−xCoxO2.75−δ samples. Lattice parameter values are taken from both RT-NPD and RT-XRD Rietveld refinements. At RT, an orthorhombic (space group Cmmm, 2√2ap × 2ap × √2ap type) → tetragonal (space group P4/mmm, ap × ap × 2ap type) → tetragonal (space group I4/mmm, 2ap × 2ap × 4ap type, “314 type”) structural transformation is observed with increasing Co doping concentration in the structure. Oxygen contents are calculated using RT-NPD refinements and Mohr salt titration methods and show a decreasing trend with increasing Co doping concentration in the structure.

With increasing Co doping in the structure, the M−T data of the sample with Co = 0.75 shows a peak around 105 K (Figure 8e). Data collected in FC mode indicate a deviation with respect to the ZFC curve occurring around 260 K. A spin glass type behavior at 105 K due to the frustration in the layered structure is observed for this sample.31 For the sample with x = 0.85, the MFC curve bifurcates from the ZFC curve at RT, consistent with the NPD measurements (observation of magnetic peak at RT, see section 3.2), and the deviation becomes significantly larger with a decrease in temperature. The irreversibility, observed at room temperature, and the temperature dependence of M FC are to some degree reminiscent of the behavior observed for “314-type” phases.22,24,30,32−34 The MZFC value, on the other hand, on further cooling below RT increases continuously until reaching a maximum at 85 ± 2 K.

temperature is found to be same for both compounds (47 K, indicated by arrows in Figure 9b,c). The ZFC magnetization (MZFC), on further cooling below 255 K for all three samples, increases continuously until it reaches a maximum at 60 ± 2 K, implying that there exists a magnetic frustration (see section 3.2). In fact, the MZFC curves are similar for all three samples. The sudden rise in the MFC curves below TN for all three samples indicates the existence of weak ferromagnetic (WFM) interactions, operating below TN. For the x = 0.5 sample, M−T measurement reveal AFM type behavior at 85 K (Figure 8d). Data collected in FC mode indicate a deviation with respect to the ZFC curve occurring at the same temperature (85 K), and this becomes significantly larger with a decrease in temperature. The M−T data for this sample illustrate a dramatic drop in magnetization in comparison with other samples. 9785


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structure, the phase transforms to a truly “314-type” novel oxygen-vacancy ordered structure. For x = 0.85, the crystal (nuclear) structure analyses using RT-XRD and NPD and the long-range magnetic ordering, similarly to the “314-type” structure, clearly demonstrate the formation of a “314-type” OV ordered structure. 4.2. Driving Force for Stabilization in the “314-Type” Novel OV Ordered Structure. To date, stabilization of the “314-type” OV ordered structure is believed to be associated with the ordering of the A-site (related to ABO3 perovskite), which in turn argues in favor of the ordering of the oxygen vacancies into the oxygen-deficient Co1−O4−δ (Co1/Fe1− O4−δ here) layer. For instance, A-site ordering between Ln and Sr (size argument) was suggested for Sr1−xLnxCoO2.63±δ (Ln = Y, Eu−Ho) samples.24,25,35 Knee et al.,32 in studies of the Sr1−xBixCoO3‑δ series, suggested that stabilization of this structure type might occur on the basis of A-site ordering due to charge effects (Sr2+ and Bi3+). Interestingly and in contrast with the previous cases, the present sample with the doped Co concentration x = 0.85 shows “314-type” OV ordering (as demonstrated by nuclear and magnetic structure analyses) for the 2ap × 2ap × 4ap unit cell without A-site ordering. This is in fact the first example of a “314-type” OV ordered structure without A-site ordering. It is worth mentioning here that the local chemical structure for the sample with x = 0.75 is also close to the “314-type” OV ordered structure. Our results then unambiguously demonstrate that Asite ordering is certainly not the driving force to stabilize the “314-type” OV ordered structure. Total oxygen contents as determined from the NPD patterns (as well as from the Mohr salt titration) for the present samples (oxygen content 2.64(2) and 2.62(2) for x = 0.75, 0.85 samples, respectively) are closer to the value of 2.625 expected for the ideal “314-phase”.30 This indeed points out that the oxygen content has a dominant role in the “314-phase” formation. The sample with x = 0.85 adopts the same G-type AFM structure exhibited by the “314-type” phases, suggesting that the magnetic properties (and magnetic lattices) of the systems are closely related. In fact, the magnetic ordering starting at RT as evidenced by NPD and the MFC and MZFC bifurcation at RT are similar to those for the other “314-phases”. For instance, the magnetic ordering temperature observed for Sr 0 . 7 5 Y 0 . 2 5 CoO 2 . 6 2 ± 0 . 0 2 3 0 is 335 K, and that for Sr0.75Y0.25CoO2.69 ± 0.02 is 290 K.30 Sr0.85Bi0.15CoO2.68 ± 0.02 shows MFC and MZFC irreversibility at RT.32 The refined magnetic moments at 10 K for the present sample (x = 0.85) are lower than the values reported for the Sr0.75Y0.25CoO2.62 ± 0.0230 sample but are similar to those for the oxidized Sr0.75Y0.25CoO2.69 ± 0.02. Nakao et al.,36 on the basis of resonant X-ray scattering techniques, highlighted the Co3+ eg orbital ordering and on that basis they proposed a ferrimagnetic structure with a Co3+ high-spin (HS, t2g4eg2) state and intermediate-spin (IS, t2g5eg1) state ordering (antiferromagnetically aligned) in the oxygen-replete layers. In a more recent study, using aberration corrected Z-contrast and annular bright field imaging combined with DFT calculations, Kishida et al.37 highlighted a similar situation that Co3+ ions in the oxygenreplete layers are antiferromagnetically aligned with alternating HS and IS, resulting in RT ferrimagnetism. Considering the similarities in the crystal structures, magnetic structures, and observed magnetic properties of the present x = 0.85 sample and the “314-phases”, we can expect a similar spin state

