Atomic and Local Electronic Structures of ... - ACS Publications

Subscriber access provided by FLORIDA STATE UNIV

Article

Atomic and local electronic structures of CaAlMnO as an oxygen storage material 2

5+#

Genki Saito, Yuji Kunisada, Kazuki Hayami, Takahiro Nomura, and Norihito Sakaguchi Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b04099 • Publication Date (Web): 08 Dec 2016 Downloaded from http://pubs.acs.org on December 10, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Atomic and local electronic structures of Ca2AlMnO5+δ as an oxygen storage material Genki Saitoa*, Yuji Kunisadaa, Kazuki Hayami a, Takahiro Nomura a, and Norihito Sakaguchia a

Center for Advanced Research of Energy and Materials, Hokkaido University, Kita 13 Nishi 8, Kitaku, Sapporo 060-8628, Japan

Abstract We investigated the atomic and local electronic structures of Ca2AlMnO5+δ to assess its potential as an oxygen storage material. High-angle annular dark-field scanning transmission electron microscopy was used to investigate structural changes in the material during oxygen storage. We found that the AlO4 tetrahedra convert to AlO6 octahedra during such a process. According to the Mn L-edge electron energy-loss near-edge structure (ELNES) measurements, the Mn oxidation state increased from +3 to +4 on oxygen storage. The observed site-resolved oxygen K-ELNES and first-principles electronic structure calculations showed that each non-equivalent oxygen site has different characteristics, corresponding to local chemical bonding and oxygen intake and release. For Ca2AlMnO5, the pre-peak intensity was higher at MnO6 octahedral sites, indicating covalent bonding between the oxygen and Mn atoms. After oxygen storage, the ELNES spectra revealed that the Jahn–Teller distortion of the Mn sites was suppressed by the increase in the Mn oxidation state; furthermore, the spectra indicate that Mn octahedron shrank in the z-direction, accompanied by an increase in Mn–O covalent bonding, thus providing sufficient space to form octahedral AlO6. Consequently, we found that the reversible oxygen storage ability is related to the canceling of the volume changes of the Mn and Al octahedra. The electrons in Mn 3d orbitals play an important role in this structural change.

*

Corresponding author: [email protected] (G. Saito)

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Graphic

Keywords: STEM–EELS, first-principles calculation, local electronic structure, HAADF, image simulation, brownmillerite, oxygen storage material


Page 2 of 30

Page 3 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction Oxygen gas is widely used industrially as a combustion improver and oxidizer, which necessitates large amounts of oxygen. Although atmospheric air contains oxygen, an energy-efficient separation technology to obtain oxygen from the air is yet to be established, despite the importance of the technique in saving energy. Usually, the cryogenic distillation method is used for oxygen separation, in which air is cooled below the boiling point of oxygen, but this process requires a large amount of electric power. The utilization of oxygen storage materials (OSMs) has recently been proposed as an efficient technology for oxygen purification1-3, wherein a large amount of oxygen can be stored and released reversibly. Transition metal oxides, such as BaYMn2O5+δ4-7, BaErMn2O5+δ8, REBaMn2O6–x9, BaLnMn2O5+δ10, Dy1–xYxMnO3+δ11, 12, RBaCo4O7+δ13, 14, YBaCo4O7+δ15, and LuFe2O4+x16, have been investigated as OSMs because they can easily change their oxygen stoichiometry with variations in temperature and oxygen partial pressure.

Among various OSMs, brownmillerite-type Ca2AlMnO5+δ is attractive because of to its high oxygen storage capability and the ready availability of the constituent elements17-19. According to an X-ray diffraction study by Motohashi et al.17, Ca2AlMnO5 (space group Ibm2 (I2bm)) transforms into Ca2AlMnO5.5 (space group

Imma) on temperature or pressure change. According to thermogravimetric (TG) analysis, Ca2AlMnO5.5 can store 3 wt.% oxygen storage19. However, the large temperature hysteresis between oxygen intake and release and the slow reaction rate require improvement. To control the oxygen storage properties, a theoretical material design based on chemical doping and an understanding of the local electronic structure are necessary. Usually, the crystal structure, coordination, and oxidation states of metals



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 30

in oxides can be analyzed by diffractometry, Mössbauer spectroscopy, and X-ray photoelectron spectroscopy. However, Ca2AlMnO5+δ contains non-equivalent atomic sites. Fig. 1 shows the atomic structure, wherein three non-equivalent oxygen sites, Mn– Mn, Al–Al, and Mn–Al bridge sites, exist in Ca2AlMnO5. For Ca2AlMnO5.5, the number of non-equivalent atomic sites increases by the layer-by-layer transformation of AlO4 tetrahedra into AlO6 octahedra, as shown in Fig. 1(b). To investigate the local electronic structure of each non-equivalent atomic site, we cannot use a classical analysis of the bulk materials; instead, atomic-resolution analysis is required.

Therefore, in this study, high-angle annular dark-field scanning transmission electron microscopy (HAADF–STEM) has been applied to investigate the atomic structure of brownmillerite-type Ca2AlMnO5+δ. In the HAADF–STEM method, an aberration-corrected fine electron probe is scanned across the sample, and scattered electrons generated from each atomic column are detected, allowing the direct imaging of atomic columns20. Moreover, HAADF–STEM imaging is incoherent, where the image contrast is proportional to the atomic number (∝Z1.6–2.0) because thermal diffuse scattering (TDS) is dominant in high-angle electron scattering. Using HAADF-STEM images with multi-slice image simulation, we can confirm a structural change from AlO4 tetrahedra to AlO6 octahedra during the oxygen storage process. To analyze the local electronic structure, electron energy-loss spectroscopy (EELS) was combined with HAADF-STEM21. In the Mn L2,3-edge electron energy-loss near-edge structure (ELNES), the shape of the L2,3 edges, chemical shifts, and the L3/L2 ratio provide a sensitive fingerprint for analyzing the Mn oxidation state22,

23

. As such, the local

electronic structure of transition metals in brownmillerite-type oxides has been previously investigated9, 24, 25. The local electronic structure of oxygen is important for


Page 5 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


understanding the origin of oxygen storage properties. Thus, we measured the site-resolved oxygen K-edge at each non-equivalent oxygen column.

