Nanoscale Ordering in Oxygen Deficient Quintuple Perovskite Sm2

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Nanoscale Ordering in Oxygen Deficient Quintuple Perovskite Sm2#Ba3+#Fe5O15-#: Implication for Magnetism and Oxygen Stoichiometry Nadezhda E Volkova, Oleg I Lebedev, Ludmila Ya. Gavrilova, Stuart Turner, Nicolas Gauquelin, Md. Motin Seikh, Vincent Caignaert, Vladimir A. Cherepanov, Bernard Raveau, and Gustaaf Van Tendeloo Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm503276p • Publication Date (Web): 07 Oct 2014 Downloaded from http://pubs.acs.org on October 10, 2014

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Chemistry of Materials

Nanoscale Ordering in Oxygen Deficient Quintuple Perovskite Sm2-εBa3+εFe5O15-δ: Implication for Magnetism and Oxygen Stoichiometry Nadezhda E. Volkova1, Oleg I. Lebedev2, Ludmila Ya. Gavrilova1, Stuart Turner3, Nicolas Gauquelin3, Md. Motin Seikh2,4, Vincent Caignaert2, Vladimir A. Cherepanov1*, Bernard Raveau2*and Gustaaf Van Tendeloo3 1

Department of Chemistry, Institute of Natural Sciences, Ural Federal University Yekaterinburg, Russia.

2

Laboratory CRISMAT UMR 6508, ENSICAEN-CNRS, Université Caen, 14050 Caen cedex, France.

3

EMAT, University of Antwerp, Groenborgenlaan, 171, B-2020, Antwerp, Belgium.

4

Department of Chemistry, Visva-Bharati University, Santiniketan-731235, West Bengal, India. * Corresponding authors ABSTRACT: The investigation of the system Sm-Ba-Fe-O in air has allowed an oxygen deficient perovskite Sm2εBa3+εFe5O15-δ (δ=0.75, ε=0.125) to be synthesized. In contrast to the XRPD pattern which gives a cubic symmetry (ap= 3.934Å), the combined HREM/ EELS study shows that this phase is nanoscale ordered with a quintuple tetragonal cell, “ap × ap × 5ap”. The nanodomains exhibit a unique stacking sequence of the A-site cationic layers along c, namely “Sm-BaBa/Sm-Ba/Sm-Ba-Sm”, and are chemically twinned in the three crystallographic directions. The nanoscale ordering of this perovskite explains its peculiar magnetic properties on the basis of antiferromagnetic interactions with spin blockade at the boundary between the nanodomains. The variation of electrical conductivity and oxygen content of this oxide versus temperature suggest potential SOFC applications. They may be related to the particular distribution of oxygen vacancies in the lattice and to the 3d5L configuration of iron.

Introduction The introduction of two sorts of cations with different valence and size, such as Ba2+ or Sr2+ and Ln3+, in the Asites of transition metal perovskite oxides has generated, besides high Tc superconductivity, numerous remarkable properties, ranging from oxygen storage in cobalt based oxides for the realization of solid oxide fuel cell (SOFC) cathodes [1-6] to colossal magneto-resistance in manganates [7-8]. In oxygen deficient perovskites, the ordering of these cations in the form of layers is at the origin of attractive magnetic and magnetoresistance properties, as for example in LnBaCo2O5+δ “112” cobaltites (see for a review ref [9]). In this respect, iron oxygen deficient perovskites, such as LnBaFe3O8+δ [6, 10-11] and LnBaFe2O5+δ [1215], isostructural to the cobalt layered perovskites, have also attracted attention as examples of compounds with double ordering in the cationic sites Ba2+/Ln3+ and in the anionic sites between oxygen and vacancies in the form of layers. Like the cobaltites, these layered ferrites exhibit attractive physical properties such as magnetoresistance and Verwey transition under oxygen loading. Nevertheless, the phase equilibria in the iron systems are not so well established compared to the Ln-Ba-Co-O systems [16-19], which were shown to be dramatically influenced by the nature of the lanthanide cation, especially in the series La to Sm. We believe that the oxygen stoichiometry and ordering as well as the Ln/Ba ordering in those ferrites are closely related to both the size difference between the Ba2+ and Ln3+ cations and the oxygen pressure used during synthesis. This suggests that layered ferrites

