Crystal Structure, Oxygen Deficiency, and Oxygen Diffusion Path of

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Crystal Structure, Oxygen Deficiency, and Oxygen Diffusion Path of Perovskite-Type Lanthanum Cobaltites La0.4Ba0.6CoO3−δ and La0.6Sr0.4CoO3−δ Yi-Ching Chen,† Masatomo Yashima,*,†,‡ Takashi Ohta,† Kenji Ohoyama,§ and Shinji Yamamoto† †

Department of Materials Science and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Nagatsuta-cho 4259, Midori-ku, Yokohama, 226-8502, Japan ‡ Department of Chemistry and Materials Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1-W4-17, O-okayama, Meguro-ku, Tokyo, 152-8551, Japan § Institute for Materials Research, Tohoku University, Katahira 2-1-1, Sendai, 980-8577, Japan S Supporting Information *

ABSTRACT: Crystal structures of perovskite-type lanthanum cobaltites, La0.4Ba0.6CoO3−δ and La0.6Sr0.4CoO3−δ, have been studied by in situ neutron powder diffractometry from 27 to 1258 °C. La0.4Ba0.6CoO3−δ has a cubic Pm3m ̅ structure in the whole temperature range, while La0.6Sr0.4CoO3−δ exhibits a phase transition from hexagonal (R3c̅ ) to cubic (Pm3m ̅ ) symmetry between 400 and 600 °C. Geometric information on oxygen diffusion is essential to understand the facile electrode reaction of perovskite-type cobaltites, but previous approaches have been limited to computational predictions. Here we provide long-awaited experimental evidence for a curved three-dimensional oxygen diffusion path in an ABO3 mixed ionic-electronic conductor, La0.4Ba0.6CoO3−δ. The oxide ions diffuse in the ⟨100⟩ direction near the stable position, while the path is along ⟨110⟩ around the middle point of the path. The connected path was not visualized in La0.6Sr0.4CoO3−δ but in La0.4Ba0.6CoO3−δ, which indicates the higher oxygen diffusivity in La0.4Ba0.6CoO3−δ. The oxygen defect concentration, bottleneck size for oxygen diffusion, and oxygen atomic displacement parameter of La0.4Ba0.6CoO3−δ are higher than those of La0.6Sr0.4CoO3−δ, which are responsible for the higher oxygen diffusivity in La0.4Ba0.6CoO3−δ.

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INTRODUCTION Fuel cells directly and efficiently convert chemical energy into electrical energy. Of various fuel cells, the solid oxide fuel cells (SOFCs) have benefits of environmentally benign power generation with fuel flexibility. However, the operating temperature of SOFCs is high (800−1000 °C). High operation temperatures cause degradation due to unfavorable reactions of adjacent cell components and sealing difficulties. The degradation shortens the lifetime of SOFCs. Therefore, it is important to reduce the operating temperature of SOFCs with high efficiency ( U(La,Sr) > U(Co), see Figure S1 in the Supporting Information), which suggests higher mobility of oxide ions than those of cations. Anisotropic atomic displacement parameters of the oxygen atom, Uij(O) (i, j = 1, 2, and 3) exhibit a larger thermal motion 5249

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Figure 3. Temperature dependence of the unit-cell parameters of La 0.4 Ba 0.6 CoO 3−δ and the reduced unit-cell parameters of La0.6Sr0.4CoO3−δ. Closed circles denote the unit-cell parameter a of La0.4Ba0.6CoO3‑δ. Open square and open circle stand for reduced unitcell parameters of ap and cp of La0.6Sr0.4CoO3−δ, where ap and cp are calculated from unit-cell parameters (aH and cH) of hexagonal La0.6Sr0.4CoO3−δ in the forms ap = aH and cp = cH/(12)0.5, respectively. aC indicates unit-cell parameters of cubic La0.6Sr0.4CoO3−δ. Blue and red solid lines denote the guides for unit-cell parameters versus temperature of La0.6Sr0.4CoO3−δ and La0.4Ba0.6CoO3−δ. Dotted line shows possible variation of the unit-cell parameters with temperature around the hexagonal-cubic phase-transition temperature.

Figure 5. Temperature dependence of oxygen concentration (3−δ) calculated from the refined occupancy factor. Open squares indicate the value in La0.6Sr0.4CoO3−δ. Closed squares stand for the value in La0.4Ba0.6CoO3−δ.