4. DISCUSSION 4.1. Structural Transformation with Co Substitution. The structural characterizations using RT-XRD and RT-NPD revealed an interesting structural transformation with Co doping (Figure 9). At RT, the Fe-rich samples with x = 0.1− 0.3 exhibit an orthorhombic (SG Cmmm, 2√2ap × 2ap × √2ap type) structure, isostructural with SrFeO2.75 with long-range vacancy ordering resulting in chains of corner-sharing octahedra separated by dimers of square pyramids. For the parent SrFeO2.75 compound, it has already been pointed out that the oxygen content and the Fe3+/Fe4+ charge ordering is the driving force to stabilize this oxygen-vacancy ordered structure. Breaking the Fe3+/Fe4+ charge ordering either by manipulating the oxygen stoichiometry or by aliovalent cation substitution immediately breaks this particular oxygen-vacancy ordering and transforms the structure in different oxygen-vacancy ordered/ disordered structures (for instance, SrFeO2.872 has a tetragonal structure and 5% Mn and Cr-doped SrFeO2.75−δ samples29 exhibit a disordered cubic structure). For the present case, the substituted Co atoms up to 30% (x = 0.3), which are randomly occupied in both Fe sites (Fe1/Co1 and Fe2/Co2 in the present case, see Figure 2b), retain the overall parent type OV ordered (SrFeO2.75-type) structure without breaking the transition-metal charge ordering. It is noteworthy that all of the compounds up to x = 0.3 have similar oxygen contents (2.75, see Figure 9). In an accordance with the RT crystal (nuclear) structure, the long-range G-type AFM ordering observed for the x = 0.25 sample is similar to that of the parent compound (SrFeO2.75), with reduced magnetic moment, however, unambiguously demonstrating the formation of a SrFeO2.75-type oxygen-vacancy ordered structure and robustness of the structure with Co doping. The charge disproportionation between Co and Fe could be the reason for the durability of the parent (SrFeO2.75) structure with Co substitution. Then, further Co substitution (from x = 0.4), which lowers the oxygen content in the structure, breaks the charge ordering due to the combined effect of increased Co content and low oxygen content (lower than 2.75) in the structure, and the sample transforms into a tetragonal superstructure, ordered along c. This superstructure could be the result of the longrange ordering of the oxygen vacancies along the c axis (occ = 0.72(2) for O3 for the x = 0.5 sample), as previously observed for the Sb- and Mo-doped derivatives.12,13 A small hike in cell parameters (b and c parameters, Figure 9) is observed for the x = 0.3 sample; this could be an indication of the phase transformation at this point. Interestingly, with a further increase of Co content in the structure, the oxygen content continuously decreases, and from x = 0.75 the structure transforms to the “314-type” novel oxygen-vacancy ordered structure. As mentioned previously, the XRD patterns of the x = 0.75 sample do not show any indication of phase transformation from the P4/mmm tetragonal model. In fact, extra peaks with respect to the tetragonal model start to appear from x = 0.8. Nevertheless, the appearance of weak magnetic reflections (discussed in section 3.2) for the x = 0.75 sample strongly suggests that the local chemical structure for the x = 0.75 phase is 314-like, in accord with the RT crystal structure analysis using the NPD pattern. It seems clear that, under these synthesis conditions, x = 0.75 is on the border of the P4/mmm to I4/mmm phase transformation. With a further increase in Co content in the 9786


