Oxygen Storage Capability of Brownmillerite-type Ca2AlMnO5+δ and

Article pubs.acs.org/cm

Oxygen Storage Capability of Brownmillerite-type Ca2AlMnO5+δ and Its Application to Oxygen Enrichment Teruki Motohashi,*,† Yuka Hirano,† Yuji Masubuchi,† Kazunori Oshima,‡ Tohru Setoyama,‡ and Shinichi Kikkawa† †

Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan Mitsubishi Chemical Group, Science and Technology Research Center, Inc., Yokohama 227-8502, Japan

‡

S Supporting Information *

ABSTRACT: The oxygen storage capability was investigated for Ca 2 (Al x Mn 1−x ) 2 O 5+δ (0.50 ≤ x ≤ 0.67) with a Brownmillerite-type structure. This oxide can store/release a large amount of excess oxygen (∼3.0 wt %) topotactically in response to variations in temperature and the surrounding atmosphere in a highly reversible manner. The capacity and response of oxygen storage are remarkable only in the vicinity of x = 0.50, that is, Ca2AlMnO5+δ, and rapidly deteriorated as the Al content increases. Owing to the high sensitivity in terms of oxygen nonstoichiometry, Ca2AlMnO5+δ exhibits oxygen intake/release ability when temperature swing between 500 and 700 °C is applied. With this characteristic feature of this oxide, a facile method for oxygen enrichment is demonstrated. KEYWORDS: oxygen storage materials, manganese oxides, Brownmillerite-type structure, oxygen enrichment

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INTRODUCTION

As alternative high-performance OSMs, transition-metal oxides are worthy of attention because of variable valence states of constituent transition metals. Recently, we found that the manganese oxide BaYMn2O5+δ with a double-perovskite structure shows remarkable oxygen storage capability at temperatures below 500 °C.8,9 This oxide rapidly stores/ releases a large amount of oxygen (3.7 wt %) in a perfectly reversible manner, involving a wide range of oxygen nonstoichiometry 0.0 ≤ δ ≤ 1.0. While our work demonstrated that manganese oxides could be promising candidates for the use in oxygen-storage applications, BaYMn2O5+δ is inactive in moderate reductive conditions in terms of oxygen intake/ release and hence inappropriate for oxygen enrichment. The present work focused on an oxygen-deficient B-site ordered perovskite, Ca2AlMnO5. This oxide crystallizes in a Brownmillerite (BM) type with a general formula of A2B2O5,17,18 in which tetrahedral BO4 and octahedral BO6 layers alternately stack with each other, as illustrated in Figure 1a. In Ca2AlMnO5, divalent Ca is located at the perovskite Asite, while trivalent Al and Mn at the B-site with a preferential formation of AlO4 and MnO6 polyhedra. The phase relation study on Ca2(AlxMn1−x)2O5 evidenced a solid solution with respect to the Al/Mn ratio ranging 0.50 ≤ x ≤ 0.67 in this oxide system.20 The postannealing effect on oxygen nonstoichiometry

Many oxides are known to exhibit oxygen nonstoichiometry which is accompanied by variations in the valence state of constituent cations. The technological importance of oxygen nonstoichiometry derives from the fact that the oxygen-content control often plays a key role in electromagnetic properties, as exemplified by the strong correlation between Tc and oxygen content in high-temperature superconducting copper oxides.1 On the other hand, oxygen nonstoichiometry itself may be regarded as “functionality” to open up the possibility for practical uses, if the oxygen intake/release capability is remarkable. Such oxides can be used as “oxygen storage materials (= OSMs)”.2−10 OSMs tend to respond to changes in temperature and the surrounding atmosphere so as to be used for regulation of oxygen partial pressure [P(O2)] in the gas phase. Various oxygen-related applications of OSMs are promising such as: (1) three-way catalysts for the effective removal of NOx, CO, and hydrocarbons from automobile exhausts to contribute to environmental protection,11 and (2) oxygen enrichment or oxygen separation to produce oxygenconcentrated atmospheres to improve the combustion efficiency of chemical plants.12−16 The first case has already been achieved with CeO2−ZrO2 solid solution, the so-called CZ. Meanwhile, a new class of OSMs is required to contribute to the second case, since such an application needs oxygen release capability without the usage of reductive gas atmospheres, which is difficult to be attainable with CZ. © 2013 American Chemical Society

Received: September 29, 2012 Revised: January 15, 2013 Published: January 15, 2013 372