La1−xSrxCoO3−δ.33,40 The valence of the Co cation in La0.6Sr0.4CoO3−δ also decreases with increasing temperature (open symbols in Figure S3 of the Supporting Information). The bond valence sum (BVS)41 (open circles in Figure S4 of the Supporting Information) agrees with the valence calculated from the refined occupancy factor (open squares in Figure S4 of the Supporting Information) within the ±2σ where the σ is the estimated standard deviation, which indicates the validity of the refined crystal structure, occupancy factor, and oxygen concentration. Crystal Structure of La0.4Ba0.6CoO3−δ. The neutron diffraction profile of La0.4Ba0.6CoO3−δ indicated the Pm3̅m symmetry in the whole temperature range from 27.0 to 1227.3 °C (Figure 1(c)). This result is consistent with the study at room temperature in the literature.42 In the Rietveld analysis for neutron diffraction data of La0.4Ba0.6CoO3−δ, La and Ba atoms were placed at 1b 1/2, 1/2, 1/2. Co and O atoms were located at 1a 0, 0, 0 and 3d 1/2, 0, 0 positions, respectively. In a preliminary analysis we refined the occupancy factors of La and Ba atoms. The refined occupancy factors of La and Ba atoms agreed with those from the ICP spectroscopy within ±3σ where the σ is the estimated standard deviation. Therefore, in the final refinement, we fixed the occupancy factors g(La) and g(Ba) to the values from ICP analysis 0.4032 and 0.5968, respectively (Table 3). The calculated diffraction profile of La0.4Ba0.6CoO3−δ agreed well with the observed one (Figure 1(c)). The refined crystallographic parameters and reliability factors are shown in Table 3. Anisotropic atomic displacement parameters of oxygen atoms Uij(O) (i, j = 1, 2, and 3) were applied for oxygen atoms because Rietveld fit was improved in comparison to the use of an isotropic atomic displacement parameter Uiso(O). For example, the Rwp = 5.18% in the Rietveld analysis of the neutron data taken at 980.6 °C using Uij(O) was lower than Rwp = 5.67% with Uiso(O). The unit-cell parameter a of Pm3m ̅ La0.4Ba0.6CoO3−δ increases with an increase of temperature due to thermal expansion (red closed circles in Figure 3). The cell parameter of La0.4Ba0.6CoO3−δ at room temperature agrees with that

Figure 4. Temperature dependence of the antiphase octahedral tilt angle in La0.6Sr0.4CoO3−δ (open circles). Solid lines were obtained by least-squares fits to experimental data with the power law. Dashed line is the extrapolations of eq (2) using optimized parameters. The transition temperature was estimated to be 489 °C.

perpendicular to the Co−O bond (U22(O) = U33(O) > U11(O)) as shown in Table 2 and by the shape of oxygen thermal ellipsoids in Figure 2b (Figure S2 in the Supporting Information). Oxygen concentration (3−δ) in La0.6Sr0.4CoO3−δ calculated from the refined oxygen occupancy factor decreases with an increase of temperature (open squares in Figure 5). The present oxygen concentration (3−δ) in La0.6Sr0.4CoO3−δ agrees with those reported in the literature.33 A similar decrease of 3−δ with temperature is observed also in other compositions of 5250

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Anisotropic atomic displacement parameters of O atoms exhibit larger thermal motion perpendicular to the Co−O bond (U22(O) = U33(O) > U11(O), Table 3) as shown by the shape of oxygen thermal ellipsoids in Figures 2(c) and S2 in the Supporting Information. Oxygen concentration (3−δ) calculated from the refined oxygen occupancy decreases with increasing temperature (Figure 5). The valence of the Co cation in La0.4Ba0.6CoO3−δ also decreases with increasing temperature (closed squares in Figure S3 of the Supporting Information). The bond valence sum (BVS)41 (closed circles in Figure S3 of the Supporting Information) agrees with the valence calculated from the refined occupancy factor (closed squares in Figure S4 of the Supporting Information) within ±0.1, which indicates the validity of the refined crystal structure, occupancy factor, and oxygen concentration. Structural Disorder and Diffusion Pathway of La 0.4 Ba 0.6 CoO 3−δ and La 0.6 Sr 0.4 CoO 3−δ . To visualize the structural disorder and diffusion pathway of oxide ions in La0.4Ba0.6CoO3−δ and La0.6Sr0.4CoO3−δ, the MEM nuclear density distribution maps on the (012) plane of hexagonal and the (100) plane of cubic structures are shown in Figure 6. The oxide ions are localized near the stable positions of R3c̅ La0.6Sr0.4CoO3−δ and Pm3̅m La0.4Ba0.6CoO3−δ at room temperature (Figures 6(a) and 6(c)), while they spread over a wider area between the stable 3d 1/2, 0, 0 positions of the Pm3̅m La0.4Ba0.6CoO3−δ at

