Crystal Structures at Atomic Resolution of the Perovskite-Related

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Crystal Structures at Atomic Resolution of the Perovskite-Related GdBaMnFeO5 and Its Oxidized GdBaMnFeO6 Susana García-Martín,*,† Keisuke Manabe,‡ Esteban Urones-Garrote,§ David Á vila-Brande,† Noriya Ichikawa,‡ and Yuichi Shimakawa*,‡ †

Departamento de Química Inorgánica, Facultad de C.C. Químicas, Universidad Complutense, 28040-Madrid, Spain Institute for Chemical Research, Kyoto University, Uji 611-0011 Japan § Centro Nacional de Microscopía Electrónica, Universidad Complutense, 28040-Madrid, Spain ‡

S Supporting Information *

ABSTRACT: Perovskite-related GdBaMnFeO5 and the corresponding oxidized phase GdBaMnFeO6, with long-range layered-type ordering of the Ba and Gd atoms have been synthesized. Oxidation retains the cation ordering but drives a modulation of the crystal structure associated with the incorporation of the oxygen atoms between the Gd layers. Oxidation of GdBaMnFeO5 increases the oxidation state of Mn from 2+ to 4+, while the oxidation state of Fe remains 3+. Determination of the crystal structure of both GdBaMnFeO5 and GdBaMnFeO6 is carried out at atomic resolution by means of a combination of advanced transmission electron microscopy techniques. Crystal structure refinements from synchrotron X-ray diffraction data support the structural models proposed from the TEM data. The oxidation states of the Mn and Fe atoms are evaluated by means of EELS and Mössbauer spectroscopy, which also reveals the different magnetic behavior of these oxides.

1. INTRODUCTION

promising for the use of the material as a solid oxide fuel cell cathode.11,12 In this study, we focus on the A-site layered ordered perovskite GdBaMn2O5 and try to substitute half of the B-site Mn by Fe for obtaining GdBaMnFeO5. When this compound is synthesized by H 2 treatment of the disordered Gd0.5Ba0.5Mn0.5Fe0.5O3‑δ, a significant amount of stacking defects, in addition to disordered domains, are observed in the crystals of the material.13 In the present study, we have succeeded in preparing the A-site highly ordered GdBaMnFeO5. Because both A-site cations and oxygen are ordered in layered manners, our primary concern is whether the Mn and Fe ions at the B site can also be ordered in a layered manner. The valence states of each B-site Mn and Fe ions are of particular interest, because the average valence state of the ions is 2.5+. GdBaMn 2.5+ Fe 2.5+ O 5 , GdBaMn 2+ Fe 3+ O 5 , and GdBaMn3+Fe2+O5 are possible charge distributions in the compound. The ordered manner in the crystal structure and the ionic charge distribution are also expected to affect the magnetic properties of the compound. We then anneal the sample in oxidizing atmosphere to oxidize GdBaMnFeO5 into GdBaMnFeO6. By using low-temperature topotactic reaction, the ordered manners of the cations remain and only the valence states of the Mn and Fe ions can be changed to the average valence of 3.5+. Structure and valence-state changes taking

Oxides with perovskite structure (chemical formula of ABO3) accept large variety of chemical substitution at both A and B cation sites. An interesting class of such compounds is formed by the double perovskites AA′B2O6 with A-site layered ordering because cation ordering has a strong impact on physical properties. In this context, magnetic properties of the A-site layered-ordered REBaMn2O6 (RE is a rare earth element) are different from those of the A-site disordered perovskites RE 0 . 5 Ba 0 . 5 MnO 3 . 1 − 4 In the A-site layered-ordered TbBaMn2 O5.75, charge ordering occurs at 457 K and antiferromagnetic interactions associated with polaron trimers are detected, whereas it does not occur when the Tb and Ba atoms occupy the A site at random.5,6 In the A-site layered-ordered AA′B2O6, oxygen vacancies are often introduced. In compounds with trivalent A and divalent A′ ions, such as GdBaMn2O5 and SmBaFe2O5, the oxygen vacancies are also located in a layered manner, only in the AO layer, producing BO5 pyramidal coordination.2,7 These compounds show interesting physical properties due to the mixed valence state of the B cation. For instance, SmBaFeO5 contains Fe2.5+ at room temperature and shows a metal-toinsulator transition at about 220 K associated with Verwey-type charge ordering from Fe2.5+ to Fe2+ and Fe3+.8 The oxygen vacancies in the layered manner also play an important role in oxide ion conduction in the compounds. Fast oxide ion conduction occurs in GdBaCo2O5+δ9,10 and this property is © XXXX American Chemical Society