Compared to the L2,3-edge ELNES of transition metals, the oxygen K-edge ELNES is challenging to analyze because of its high sensitivity to the exact electronic structure; in other words, however, oxygen K-edge ELNES can be used to clarify the local electronic structure. Recently, the interpretation of oxygen K-edge ELNES spectra using first-principles band structure calculations has become a common method for analyzing local electronic and bonding structures, and the measured ELNES spectra agree well with the calculated partial density of state (PDOS) in perovskite-related structures including YBa2Cu3O7−x26, Ca(Sr)FeO2.527, and Ca2B’xFe2-xO5 (B’=Al, Ga)28. Therefore, in this study, we investigated the electronic structural changes between Ca2AlMnO5 and Ca2AlMnO5.5. Because Mn3+ is a Jahn–Teller ion, the MnO6 octahedron is distorted, and this distortion might affect its bonding with oxygen atoms. We examined the origin of the oxygen storage ability by analyzing the site-resolved ELNES spectra with the assistance of first-principles electronic structure calculations.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 1. Atomic structure models of brownmillerite-type Ca2AlMnO5 and Ca2AlMnO5.5.

2. Methods Ca2AlMnO5+δ was prepared by combining solution combustion synthesis with heat treatment19. The raw materials were Ca(NO3)2·4H2O, Al(NO3)·9H2O, and Mn(NO3)2. Glycine (C2H5O2N) was used as the combustion fuel. The sol-gel mixture was combustion synthesized at 400 °C in air. The obtained precursor was annealed at 1250 °C in a flow of air and N2 gas. Oxygen intake and release were performed using a TG analyzer (Mettler Toledo TG-DSC1) at a scan rate of ±2 °C/min in flowing O2 gas. Powder X-ray diffraction (XRD, Rigaku Miniflex, Cu Kα) was carried out to determine the crystal structures of the synthesized materials. Structural refinements were also performed using PDXL software (Rigaku). To confirm the electron diffraction patterns, we placed ethanol droplets containing a suspension of the sample powders onto a collodion-coated Cu microgrid (EM Japan Co., Ltd.). The deposited suspension was subsequently dried at 60 °C in an oven before TEM observation. Electron diffraction


Page 6 of 30

Page 7 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and high-resolution images were characterized using a 200-kV transmission electron microscope (TEM, JEOL, JEM-2010F). To prepare thin TEM samples, we mixed the synthesized powder with resin and cotton and placed this mixture between two silicon wafers. The wafer was then heated at 140 °C for 15 min. The obtained block was sliced, mechanically polished to a thickness of less than 20 µm, and then ion-milled with 5–1 kV Ar ions (PIPS model 691, Gatan Inc.). To decrease the thickness of the damaged layer, we applied a 1 kV in the final Ar-ion milling step. We chose a thick area of the specimen for observation to avoid the influence of the surface damaged layer. HAADF– STEM (Titan3 G2 60-300, FEI Company) was performed at 300 kV. The inner collection semi-angle of the HAADF detector and the convergence semi-angle of the electron probe were 64 and 22 mrad, respectively. To avoid image drift, we acquired cumulatively 20 images of 512 × 512 pixel resolution within a short time and integrated after drift correction. HAADF–STEM images were also calculated with the Dr. Probe version 1.7 multi-slice image simulator29, which uses the frozen phonon approximation to incorporate the TDS influence. In these image simulations, we used a Lorentz function with a full width at half maximum (FWHM) of 0.14 nm for the electron probe intensity distribution. The defocus spread was set to 3 nm. In selected area electron diffraction (SAED) mode, electron energy loss spectra (EELS) were acquired at 300 kV using a Titan3 G2 60-300, in which the selected area of thin sample (around 80 nm) was irradiated with the monochromator-excited electron beam with a beam current of 0.2– 0.3 nA, collection semi-angle of 3.3 mrad, and FWHM energy spread of 0.2 eV. Site-resolved EELS were also measured in STEM mode in the [101] direction at 80 kV with a FWHM of 0.5 eV using a spherical aberration corrected STEM (JEM ARM-200F). The collection semi-angles in STEM mode were 67.7 mrad and 131.3 mrad for Ca2AlMnO5 and Ca2AlMnO5.5 respectively. The acquired area was 128 × 88–



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

148 pixels with a step of 0.01 nm/pixel in the line scan mode. The energy dispersion of EELS was 0.1 eV/channel with a probe current of approximately 50 pA. Structural models and calculated electron diffraction patterns were drawn using CrystalMaker® (Version 9.2.5 for Windows, CrystalMaker Software Ltd.).

First-principles electronic structure calculations were performed based on spin-polarized density functional theory (DFT), using the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional30, 31 for the exchange–correlation energy, as implemented in the plane-wave and projector augmented wave method using the Vienna Ab-initio Simulation Package (VASP 5.3.3)32-37. We applied a 600-eV cutoff to limit the plane-wave basis set without compromising computational accuracy. The on-site coulomb interaction for Mn 3d electrons was treated based on the formalisms proposed by Dudarev et al.38, using an effective on-site coulomb interaction parameter of 2.0 eV that reproduces the experimentally reported oxygen storage properties17. We used the unit cells of Ca2AlMnO5 and Ca2AlMnO5.5. A 3 × 3 × 3 Monkhorst–Pack special k-point grid39 was used for the first Brillouin zone sampling with a Gaussian smearing model of σ = 0.05 eV for the atomic structure optimization and σ = 0.2 eV for the PDOS calculations. We also calculated the O K-edge ELNES based on the Z+1 approximation (e.g., replace O with F) to model the corresponding core–hole effect. The calculated lattice constants of Ca2AlMnO5 and Ca2AlMnO5.5 were a = 5.2760 Å, b = 15.1641 Å, and c = 5.5228 Å, and a = 5.3035 Å, b = 29.5476 Å, and c = 5.4310 Å, respectively. The antiferromagnetic magnetic structure was treated as previously reported40. Atomic positions were optimized until the forces on each atom were smaller than 0.02 eV/Å.