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compositions of the resultant products are close to the nominal values (see Supporting Information). Phase purity and lattice parameters were checked for the resultant products by means of X-ray powder diffraction (XRD; Rigaku Ultima IV; CuKα radiation). The diffraction data were collected over an angular range of 5−140° and a counting time of 10 s per step. XRD patterns for the as-prepared products (i.e., N2-annealed at 1250 °C for 12 h) are displayed in Figure 2. The x = 0.60 and 0.67 products are the

Figure 1. Schematic illustration of the crystal structures of (a) Brownmillerite-type Ca2AlMnO5 and (b) its fully oxygenated form Ca2AlMnO5.5. The illustration was drawn with VESTA software19 based on the structural model reported in the literature.

was examined for Ca2AlMnO5 (i.e., x = 0.50):21 it appeared that the sample absorbs excess oxygen at 600 °C under a 30 MPa oxygen pressure to form Ca2AlMnO5.5 involving oxidation of Mn3+ to Mn4+. Note that this result implies that oxygen storage capacity (OSC; the amount of excess oxygen being stored in the crystal lattice) of this oxide reaches 3.3 wt %, being even larger than the maximum value for the conventional OSM, CZ (2.8 wt %). Nevertheless, the oxygen intake/release characteristics against temperature and the surrounding atmosphere were not studied. Here, we report the oxygen storage capability of BM-type Ca2(AlxMn1−x)2O5+δ (0.50 ≤ x ≤ 0.67). This oxide can store/ release a large amount of excess oxygen (OSC ∼ 3.0 wt %) topotactically in response to small variations in temperature and the surrounding atmosphere in a highly reversible manner. The capacity and response of oxygen storage are found to be remarkable only in the vicinity of x = 0.50, that is, Ca2AlMnO5+δ, and rapidly deteriorated as the Al content increases. We emphasize that Ca2AlMnO5+δ consists only of “rock-forming elements” which are abundantly distributed in the earth crust, making this oxide a potential candidate for large-scale applications to oxygen storage. With use of this oxide, a facile method for oxygen enrichment is demonstrated in the present work.

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Figure 2. X-ray powder diffraction patterns for the as-prepared (i.e., N2-annealed at 1250 °C) products of Ca2(AlxMn1−x)2O5+δ with x = 0.50, 0.55, 0.60, and 0.67. Diffraction peaks for the products are indexed based on an orthorhombic unit cell. Peaks due to (Ca,Mn)O are marked with circles. single phase of the orthorhombic BM-type structure. The x = 0.50 and 0.55 products are well crystallized but containing a trace amount of rock-salt type (Ca1−yMny)O (∼1.6 and ∼0.8 wt %, respectively). To determine the actual Al/Mn ratio in the BM-type phase, structural refinements were carried out using RIETAN-FP software.22 The XRD data for all the as-prepared products were analyzed assuming the oxygen stoichiometric structure (δ = 0) with orthorhombic Ibm2 symmetry as in the previous study.17 Details in the refinements are given in the Supporting Information. The refined Al contents (x) essentially agree with the nominal values within the errors, indicating that compositional deviations due to the impurities are negligible. The oxygen intake/release characteristics were investigated by means of thermogravimetry (TG; Rigaku TG8120GH). Isothermal TG data were measured at 400 and 500 °C to estimate equilibrium oxygen contents under various P(O2) atmospheres. The measurements were carried out for 30 mg specimens after switching the gas flow (200 mL/min) from N2 to O2/N2 gas mixtures. Oxygen concentration in the flowing gas was precisely controlled utilizing mass-flow controllers and commercial gas mixtures. The temperature scan was also performed up to 900 °C with heating/cooling rates of ±1 °C/min. In addition, the following experiments were carried out to investigate the response and reversibility of the oxygen intake/release processes: (1) TG measurements in flowing air and O2 gas with temperature swing between 500 and 700 °C every 2 h, (2) isothermal TG measurements at 500 °C upon switching the gas flow from N2 to O2 and vice versa every 2 h. Oxygen enrichment with the use of Ca2AlMnO5+δ was demonstrated. A 1.0 g portion of the powder product was placed in a quartz microreactor with 4 mmϕ inner diameter. The specimen was first heated at 500 °C in flowing air for 2 h to store excess oxygen, and subsequently heated at 700 °C for 1 h to release oxygen. The evolved gas during the second process was collected in a Tedlar bag in which