Table 3. Refined Crystal Parameters and Reliability Factors in Rietveld Analysis of Neutron Diffraction Data of La0.4Ba0. 6CoO2.55(2) (980.6 °C)a atoms

site b

g

x

y

1b 1.0 1/2 1/2 La0.4032Ba0.5968 Co 1a 1.0 0 0 O 3d 0.849(7) 1/2 0 Reliability Factors: Rwp = 5.18%, RI = 5.55%, RF =

z

U (Å2)

1/2 0 0 4.86%,

0.0389(15) 0.041(3) 0.0649c S = 3.69

a Note: Cubic space group Pm3̅m; number of chemical formula units of La0.4Ba0.6CoO3−δ in a unit cell: Z = 1. Unit-cell parameters: a = b = c = 4.0179(3) Ǻ , α = β = γ = 90°. Unit-cell volume: 64.861(9) Ǻ 3. g: Occupancy. bThe chemical composition is determined by ICP spectroscopy. cEquivalent isotropic atomic displacement parameters. Anisotropic atomic displacement parameters of oxygen atom: U11(O) = 0.039(2) Å2, U22(O) = U33(O) = 0.077(1) Å2.

reported in the literature.42 The average thermal expansion coefficient is estimated to be 25.9 ± 0.3 × 10−6 °C−1 from the refined unit-cell parameter. Crystal structure of Pm3̅m La0.4Ba0.6CoO3−δ consists of CoO6 octahedron and (La,Ba) cation (Figure 2(c)). The CoO6 octahedron in Pm3̅m La0.4Ba0.6CoO3−δ exhibits no tilting (a0a0a0). The equivalent isotropic atomic displacement parameter of the O atom, Ueq(O), is larger than those of cations (Ueq(O) > U(La,Ba) and Ueq(O) > U(Co)) (Table 3; Figure S4 in the Supporting Information).

Figure 6. Distribution of scattering-length densities on (a) the (012) plane of R3̅c La0.6Sr0.4CoO3−δ at 27.5 °C, (b) (100) plane of Pm3̅m La0.6Sr0.4CoO3−δ at 1258.0 °C, and (100) plane of Pm3̅m La0.4Ba0.6CoO3−δ at (c) 27.0 °C and (d) 1227.3 °C, with contours in the range from 0.2 to 2 fm Å−3 (0.2 fm Å−3 step). The thick black dashed lines and thin black straight lines in (a) show the R3̅c and pseudo Pm3̅m unit cells, respectively. The thick black dashed line with arrow in (d) denotes the visualized oxygen diffusion path in La0.4Ba0.6CoO3−δ. The oxide ions move from the position P1 to P3 through P2 point. The thick black dashed line with arrow in (b) denotes a possible oxygen diffusion path in cubic La0.6Sr0.4CoO3−δ. 5251