still offer a wide field for the investigation of magnetic properties. Based on the above considerations, we have revisited the pseudo ternary system Fe2O3-BaO-Sm2O3, working in air, bearing in mind that in contrast to the double ordered perovskite SmBaFe2O5+δ [12-14], the oxygen deficient perovskite SmBaFe3O8+δ [6,11] was shown to be disordered or only partially ordered, with a cubic symmetry. We report herein on an unusual oxygen deficient perovskite Sm2εBa3+εFe5O15-δ. Although this oxide is found to be cubic from X-ray diffraction (XRD), our combined electron diffraction (ED) high resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) study reveals that it is a nanoscale ordered layered perovskite, whose nanodomains consist of quintuple perovskite cells “ap × ap × 5ap” corresponding to the stacking sequence “Sm-Ba-Sm/Ba-Sm/Ba-Ba-Sm”, with a preferential location of the anionic vacancies at the vicinity of the mixed Ba/Sm layers. It is also shown that these nanodomains are three-dimensionally twinned, explaining why only a cubic symmetry can be detected by X-ray diffraction. The impact of this nanoscale ordering upon magnetism and oxygen mobility is shown. The peculiar magnetic properties of this oxide are explained in terms of antiferromagnetic interactions, with a spin blockade at the boundary of the nanodomains. The close relationships between electrical conductivity and oxygen stoichiometry indicate potential SOFC applications. This effect may be due to the special distribution of the oxygen vacancies in the nanostructure and to 3d5L configuration of iron.

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Experimental For the investigation of the phase formation within the composition range Sm1-xBaxFeO3-y in air, the samples were prepared using a glycerin nitrate technique. Samarium oxide Sm2O3 (99.99% purity), BaCO3 (special purity grade) and iron oxalate FeC2O4×2H2O (‘‘pure for analysis’’ grade) were used as starting materials. Before weighing, the starting materials (samarium oxide and barium carbonate) were preliminary heated in order to remove adsorbed gases and water. The starting compounds taken in appropriate ratios were dissolved in 4.5 М nitric acid, after which glycerin was added in the required amount for a complete reduction of the nitrates. After heating, the formation of a viscous gel was first observed, that was subsequently transformed to a brown powder. Finally this powder was annealed at 1100°C for 120 h in air, with intermediate grinding steps. For the exploration of the phase equilibria in the Sm-Ba-Fe-O system, all the samples were quenched to room temperature at a cooling rate of ~ 500 ºC /min. Then, when the single phase was identified from X-ray diffraction the same experimental protocol was performed except that the sample was not quenched, but slowly cooled, (cooling rate of about 10 ºC /min), in order to enter the maximum oxygen content into the structure. X-ray diffraction measurements were performed using a DRON-6 diffractometer, working with CuKα-radiation, within the angular range 2Θ = 20°–120˚, in steps of 0.010.04˚, 10 s/step and an Equinox-3000 diffractometer, with CuKα-radiation, equipped with a curved position-sensitive detector CPS-590 in the angle interval 2Θ=10°–90°and acquisition time of 1-2 s, step 0.012˚. Transmission electron microscopy, including high resolution (HRTEM) and electron diffraction (ED) experiments were carried out on a FEI Tecnai G2 30 UT microscope operated at 300 kV. High resolution HAADF-STEM imaging was performed on a JEOL ARM-200F cold FEG double aberration corrected microscope operated at 200 kV and equipped with a large solid-angle CENTURIO EDX detector. High resolution ADF-STEM/ABF-STEM and electron energy-loss spectroscopy (EELS) experiments were performed on a Titan “cubed” electron microscope equipped with an aberration corrector for the probe-forming lens as well as a high-brightness gun, electron monochromator and a high resolution EELS spectrometer (Gatan Enfinium) operated at 120 kV for the EELS experiments and 200 kV acceleration voltage for ABF imaging. High resolution EELS experiments were carried out with the electron monochromator excited to provide 250 meV energy resolution. The STEM convergence semi-angle used was 22 mrad at 120 kV and 200 kV, providing a probe size of ~1.0 Å at 120kV and ~0.8 Å at 200kV. The convergence angle was lowered to ~18 mrad when using the electron monochromator, yielding a probe size of ~ 1.5 Å. The inner collection semi-angle β for ABF-STEM imaging at 300kV was ~11 mrad. The EELS collection semi-angle β at 120 kV was 61 mrad. Proper beam intensity (approx. 60 pA), pixel sampling and dwell time (approx. 0.05 s/pixel) were cho-