Page 8 of 30

Page 9 of 30


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3. Results and discussion XRD was used to determine crystal structures of the synthesized powders (Fig. S1 in the supporting information). According to the results of Rietveld structural refinement, crystals of Ca2AlMnO5 are orthorhombic with lattice constants of a = 5.2425 Å, b = 15.0051 Å, and c = 5.4721 Å in the I2bm space group. The crystal structure of Ca2AlMnO5.5 was determined to the lattice parameters a = 5.2547 Å, b = 29.4266 Å, and c = 5.3731 Å in the Imma space group. Detailed structural parameters are listed in Tables S1 and S2. These crystal structures agree with those reported17, 40, 41, in which small amount of a secondary-phase-like defect structure or unreacted phase were observed in Ca2AlMnO5.5. As shown in Fig. 1, the AlO6 octahedral sites are present in every other Al layer in Ca2AlMnO5.5, resulting in a doubling in the periodicity along the b-axis, in which the crystal lattice shrank from 15.0051 to 14.7133 Å (29.4266/2) along the b-axis. Single-crystal electron diffraction patterns were also obtained using TEM. Fig. 2(a) shows the calculated electron diffraction patterns along the [101] direction, for which a decrease in the distance of (020) revealed an increase in periodicity along the b-axis. The observed SAED patterns shown in Fig. 2(b) match well with the calculated results. For Ca2AlMnO5.5, diffuse streaks appeared along the [010] direction, suggesting the existence of stacking faults, such as lattice defects, in the crystal. The acquired high-resolution TEM (HR–TEM) image in Fig. 2(c) confirms the ordered layer structure corresponding with the structure models. However, determining the atomic position of each element from the HR–TEM image was difficult.


Page 10 of 30

Page 11 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 2. (a) Calculated electron diffraction patterns, (b) observed electron diffraction patterns, and (c) HR–TEM images of Ca2AlMnO5 and Ca2AlMnO5.5 along the [101] direction.

To elucidate the crystal structures of the synthesized materials at the atomic scale, we used HAADF-STEM. Fig. 3(a) shows the atomic structure model of Ca2AlMnO5 and Ca2AlMnO5.5 along the [101] direction. The corresponding HAADF–STEM image in Fig. 3(b) was calculated using multi-slice image simulation with a sample thickness of approximately 20 nm. Bright dots correspond to the atomic columns and atomic positions, which agree with the structural model shown in Fig. 3(a). The Mn columns had a brighter contrast than the Al columns, which have a relatively low atomic number. Similarly, the oxygen columns were not clear owing to the difference in atomic number. While all Al atoms were observed AlO4 tetrahedra (Alt) in Ca2AlMnO5, AlO6 octahedra (Alo) were observed in every other Al layer in Ca2AlMnO5.5. Image simulation results revealed that Al columns in Alo sites had a brighter contrast than those in Alt sites. Therefore, these two different Al sites could be distinguished by the contrast. This contrast variation was caused by differences in electron beam channeling. At the Alt



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

sites, the arrangement of Al atoms is not straight but zigzagged along the [101] direction (see Fig. S2 in the supporting information). In such an arrangement, electron beam channeling along the atomic column is not enhanced, resulting in a decrease in contrast. At the Alo sites, the Al atoms are arranged in an almost straight line, which increases the contrast for these columns. Similar phenomena have been observed in other materials27, 42

. The observed HAADF–STEM images shown in Fig. 3(c) were well matched to the

simulated images shown in Fig. 3(b). Atomic columns of Mn, Ca, and Al were clearly observed, with contrast varying in accordance with the atomic number. For site-resolved analysis, it was necessary to distinguish the Alt and Alo sites. The graphs on the right-hand side of Fig. 3 show the intensity profiles corresponding to the vertical lines shown in Fig. 3(c). The intensity varies significantly between the Alt and Alo sites owing to the different channeling behaviors. Based on these results, we succeeded in directly observing the ordered layer-by-layer structure of Ca2AlMnO5.5. In addition, the planar fault in Ca2AlMnO5.5, in which two AlO4 tetrahedral planes are continuously arranged as a stacking error, was rarely observed (Fig. S3 in the supporting information). The presence of these stacking faults was found to cause diffuse streaks in the electron diffraction pattern, as shown in Fig. 2(b). These stacking faults also affect the oxygen storage ability.


Page 12 of 30

Page 13 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 3. (a) Model of the atomic structure of Ca2AlMnO5 and Ca2AlMnO5.5 in the [101] direction. (b) Multi-slice image simulation and (c) observed HAADF-STEM images. The graphs on the right-hand side show the intensity profiles corresponding to the vertical lines shown in Fig. 3(c).

Before site-resolved analysis, an EEL spectrum utilizing parallel beam illumination was acquired. During measurement, the crystal structures did not change. Fig. 4(a) shows the O K–edge ELNES, which provides information on the excitations of O 1s electrons to 2p bands. After oxygen storage, the pre-peak intensity around 529 eV increased significantly, which might be caused by a change in the local chemical bonding around oxygen. In general, an increased O K–edge ELNES peak suggests an increase in the unoccupied antibonding hybrid orbitals of oxygen and the cation27.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Because of the non-equivalent oxygen sites, including Mn–Mn, Al–Al, and Mn–Al bridge sites, further site-resolved analysis is required to discuss the local electronic structure.