EXPERIMENTAL SECTION

Polycrystalline samples of Ca2(AlxMn1−x)2O5+δ with x = 0.50, 0.55, 0.60, and 0.67 were synthesized by a coprecipitation method. Ca(NO3)2·4H2O (99.9%, Kanto Chemical), Al(NO3)3·9H2O (99.9%, Wako Pure Chemicals), and Mn(NO3)2·6H2O (99.9%, Wako Pure Chemicals) were used as starting materials. Stoichiometric amounts of these reagents were dissolved in distilled water and subsequently coprecipitated in a 10−28% ammonia solution. The resultant slurry was stirred and heated in a ceramic crucible to evaporate water, and then prefired at 450 °C in air for 1 h, resulting in a porous solid residue. This solid was ground and pressed into cylindrical pellets in 10 mmϕ. The pellets were fired at 1250 °C in air for 24 h, followed by post annealing at 1250 °C in flowing N2 gas (99.99% purity) for 12 h to minimize the amount of excess oxygen in the Ca2(AlxMn1−x)2O5+δ phase. The chemical analysis by inductively coupled plasma-atomic emission spectroscopy (ICP-AES; Seiko Instruments Inc. SPS-3500 and Shimadzu ICPE-9000) indicated that the actual cationic 373

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100 mL of air (i.e., O2/N2/Ar = 21/78/1 in a molar ratio) was injected prior to the experiment. This temperature cycle was repeated four times. At the end of each cycle, the O2/N2 ratio in the Tedlar bag was analyzed with gas chromatography (Inficon 3000 Micro GC). Details of the equipment will be given in the next section.

exhibit common features typically seen for perovskite-related structures, distinct changes are visible between these products. In fact, the XRD pattern for the oxygenated product (O2annealed at 400 °C for 12 h) can be indexed on the basis of the Imma space group as for the fully oxygenated Ca2AlMnO5.5 product.21 The lattice parameters of our oxygenated product are determined to be a = 0.5255 nm, b = 2.942 nm, and c = 0.5373 nm. These values are slightly smaller than those of the fully oxygenated δ = 0.5 product:21 a = 0.5286 nm, b = 2.953 nm, and c = 0.5403 nm. This result is rather unexpected, as the incomplete oxygenation should lead to a larger unit cell volume than that of the fully oxygenated product. The similarity in XRD profiles between the pristine and the oxygen-excess phases indicates that the oxygenation reaction takes place topotactically. The structural model of Ca2AlMnO5.5 consists of alternating tetrahedral AlO4 and octahedral AlO6 blocks, with each block being separated by octahedral MnO6, as schematically drawn in Figure 1b. This implies that the oxygen intake process is achieved by partial oxygenation of the tetrahedral AlO4 block in the original BM-type structure. Such a topotactic transformation may be favorable to attain high durability in the redox cycle, which is strictly required for practical uses. It is also worthwhile to point out that half of the oxygen-deficient AlO4 blocks are intact even in the fully oxygenated form Ca2AlMnO5.5. This structural feature is likely linked with the fast oxygen migration. As demonstrated in Figure 3, the oxygen storage characteristics significantly depend on the Al content (x). The saturated weight gain Δwmax is the highest at x = 0.50, and decreased as x increases. In addition, the oxygen intake rate rintake, which is defined as a slope of the Δw vs time plot at 50% of Δwmax, is noticeably deteriorated for Al-rich products (inset of Figure 3). It should be noted that the oxygen storage capability is absent at x = 0.67: this result is somewhat surprising, because manganese ions in this product are expected to involve oxygen nonstoichiometry as for the x = 0.50−0.60 products. It is now evident that the remarkable oxygen storage capability of Ca2(AlxMn1−x)2O5+δ appears only in the vicinity of x = 0.50, despite a wide solubility range of Al (x). This finding emphasizes the importance of compositional control for achieving the highest efficiency in oxygen storage applications. To interpret the contrastive behaviors between x = 0.50 and 0.67, these products are compared from the crystallographic point of view. While most of the atomic distances are similar between the two products, the Al/Mn(1)−O(2) distance is noticeably shorter for x = 0.67 (d = 0.220 nm) than x = 0.50 (0.228 nm), where Al/Mn(1) and O(2) are the octahedral cation site and its apical oxygen, respectively. Our structural refinements indicate that the octahedral site is preferentially occupied by Mn for x = 0.50, while one-third of Mn at this site is randomly replaced by Al for x = 0.67 (it might be more appropriate to define the composition as Ca2Al(Mn1−yAly)O5+δ with y = 1/3). In Ca2AlMnO5 (i.e., x = 0.50), the octahedral coordination is elongated along the apical oxygen direction because of the Jahn−Teller (JT) effect of trivalent Mn (3d4). Replacement of Mn3+ by non-JT Al3+ forms undistorted AlO6 octahedra among the MnO6 octahedra, and thereby leads to the shortened Al/Mn(1)−O(2) distance in the averaged crystal structure. Since MnO6 and AlO6 octahedra are distributed statistically, the position of apical oxygen is necessarily displaced in Al-rich compositions. We believe that such a disordered nature of the local structure significantly suppresses the oxide-ion absorption, leading to the deteriorated oxygen