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sen to maximize the EELS signal while ensuring minimal ionization damage. The oxygen content in the single phase oxide was determined by the TGA method (STA 409PC, Netzsch Gmbh). The samples were placed in the TGA cell, heated up to 1100 °C and equilibrated in air at this temperature during 8 h. The measurements were performed in the static mode (dwelling at constant temperature during 8 h) or dynamic mode (cooling/heating rate 1 K/min). The absolute value of the oxygen content was determined using direct reduction of the sample in a hydrogen flow at 1100 °C inside the TGA cell. This determination was based on XRD measurements carried out just after the reduction process, showing the complete disappearance of the perovskite phase and the only presence of Sm2O3, BaO and Fe, as final products after reduction. The dc magnetization measurements were performed using a superconducting quantum interference device (SQUID) magnetometer equipped with a variable temperature cryostat (Quantum Design, San Diego, USA) in the range of 5 to 300 K. All the dc measurements were carried out in an applied field of 0.3 T on dense ceramic bars of dimensions ~ 4 × 2 × 2 mm3. The ac susceptibility, χac(T) was measured with a PPMS (Quantum Design, San Diego, USA) with frequencies ranging from 10 Hz to 10 kHz. Total conductivity was measured by the 4-probe DC technique. The powder samples were pressed into the bars with the sizes about 4 × 2 × 25 mm3 and sintered at 1350 ˚C in air during 20 h. Temperature control and data collection were performed with the Zirconia 318. Results and Discussion In order to investigate the possibility to synthesize new oxygen deficient perovskites with general formula Sm1-xBaxFeO3-y, a number of samples within a compositional range of 0< x< 1 were prepared, working at 1100°C in air as described above. It was first observed that under these experimental conditions, neither the ordered oxygen deficient perovskite SmBaFe2O6-δ (x=0.5) nor the partially ordered perovskite SmBa2Fe3O8-δ (x=0.667) could be obtained. These results are in perfect agreement with the literature; indeed, SmBaFe2O6-δ could only be prepared by several authors [13-15] in a reducing atmosphere ( PO = 10-12.44 - 10-29.4 bar), whereas the synthesis of 2

SmBa2Fe3O8-δ required an oxygen flow at 1000°C [10-12]. In contrast, we show here that a single phase perovskite can be synthesized in air for the nominal composition corresponding to the formula Sm0.375Ba0.625FeO3-y (x = 0.625). All other samples Sm1-xBaxFeO3-y are found to be biphasic, containing, besides this new perovskite, either SmFeO3 (e.g. for x=0, 0.05, 0.1, 0.5, 0.6, 0.61, 0.62) or BaFeO3-δ (e.g. for x= 0.63, 0.64, 0.65, 0.7, 0.75, 0.9, 0.95, 1.0). Nanoscale ordering in the perovskite Sm2-εBa3+εFe5O15-δ The powder XRD pattern of Sm0.375Ba0.625FeO3-y (Fig.1) is characteristic of a cubic perovskite and can be indexed according to the Pm3m space group, with a = 3.934(1) Å.