Received: October 13, 2016

A

DOI: 10.1021/acs.inorgchem.6b02472 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

3. RESULTS AND DISCUSSION Figure 1a−c shows SAED patterns of the GdBaMnFeO5 crystal along three zone axes, which are indexed with the cubic

place by the oxidation in the B-site cations are investigated in details. A new modulated structure associated with Gd displacement in the A site has also been found in the oxidized GdBaMnFeO6. The structures and properties are significantly affected by the valence states of cations and their ordering manner in the oxides. The present results reveal the importance of controlling the valence states and the ordering of cations in transition-metal oxides.

2. EXPERIMENTAL SECTION GdBaMnFeO5 was prepared by conventional solid-state reaction. Stoichiometric amounts of Gd2O3, BaCO3, Fe2O3, and MnCO3 were well mixed and fired at 1300 °C for 12 h in argon atmosphere. To establish an appropriate reducing condition, a Zr-metal sponge was placed close to the sample. GdBaMnFeO6 was obtained from the prepared GdBaMnFeO5 by heating at 500 °C for 1 h in air. Note that the 500 °C annealing temperature is low enough to retain the cation ordering. Crystal structures of the compounds were analyzed by combination of selected-area electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM), high-angle annular dark field (HAADF), and annular bright field (ABF) scanning TEM (STEM), electron energy-loss spectroscopy (EELS) and exit wave reconstruction (EWR). For TEM studies the compounds were ground in n-butyl alcohol and ultrasonically dispersed. A few drops of the resulting suspension were deposited on a carbon-coated grid. SAED, HRTEM, and EELS experiments were performed with a JEOL JEM 3000F microscope operating at 300 kV (double tilt ±20°) (point resolution 0.17 nm), fitted with X-ray energy dispersive spectroscopy (XEDS) microanalysis system (OXFORD INCA), and ENFINA spectrometer with an energy resolution of 1.3 eV. The atomic ratio of the metals was determined by XEDS analyses finding good agreement between analytical and nominal composition in all the crystals. Average oxidation state of Fe atoms of the oxide was determined by EELS.14 When necessary, plural-scattering effects were removed with a Fourierratio deconvolution method.15 The spectra were acquired in diffraction mode, with a dispersion of 0.1 eV/channel, a collection angle β ≈ 5.3 mrad and an acquisition time of 2 s. The exit-plane wave was reconstructed and residual aberrations were numerically corrected for the complex valued exit-plane wave using the IWFR software reaching the information limit of the JEM 3000F microscope (0.11 nm) .16 The phase image was digitally processed with iMTools electron microscope processing software.17 HAADF and ABF STEM experiments and EELS mapping were performed on an ARM 200cF microscope, fitted with a condenser lens aberration corrector (point resolution in STEM mode of 0.08 nm) and GIF Quantum-ER spectrometer. HAADF images were acquired with an inner acceptance angle of 90 mrad and ABF ones with a collection angle of 11 mrad. EELS mapping was performed with a collection semiangle β ≈ 30 mrad, 0.5 eV/channel dispersion and collection time for each spectrum of 0.09 s. Gd-M4,5, Ba-M4,5, Fe-L2,3, and Mn-L2,3 edge signals were chosen for mapping. The crystal structures were refined with synchrotron X-ray diffraction (SXRD) data taken at the beamline BL02B2 in SPring-8. The room-temperature SXRD pattern obtained with a wavelength of 0.0774939 or 0.0775438 nm was recorded on the image plate of a large Debye−Scherrer camera. The powder sample was put into a 0.1 mm glass capillary tube to minimize absorption and rotated during the measurement. The obtained data were analyzed with the Rietveld method by using the RIETAN-FP software package.18 The Fe oxidation state was estimated from 57Fe Mössbauer spectrum of the sample at room temperature. The spectra were observed in transmission geometry in combination with a constantacceleration spectrometer using 57Co/Rh as a radiation source. α-Fe was used as a control for velocity calibration and isomer shift. The obtained spectra were fitted by a least-squares method with Lorentzian functions. The magnetic properties were also measured with a commercial Quantum Design MPMS SQUID magnetometer. Temperature dependence of the magnetic susceptibility was measured at 5−400 K in an external magnetic field of 10 kOe.

