Experimental Visualization of the Diffusional Pathway of Oxide Ions in

Article pubs.acs.org/cm

Experimental Visualization of the Diffusional Pathway of Oxide Ions in a Layered Perovskite-type Cobaltite PrBaCo2O5+δ Yi-Ching Chen,† Masatomo Yashima,*,†,‡ Juan Peña-Martínez,§ and John A. Kilner§,∥ †

Department of Materials Science and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1-W4-17, O-okayama, Meguro-Ku, Tokyo 152-8551, 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 § Department of Materials, Imperial College London, London SW7 2AZ, U.K. ∥ International Institute for Carbon-Neutral Energy Research (I2CNER), Kyushu University, 744 Motooka, Nishi-Ku, Fukuoka 819-0395, Japan ABSTRACT: Layered perovskite-type cobaltite PrBaCo2O5+δ is an oxide-ion and electronic mixed conductor with high oxide-ion conductivity and is promising for potential use in SOFC (solid oxide fuel cell) cathodes and oxygen membranes. The crystal structure and oxide-ion diffusional pathway of PrBaCo2O5+δ at 596 and 1000 °C have been investigated by in situ high-temperature neutron powder diffraction and a maximum-entropy method (MEM)-based nucleardensity analysis. The oxide ions of PrBaCo2O5+δ two-dimensionally and anisotropically diffuse through equatorial and deficient apical oxygen sites along the ⟨101⟩ directions in the Pr−Co−O ion-conducting slab. KEYWORDS: oxide-ion diffusion, crystal structure, layered perovskite, cobaltite, PrBaCo2O5+δ, neutron diffraction

■

INTRODUCTION

Mixed oxide ionic and electronic conducting ceramics (MIECs) are attracting much interest for their wide applications, such as oxygen separation membranes and cathodes in solid oxide fuel cells (SOFCs). AA′B2O5+δ-based oxides with a layered (A and A′ cation-ordered) perovskite-type structure have extensively been studied as new mixed ionic−electronic conductors, where the A = rare earth, A′ = Ba, and B = Co, and 5 + δ is the oxygen concentration.1−11 The cation ordering is believed to enhance the oxide-ion diffusivity.1 The development of improved MIECs requires a better understanding of the oxygen diffusion mechanism in their crystal structures.6−9,11−13 The mechanism of oxide-ion diffusion in cation-ordered AA′B2O5+δ-based materials is an important issue. In this work, we have chosen the chemical composition PrBaCo2O5+δ, because PrBaCo2O5+δ has the highest oxide-ion self-diffusion coefficient among AA′B2O5+δ-based oxides (A = La, Pr, Nd, Sm, Gd, and Y; A′ = Ba; and B = Co).4 Anisotropic oxide-ion diffusion of PrBaCo2O5+δ was studied by SIMS experiments.7 The diffusional pathway of mobile oxide ions in PrBaCo2O5+δ was examined by a molecular dynamics (MD) computer simulation.8 However, the oxide-ion diffusion path of PrBaCo2O5+δ has not still been determined by experiments. Here, we report the first visualization of the oxide-ion diffusion path in PrBaCo2O5+δ, through high-temperature neutron powder diffraction experiments and the maximum-entropy method (MEM). © XXXX American Chemical Society

Figure 1. Rietveld pattern of neutron powder diffraction data of PrBaCo2O5.357 taken at 1000 °C in air.