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Fig.1: Powder XRD pattern of Sm2-εBa3+εFe5O15-δ The oxygen content in the single phase sample, obtained from TGA upon slow cooling of the sample to room temperature, indicates an oxygen deficiency y of 0.15 ±0.01. These observations seemingly imply a statistical distribution of the Ba2+ and Sm3+ cations in the perovskite cages and suggest that the vacancies are randomly distributed over the anionic sites. In fact, a detailed transmission electron microscopy (TEM) study shows that the structure is perfectly ordered at the nanoscale. The electron diffraction (ED) patterns of this phase (Fig.2a) show that whatever the crystallographic direction, [001], [010] or [100] a single ap parameter in most of the cases cannot be observed, but instead superstructure spots, corresponding to an “ap × ap × 5ap” tetragonal cell are always identified. The HRTEM images (Fig.2 b-c) show that these tetragonal nanodomains are chemically twinned (Fig.2c) along the three cube directions of the perovskite basic structure (Fig.2c). Bearing in mind that one cubic cell corresponds to the formula Sm0.375Ba0.625FeO2.85, these tetragonal nanostructures can be formulated as Sm2-εBa3+εFe5O15-δ (δ~0.75, ε~0.125). They consist of 5 SmO/BaO layers stacked alternately with 5 FeO2 layers along c. A high angle annular dark field scanning TEM (HAADF-STEM) image of the Sm2-εBa3+εFe5O15-δ structure along the [100] direction is presented in Fig.3a. As the image contrast in HAADFSTEM is sensitive to the atomic number Z~2, it is clearly established from the contrast segregation that the Ba2+ and Sm3+ cations are ordered in (001) layers along the caxis. One indeed observes rows of bright dots perpendicular to c, which corresponds to three sorts of Sm or Ba cationic layers, judging from their intensity: bright, intermediate and less bright intensity layers are tentatively attributed to pure Sm, mixed Ba/Sm and pure Ba layers respectively.

Fig.2: Electron diffraction and HRTEM imaging of Sm2Ba ε 3+εFe5O15-δ. (a) Electron diffraction patterns along the [100], [110] and [111] zone axis orientation. (b) HRTEM image of the structure along the [100] zone axis orientation. The superstructure is c-axis oriented, with a periodicity of 1.95 nm (5 perovskite unit cells). (c) Overview HRTEM image showing twinned domains, with a 90° rotation between the c1 and c2 orientation respectively.



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ers depicted with white and yellow lines respectively. Dark contrast lines are clearly visible in the middle of each repetition between two successive Sm/Ba layers. (c)&(d) HAADF-STEM and simultaneously acquired ABFSTEM images along the (c) [100] and (d) [110] zone axis orientation, showing the oxygen lattice in ABF-STEM image. Enlargement image and corresponding structural model is given as insert. The oxygen positions close to the Sm columns are severely displaced (forming a “zigzag chain”) with respect to the standard octahedral symmetry positions. Thus the HAADF-STEM image of each tetragonal domain can be interpreted by the following periodic stacking sequence of the A cationic layers along the c axis: “Sm-Ba-Sm/Ba-Sm/Ba-Ba-Sm”. It is this ordering which dictates the quintupling of the perovskite structure along one direction. Importantly, the presence of a dark line between two successive mixed Sm/Ba layers can be observed. It also appears from the measurement of interplanar distances that the “Sm/Ba-Sm/Ba” interval between two mixed layers is larger than the other interlayer distances (“Sm-Ba” or “Ba-Sm/Ba”). Such contrast and distance variations can be attributed to the presence of oxygen vacancies and were observed in similar oxygen deficient perovskites [20, 21]. The oxygen lattice is imaged using atomic resolution annular bright-field (ABF) imaging in Figure 3c,d. It appears from the images along [100] and [110] orientations of a single Sm2-εBa3+εFe5O15-δ domain, that the oxygen positions in all the layers are close to the ideal octahedral positions. However, a closer inspection of the images reveals that the oxygen columns in the equatorial positions close to the Sm layer deviate from their ideal octahedral position, and lie closer to the Sm3+ cations yielding a “zigzag” contrast along [100] and [110] (shown with blue circles in the bottom section of Figure 3). No vacant oxygen sites are visible from the ABF imaging. This is however to be expected in the case of randomly distributed oxygen vacancies, due to the projected nature of the images. This chemical ordering is clearly confirmed by elemental electron energy-loss spectroscopy (EELS) mapping (Fig.4), which demonstrates that the Sm layers, spaced by 5ap along the c axis, are practically pure and that the surrounding Ba layers are in fact not totally Sm free, but intermixed with a small amount of Sm. Fig.3: High resolution HAADF-STEM imaging of the Sm2-εBa3+εFe5O15-δ structure. (a) HAADF-STEM image along the [100] zone axis orientation, showing a 5 perovskite unit cell contrast periodicity along the c-axis. The vertical arrows represent the direction of the pure Sm and mixed Sm/Ba layers stacked perpendicular to the c direction (b) Line intensity( increasing from the left to the right due to the increasing thickness of the sample) profile over the cdirection of the HAADF-STEM image in (a); the intensity of the peaks indicates a Sm-Ba-Ba/Sm-Ba/Sm-Ba-Sm ordering, where peaks correspond to Sm and Sm/Ba lay-