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RESULTS AND DISCUSSION Oxygen Storage Capability. The oxygen intake behaviors of Ca2(AlxMn1−x)2O5+δ were investigated by means of isothermal TG. The result shown in Figure 3 reveals a

Figure 3. Isothermal TG curves (400 °C) for the x = 0.50, 0.55, 0.60, and 0.67 products after switching the gas flow (200 mL min−1) from N2 (99.99% purity) to O2. The inset shows the oxygen intake rate rintake against x. The rintake values are defined as a slope of the Δw vs time plot at 50% of Δwmax.

remarkable oxygen storage capability of this oxide system. For x = 0.50, the sample weight immediately starts to increase when the gas flow is switched from N2 to O2. Hence, the weight gain is attributed to the increase in the oxygen content. The saturated weight gain (= OSC) reaches 2.9 wt %, corresponding to δ = 0.44. This value is 88% of the maximum oxygen content (δ = 0.50) that is achieved by post annealing under a 30 MPa oxygen pressure.21 In Figure 4, XRD patterns for the as-prepared and oxygenated products are compared. While both of the patterns

Figure 4. X-ray powder diffraction patterns for the as-prepared and oxygenated Ca2AlMnO5+δ products. These patterns are indexed based on orthorhombic unit cells with different cell sizes. Peaks due to (Ca,Mn)O are marked with circles. 374

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intake rate and finally the disappearance of oxygen storage capability in the x = 0.67 product. Oxygen Intake/Release Characteristics. Thermogravimetric behaviors were investigated for x = 0.50, that is, Ca2AlMnO5+δ, up to 900 °C (Figure 5). In flowing N2 gas, the

Figure 6. Isothermal TG curves (500 °C) for Ca2AlMnO5+δ under various P(O2) atmospheres. In this figure, the sample weight is plotted against logarithmic time in flowing O2/N2 gas mixtures. The inset shows the equilibrium weight gain (Δweq) as a function of P(O2). Note that error bars at P(O2) = 0.05 and 0.10 atm are pretty large, as the weight variation was not saturated within 24 h (= 1,440 min). Figure 5. TG curves for the x = 0.50 product up to 900 °C. The data were measured in flowing O2 (solid curves) and N2 (broken curve) atmospheres with a scan rate of ±1 °C/min.

via small temperature variations even under a fixed atmosphere. In fact, as demonstrated in Figure 7a, Ca2AlMnO5+δ can store/ release a large amount of oxygen reversibly when a temperature swing between 500 and 700 °C is applied. The oxygen intake rate is much slower in air than in O2 such that the weight gain is not saturated within 2 h in air, resulting in smaller OSC = 2.1 wt % than 2.7 wt % in O2 atmosphere. It was confirmed that such a reversible oxygen intake/release cycle can also be achieved upon switching the gas flow from N2 to O2, and vice versa at a constant temperature, as demonstrated in Figure 7b. It should be noted that the oxygen intake/release behaviors of Ca2AlMnO5+δ are similar to those of a complex cobalt oxide YBaCo4O7+δ.4,5,23 In fact, both of these oxides exhibit common features such as oxygen storage capability under oxygen rich atmospheres and single-step-like oxygen release at elevated temperatures. The similarities imply that the mechanism underlying these features may be essentially identical. We attribute the unusual oxygen intake/release in these oxides to a subtle balance of competing valence states of constituent transition metals. The redox equilibria of Mn3+/Mn4+ and Co2+/Co3+ are easily attained such that valence changes and thereby oxygen intake/release can be induced by small variations in temperature. A recent work by Remsen and Dabrowski have reported oxygen storage capacities of hexagonal Dy1−xYxMnO3+δ.10 We suggest that the origin of its oxygen nonstoichiometry could also be identical because of the similar thermogravimetric behaviors. There are different features among these compounds. From the structural point of view, excess oxygen in Ca2AlMnO5+δ is coordinated not to Mn but to Al, being in contrast to the case of YBaCo4O7+δ, where additional oxide ions are located in the vicinity of the Co site that is oxidized.24 The structural aspect of the oxygen-excess Ca2AlMnO5.5 phase was discussed by Palmer et al.21 Upon oxygen intake, it would be expected that the Mn− O bonds decrease in length as the Mn cation is oxidized from 3+ to 4+, while the change from tetrahedral to octahedral geometry causes an expansion in the relevant Al−O bonds. Indeed, both of these structural changes were observed: the bond valence sum (BVS)25 calculation gives a substantial increase from +3.29 to +3.81 for the Mn site, and almost