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1227.3 °C (Figure 6(d)), which is consistent with the higher oxygen permeation rates of lanthanum barium cobaltites at higher temperatures.14,16 At high temperatures, the nuclear density distributions of Pm3m ̅ La0.6Sr0.4CoO3−δ and La0.4Ba0.6CoO3−δ exhibit larger thermal motion of O atoms (Figures 6b and 6d). The MEM results reveal that the oxide ions in the cubic structure exhibit a large thermal motion perpendicular to the Co−O bond (Figures 6(b)−(d)), corresponding to large anisotropy of the atomic displacement parameters (U22(O) = U33(O) > U11(O), Figures 2(b) and 2(c), Tables 2 and 3). The most striking feature is the visualization of diffusion paths of the oxide ions in La0.4Ba0.6CoO3−δ at 1227.3 °C (Figure 6(d)). The diffusion path does not follow the edge of the CoO6 octahedron but displays an arc shape away from the B-site Co cation (the curved dashed line with arrows in Figure 6(d)). The oxide ions diffuse along the ⟨100⟩ and ⟨110⟩ directions around the stable 3d position (P1 and P3 in Figure 6(d)) and at the center of the path (P2 in Figure 6(d)), respectively. The oxide ions three-dimensionally diffuse on the {100} planes of the cubic perovskite-type La0.4Ba0.6CoO3−δ at 1227.3 °C (Figure S5 in the Supporting Information). As Cherry et al. previously noted,17 it has been commonly assumed that the migrating anion in the ABO3−δ perovskitetype structure takes a direct linear path along the ⟨110⟩ edge of the BO6 octahedron. On the contrary, based on the results of theoretical works, a curved diffusion path of the oxide ions was proposed in the literature.17,18 We confirmed an oxide ion diffusion pathway in a pure ionic conductor, lanthanum gallatebased material La0.8Sr0.2Ga0.8Mg0.15Co0.05O3−δ, where the iontransfer-number is nearly equal to unity.43 We reported possible oxide ion diffusion pathways in the mixed ionic-electronic conductors of cathode materials as La0.6Sr0.4CoO3−δ19 and La0.6Sr0.4Co0.8Fe0.2O3−δ.11 Itoh et al.44 investigated the nucleardensity distribution of a mixed conductor Ba0.5Sr0.5Co0.8Fe0.2O3−δ. However, the diffusion pathways were not visualized as connected probability density of oxide ions in these ABO3 perovskite-type mixed ionic-electronic conductors.11,19,44 On the basis of the results in the present work, we have experimentally confirmed the connected and curved diffusion pathways of the oxide ions in the ABO3 perovskite-type mixed ionicelectronic conductor for the first time (Figure 6(d)). Crystal Structure and Oxygen Diffusivity: Comparison Between La0.4Ba0.6CoO3−δ and La0.6Sr0.4CoO3−δ. In this section, we compare the correlation between crystal structure and oxygen diffusivity in La0.4Ba0.6CoO3−δ with La0.6Sr0.4CoO3−δ because higher oxygen diffusivity can provide higher ionic conductivity for cell applications. According to eq (1), D = 2eCμ, higher oxygen diffusivity is caused by higher carrier concentration C and higher oxygen mobility μ. Larger unit-cell parameters, larger bottleneck size, and higher atomic displacement parameters of oxygen atoms in La0.4Ba0.6CoO3−δ would make its oxygen mobility μ higher than that of La0.6Sr0.4CoO3−δ at the same temperature (Figures 3, 7, and 8), which leads to higher oxygen diffusivity D in La0.4Ba0.6CoO3−δ. Here, the bottleneck size is defined as the area of neighboring triangle (La,Ba)−Co−(La,Ba) (see the triangle described by the dashed lines in Figure 2(c)). The oxygen atoms pass through the bottleneck area to link with adjacent anion sites as shown by the dashed line with arrows in Figure 2(c). The bottleneck size is obtained from unit-cell parameter a, as bottleneck size = √2a2/4. As shown in Figure 7, the bottleneck size increases with increasing temperature. Larger bottleneck size at higher temperatures provides a larger space for the passing of oxygen atoms. It should be noted that the bottleneck size in

Figure 7. Temperature dependence of bottleneck size for diffusion path. Red closed circles stand for the bottleneck sizes for the oxygen diffusion path in La0.4Ba0.6CoO3‑δ. Blue open circles denote the bottleneck sizes for oxygen diffusion in Pm3̅m La0.6Sr0.4CoO3−δ. Red line was obtained by a least-squares fit to experimental data of La0.4Ba0.6CoO3−δ: (Bottleneck size) = 2.88 × 10−4 T (°C) + 5.4196. Blue line was obtained by a least-squares fit to experimental data of Pm3̅m La0.6Sr0.4CoO3−δ: (Bottleneck size) = 3.27 × 10−4T (°C) + 5.0563. Each estimated standard deviation is smaller than the symbol.

Figure 8. Temperature dependence of equivalent isotropic atomic displacement parameters (ADP) of oxygen atoms in La0.4Ba0.6CoO3−δ (red closed circles) and La0.6Sr0.4CoO3−δ (blue open circles). The red and blue lines were obtained by polynomial fits, ADP = 3.5708 × 10−8T2 (°C) + 3.2218 × 10−6T (°C) + 0.012729 and ADP = 3.2693 × 10−8T2 (°C) + 1.3931 × 10−5T (°C) + 0.021687, respectively.