Figure 1. Experimental SAED patterns of a crystal of GdBaMnFeO5 along the (a) [001]p, (b) [01̅1]p, and (c) [1̅10]p zone axes. The patterns are indexed on the basis of a cubic perovskite structure. (d) Experimental HRTEM image of a crystal along the [1̅10]p zone axis.

perovskite structure. Strong Bragg reflections characteristic of the perovskite structure with extra reflections at Gp ± 1/2(001)* indicate a 2-fold structure modulation along the [001]p direction. The unit cell of the crystal structure deduced from the SAED patterns is ap × ap × 2ap. The HRTEM image of a crystal oriented along the [−110]p zone axis (Figure 1d) shows clear contrast differences in agreement with the 2ap periodicity. This superstructure formation is associated with layered-type ordering of the Gd and Ba ions, as it is confirmed by HAADF-STEM in combination with EELS mapping. The HAADF-STEM image taken along the [010]p zone axis (Figure 2a) also shows the layered-type 2a p periodicity along the [001]p direction. The EELS mapping taken in this image (Figure 2b) confirms that (001)p planes of Gd alternate with (001)p planes of Ba. On the contrary, EELS mapping of the Mn and Fe ions, indicates that the Mn and Fe ions do not present the layered-manner ordering. Note also

Figure 2. (a) Experimental HAADF-STEM image of a crystal of GdBaMnFeO5 along the [010]p zone axis. (b) EELS maps of the area of the crystal indicated in yellow in panel a. Yellow arrows indicate cation-layers of Gd and Ba. Columns of Gd atoms are indicated in red, Ba atoms in green, and columns of Mn and Fe atoms in blue. B


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Inorganic Chemistry

Figure 3. (a) Experimental ABF-STEM image of a crystal of GdBaMnFeO5 along the [010]p zone axis. Columns of oxygen atoms are indicated in yellow. Columns of Gd atoms are indicated in red, Ba atoms in green and columns with both Fe and Mn atoms are indicated in blue. (b) Reconstructed phase image of the exit wave along the [010]p zone axis. Drawing of the crystal structure determined with the HAADF-STEM, ABFSTEM, and EWR results projected along the [010]p zone axis.

that the possible rock-salt-type ordering of the B-site cations, which would give a superlattice reflection at Gp ± 1/2(111)* in the SAED patterns,19 is excluded. Therefore, the Mn and Fe ions are randomly located at the B site of the structure. The anion sublattice information has been obtained by ABF-STEM in combination with EWR. Figure 3a shows the ABF-STEM image of a crystal of GdBaMnFeO5 along the [010]p zone axis. Figure SI-1 shows both HAADF-STEM and ABF-STEM images for comparison. The [010]p zone axis projection of the structure can isolate each column of Ba, Gd, and oxygen atoms. Four columns of oxygen atoms are clearly seen around the columns of the Ba atoms. On the contrary, the absence of dark contrast within the Gd columns along the [100] p direction, marked by yellow arrows, suggests absence of oxygen at those positions. Figure 3b shows the reconstructed phase image of the [010]p zone axis. The bright dots correspond to the projected potential of the different atomic columns identified by dark contrast in the ABF-STEM image. The intensity profile performed along the [001]p direction in the row of columns of atoms marked by the red arrow demonstrates that the oxygen vacancies are located within the Gd layers. The peaks with higher intensity are associated with the (Mn/Fe)O columns and those with lower intensity to the columns of oxygen atoms. The signal intensity profile does not show peak associated with oxygen at the sites located in the Gd planes. From these results, an oxygen-deficient perovskite-type model of the crystal structure, which consists of the layered ordering of Ba and Gd atoms and the (Fe/Mn)O5 pyramids is proposed, as seen in the model superimposed in the reconstructed image. It is noted that the present crystals do not show stacking defects or nonordered domains, as it is found when this material is prepared by H 2 treatment of Gd0.5Ba0.5Mn0.5Fe0.5O3‑δ.13 On the basis of this model, the crystal structure was refined by the Rietveld method with the SXRD data (Figure 4a). Because Fe and Mn are difficult to be distinguished from the SXRD data due to their very close X-ray scattering amplitudes, in the structure refinement Fe/Mn occupancies at 0.5/0.5 were fixed according to the results of the EELS mappings. As evidently seen in the superstructure diffraction peak at 2θ = 5.8°, which corresponds to d = 0.76 nm ∼2ap, the refinement results confirm that the Ba and Gd atoms are ordered in the tetragonal P4/mmm structure (structure parameters are listed in Table SI-1). The refined occupancy for the O3 site was 0.05(1), confirming that oxygen sites in the Gd layers are