sample weight and accordingly the oxygen content remain practically unchanged. Upon heating under O2 atmosphere, the sample weight starts to increase at about 200 °C and then shows the maximum at 300 °C with a 3.0 wt % weight gain, corresponding to the oxygen content δ = 0.45. When further increasing temperature, an abrupt weight drop is observed at 650 °C, resulting in the sample weight being nearly identical to the initial value. In the subsequent cooling scan in flowing O2 gas, a large weight gain by oxygen intake occurs at 550 °C. The sample weight is saturated at 2.7 wt % (δ = 0.41) below 500 °C without oxygen release upon further cooling. The thermal hysteresis in the oxygen content may be due to the different oxygen intake/release kinetics during the heating/cooling processes, but its details are still unclear. To estimate equilibrium oxygen contents as a function of P(O2), isothermal TG data were measured at 500 °C under various atmospheres ranging P(O2) = 2 × 10−5 ∼ 1 atm. As shown in Figure 6, the sample weight immediately increases and reaches 2.7−2.9 wt % at P(O2) ≥ 0.21 atm, indicating the formation of the oxygen-excess phase. A large weight gain is also seen at P(O2) = 0.05 and 0.10 atm, although the lowered oxygen-intake rate hinders the saturated weight within 24 h (= 1,440 min). On the other hand, the sample weight is essentially constant at P(O2) ≤ 0.02 atm, suggesting the disappearance of oxygen storage capability in diluted oxygen atmospheres. In the inset of Figure 6, the equilibrium weight gain (Δweq) is plotted as a function of P(O2). The Δweq vs P(O2) plot clearly shows a discontinuous jump of oxygen content between P(O2) = 0.02 and 0.05 atm. The discontinuous nature suggests that the oxygen intake/release behaviors are accompanied by a firstorder phase transformation between oxygen-excess (δ ≈ 0.4) and oxygen-stoichiometric (δ ≈ 0) phases. From the TG data in Figure 5, it appears that this oxide changes its oxygen content drastically in a narrow temperature window between 550 and 650 °C to cause either oxygen-excess or oxygen-stoichiometric form depending on temperature. Thus, remarkable oxygen intake/release behaviors are expected 375

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Figure 7. (a) TG curves for the x = 0.50 product in flowing O2 gas (red curve) and flowing air (black curve) upon temperature swing between 500 and 700 °C. The sample temperature is also shown with a blue curve. (b) Isotheral TG curve for the x = 0.50 product at 500 °C in flowing N2, then O2, and vice versa.