The L2,3–edge ELNES of transition metals in oxides shows characteristic white lines due to the transitions of excited 2p core electrons into unoccupied d-orbitals22. To evaluate the oxidation state of Mn, the Mn L2,3–edge ELNES was analyzed, as shown in Fig. 4(b). The peak positions and multiple scattering effects were corrected using zero-loss spectra acquired in dual EELS mode, in which low-loss spectra and core-loss spectra are automatically acquired at the same time for calibration of the energy scale. Using Fig. 4, we identified the following features in the Ca2AlMnO5.5 spectrum compared to that of Ca2AlMnO5: (i) The Mn L2,3–peaks shifted to a slightly higher energy, (ii) the white-line intensity L3/L2 ratio decreased, and (iii) the Mn L3–peak split. According to the literature23, 43, when the Mn oxidation state increases, the energy loss of the Mn L3–edge increases owing to a decrease in coulomb repulsion between the 2p core electrons and valence electrons arising from the charge transfer from Mn to O. The peak shift to higher energies and decreased L3/L2 ratio indicate an increase in the Mn oxidation state. Kurata et al. reported the ELNES of various Mn oxides22, in which splitting of the Mn L3 energy appeared in MnO2 and BaMnO4 owing to the ligand-field splitting of the Mn 3d orbitals. Assuming that the valences of Ca, Al, and O are +2, +3, and –2, respectively, the formal Mn valences in Ca2AlMnO5 and Ca2AlMnO5.5 are +3 and +4, respectively. The obtained results agreed with this Mn valence change. As mentioned, although oxygen atoms are absorbed into the Al layers in the stored oxygen states, the oxygen intake results in an increase in the Mn oxidation state from +3 to +4.


Page 14 of 30

Page 15 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 4. EEL spectra of (a) O K–edge and (b) Mn L–edge ELNES for Ca2AlMnO5 and Ca2AlMnO5.5, acquired by parallel beam illumination in SAED mode.

To evaluate the local electronic structure of oxygen at different non-equivalent sites, STEM–EELS analysis was carried out. For site-resolved EELS, the 300 kV applied voltage is not suitable because of the delocalization effect. Compared to the resolution of the HAADF–STEM imaging, the spatial resolution of EELS is lower because the irradiated focused electron beam excites surrounding electrons, resulting in the wide generation of EELS signals. The delocalization factor, dE, of inelastic electrons losing an amount of energy, ∆E, can be described using the following equation44, 45: dE =

λ  2E    2  ∆E 

3/4

=

λ 2θ E3 / 4

,


(1)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

where λ and E are the wavelength and the incident electron beam energy, respectively, and θE (= ∆E/(2E)) is the characteristic inelastic scattering angle. When the beam energy was 300 kV, the dE of the O K–edge spectra (532 eV) corresponded to 0.192 nm, which might be insufficient to distinguish between different oxygen sites. Therefore, in this study, we applied a voltage of 80 kV for STEM–EELS analysis, which decreased the dE to 0.151 nm. We have also measured the Al L2,3–edge ELNES spectra (around 73 eV) because these spectra are sensitive to the Al coordination number. However, it was difficult to acquire clear spectra owing to the overlap with the plasmon response and multiple scattering. Furthermore, site resolved analysis was also difficult due to increased Al L2,3–edge delocalization factor of 0.669 nm. Based on these results, we focused on the O K–edge ELNES.

The model of the atomic structure obtained by XRD structural refinement and the experimental site-resolved O K–edge ELNES spectra, in which the focused electron beam was irradiated in STEM mode at 80 kV, are shown in Fig. 5. HAADF images were recorded at the same time to confirm the atomic positions. Ca2AlMnO5 has three non-equivalent oxygen sites at the MnO6 octahedra (Mn–Mn), AlO4 tetrahedra (Al–Al), and the linking site between Mn and Al (Mn–Al), in which the number of oxygen atoms at the Mn–Al sites was lower to those at the Mn–Mn and Al–Al sites. As shown in the EEL spectrum of Ca2AlMnO5 shown in Fig. 5(a), a pre-peak is clearly observed for Mn–Mn oxygen. The Mn–Al oxygen also had small pre-peak, although the pre-peak intensity was significantly decreased at the Al–Al site. From these results, we found that the observed pre-peak in Fig. 4(a) relates to the chemical bonding states of the Mn–Mn site oxygen. In the case of Ca2AlMnO5.5, the number of non-equivalent oxygen sites increased to five because of the formation of AlO6 octahedra (Alo). The pre-peaks were


Page 16 of 30

Page 17 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


observed at Mn–Mn and Mn–Alo sites, and these are caused by the different chemical bonding states at each oxygen site. The O K–edge ELNES depends on 1s-2p transitions, and while the isolated O2– ion had a closed-shell electronic structure without unoccupied 2p states, the 1s core electrons of the O2– ion in Ca2AlMnO5 and Ca2AlMnO5.5 can be excited to the unoccupied 2p orbitals formed by hybridization with other cations, such as Mn, Al, and Ca. Therefore, the observed O K–edge ELNES reflect the chemical bonding states between oxygen and the cations. Here, we should mention that the irradiated electrons caused significant excitation of the surrounding electrons owing to delocalization. Therefore, the site-resolved spectra also contain some information concerning nearby sites.

Fig. 5. Structural models and experimental O K–edge ELNES of (a) Ca2AlMnO5 and (b) Ca2AlMnO5.5 at non-equivalent sites of MnO6 octahedra (Mn), AlO4 tetrahedra (Alt),



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and AlO6 octahedra (Alo).

To further characterize the observed site-resolved O K–edge EEL spectra, we calculated the PDOS and ELNES of each non-equivalent oxygen and Mn atom using first-principles calculations, where, for convenience, the b-axis was defined as the z-axis direction in the calculation. Fig. 6 shows the calculated PDOS of the O 2p and Mn 3d orbitals. Here, we denote spin up as the majority spin in these figures. A positive PDOS value in Fig. 6 denotes a majority (up) spin, while a negative value corresponds to a minority (down) spin. For our interpretation of the observed EEL spectra, first, we focused on the Mn oxidation state. As discussed above, the Mn oxidation states in Ca2AlMnO5 and Ca2AlMnO5.5 have been calculated to be +3 and +4, respectively. Owing to the Jahn-Teller effect, the MnO6 octahedra in Ca2AlMnO5 (Mn3+) are distorted in the z-direction, as shown in inset scheme in Fig. 6. In an octahedral geometry, the five 3d orbitals split into the higher eg and lower t2g orbitals. For Mn3+, four electrons occupy the Mn 3d orbitals (dyz, dzx, dx2-y2, and dz2), and the 3dz2 orbital (labeled “b”) of eg symmetry is occupied. In the calculated PDOS of the Mn d orbitals shown in Fig. 6(a), the majority spin of the dz2 state was occupied. Because Mn connects to the Mn–Al oxygen atoms in the z-direction, the pz orbital in the Mn–Al bridge is strongly hybridized with the Mn 3dz2 orbital. However, this feature was not observed in the EEL spectra at the Mn–Al sites because the 3dz2 orbital is occupied. Owing to the delocalization effect, information concerning nearby Mn–Mn oxygen atoms might be mixed into the Mn–Al spectra. In contrast to the Mn–Al oxygen atoms, the Mn–Mn oxygen atoms are located at the sides of Mn atoms, where the Mn dxy orbitals are strongly hybridized with the unoccupied oxygen 2px and 2py orbitals labeled “c” in Fig. 6(a). Additionally, the Mn 3dx2, 3dzy, and 3dxz orbitals bind to the pz orbital of the Mn–