Figure 4. Synchrotron X-ray powder diffraction profiles of (a) GdBaMnFeO5 and (b) GdBaMnFeO6 (λ = 0.0774939 nm) and the results of Rietveld structure analysis. The dots and solid lines represent the experimentally observed and calculated profiles, and the solid lines below corresponds to the intensity difference. Ticks indicate the Bragg reflection positions for P4/mmm (a) and Pm (b) crystal structures.

vacant. The results are consistent with those obtained from SAED, HRTEM, STEM, EELS, and EWR. Therefore, the crystal structure of GdBaFeMnO5 is an A-site layered-ordered oxygen deficient perovskite with the (Fe/Mn)O5 pyramids. The oxidation states of Mn and Fe of the prepared GdBaFeMnO5 determined by EELS were 2.3 and 3.0, respectively (Figure SI-2). The Mössbauer spectrum at room temperature (Figure 5a) shows a magnetically ordered sextet with the isomer shift of 0.35 mm/sec, which is a typical value for Fe3+, consistent with the EELS analysis. Considering the chemical composition of GdBaFeMnO5 with the complete oxygen vacancies in the Gd layer, we can conclude that the ionic configuration of the compound is Gd3+Ba2+Mn2+Fe3+O5. It is interesting to note again that the Gd3+ and Ba2+ ions are ordered at the A site in a layered manner, while the Fe3+ and Mn2+ ions are located randomly at the pyramidal coordinated B-site. When the as-prepared GdBaFeMnO5 sample is heated in air, a weight-increase corresponding to one mole of oxygen atoms per formula unit was observed in the thermogravimetric analysis (Figure SI-3), suggesting that GdBaMnFeO5 changed to GdBaMnFeO6. Figure 6 shows SAED patterns of a crystal of GdBaMnFeO6 along three zone axes and the HRTEM image of a crystal oriented along the [1̅10]p zone axis. Significant differences are found in the patterns compared to those of GdBaMnFeO5. In addition to the Bragg reflections of the perovskite structure, superlattice reflections at Gp ± 1/2(001)* associated with the layered-type ordering of the Ba and Gd atoms and reflections at Gp ± 1/2(110)* and Gp ± 1/2(111)* C