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ferromagnetic (WFM) component due to the spin canting in the Fe1/Co1 sublattice and the paramagnetic (PM) behavior of doped Co ions under the influence of negative internal field due to the AFM. Such a type of MR has already been observed in Bi0.3Ca0.7Mn0.75Cr0.25O338and LaCr0.85Mn0.15O339 materials. The lack of a substantial amount of Co ions to behave as PM entities in the structure could be a reason for not observing the MR in the samples with x = 0−0.15. On the other hand, the magnetic frustration due to the existence and competition of WFM and AFM interactions in the layered perovskite-type structure could be the origin of low-temperature peaks (at 60 K in MZFC for x = 0.15, 0.25, 0.3 samples) seen in the ZFC magnetization data. With increasing Co content in the structure, the crystal structure transforms into a tetragonal superstructure. At the same time, the sample having equal contents of Co and Fe in the structure shows AFM type M−T behavior without any long-range magnetic ordering. In addition, a much smaller magnetization value in comparison to those for the previous samples is observed. The equal and random distribution of Fe and Co in the structure could be a reason the magnetic interactions are weakened. Finally, the sample with x = 0.85 having the maximum Co content in the structure (among all the samples in the present study) shows long-range magnetic ordering starting at RT. This is in fact evidenced from both the NPD patterns and M−T behavior. Similar to the case for the other “314-phases”, the Co3+ spin state ordering is likely the origin of this RT ferrimagnetism observed for the x = 0.85 sample.

5. CONCLUSIONS The effect of Co substitution in the SrFe1−xCoxO2.75−δ (x = 0.1−0.85) system has been explored with a complete range of x. The introduction of Co in the structure revealed an interesting structural transformation with Co doping. At RT, the parent-type orthorhombic crystal structure (SG Cmmm) transforms to a tetragonal (SG P4/mmm) ap × ap × 2ap type oxygen-vacancy ordered superstructure with doped Co concentration x = 0.4 and finally from x = 0.75 (Co content) transforms to a “314-type” novel OV ordered structure. The nuclear and magnetic structures and the magnetization measurements (appearance of RT MFC and MZFC irreversibility) confirm the formation of “314-type” oxygen-vacancy ordering for the sample with x = 0.85, which is the first case of “314type” oxygen-vacancy ordering without A-site ordering. The samples with x = 0.25, 0.3 show temperature-induced magnetization reversal, and the negative magnetization starts to appear at a compensation temperature of 47 K measured at 100 Oe magnetic field for both samples. The competition between the WFM component due to the spin canting in Fe ions and the paramagnetic behavior of a considerable amount of doped Co ions under the influence of negative internal field give rise to the MR. The Co-rich sample (x = 0.85) shows long-range magnetic ordering (G-type AFM). Similar to the case for the other “314-phases”, the Co3+ spin state ordering is likely the origin of this RT ferrimagnetism observed for the x = 0.85 sample. Moreover, our experimental results highlight that the stabilization of the “314-type” complex superstructure is related to the ordering of oxygen vacancies in the oxygen-deficient Co−O layers. Our experimentally observed different spin state ordering in different Co/Fe sites for the x = 0.85 sample demonstrates a direct correlation of the spin and charge 9787