Page 18 of 30

Page 19 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Mn oxygen. From these results, we concluded that the origin of the observed pre-peak from the Mn–Mn oxygen arose from the hybridization of oxygen 2p and Mn 3d antibonding orbitals. In contrast, the Al–Al oxygen atoms bind only to Ca and Al atoms, which form unoccupied hybrid orbitals far above the Fermi level. Consequently, the intensity of the pre-peaks at Al–Al oxygen atoms significantly decreased. The calculated EELS of O K-edge shown in Fig. S4 in the Supporting Information also suggests a similar trend.

Next, we interpreted the site-resolved O K–edge EEL spectrum of Ca2AlMnO5.5 shown in Fig. 5(b). As with the spectrum of Ca2AlMnO5, the pre-peak intensity was significantly increased at Mn–Mn and Mn–Alo sites (highlighted by asterisks). After oxygen storage, one electron was lost from the Mn 3d orbitals, which caused the Jahn– Teller distortion to disappear. Because the occupied electron in the 3dz2 orbitals was lost, the 3dz2 orbital, which is the highest occupied molecular orbital (HOMO), was shifted above the Fermi level, becoming the lowest unoccupied molecular orbital (LUMO). The calculated PDOS for Mn and oxygen reflect this electronic change (see blue arrows in Fig. 6). This electron loss is the main reason for increased pre-peak intensity shown in Fig. 4(a) and Fig. 5(b). Owing to the disappearance of the Jahn–Teller distortion, the Mn 3dz2 and 3dxy orbitals were degenerate, and the Mn octahedron shrank in the z-direction. The origin of pre-peak in ELNES can be explained by this degeneration, and the X-ray diffraction measurements also confirmed the slight shrinkage of the crystal lattice along the b-axis. Because the px and py orbitals are strongly hybridized with the Mn 3dxy orbitals, the Mn–Mn oxygen had a higher pre-peak intensity. Furthermore, at the Mn–Alt and Mn–Alo sites in the z-direction to the Mn atom, the pz orbitals are hybridized with the Mn 3dz2 orbitals. As shown in the structural model in



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 5(b), the distance between Mn and the Mn–Alo oxygen is 1.806 Å, shorter than the distance to the Mn–Alt oxygen (2.134 Å). Because of this short bonding distance, the 2pz orbital in the Mn–Alo oxygen is strongly hybridized with the Mn 3dz2 orbitals, which might cause the increased pre-peak intensity of the Mn–Alo site.

Based on these results, we discuss the origin of the oxygen storage ability of Ca2AlMnO5+δ. Firstly, we focus on the relationship between the Mn oxidation state and the structural changes. As previously discussed, the Mn oxidation states were estimated to be +3 and +4 in Ca2AlMnO5 and Ca2AlMnO5.5, respectively. Ca2AlMnO5 consists of MnO6 octahedra and AlO4 tetrahedra, where the MnO6 octahedron of Ca2AlMnO5 are distorted in the z-direction owing to the Jahn-Teller effect. After oxygen storage, the Mn octahedron shrank along the z-direction owing to the disappearance of Jahn–Teller distortion, accompanied by an increase in Mn–O covalent bonding, which also provided sufficient space to form AlO6 octahedra from the AlO4 tetrahedra. In other words, the formation of AlO6 octahedra reduced the distance between oxygen and the Mn atom in the z-direction, resulting in an increase in the Mn valence to +4. This is a proposed mechanism for the structural changes observed during oxygen storage. Secondly, we discuss the change in the electronic structure using first-principles electronic structure calculations and site-resolved EELS. When the Mn oxidation state increased from +3 to +4 after oxygen storage, an electron is lost from the Mn 3d orbitals, resulting in the formation of AlO6 octahedra from AlO4 tetrahedra. In the case of Ca2AlMnO5, the electron in the Mn 3dz2 orbital (the HOMO) plays an important role in this structural change. After oxygen storage, unoccupied Mn 3dz2 electrons are generated just above the Fermi level. The increased pre-edge intensity at the O K–edge ELNES is shown in Fig. 4, and site-resolved EELS clearly confirmed this electronic variation. Based on this


Page 20 of 30

Page 21 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


discussion, we conclude that reversible oxygen storage ability is achieved by the canceling of the volume changes for the Mn and Al octahedra, where the HOMO-LUMO transitions of the Mn 3d orbitals are related to this structural change.

Fig. 6. Calculated PDOS of O 2p and Mn 3d orbitals at different sites in (a) Ca2AlMnO5 and (b) Ca2AlMnO5.5. Except for Mn–Al, the PDOS minority spin was abbreviated because Mn–Mn and Al–Al were non-spin-polarized, while Mn–Al was spin-polarized. The inset figure shows a schematic illustration of the crystal field splitting of the Mn 3d orbitals.