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Inorganic Chemistry

A-site layered ordering of Gd and Ba and the B-site location at random of Mn and Fe remain by the annealing in the oxidizing atmosphere. Note that a close inspection of the cation distances in the image reveals modulation of the crystal structure associated with the Gp ± 1/2(110)* and Gp ± 1/2(111)* reflections. As shown in the line intensity profiles along the [110]p direction in the BaO4 and GdO4 layers (Figure 7c), this modulation is caused by periodic shifting of Gd atoms along the [110]p direction. While a constant Ba−Ba distance of 0.27 nm is found, alternated Gd−Gd distances of 0.23 and 0.31 nm are observed. On the contrary, there are not different (Mn/Fe)(Mn/Fe) distances along [110]p or [001]p direction, neither Ba−Gd distances along [001]p. Taking into account the A-site layered-type ordering in combination with Gd shifting along the [110]p direction, we have constructed a √2ap × √2ap × 2ap structural model of GdBaMnFeO6 and the crystal structure has been refined with the SXRD data (Figure 4b). The results of the refinement are listed in Table SI-2. Note that the refined oxygen occupancies in the Gd layer were 1.02(3), confirming that the oxygen atoms are fully incorporated into the Gd layer making the (Mn/Fe)O6 octahedra. The crystal structure of GdBaMnFeO6 is monoclinic with space group Pm and significantly distorted (Mn/Fe)O6 octahedra, as seen in the crystal structure representation in Figure 8. The average (Mn/Fe)-O bond length in the octahedra is 0.195 nm, which decreases from 0.202 nm in GdBaMnFeO5, indicating that the average valence states of the B-site (Mn/Fe) ions increase by the oxidation. The oxidation states of Mn and Fe in GdBaMnFeO6 determined by the EELS experiments were 3.8 and 3.0, respectively (Figure SI-4). The Mössbauer spectrum for GdBaMnFeO6 at room temperature (Figure 5b) shows a doublet component with the isomer shift of 0.36 mm/sec, which is almost the same as that of Fe3+ observed in GdBaMnFeO5, confirming the 3+ state of Fe in GdBaMnFeO6. Considering that the oxygen sites are fully occupied by the annealing, the most probable ionic configuration of the compound should be Gd3+Ba2+Mn4+Fe3+O6. Interestingly, the average oxidation state of the B-site Mn/Fe increases from 2.5+ in GdBaMnFeO5 to 3.5+ in GdBaMnFeO6, but only the Mn oxidation state increases from 2+ to 4+, whereas the Fe oxidation state remains at 3+. The result shows a contrast with the recent observation in the topotactic change from the brownmillerite Ca2Mn3+Fe3+O5 to the fully oxygenated double perovskite Ca2Mn4+Fe4+O6,21,22 where the cation oxidation states of both Mn3+ and Fe3+ increase to Mn4+ and Fe4+, respectively. It is also important to note that the magnetic behavior changes with the oxidation although the B-site Fe and Mn cation arrangement is maintained during the topotactic oxidation. As we see in the sextet Mössbauer spectrum of Fe3+, GdBaMnFeO5 shows antiferromagnetism of Mn2+ and Fe3+ at room temperature. Because the Mn2+ and Fe3+ ions have isoelectronic d5 electron configurations, the antiferromagnetic spin structure could be stabilized even with the random distribution of those ions in the perovskite structure. GdBaMnFeO6, on the other hand, shows paramagnetism of Mn4+ and Fe3+ at room temperature. Although it is difficult to determine the precise magnetic transition temperature due to the large magnetic moment of Gd3+ (Figure SI-5), the magnetic transition temperature seems to be lower than or very close to room temperature. Note that the oxidized Mn4+ with d3 electron configuration in GdBaMnFeO6 can induce the

Figure 5. Mö s sbauer spectra at room temperature of (a) GdBaMnFeO5 and (b) GdBaMnFeO6.

Figure 6. Experimental SAED patterns of a crystal of GdBaMnFeO6 along the (a) [001]p, (b) [010]p, and (c) [11̅ 0]p zone axes. The patterns are indexed on the basis of a cubic perovskite structure. (d) Experimental HRTEM image of a crystal along the [11̅ 0]p zone axis.

are observed. Construction of the reciprocal lattice from the SAED results indicates a √2ap × √2ap × 2ap or a 2ap × 2ap × 2ap unit cell of the crystal structure. The contrast differences in the HRTEM image of the [−110]p zone axis, on the other hand, indicate a √2ap × 2ap periodicity in agreement with the √2ap × √2ap × 2ap unit cell. The Gp ± 1/2(110)* and Gp ± 1/2(111)* reflections can be associated with tilting of the (Mn/Fe)O6 octahedra.20 However, Gp ± 1/2(111)* reflections can also be associated with rock-salt type ordering of the Mn and Fe atoms.19 Figure 7 shows the HAADF-STEM image and EELS mapping along the [1̅10]p zone axis of a crystal of GdBaMnFeO6. The HAADF-STEM image agrees with layered ordering of the Ba and Gd atoms. The EELS maps of Gd and Ba confirm the layered ordering. However, the EELS maps of the Fe and Mn atoms indicate that these two different atoms are located at random within the B positions of the structure discarding the rock-salt type ordering. Therefore, the D