identical values of +2.89 and +2.87 for the AlO4 and AlO6 polyhedra, respectively. We thus suggest that the relaxation of local polyhedral tilting triggered by the shrinking Mn−O and expanding Al−O bonds would be a possible driving force of the topotactic oxygenation of Ca2AlMnO5. Demonstration of Oxygen Enrichment. The present work has evidenced that Ca2AlMnO5+δ exhibits its remarkable oxygen intake/release capability in moderate redox conditions. In fact, this oxide can release oxygen even under oxygen-rich atmospheres: this is in sharp contrast to the case of CZ, where the oxygen release process always requires strongly reductive conditions. Surely even simple metal oxides such as Co3O4 and CuO undergo a significant loss of oxygen content in moderateto-high oxygen partial pressures. Nevertheless, the oxygen release process in such oxides involves a drastic cation rearrangement which inevitably causes deteriorated cyclic performance. Meanwhile, the oxygen intake/release takes place topotactically in Ca2AlMnO5+δ, such that the change in the cationic framework is minimized. This characteristic feature enables us to use this oxide as an oxygen pump for regulation of oxygen partial pressure. Here, we show a facile method for oxygen enrichment with a use of Ca2AlMnO5+δ, that effectively works even with temperature swing in a narrow window. A schematic illustration of the equipment is shown in the inset of Figure 8. The quartz microreactor is connected to two lines via switching valves: Line #1 for flowing air as oxygen source, and Line #2 for collecting evolved gas in the Tedlar bag. Line #1 was open in the oxygen intake process at 500 °C for 2 h, then the valves were switched to Line #2 in the subsequent oxygen release process at 700 °C for 1 h. The evolution of O2 concentration in the Tedlar bag is presented in Figure 8. The O2 concentration linearly increases when the temperature cycle is repeated, reaching 50% at the fourth cycle. From the result of Figure 7a, we anticipate that the specimen stores/releases 2.1 wt % of oxygen in each cycle. In this figure, ideal values for O2 concentration are also plotted, under an assumption that the evolved O2 gas was collected in the Tedlar bag without any loss (for more detailed considerations, see the Supporting Information). The experimental values are comparable to the ideal ones, leading to the operating efficiency as high as 97% at the fourth cycle. It should be emphasized that the simple operating principle, designed for the unique oxygen intake/ release characteristics of this oxide, contributes to the high

Figure 8. Result of the oxygen enrichment experiment. The data plots with red symbols are O2 concentration values in the collected gas at each temperature cycle. The ideal values for O2 concentration are also plotted with black symbols. The inset shows a schematic illustration of the equipment (see the main text).

efficiency of our equipment. The present result implies that 20 g of pure oxygen can be produced by 1.0 kg of Ca2AlMnO5+δ with a single temperature cycle, corresponding to 14 L of O2 gas in the standard ambient temperature and pressure. Such high efficiency ensures the future prospect of this oxide for practical uses in the oxygen enrichment technology.

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CONCLUSIONS The present work revealed the remarkable oxygen storage capability of Ca2 AlMnO 5+δ with a Brownmillerite-type structure. This oxide can store/release a large amount of excess oxygen (∼ 3.0 wt %) topotactically depending on temperature and the surrounding atmospheres in a highly reversible manner. This oxide is featured with its high sensitivity in terms of oxygen nonstoichiometry in moderate redox conditions, which enables us to induce oxygen intake/ release even with small variations in temperature. Such a characteristic feature may open the possibility of various future applications to oxygen-storage technologies such as oxygen enrichment/oxygen separation12−16 and cathode materials in solid oxide fuel cells,26,27 which are difficult to be attainable with the conventional oxygen storage material. It is noteworthy 376

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that this oxide consists only of “rock-forming elements” which are abundantly distributed in the earth crust, being quite favorable for large-scale practical uses.

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(21) Parmer, H. M.; Snedden, A.; Wright, A. J.; Greaves, C. Chem. Mater. 2006, 18, 1130. (22) Izumi, F.; Momma, K. Solid State Phenom. 2007, 130, 15. (23) Hao, H.; Cui, J.; Chen, C.; Pan, L.; Hu, J.; Hu, X. Solid State Ionics 2006, 177, 631. (24) Chmaissem, O.; Zheng, H.; Huq, A.; Stephens, P. W.; Mitchell, J. F. J. Solid State Chem. 2008, 181, 664. (25) Brown, I. D.; Altermatt, D. Acta Crystallogr. 1985, B41, 244. (26) Brett, D. J. L.; Atkinson, A.; Brandon, N. P.; Skinner, S. J. Chem. Soc. Rev. 2008, 37, 1568. (27) Kim, J.-H.; Manthiram, A. Chem. Mater. 2010, 22, 822.

ASSOCIATED CONTENT

S Supporting Information *

Cationic compositions analyzed by ICP-AES, Rietveld refinement details for the Ca2(AlxMn1−x)2O5+δ products, and detailed considerations of the result of the oxygen enrichment experiment. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +81(0)11 706 6741. Fax: +81(0)11 706 6740. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Professor W. Ueda (Hokkaido University) and Professor R. Abe (Kyoto University) for their discussion and comments. The present work was supported by a Grant-inAid for Science Research (Contract No. 22750181) from the Japan Society for the Promotion of Science. T.M. acknowledges financial support from the Inamori Foundation.

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REFERENCES

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