As an oxygen storage material, Ca2AlMnO5 still has some issues, including a large temperature hysteresis between oxygen intake and release, slow reaction rate, and high working temperature. To control the oxygen storage properties, theoretical material



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

design by chemical doping was considered. From the results obtained in this study, several key factors were identified: (i) the initial crystal structure, (ii) Mn oxidation state, (iii) lattice parameters and bond distances, (iv) Mn–O covalent bond strength, and (v) the HOMO-LUMO transition between Mn 3d orbitals. The brownmillerite structure has several variants, including I2bm, Pnma, Imma, Pcmb, and C2/c, in which the configuration and stacking order of twisting of tetrahedral chains are different41, 46, 47. For example, Sr2MnAlO5 has an Imma structure48, Ca2FeMnO5 a Pnma structure49, and Ca2AlMnO5 an I2bm structure. The structural stabilities are determined by the tetrahedral layer separation and the distortion angle of the tetrahedral chains46. It is possible that the oxygen storage behavior depends on these structural variants. Regarding chemical doping, a large number of possible elemental combinations and doping ratios exist; thus, it would be difficult to explore each composition experimentally. In such a situation, theoretical material design becomes a powerful method. Chen et al. systematically calculated the reaction enthalpies, free energies, and substitutional energies of chemically doped Ca2AlMnO5 assuming an I2bm structure and found that the ionic size or cell volume strongly affected the oxygen release properties and that a higher oxygen p-band position led to a lower reaction enthalpy18. To develop oxygen storage materials with enhanced properties, collaborative study together with material synthesis, oxygen storage characterization, structural analysis, and theoretical calculations are vital.


Page 22 of 30

Page 23 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4. Conclusions In this study, we investigated the atomic and local electronic structures of Ca2AlMnO5 and Ca2AlMnO5.5 for use as oxygen storage materials. A structural change from AlO4 tetrahedra to AlO6 octahedra in every other layer was determined, and we found that the Mn oxidation state increased from +3 to +4 using Mn L–edge EELS. Owing to the Jahn-Teller effect, the MnO6 octahedron of Ca2AlMnO5 (Mn3+) was distorted in the z-direction. The observed pre-peaks in the O K–edge EELS at Mn–Mn oxygen sites were attributed to the antibonding orbital hybridization of oxygen 2p and Mn 3d. After oxygen storage, the 3dz2 orbital was occupied, and the HOMO became the LUMO, resulting in an increased pre-peak intensity in the O K–edge EELS. Owing to the disappearance of the Jahn–Teller distortion, the Mn 3dz2 and 3dxy orbitals were degenerate, and the Mn octahedron shrank along the z-direction with increased Mn–O covalent bonding, which also provided enough space to form the AlO6 octahedra from AlO4 tetrahedra with a slight change in volume. We found that the reversible oxygen storage ability is related to this volume change canceling mechanism for the Mn and Al octahedra, and the HOMO-LUMO transitions between Mn 3d orbitals play an important role for this structural change. This study also demonstrated that the experimentally acquired site-resolved O–K edge ELNES at each non-equivalent site can be interpreted clearly by the first-principles electronic structure calculations, which provide valuable information for further improvement of oxygen storage materials.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Acknowledgments The authors would like to thank Ms. N. Sheng and Ms. K. Miyazaki for providing the Ca2AlMnO5 powders. We also gratefully acknowledge the technical support provided by Mr. K. Ohkubo, Mr. R. Oota, Mr. T. Tanioka, Ms. Y. Yamanouchi, Ms. E. Obari, Dr. Y. Matsuo, and Ms. N. Hirai for the TEM measurements. We also thank Prof. T. Akiyama for the many helpful discussions. Part of this work was conducted at Hokkaido University and supported by the “Nanotechnology Platform” Program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Calculations were performed using computer facilities at the ISSP Super Computer Center (University of Tokyo).

References 1.

Machida, M.; Kawamura, K.; Ito, K.; Ikeue, K., Large-Capacity Oxygen

Storage by Lanthanide Oxysulfate/Oxysulfide Systems. Chem. Mater. 2005, 17, 1487– 1492. 2. Readman, J. E.; Olafsen, A.; Larring, Y.; Blom, R., La0.8Sr0.2Co0.2Fe0.8O3-δ as a Potential Oxygen Carrier in a Chemical Looping Type Reactor: An In-Situ Powder X-ray Diffraction Study. J. Mater. Chem. 2005, 15, 1931–1937. 3. Karppinen, M.; Yamauchi, H.; Otani, S.; Fujita, T.; Motohashi, T.; Huang, Y. H.; Valkeapää, M.; Fjellvåg, H., Oxygen Nonstoichiometry in YBaCo4O7+δ: Large Low-Temperature Oxygen Absorption/Desorption Capability. Chem. Mater. 2006, 18, 490–494. 4.

Motohashi, T.; Ueda, T.; Masubuchi, Y.; Kikkawa, S., Enhanced Oxygen

Intake/Release Kinetics of BaYMn2O5+δ Fine Powders Prepared by a Wet-Chemical Route. J. Ceram. Soc. Jpn. 2011, 119, (1395), 894–897. 5. Gilleßen, M.; Lumeij, M.; George, J.; Stoffel, R.; Motohashi, T.; Kikkawa, S.; Dronskowski, R., Oxygen-Storage Materials BaYMn2O5+δ from the Quantum-Chemical Point of View. Chem. Mater. 2012, 24, 1910–1916. 6. Motohashi, T.; Ueda, T.; Masubuchi, Y.; Kikkawa, S., Oxygen Intake/Release Mechanism of Double-Perovskite Type BaYMn2O5+δ (0 ≤ δ ≤ 1). J. Phys. Chem. C 2013, 117, 12560–12566.


Page 24 of 30

Page 25 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


7.