In contrast, the median mixed Sm/ Ba layers are found to contain approximately equal amounts of each element. The Fe layers that are sandwiched in between the “Sm, Ba” layers are clearly identified from the elemental maps. The Fe sublattice is not influenced by the ordering, and runs undisturbed throughout the full superstructure. Using all the acquired electron microscopy data, an idealized model for this quintuple tetragonal perovskite is proposed in Figure 5.


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Fig.4: EELS elemental mapping of Sm2-εBa3+εFe5O15- δ . (a) Overview HAADF-STEM image (b) Fe-L2,3 map (c) Sm-M4,5 map (d) Ba-M4,5 map (e) O-K map (f) Color overlay with Sm in yellow , Ba in red, Fe in grey and O in blue. To further investigate the presence of oxygen vacancies in the structure (which were not visible in ABF-STEM imaging due to the projected nature of the image), we used spatially resolved electron energy-loss at high energy resolution. It is known from literature that the fine structure of EELS edges is sensitive to the local environment of the probed element. In this case, the O-K and Fe-L2,3 edges were studied to investigate the valency and bonding coordination of the Fe cations at atomic resolution [2022]. These spatially resolved EELS data show that the O-K edge spectra corresponding to the “FeO2” planes (labeled a,b,c) exhibit different intensity ratios of the two prepeaks to the O-K edge, prepeak1/ prepeak2 at approximately 529/531 eV, depending on the nature of the surrounding “Sm,Ba” layers (Fig.6a).

Fig.5: HAADF-STEM image of twinned area and corresponding schematic representation of the nanoscale ordered structure of Sm2-εBa3+εFe5O15- δ. Two tetragonal regions showing a quintuple ordering of the Ba and Sm layers are chemically twinned.

The O-K fine structure in the Sm plane is very similar to that of plane a, whereas those of the Ba and Ba/Sm planes are similar to b and c planes respectively. A first observation is that pre-peak 1 at ~529 eV is less intense for oxygen anions close to Sm cations (SmO layers as well as a layers). According to the literature, the height of this pre-peak is generally rather independent of the rare earth element and should be around the same height as prepeak 2 [23]. Pre-peak 2, related to Fe3d eg – O2p hybridized states seems invariant in the structure, apart from the c plane where it is slightly subdued, accompanied by an increase of pre-peak 1 below 530eV. Pre-peak 1 can be attributed to a charge transfer from the eg to the t2g band of Fe (the eg band is usually empty for Fe3+). This increase of pre-peak 1 related to Fe3d t2g – O2p hybridized states is also visible in the Ba/Sm mixed layers, and can be linked to the presence of oxygen vacancies in those planes. This peak is stronger in the c plane suggesting the presence of more vacancies in this plane.