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Inorganic Chemistry

Figure 7. (a) Experimental HAADF-STEM image of a crystal of GdBaMnFeO6 along the [11̅ 0]p zone axis. Red and green lines indicate cation-layers of Gd and Ba, respectively. (b) EELS maps of the area of the crystal indicated in yellow in panel a. Columns of Gd atoms are indicated in red, Ba atoms in green, Fe atoms in brown and columns of Mn atoms in blue. (c) Intensity line-profile along the [110]p direction collected for the columns of atoms of Gd (red) and Ba (green). Distance between columns of Ba atoms along the [110]p direction is 0.27 nm; two alternated distances of 0.23 and 0.31 nm between columns of Gd atoms along the [110]p direction.

oxidized phase GdBaMnFeO6 is obtained by low-temperature topotactic reaction, which maintains the layered ordering. In addition to charge variations of the B-site cations, oxidation of GdBaMnFeO5 reveals interesting modifications in the magnetic properties and crystal structure of the oxidized compound. The average oxidation state of the B cations increases from 2.5+ in GdBaMnFeO5 to 3.5+ in GdBaMnFeO6 only due to oxidation of Mn from 2+ to 4+ while the oxidation state of Fe remains 3+. This variation of the oxidation state of Mn also affects the magnetic properties of the oxide: while GdBaMnFeO5 shows antiferromagnetism of Mn2+ and Fe3+ at room temperature, GdBaMnFeO6 is a paramagnetic compound. Determination of the crystal structure of GdBaMnFeO5 indicates layered-type ordering of Gd and Ba atoms at the A sites but Mn and Fe located at random at the B sites. The coordination polyhedral of these B atoms consists of (Fe/ Mn)O5 pyramids due to location of the “anion vacancies” within the Gd (001)p-planes. Oxidation of GdBaMnFeO5 implies filling of the anion positions of the GdO-planes, which drives modulation of the crystal structure associated with periodic displacement of the Gd atoms along the [110]p direction ([100] considering the √2ap × √2ap × 2ap unit cell) to optimize the Gd−O bonding distances.

Figure 8. Crystal structure of GdBaMnFeO6. Purple, green, and red spheres represent Gd, Ba, and oxygen atoms, respectively. Randomly distributed Mn and Fe atoms form distorted octahedra.

ferromegnetic superexchange interaction with Fe3+ (d5) according to the Kanamori−Goodenough rule. Although it is difficult to discuss the details of the superexchange interactions of the randomly distributed Mn and Fe ions at the B sites of the perovskite structure, the Mn4+−Fe3+ ferromagnetic interaction could reduce the antiferromagnetic interactions of Fe3+−Fe3+ and Mn4+−Mn4+, leading to the decrease in the magnetic transition temperature.

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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.6b02472. Experimental HAADF-STEM and ABF-STEM images of GdBaMnFeO5, EELS spectra of GdBaMnFeO5 and GdBaMnFeO 6 , thermogravimetric analysis for GdBaMnFeO5, tables containing the structural parame-

4. CONCLUSIONS GdBaMnFeO5 with complete A-site layered-type ordering of the Gd and Ba atoms has been obtained for the first time by using the appropriate reducing conditions of synthesis. The E


Article

Inorganic Chemistry

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ters of GdBaMnFeO5 and GdBaMnFeO6 obtained from Rietveld analysis, and temperature dependence of magnetic susceptibility of GdBaMnFeO5 and GdBaMnFeO6 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Susana García-Martín: 0000-0003-0729-4892 David Á vila-Brande: 0000-0003-0452-2482 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. The authors contributed equally. Funding

JST-MINECO Strategic International Research Cooperative Program on Multidisciplinary Materials Science (Project PIB2010JP-00181). Grants-in-Aid for Scientific Research (Nos. 16H00888, and 16H02266), grant for the Joint Project of Integrated Research Consortium on Chemical Sciences from MEXT, grant from JST-CREST program of Japan, grant from JSPS Core-to-Core program(A) Advanced Research Networks, MINECO Project MAT2013-46452-C4-4-R, and CM project MATERYENER3CM-S2013/MIT-2753. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank T. Saito and D. Kan for their help during this study. The synchrotron radiation experiments at SPring-8 were performed with the approval of the Japan Synchrotron Radiation Research Institute (proposal Nos: 2013B1226 and 2014A1474). We thank the ICTS Centro Nacional de Microscopiá Electrónica of U.C.M. for technical assistance.

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REFERENCES

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Crystal Structures at Atomic Resolution of the Perovskite-Related

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