Motohashi, T.; Takahashi, T.; Kimura, M.; Masubuchi, Y.; Kikkawa, S.;

Kubota, Y.; Kobayashi, Y.; Kageyama, H.; Takata, M.; Kitagawa, S.; Matsuda, R., Remarkable Oxygen Intake/Release of BaYMn2O5+δ Viewed from High-Temperature Crystal Structure. J. Phys. Chem. C 2015, 119, 2356–2363. 8. Świerczek, K.; Klimkowicz, A.; Zheng, K.; Dabrowski, B., Synthesis, Crystal Structure and Electrical Properties of A-site Cation Ordered BaErMn2O5 and BaErMn2O6. J. Solid State Chem. 2013, 203, 68–73. 9. Ávila-Brande, D.; King, G.; Urones-Garrote, E.; Subakti; Llobet, A.; García-Martín, S., Structural Determination and Imaging of Charge Ordering and Oxygen Vacancies of the Multifunctional Oxides REBaMn2O6-χ (RE = Gd, Tb). Adv. Funct. Mater. 2014, 24, 2510–2517. 10. Klimkowicz, A.; Świerczek, K.; Takasaki, A.; Molenda, J.; Dabrowski, B., Crystal Structure and Oxygen Storage Properties of BaLnMn2O5+δ (Ln: Pr, Nd, Sm, Gd, Dy, Er and Y) oxides. Mater. Res. Bull. 2015, 65, 116–122. 11. Abughayada, C.; Dabrowski, B.; Avdeev, M.; Kolesnik, S.; Remsen, S.; Chmaissem, O., Structural, Magnetic, and Oxygen Storage Properties of Hexagonal Dy1−xYxMnO3+δ. J. Solid State Chem. 2014, 217, 127–135. 12. Remsen, S.; Dabrowski, B., Synthesis and Oxygen Storage Capacities of Hexagonal Dy1–xYxMnO3+δ. Chem. Mater. 2011, 23, 3818–3827. 13. Kadota, S.; Karppinen, M.; Motohashi, T.; Yamauchi, H., R-Site Substitution Effect on the Oxygen-Storage Capability of RBaCo4O7+δ. Chem. Mater. 2008, 20, 6378–6381. 14.

Motohashi, T.; Kadota, S.; Fjellvåg, H.; Karppinen, M.; Yamauchi, H.,

Uncommon oxygen intake/release capability of layered cobalt oxides, REBaCo4O7+δ: Novel Oxygen-Storage Materials. J. Mater. Sci. Eng. B 2008, 148, 196–198. 15. Parkkima, O.; Yamauchi, H.; Karppinen, M., Oxygen Storage Capacity and Phase Stability of Variously Substituted YBaCo4O7+δ. Chem. Mater. 2013, 25, 599–604. 16. Hervieu, M.; Guesdon, A.; Bourgeois, J.; Elkaïm, E.; Poienar, M.; Damay, F.; Rouquette, J.; Maignan, A.; Martin, C., Oxygen Storage Capacity and Structural Flexibility of LuFe2O4+x (0 ≤ x ≤ 0.5). Nat. Mater. 2014, 13, 74–80. 17. Motohashi, T.; Hirano, Y.; Masubuchi, Y.; Oshima, K.; Setoyama, T.; Kikkawa, S., Oxygen Storage Capability of Brownmillerite-Type Ca2AlMnO5+δ and Its Application to Oxygen Enrichment. Chem. Mater. 2013, 25, 372–377. 18. Ling, C.; Zhang, R.; Jia, H., Quantum Chemical Design of Doped Ca2MnAlO5+δ as Oxygen Storage Media. ACS Appl. Mater. Interfaces 2015, 7, 14518– 14527.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

19.

Page 26 of 30

Nomura, T.; Zhu, C.; Sheng, N.; Murai, R.; Akiyama, T., Solution Combustion

Synthesis of Brownmillerite-Type Ca2AlMnO5 as an Oxygen Storage Material. J. Alloys Compd. 2015, 646, 900–905. 20. Urban, K. W., Studying Atomic Structures by Aberration-Corrected Transmission Electron Microscopy. Science 2008, 321, 506–510. 21. Muller, D. A.; Sorsch, T.; Moccio, S.; Baumann, F. H.; Evans-Lutterodt, K.; Timp, G., The Electronic Structure at the Atomic Scale of Ultrathin Gate Oxides. Nature 1999, 399, 758–761. 22. Kurata, H.; Colliex, C., Electron-Energy-Loss Core-Edge Structures in Manganese Oxides. Phys. Rev. B 1993, 48, 2102–2108. 23. Riedl, T.; Gemming, T.; Wetzig, K., Extraction of EELS White-Line Intensities of

Manganese

Compounds:

Methods,

Accuracy,

and

Valence

Sensitivity.

Ultramicroscopy 2006, 106, 284–291. 24. Turner, S.; Verbeeck, J.; Ramezanipour, F.; Greedan, J. E.; Van Tendeloo, G.; Botton, G. A., Atomic Resolution Coordination Mapping in Ca2FeCoO5 Brownmillerite by Spatially Resolved Electron Energy-Loss Spectroscopy. Chem. Mater. 2012, 24, 1904–1909. 25.

Yao, L.; Majumdar, S.; Äkäslompolo, L.; Inkinen, S.; Qin, Q. H.; van Dijken,

S., Electron-Beam-Induced Perovskite–Brownmillerite–Perovskite Structural Phase Transitions in Epitaxial La2/3Sr1/3MnO3 Films. Adv. Mater. 2014, 26, 2789–2793. 26. Mizoguchi, T.; Varela, M.; Buban, J. P.; Yamamoto, T.; Ikuhara, Y., Site Dependence and Peak Assignment of YBa2Cu3O7-x O K-Edge Electron Energy Loss Near-Edge Fine Structure. Phys. Rev. B 2008, 77, 024504. 27. Haruta, M.; Kurata, H.; Matsumoto, K.; Inoue, S.; Shimakawa, Y.; Isoda, S., Local

Electronic

Structure

Analysis

for

Brownmillerite

Ca(Sr)FeO2.5

using

Site-Resolved Energy-Loss Near-Edge Structures. J. Appl. Phys. 2011, 110, 033708. 28. Grosvenor, A. P.; Ramezanipour, F.; Derakhshan, S.; Maunders, C.; Botton, G. A.; Greedan, J. E., Effects of Bond Character on the Electronic Structure of Brownmillerite-Phase Oxides, Ca2B′xFe2-xO5 (B′ = Al, Ga): An X-ray Absorption and Electron Energy Loss Spectroscopic Study. J. Mater. Chem. 2009, 19, 9213–9220. 29. Barthel, J. Dr. Probe-STEM Simulation Software, http://www.er-c.org/barthel/drprobe/. 30.

Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation

Made Simple. Phys. Rev. Lett. 1996, 77, 3865–3868. 31. Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple [Phys. Rev. Lett. 77, 3865 (1996)]. Phys. Rev. Lett. 1997, 78, 1396–1396.