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Fig.6: EELS fine structure of Sm2-εBa3+εFe5O15-δ; (Left panel) O-K edge fine structure signatures from the regions indicated in the central panel. (Center panel) Structural model with indicated EELS integration areas. (Right panel) Fe-L2,3 fine structure signatures from the regions indicated in the central panel with references for 4, 5 and 6-fold coordinated Fe3+, and a simultaneously acquired and energy-shifted Ba M5 edge. In the Sm plane, one peak at around 531eV can be observed, together with a small shoulder at slightly lower energy than the Fe3d t2g – O2p related peak (pre-peak 2) which might be linked to the presence of Zhang-Rice singlet like states, implying for iron a 3d5L configuration, as observed previously for copper in the high Tc superconductors [21]. A further hypothesis for the reduced prepeak 1 is that this might be related to the distortion of the octahedron through a shift of oxygen towards the Sm plane. The spatially resolved EELS spectra of the Fe-L2,3 edge are plotted in Figure 6b, together with references for Fe3+ in a 6-fold, 5-fold and 4-fold coordination [20, 24]. The Fe L3 and L2 “white lines” arise from transitions of 2p3/2 → 3d3/23d5/2 (L3) and 2p1/2 → 3d3/2 (L2) and are known to be sensitive to valence and coordination, as their intensity is related to the density of unoccupied states in the 3d bands [25]. For example, the reference data for Fe3+ in Oh octahedral coordination displays a clear splitting of the L3 peak, which results from the crystal field splitting of the energy levels in the Fe 3d unoccupied states into t2g and eg levels. The pre-peak to L3 at 708 eV is associated with transitions from 2p3/2 → t2g. The main L3 maximum at 709.5 eV is associated with a transition from 2p3/2 → eg. [26] The data presented in Figure 6b clearly shows that the a and b FeO2 planes exhibit very similar Fe-L2,3 edges, with an L3 peak maximum at 709.5 eV, and a pre-peak to L3, even if faint at 708 eV. The energy position of the L3 maximum, together with the shape and positions of the L3 and L2 are then indicative of Fe3+ in an octahedral coordina-

tion. No signal that could point towards the presence of Fe4+is detected. However, all the acquired Fe L3 edges are significantly broadened with respect to the plotted references for 6-fold, 5-fold and 4-fold coordinated Fe3+. This broadening can be explained by a change in coordination of the Fe atoms within the probed atomic columns (along the beam-direction). Bearing in mind that the measured oxygen stoichiometry is 14.25, instead of 15, this suggests that the iron coordination is mainly 6, i.e. octahedral, but may also be mixed with the presence of some FeO5 pyramids in those layers. To ensure the broadening is not an instrumental effect, the simultaneously acquired Ba M5 edge is plotted together with the Fe L2,3 edges. The width of this peak is of the same order of magnitude as the Fe L2,3 references, ensuring the observed peak broadening is physical in nature, and not instrumental. The Fe L2,3 spectrum from a single FeO2 column in an “a” plane is also plotted in the right panel (labelled “D”). In this spectrum, the pre-peak to the L3 edge, indicative of octahedrally coordinated Fe3+, is well defined, meaning the density of oxygen vacancies is lower in this atomic column. In contrast, the Fe-L2,3 edge of the c plane is different: the prepeak to L3 has disappeared, indicating without any ambiguity that the coordination of iron located in between the mixed Sm/Ba layers is lowered to a 5-fold (pyramidal) or even 4-fold (tetrahedral) coordination. This confirms the hypothesis discussed earlier that a great part of the oxygen vacancies are distributed around the median iron, in the FeO2 equatorial c plane, but also at the apical sites in the Sm/Ba layers. This is coherent with the dark contrast seen in the HAADF-STEM images in between the Sm/Ba


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layers (Fig.3a,b), which can also indicate an increased presence of oxygen vacancies. Magnetic properties of Sm2-εBa3+εFe5O15-δ Fig. 7 shows the zero-field-cooling (ZFC) and fieldcooling (FC) curves of Sm2-εBa3+εFe5O15-δ in an applied magnetic field of 0.3 T in the range 5 to 300 K.

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been detected, in contrast to the preliminary X-ray observations. Such a nanoscale phase ordering has a pronounced impact on the magnetic properties of the system, as previously demonstrated for the nanoscale ordered double perovskite LaBaCo2O6 [27]. The latter, differently from the two other forms- disordered cubic “ap” and ordered tetragonal “ap × ap × 5ap” - was shown to exhibit spin blocking due to the presence of 90° oriented nanodomains, resulting in a strong magnetic anisotropy and high coercitivity [27]. The present data may be correlated to the possible existence of intra- and interdomain antiferromagnetic interactions throughout the measured temperature range. Such speculation is supported by the fact that we do not observe any linear dependence of inverse susceptibility (see Fig. 8b). 0.3