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(10) Troyanchuk, I. O.; Karpinsky, D. V.; Sazonov, A. P.; Sikolenko, V.; Efimov, V.; Senyshyn, A. J. Mater. Sci. 2009, 44, 5900−5908. (11) Toulemonde, O.; Abel, J.; Yin, C.; Wattiaux, A.; Gaudin, E. Chem. Mater. 2012, 24 (6), 1128−1135. (12) Aguadero, A.; Perez-Coll, D.; Alonso, J. A.; Skinner, S. J.; Kilner, J. Chem. Mater. 2012, 24, 2655−2663. (13) Aguadero, A.; de la Calle, C.; Alonso, J. A.; Escudero, M. J.; Fernández-Díaz, M. T.; Daza, L. Chem. Mater. 2007, 19, 6437−6444. (14) Li, Y.; Kim, Y. N.; Cheng, J.; Alonso, J. A.; Hu, Z.; Chin, Y.-Y.; Takami, T.; Fernandez-Diaz, M. T.; Lin, H.-J.; Chen, C.-T.; Tjeng, L. H.; Manthiram, A.; Goodenough, J. B. Chem. Mater. 2011, 23, 5037− 5044. (15) Motohashi, T.; Ueda, T.; Masubuchi, Y.; Takiguchi, M.; Setoyama, T.; Oshima, K.; Kikkawa, S. Chem. Mater. 2010, 22, 3192− 3196. (16) MacChesney, J. B.; Sherwood, R. C.; Potter, J. F. J. Chem. Phys. 1965, 43, 1907−1913. (17) Takeda, T.; Yamaguchi, Y.; Watanabe, H. J. Phys. Soc. Jpn. 1972, 33, 967−969. (18) Schmidt, M.; Campbell, S. J. Solid State Chem. 2001, 156, 292− 304. (19) DHondt, H.; Abakumov, A. M.; Hadermann, J.; Kalyuzhnaya, A. S.; Rozova, M. G.; Antipov, E. V.; Tendeloo, G. V. Chem. Mater. 2008, 20, 7188−7194. (20) Le Toquin, R.; Paulus, W.; Cousson, A.; Prestipino, C.; Lamberti, C. J. Am. Chem. Soc. 2006, 128, 13161−13174. (21) Takeda, Y.; Kanno, R.; Yamamoto, O.; Takano, M.; Bando, Y. Z. Z. Anorg. Allg. Chem. 1986, 540, 259−270. (22) Istomin, S. Y.; Grins, J.; Svensson, G.; Drozhzhin, O. A.; Kozhevnikov, V. L.; Antipov, E. V.; Attfield, J. P. Chem. Mater. 2003, 15, 4012−4020. (23) Withers, R.; James, M.; Goossens, D. J. Solid State Chem. 2003, 174, 198−208. (24) Fukushima, S.; Sato, T.; Akahoshi, D.; Kuwahara, H. J. Appl. Phys. 2008, 103, 07F705−07F705−3. (25) Fukushima, S.; Sato, T.; Akahoshi, D.; Kuwahara, H. J. Phys. Soc. Jpn. 2009, 78, 064706−064712. (26) James, M.; Avdeev, M.; Barnes, P.; Morales, L.; Wallwork, K.; Withers, R. J. Solid State Chem. 2007, 180, 2233−2247. (27) Van Doorn, R.; Burggraaf, A. Solid State Ionics 2000, 128, 65− 78. (28) Rodriguez-Carvajal, J. An introduction to the program FULLPROF; Labratoire Leon Brillouin (Saclay: CEA-CNRS), Saclay, France, 2001. (29) Ramezanipour, F.; Greedan, J. E.; Cranswick, L. M. D.; Garlea, V. O.; Siewenie, J.; King, G.; Llobet, A.; Donaberger, R. L. J. Mater. Chem. 2012, 22, 9522−9538. (30) Sheptyakov, D. V.; Pomjakushin, V. Yu.; Drozhzhin, O. A.; Istomin, S. Ya.; Antipov, E. V.; Bobrikov, I. A.; Balagurov, A. M. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80, 024409. (31) Toulemonde, O., et al. Oxygen vacancies ordering in SrFe0.25Co0.75O2.64 inducing giant magnetic exchange coupling effect (unpublished). (32) Knee, C. S.; Lindberg, F.; Khan, N.; Svensson, G.; Svedlindh, P.; Rundlof, H.; Eriksson, S. G.; Borjesson, L. Chem. Mater. 2006, 18, 1354−1364. (33) Kobayashi, W.; Ishiwata, S.; Terasaki, I.; Takano, M.; Grigoraviciute, I.; Yamauchi, H.; Karppinen, M. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72, 104408. (34) Fleck, C. L.; Balakrishnan, G.; Lees, M. R. J. Mater. Chem. 2011, 21, 1212−1217. (35) James, M.; Cassidy, D.; Goossens, D.; Withers, R. J. Solid State Chem. 2004, 177, 1886−1895. (36) Nakao, H.; Murata, T.; Bizen, D.; Murakami, Y.; Ohoyama, K.; Yamada, K.; Ishiwata, S.; Kobayashi, W.; Terasaki, I. J. Phys. Soc. Jpn. 2011, 80, 023711−023715. (37) Kishida, T.; Kapetanakis, D. M.; Yan, J.; Sales, B. C.; Pantelides, S. T.; Pennycook, S. T.; Chisholm, M. F. Sci. Rep. 2016, 6, 19762− 19762−6.

ordering in these system types. Considering the importance of cobaltates and the 314-structure (with composition Sr0.7Y0.3CoO2.65−δ) as a cathode material for an IT-SOFC, our results mark an important step forward into an understanding of manipulating the A or B site (Sr and Co site) retaining the structure (“314-type”) unchanged to design new cathode materials (cobaltate) for IT-SOFCs.

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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.6b01554. Room-temperature and low-temperature neutron powder diffraction patterns (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for S.M.: [email protected]. Present Address †

Department of Physics, Siddaganga Institute of Technology, BH Road, Tumakuru - 572103, India Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are indebted to the CNRS, Université Bordeaux, and SOPRANO project (Seventh Framework Programme FP7/ 2007−2013 under Grant Agreement No. 214040) for funding this work. S.M. thanks Campus France for the Prestige & Marie Curie cofinancial grant, and this work has received funding from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7/ 2007-2013) under REA grant agreement no. PCOFUND-GA2013-609102, through the PRESTIGE programme coordinated by Campus France. This work is based on experiments performed at the Swiss spallation neutron source SINQ, Paul Scherrer Institute, Villigen, Switzerland. The authors also thank Dr. Vladimir Pomjakushin for his help in collecting the neutron powder diffraction patterns.

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REFERENCES

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