Page 27 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


32.

Kresse, G.; Hafner, J., Ab Initio Molecular Dynamics for Liquid Metals. Phys.

Rev. B 1993, 47, 558–561. 33. Kresse, G.; Hafner, J., Ab Initio Molecular-Dynamics Simulation of the Liquid-Metal–Amorphous-Semiconductor Transition in Germanium. Phys. Rev. B 1994, 49, 14251–14269. 34.

Kresse, G.; Furthmüller, J., Efficiency of Ab-initio Total Energy Calculations

for Metals and Semiconductors using a Plane-wave Basis Set. Comput. Mater. Sci. 1996, 6, 15–50. 35. Blöchl, P. E., Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953–17979. 36.

Kresse, G.; Furthmüller, J., Efficient Iterative Schemes for Ab Initio

Total-Energy Calculations using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169–11186. 37.

Kresse, G.; Joubert, D., From Ultrasoft Pseudopotentials to the Projector

Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758–1775. 38. Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P., Electron-Energy-Loss Spectra and the Structural Stability of Nickel Oxide: An LSDA+U study. Phys. Rev. B 1998, 57, 1505–1509. 39. Monkhorst, H. J.; Pack, J. D., Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188–5192. 40. Palmer, H. M.; Snedden, A.; Wright, A. J.; Greaves, C., Crystal Structure and Magnetic Properties of Ca2MnAlO5.5, an n = 3 Brownmillerite Phase. Chem. Mater. 2006, 18, 1130–1133. 41. Wright, A. J.; Palmer, H. M.; Anderson, P. A.; Greaves, C., Structures and Magnetic Ordering in the Brownmillerite Phases, Sr2MnGaO5 and Ca2MnAlO5. J. Mater. Chem. 2002, 12, 978–982. 42. Haruta, M.; Kurata, H.; Komatsu, H.; Shimakawa, Y.; Isoda, S., Effects of Electron Channeling in HAADF-STEM Intensity in La2CuSnO6. Ultramicroscopy 2009, 109, 361–367. 43.

Wang, Z. L.; Yin, J. S.; Jiang, Y. D., EELS Analysis of Cation Valence States

and Oxygen Vacancies in Magnetic Oxides. Micron 2000, 31, 571–580. 44. Haruta, M.; Kurata, H.; Komatsu, H.; Shimakawa, Y.; Isoda, S., Site-Resolved Oxygen K-Edge ELNES of the Layered Double Perovskite La2CuSnO6. Phys. Rev. B 2009, 80, 165123. 45. Egerton, R. F., Inelastic Scattering and Energy Filtering in the Transmission Electron Microscope. Philos. Mag. 1976, 34, 49–65.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

46.

Parsons, T. G.; D’Hondt, H.; Hadermann, J.; Hayward, M. A., Synthesis and

Structural Characterization of La1−xAxMnO2.5 (A = Ba, Sr, Ca) Phases: Mapping the Variants of the Brownmillerite Structure. Chem. Mater. 2009, 21, 5527–5538. 47. Grosvenor, A. P.; Greedan, J. E., Analysis of Metal Site Preference and Electronic Structure of Brownmillerite-Phase Oxides (A2B′xB2−xO5; A = Ca, Sr; B′ = Al, Mn, Fe, Co) by X-ray Absorption Near-Edge Spectroscopy. J. Phys. Chem. C 2009, 113, 11366–11372. 48.

Hadermann, J.; Abakumov, A. M.; D'Hondt, H.; Kalyuzhnaya, A. S.; Rozova,

M. G.; Markina, M. M.; Mikheev, M. G.; Tristan, N.; Klingeler, R.; Buchner, B.; Antipov, E. V., Synthesis and Crystal Structure of the Sr2Al1.07Mn0.93O5 Brownmillerite. J. Mater. Chem. 2007, 17, 692–698. 49. Ramezanipour, F.; Cowie, B.; Derakhshan, S.; Greedan, J. E.; Cranswick, L. M. D., Crystal and Magnetic Structures of the Brownmillerite Compound Ca2Fe1.039(8)Mn0.962(8)O5. J. Solid State Chem. 2009, 182, 153–159.


Page 28 of 30

Page 29 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure captions Fig. 1. Atomic structure models of brownmillerite-type Ca2AlMnO5 and Ca2AlMnO5.5. Fig. 2. (a) Calculated electron diffraction patterns, (b) observed electron diffraction patterns, and (c) HR–TEM images of Ca2AlMnO5 and Ca2AlMnO5.5 along the [101] direction. Fig. 3. (a) Model of the atomic structure of Ca2AlMnO5 and Ca2AlMnO5.5 in the [101] direction. (b) Multi-slice image simulation and (c) observed HAADF-STEM images. The graphs on the right-hand side show the intensity profiles corresponding to the vertical lines shown in Fig. 3(c). Fig. 4. EEL spectra of (a) O K–edge and (b) Mn L–edge ELNES for Ca2AlMnO5 and Ca2AlMnO5.5, acquired by parallel beam illumination in SAED mode. Fig. 5. Structural models and experimental O K–edge ELNES of (a) Ca2AlMnO5 and (b) Ca2AlMnO5.5 at non-equivalent sites of MnO6 octahedra (Mn), AlO4 tetrahedra (Alt), and AlO6 octahedra (Alo). Fig. 6. Calculated PDOS of O 2p and Mn 3d orbitals at different sites in (a) Ca2AlMnO5 and (b) Ca2AlMnO5.5. Except for Mn–Al, the PDOS minority spin was abbreviated because Mn–Mn and Al–Al were non-spin-polarized, while Mn–Al was spin-polarized. The inset figure shows a schematic illustration of the crystal field splitting of the Mn 3d orbitals.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Supporting Information Supporting Information Available: XRD patterns of the synthesized powders, structural parameters from Rietveld refinement, scheme of electron beam channeling, observed planar fault and Calculated O K-edge ELNES.


Page 30 of 30

Atomic and Local Electronic Structures of ... - ACS Publications

Recommend Documents