■

MATERIALS AND METHODS

Sample Preparation and Characterization. The polycrystalline PrBaCo2O5+δ sample was prepared by a conventional solid-state reaction method. High-purity powders of Pr6O11, BaCO3, and Co3O4 were ball-milled and then calcined at 1000 °C for 10 h. The mixtures were then pressed into pellets at about 100 MPa, annealed at 1150 °C for 48 h in air, and cooled down to room temperature (1 °C/min) with several intermediate grindings.7 The laboratory-based X-ray, synchrotron X-ray and neutron powder diffraction patterns indicated Received: March 26, 2013 Revised: June 1, 2013

A

dx.doi.org/10.1021/cm4010618 | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

Article

maximum-entropy methods16,17 using the computer programs RIETANFP16 and PRIMA17 (40 × 40 × 80 pixels), respectively. The following neutron scattering lengths were used in the Rietveld analysis: Pr, 4.58; Ba, 5.07; Co, 2.49; O, 5.803 fm. The crystal structure and nuclear-density distribution were visualized by the VESTA program.18

that the sample consists of a single orthorhombic Pmmm phase without impurities. The inductively coupled plasma (ICP) analysis revealed that the chemical composition of the final product (Pr1.014(5)Ba0.992(6)Co2.000(6)O5.75) was equal to PrBaCo2O5.75 at room temperature within 3 times of the standard deviation, where the number in parentheses is the standard deviation in the last digit. Here, the oxygen content (5 + δ) of the PrBaCo2O5+δ sample was determined by thermogravimetric analysis (TGA) in air and in a 5% H2 and 95% N2 gas mixture. Neutron Diffraction and Data Analysis. Neutron powder diffraction data of PrBaCo2O5+δ were collected in air at 596 and 1000 °C by the step interval of 0.1° in the 2θ range from 7° to 157° using a 150 detector system HERMES14 with a fixed wavelength of 1.84843 Å using a furnace.15 Neutron-diffraction data were analyzed by the Rietveld and

■

RESULTS AND DISCUSSION Rietveld refinements of the neutron-diffraction data of PrBaCo2O5+δ at 596 and 1000 °C were successfully performed on the basis of the tetragonal structure with P4/mmm space group symmetry (Figures 1, 2, and 3a and Table 1). In a preliminary analysis, the refined occupancy factors of Pr at the 1d site and Ba at the 1c site were larger than 1; thus, they were fixed to be 1 in the final refinement. In another preliminary analysis, the refined occupancy factor of oxygen at the O1 site Table 1. Refined Structural Parameters of PrBaCo2O5.357 Obtained by the Rietveld Analysis of Neutron Diffraction Data Taken in Air at 1000.3(8) °Ca atom site

g

x

y

z

U/10−2 Å2

Pr 1d Ba 1c Co 2g O1 1a O2 1b O3 4i

1 1 1 1 0.375(11)b 0.996(3)b

1/2 1/2 0 0 0 0

1/2 1/2 0 0 0 1/2

1/2 0 0.2522(6) 0 1/2 0.29163(19)

2.80(17) 2.60(13) 2.21(12) 3.29c 5.40c 4.55c

a

Space group: tetragonal P4/mmm (No. 123). Unit-cell parameters: a = 4.00306(15) Å, c = 7.7865(3) Å. g: Occupancy. x, y, z: Fractional coordinate. U: Atomic displacement parameter (ADP). Reliability factors: Rwp = 4.180%, GoF = 2.693, RB = 4.155%, RF = 2.746%. b Linear constraint: g(O2) = 4.3569 − 4g(O3), which is required for the chemical composition PrBaCo2O5.357 determined by TG analysis. c Equivalent isotropic ADP where the anisotropic ADPs/10−2 Å2: U11(O1) = U22(O1) = 4.1(2), U33(O1) = 1.7(2), U11(O2) = U22(O2) = 7.5(7), U33(O2) = 1.2(8), U11(O3) = 4.30(18), U22(O3) = 3.53(15), U33(O3) = 5.81(14), Uij = 0 (i ≠ j).

Figure 2. Refined crystal structure of PrBaCo2O5.357 at 1000 °C drawn with blue CoO6 octahedra. Yellow, green, and blue spheres denote Pr, Ba, and Co atoms, respectively. Red and red/white spheres stand for occupied and defective oxygen sites, respectively. For simplicity, we did not use the thermal ellipsoids in this figure, although the refinement was carried out with anisotropic ADPs. Blue and red arrows denote the cation−anion and anion−anion distances, respectively.