La0.4Ba0.6CoO3−δ is larger than that in La0.6Sr0.4CoO3−δ at a temperature due to larger ionic radius of Ba than that of Sr, which would lead to higher oxygen mobility μ in La0.4Ba0.6CoO3−δ than that in La0.6Sr0.4CoO3−δ. Figure 8 shows higher equivalent isotropic atomic displacement 5252

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The Journal of Physical Chemistry C parameters of oxygen atoms in La0.4Ba0.6CoO3−δ than those in La0.6Sr0.4CoO3−δ at a temperature. These results strongly suggest higher oxygen mobility μ in La0.4Ba0.6CoO3−δ, leading to higher diffusivity D. The present neutron-diffraction study has demonstrated a higher oxygen defect concentration δ in La0.4Ba0.6CoO3−δ (Figure 5), which would lead to higher mobile carrier concentration C and higher diffusivity D. These evidences strongly suggest a higher oxygen diffusivity D in La0.4Ba0.6CoO3−δ. In fact, the nuclear-density maps in Figure 6 have indicated that the spatial distribution of oxide ions in La0.4Ba0.6CoO3−δ is larger than that in La0.6Sr0.4CoO3−δ at a temperature. The diffusional pathway was visualized as a connected nuclear density in La0.4Ba0.6CoO3−δ. On the contrary, the path and connected density were not observed in the nuclear-density map in La0.6Sr0.4CoO3−δ. These results indicate higher oxygen diffusivity D in La0.4Ba0.6CoO3−δ. Here we have demonstrated a simple picture on the structure-oxygen diffusivity correlation. To discuss more quantitatively the correlation, further studies might be required. However, the present simple picture would be a useful guideline to understand the correlation between the crystallographic information and oxygen diffusivity.

CONCLUSIONS We have studied the crystal structure and oxygen diffusional pathway of perovskite-type lanthanum cobaltites, La0.4Ba0.6CoO3−δ and La0.6Sr0.4CoO3−δ, by Rietveld and MEM analyses of in situ neutron powder diffraction data taken from 27.0 to 1258.0 °C. Geometric information on oxygen diffusion is essential to understand the facile electrode reaction of perovskite-type cobaltites, however, the previous approaches have been limited to computational predictions. Here we have provided longawaited experimental evidence for a curved three-dimensional oxygen diffusion path in an ABO3 mixed ionic-electronic conductor La0.4Ba0.6CoO3−δ (Figures 6 and S5 in the Supporting Information). The oxide ions diffuse in the ⟨100⟩ direction near the stable position, while the path is along ⟨110⟩ around the middle point of the path. The connected path was not visualized in La0.6Sr0.4CoO3−δ but in La0.4Ba0.6CoO3−δ, which indicates the higher oxygen diffusivity in La0.4Ba0.6CoO3−δ. The present work has demonstrated that the oxygen defect concentration δ and oxygen atomic displacement parameters of La0.4Ba0.6CoO3−δ are higher than those of La0.6Sr0.4CoO3−δ. This work has shown that the bottleneck size for oxygen diffusion in La0.4Ba0.6CoO3−δ is larger than that of La0.6Sr0.4CoO3−δ. These results are responsible for the higher oxygen diffusivity in La 0 . 4 Ba 0 . 6 CoO 3 − δ . This evidence suggests that the La0.4Ba0.6CoO3−δ is a potential cathode material for SOFCs, compared with the conventional La0.6Sr0.4CoO3−δ.

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ASSOCIATED CONTENT

S Supporting Information *

Temperature dependence of atomic displacement parameters in La0.6Sr0.4CoO3−δ and in La0.4Ba0.6CoO3−δ. Three-dimensional network of diffusional paths in La0.4Ba0.6CoO3−δ. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

We express our special thanks to Mr. T. Yamada, Dr. K. Hosoi, and Dr. N. Komada (Mitsubishi Materials Co.) for sample preparation, Mr. M. Ohkawara (Tohoku Univ.) for arranging the neutron diffraction experiments, and Dr. T. Wakita for the arrangement of ICP measurements. We also thank Mr. T. Tsuji, Mr. K. Omoto, Mr. Y. Yonehara, Mr. D. Sato, Ms. M. Saito, Mr. H. Yamada, and Mr. T. Takizawa (Tokyo Inst. Tech.) for experimental assistance and useful discussions. A part of this work was financially supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan, through a Grant-in-Aid for Scientific Research (B) No. 21360318 and Challenging Exploratory Research No. 23655190. The neutron diffraction measurements were performed at the T1-3 beamline of the JRR-3M research reactor of JAEA under the project Nos. 10766, 10768, 9726, 8767, and 5682. The synchrotron experiments were done at the BL-4B2 beamline of the Photon Factory under the project Nos. 2011G185, 2010G144, 2005G157, and 2009G072. The synchrotron diffraction measurements were performed also at the BL02B2 beamline of SPring-8 under the project Nos. 2010B1788, 2011B1995, and 2011A1442. Correspondence and requests should be addressed to M. Y.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.Y.). Notes

The authors declare no competing financial interest. 5253

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