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Fig.7: Temperature dependent MZFC and MFC magnetization of Sm2-εBa3+εFe5O15-δ measured under an applied field of 0.3 T. Inset shows the difference MZFC - MFC as a function of temperature. For both the ZFC and FC curves, the magnetic moment increases with decreasing temperature. However, even at 300 K, there exists a noteworthy irreversibility between ZFC and FC. The inset in Fig. 7 shows the difference between ZFC and FC magnetization as a function of temperature revealing a sharp increase in the difference below 50 K. More interestingly, the ZFC curve shows a pronounced bump around 50 K which is insignificant for the FC branch data. Such a behavior in magnetic susceptibility is commonly observed for magnetic glassy materials or for superparamagnetism. It is known that frequency dependent susceptibility measurements are an effective method to investigate magnetic glass type characteristics. Bearing in mind such a possibility we have carried out ac susceptibility measurements at three different frequencies (data not shown here) and did not detect any spin glass behavior. In the case of superparamagnetism the blocking temperature should correspond to the peak in the ZFC curve at 50 K. A broad hump at blocking transition is expected for a wide size distribution of the superparamagnetic particles. We do however not see such a feature. In contrast, the ZFC data keep increasing, leaving a small hump at 50 K (Fig. 7). Moreover, the M(H) curve (Fig. 8a) exhibits a linear behavior at lower temperature, allowing the presence of superparamagnetism to be discarded. In order to understand the peculiar magnetic behaviour of this oxide system, we have to consider the prevalence of antiferromagnetic ordering. Our combined HRSTEM/EELS study reveals that this phase is ordered in the form of oxygen deficient perovskite tetragonal nanostructures "ap × ap × 5ap" that are twinned at 90° three-dimensionally. Importantly, no cubic region has

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Fig. 8: Isothermal field dependent magnetization at 5 K (a) and inverse susceptibility (χ-1) plot as function of temperature (b) for Sm2-εBa3+εFe5O15-δ. This non-Curie-Weiss behavior reflects the absence of free spins up to room temperature in this system. A critical look to the M(H) curve measured at 5 K (Fig. 8a) reveals a significant opening of the loop in both the upper and lower quadrants and practically no opening at lower field. This suggests the occurrence of a metamagnetic transition, demonstrating that the sample returns to the antiferromagnetic ground state when the external field is decreased back to zero. A metamagnetic transition is commonly observed in systems with competing ferromagnetic and antiferromagnetic interactions as in the perovskite manganites and cobaltites [28-31]. However, we do not see here any convincing evidence of a ferromagnetic component. We therefore believe that the spin



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canting that should occur at the boundary between the nanodomains is blocked, due to this exceptional chemical twinning, similarly to LaBaCo2O6 [27]. The application of a much higher magnetic field would be necessary to realize a canted spin state. The observed peak around 50 K in the ZFC curve may originate from a spin rotation related to rare-earth iron interaction (Sm-Fe) arising in the magnetic domain boundaries. Such type of spin rotation transition is also observed in perovskites manganites [32, 33].

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in this oxide are most probably linked to the distribution of the oxygen vacancies around the median iron plane of the structure and closely related to the particular nature of the Fe-O bonds involving a 3d5L configuration. Taking into account that in contrast to SmBaFe2O6-δ the nanoscale ordered Sm2-εBa3+εFe5O15-δ is prepared and stable in air the obtained results show that this perovskite can be considered as a potential candidate for the purpose of SOFC’s cathode usage.

Electrical properties and oxygen mobility in Sm2εBa3+εFe5O15-δ

The temperature dependence of the total conductivity of the Sm2-εBa3+εFe5O15-δ in air is shown in Fig. 9. The behavior of total conductivity is similar to that obtained for the double perovskites SmBaCo2-xFexO6-δ [34], but the absolute values of the variations are approximately two orders of magnitude smaller. However, the value of total conductivity of Sm2-εBa3+εFe5O15-δ is one order higher in comparison to the undoped SmBaFe2O6-δ at least at room temperature [13].