Figure 3. (a) Refined crystal structure of PrBaCo2O5.357 at 1000 °C drawn with blue CoO6 octahedra and red oxygen thermal ellipsoids. Yellow and green spheres denote Pr and Ba atoms, respectively. (b) Nuclear-density distributions on the bc and ca planes and yellow isosurface of nuclear density at 0.085 fm Å−3 of PrBaCo2O5.357 at 1000 °C. B

dx.doi.org/10.1021/cm4010618 | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

Article

along the c axis make different cation−anion distances (Figure 2). The O3 displacement along the c axis leads to the shorter O3−O2 distance and the longer O1−O3 one (Figure 2 and Table 2), which suggests that the oxide-ion diffusion occurs via the shorter O3−O2 pathway. The equivalent isotropic ADPs of oxygen atoms at O2 and O3 sites are higher than that at the O1 site (Table 1). The bond valence sums (BVS) of O2 and O3 are lower than the BVS of O1 at 1000 °C (Table 2), which indicates that the O2 and O3 are underbonded and weakly bound compared with O1. These results suggest that the O2 and O3 are more mobile than O1. The O2 site is highly defective. As a result, we expect that the oxide ions diffuse through the O2 and O3 sites (arrows in Figure 3a). Figure 3b shows the nuclear-density distribution on the bc and ca planes of PrBaCo2O5.357 at 1000 °C. The striking feature is the experimental visualization of the anisotropic and two-dimensional oxide-ion diffusional pathways along the ⟨101⟩ directions in the Pr−Co−O ion-conducting slab. This anisotropic feature is consistent with the results of (1) SIMS experiments7 and (2) molecular dynamics calculations.8 Figure 4 shows the nuclear-density distributions on the ca plane of PrBaCo2O5+δ at (a) 596 °C and (b) 1000 °C. The spatial distribution of oxide ions at a higher temperature, 1000 °C, is larger than that at 596 °C. A connected distribution between the O2 and O3 sites is not observed at 596 °C, but is at 1000 °C. This is consistent with higher oxygen diffusion coefficient at higher temperatures.3,4,7,8

g(O1) was 1.00, within 3 times of the estimated standard deviation; thus, we fixed g(O1) to 1 in the final refinement. The refinement with anisotropic atomic displacement parameters (ADPs) for anions gave a better fit (lower Rwp = 4.180%, Figure 3) compared with the isotropic ADPs (higher Rwp = 4.420%). Thus, we used the anisotropic ADPs in the final refinement. All the figures (Figures 1−4) and Tables 1 and 2 were obtained by using the crystallographic parameters in the final refinement with anisotropic ADPs. PrBaCo2O5+δ has a tetragonal (P4/mmm, ap × ap × 2cp) layered perovskite-type structure consisting of alternating Ba−O1 planes and Pr−Co−O slabs (Figures 2 and 3a) along the c axis at 596 and 1000 °C. The Pr−Co−O slab has the Pr−(defective O2) and two Co−O3 planes. The Co cation is displaced along the c axis to the Ba−O1 plane, while the O3 anion shifts along the c axis to the Pr−O2 plane. These displacements are attributable to the electrostatic forces. The Ba−O1 plane (total valence = +2 + (−2) = 0) is more negative compared with the more positive Pr−O2 plane (total valence = +3 + (−2)·0.375 = +2.25. The displacements of Co and O3 atoms

■

CONCLUSIONS AND DISCUSSION In conclusion, we have addressed an unresolved issue on the experimental visualization of the oxide-ion diffusional pathway of PrBaCo2O5+δ. The oxide-ion diffusion is highly anisotropic, occurring only in the Pr−(defective O2) and adjacent Co−O3 planes. The A−A′ (= Pr−Ba) cation ordering makes (i) lower BVS values of O2 and O3 compared with that of O1 and (ii) a higher oxygen vacancy concentration at the O2 site, which leads to the −O2−O3−O2− highway of mobile oxide ions in the conducting Pr−Co−O slab. The ⟨110⟩ oxygen diffusion path along the edge of the CoO4.734 octahedron of PrBaCo2O5+δ (BO6 in AA′B2O5+δ) is similar with that in cubic perovskite-type ABO3 oxides. The oxygen diffusion in cubic perovskite-type ABO3 is isotropic and three-dimensional.12,19−21 On the

Figure 4. Nuclear-density distributions on the ca plane at y = 0 of PrBaCo2O5+δ at (a) 596 °C and (b) 1000 °C.

Table 2. Bond Valence Sum (BVS), Coordination Number (CN), and the Bond Length of the Refined Structure of P4/mmm PrBaCo2O5.357 at 1000 °C, Obtained by the Rietveld Analysis of Neutron Data atom

BVS*

CN

bond length between X and Y atoms r(X−Y) × number of bonds

Ba

2.02

12

Pr

2.65

12

Co

2.35

6

O1

1.90

6

O2

1.69

6

O3

1.71

6

r(Ba−O3) = 3.0270(11) Å × 8 r(Ba−O1) = 2.8306(1) Å × 4 r(Pr−O2) = 2.8306(1) Å × 4 r(Pr−O3) = 2.5765(9) Å × 8 r(Co−O1) = 1.964(5) Å × 1 r(Co−O2) = 1.930(5) Å × 1 r(Co−O3) = 2.0250(7) Å × 4 r(O1−Ba) = 2.8306(1) Å × 4 r(O1−Co) = 1.964(5) Å × 2 r(O2−Pr) = 2.8306(1) Å × 4 r(O2−Co) = 1.930(5) Å × 2 r(O3−Ba) = 3.0270(11) Å × 2 r(O3−Pr) = 2.5765(9) Å × 2 r(O3−Co) = 2.0250(7) Å × 2

*

Bond valence parameters for Ba, Pr, and Co are 2.29, 2.135, and 1.700.23 C

dx.doi.org/10.1021/cm4010618 | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

Article

(11) Cox-Galhotra, R. A.; Huq, A.; Hodges, J. P.; Kim, J.-H.; Yu, C.; Wang, X.; Jacobson, A. J.; McIntosh, S. J. Mater. Chem. A 2013, 1, 3091−3100. (12) Chen, Y.-C.; Yashima, M.; Ohta, T.; Ohoyama, K.; Yamamoto, S. J. Phys. Chem. C 2012, 116, 5246−5254. (13) Yashima, M.; Yamada, H.; Nuansaeng, S.; Ishihara, T. Chem. Mater. 2012, 24, 4100−4113. (14) Ohoyama, K.; Kanouchi, T.; Nemoto, K.; Ohashi, M.; Kajitani, T.; Yamaguchi, Y. Jpn. J. Appl. Phys., Part 1 1998, 37, 3319−3326. (15) Yashima, M. J. Am. Ceram. Soc. 2002, 85, 2925−2930. (16) Izumi, F.; Momma, K. Solid State Phenom. 2007, 130, 15−20. (17) Izumi, F.; Dilanian, R. A. Recent Res. Dev. Phys. 2002, 3, 699− 726. (18) Momma, K.; Izumi, F. J. Appl. Crystallogr. 2011, 44, 1272−1276. (19) Cherry, M.; Islam, M. S.; Catlow, C. R. A. J. Solid State Chem. 1995, 118, 125−132. (20) Yashima, M.; Tsuji, T. J. Appl. Crystallogr. 2007, 40, 1166−1168. (21) Yashima, M.; Kamioka, T. Solid State Ionics 2008, 178, 1939− 1943. (22) Ali, R.; Yashima, M.; Izumi, F. Chem. Mater. 2007, 40, 1166− 1168. (23) Brese, N. E.; O’Keeffe, M. Acta Crystallogr., Sect. B 1991, 47, 192−197.