Fig. 10. Variation of oxygen content versus temperature for Sm2-εBa3+εFe5O15-δ in air determined by TGA. Points represent data obtained at constant temperature (isothermal dwells for 10 h), continuous line corresponds to data obtained in dynamic mode (heating rate 5 K/min). Conclusion

Fig. 9. Variation of the total conductivity of Sm2in air versus temperature.

εBa3+εFe5O15-δ

The existence of a maximum in the electrical conductivity dependence versus temperature around ~380°C can be understood on the basis of the variation of the oxygen content (Fig.10). From 293°C to 38o°C, the oxygen content does not vary significantly, and consequently the increase of conductivity with temperature from in this regime can be explained by the charge disproportionation reaction × • . This statement is supported by the ′ + Fe Fe 2 Fe Fe ↔ Fe Fe value of activation energy, estimated as 20 eV from the linear part in the Arrhenius coordinates (see insert in Fig. 9), which is typical for the hopping conductivity mechanism. Beyond 380°C, further increase of the temperature leads to a significant increase of oxygen vacancies ( ) (Fig.10) that prevents the formation of most mobile electron holes ( and consequently the conductivity decreases (Fig.9). The conductivity and the oxygen mobility

The perovskite Sm2-εBa3+εFe5O15-δ shows a unique Asite layered ordering of the Ba2+ and Sm3+ cations, that has never been observed before in this structural family. Though it is a crucial factor, the size difference between Ba2+and Sm3+ is not sufficient to explain the origin of this quintuple periodicity. In this respect, the oxygen deficiency, and especially the location of the oxygen vacancies at the vicinity of the median FeO2 plane may play a role in the stabilization of the structure, relaxing the strains. Most important is the fact that this ordering is only stabilized at a nanoscale and is extended three-dimensionally to the bulk through chemical twinning. It results in a peculiar magnetic behaviour of this compound, which can be interpreted on the basis of antiferromagnetic interactions with spin blockade at the boundary between nanodomains. EELS also shows that this ferrate cannot be considered as a single mixed valent, Fe3+/Fe4+oxide, but rather as hybridized, Fe3+ /Fe3d5L, similar to copper in the high Tc superconductors. This raises the issue of the influence of the electronic distribution upon magnetism and conductivity in this new structural family. The impact of oxygen content upon electrical conductivity together with its stability in air at relatively high temperature make it also a potential candidate for SOFC applications.


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These results open the route to the investigation of numerous perovskite-like ferrates containing barium and a lanthanide, whose apparently simple structure is more complex than expected and may generate attractive properties in view of various applications. Acknowledgements The UrFU authors were financially supported by the Ministry of Education and Science of Russian Federation (project N 4.1039.2014/K) and by UrFU under the Framework Program of development of UrFU through the «Young scientists UrFU» competition. The CRISMAT authors gratefully acknowledge the EC, the CNRS and the French Minister of Education and Research for financial support through their Research, Strategic and Scholarship programs. This work was supported by funding from the European Research Council under the Seventh Framework Program (FP7), ERC grant N°246791 – COUNTATOMS. S.T. gratefully acknowledges the fund for scientific research Flanders for a post-doctoral fellowship and for financial support under contract number G004413N. N.G. acknowledges funding from the European Research Council under the 7th Framework Program (FP7), ERC starting grant number 278510 – VORTEX. References (1) Chang, A.; Skinner, S.J.; Kilner, J.A. Solid State Ionics 2006, 177, 2009. (2) Kim, J.-H.; Manthiram A. J. Electrochem. Soc. 2008, 155, B385. (3) Motohashi, T.; Ueda, T.; Masubuchi, Y.; Takiguchi, M.; Seyotama, T.; Oshima, K; Kikkawa, S. Chem. Mater. 2010, 22, 3192. (4) Chen, D.; Wang, F.; Shi, H.; Ran, R.; Shao, Z. Electrochimica Acta 2012, 78, 466. (5) Tsipis, E.V.; Kharton V.V. J. Solid State Electrochem. 2008, 12, 1367.

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Nanoscale Ordering in Oxygen Deficient Quintuple Perovskite Sm2

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