contrary, this work has demonstrated that the cation-ordered AA′B2O5+δ exhibits a highly anisotropic and two-dimensional network of oxide-ion diffusion paths. A similar, but different, two-dimensional and anisotropic diffusional pathway of oxide ions through the equatorial −O3−O3− sites was reported in A site deficient double-perovskite-type La0.64(Ti0.92Nb0.08)O2.99 (= AA′B2O5+δ; A = La, A′ = La0.271, B = (Ti0.92Nb0.08), and δ = 0.98).22

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS 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 (A) No. 24246107 and the Takahashi Industrial and Economic Research Foundation. J.P.-M. acknowledges financial support from the Spanish Government through the “Juan de la Cierva” and “José Castillejo” fellowship programs. We thank Prof. K. Ohoyama, Prof. T. Ishigaki, and Mr. M. Ohkawara for arranging the neutron diffraction experiments; Dr. J. Kim and Dr. K. Osaka for settings of the synchrotron diffraction experiments; and Dr. T. Wakita for the arrangement of ICP measurements. We also thank Mr. K. Omoto, Ms. M. Saito, Mr. Y. Yonehara, and Mr. D. Sato for their help with the neutron-diffraction experiments. We thank Dr. K. Fujii, Ms. E. Kitagawa, Mr. H. Kato, Mr. S. Matsuyama, Mr. U. Fumi, Mr. N. Kaneko, Mr. Y. Kubo, Mr. T. Sekikawa, and Mr. D. Haratake (Tokyo Inst. Tech.) for experimental assistance and useful discussions. The neutron-diffraction measurements were carried out as projects approved by the Neutron Science Laboratory, Institute for Solid State Physics, University of Tokyo (Proposal Nos. 10767, 10768, 9723, 8767). Neutron-diffraction data at room temperature were measured also by the iMateria diffractometer installed at the J-Parc facility, Tokai, Japan (Proposal Nos. 2012B0217 and 2013A0136). The synchrotron experiments were performed as projects approved by the Photon Factory of KEK (Proposal Nos. 2011G18, 2011G640) and by the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2012B1696, 2011B1995, 2012A1415 and 2011A1442).

■

REFERENCES

(1) Taskin, A. A.; Lavrov, A. N.; Ando, Y. Appl. Phys. Lett. 2005, 86, 091910. (2) Tarancón, A.; Skinner, S. J.; Chater, R. J.; Hernández-Ramírez, F.; Kilner, J. A. J. Mater. Chem. 2007, 17, 3175−3181. (3) Kim, G.; Wang, S.; Jacobson, A. J.; Reimus, L.; Brodersen, P.; Mims, C. A. J. Mater. Chem. 2007, 17, 2500−2505. (4) Zhang, K.; Ge, L.; Ran, R.; Shao, Z.-P.; Liu, S. M. Acta Mater. 2008, 56, 4876−4889. (5) Orea, A.; Slater, P. R. Chem. Mater. 2010, 22, 675−690. (6) Parfitt, D.; Chroneos, A.; Tarancón, A.; Kilner, J. A. J. Mater. Chem. 2011, 21, 2183−2186. (7) Burriel, M.; Pena-Martinez, J.; Chater, R. J.; Fearn, S.; Berenov, A. V.; Skinner, S. J.; Kilner, J. A. Chem. Mater. 2012, 24, 613−621. (8) Seymour, I. D.; Tarancón, A.; Chroneos, A.; Parfitt, D.; Kilner, J. A.; Grimes, R. W. Solid State Ionics 2012, 216, 41−43. (9) Hu, Y.; Hernandez, O.; Broux, T.; Ritter, C.; Geneste, G.; Dezanneau, G. J. Mater. Chem. 2012, 22, 18744−18747. (10) Liu, J.; Collins, G.; Liu, M.; Chen, C.-L.; He, J.; Jiang, J.-C.; Meletis, E. I. Appl. Phys. Lett. 2012, 100, 193903. D

dx.doi.org/10.1021/cm4010618 | Chem. Mater. XXXX, XXX, XXX−XXX