Evidence of Intragranular Segregation of Dopant ...

Electrochemical and Solid-State Letters, 10 共1兲 P1-P3 共2007兲

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1099-0062/2006/10共1兲/P1/3/$20.00 © The Electrochemical Society

Evidence of Intragranular Segregation of Dopant Cations in Heavily Yttrium-Doped Ceria Ding Rong Ou,a,z Toshiyuki Mori,a Fei Ye,a Jin Zou,b,c Graeme Auchterlonie,c and John Drennanc a

Fuel Cell Materials Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan School of Engineering, and cCentre for Microscopy and Microanalysis, The University of Queensland, St. Lucia, Queensland 4072, Australia b

The intragranular segregation of dopant cations in 25 atom % yttrium-doped ceria was explored by energy filtering transmission electron microscopy. Through the comparison between samples sintered at different temperatures, the correlation between the intragranular segregation of dopant cations and the formation of nanosized domains in heavily doped ceria is clearly shown. It is suggested that the segregation can lead to the domain formation, and the oxygen vacancies may also segregate into the domains with the dopant cations. © 2006 The Electrochemical Society. 关DOI: 10.1149/1.2372224兴 All rights reserved. Manuscript submitted June 23, 2006; revised manuscript received August 29, 2006. Available electronically November 1, 2006.

Rare earth-doped ceria with a fluorite structure is a promising ionic conductor that can be used in the intermediate-temperature solid oxide fuel cells 共SOFCs兲.1,2 Rare earth dopants, such as samarium, gadolinium, and yttrium, could have a high solubility in doped ceria.3-6 This was traditionally regarded as an advantage because it was expected that the conductivity should increase with increasing the doping concentration within the limit of solubility. However, recent studies showed that the conductivity increases with increasing the doping concentration and reaches a maximum at a critical concentration of 10–20 atom %, which is much lower than the reported solubility 共⬎35–40 atom %兲.3,6-8 Beyond that critical concentration, the conductivity decreased with further increasing the doping concentration. One explanation for this phenomenon is the clustering of dopant cations and oxygen vacancies.3,6 With increasing the doping concentration, the density and size of clusters increase, resulting in enhanced traps for the oxygen vacancies and, in turn, in the decrease in conductivity.9 In addition to the clusters, it has been demonstrated that nanosized domains can form inside the grains of heavily doped ceria and these nanosized domains contribute to the decrease in conductivity with increasing the doping concentration.7,8,10,11 However, due to the lack of strong evidence of the existence of nanosized domains and their indistinct structural and compositional characteristics, the formation mechanism of nanosized domains and their influence on the conductivity remains unclear, which led to the argument that the domains can exist and reduce the conductivity as an open question. To clarify this question, the compositional characteristic of nanosized domains must be determined. To study chemistry of nanosized inclusions, energy filtering transmission electron microscopy 共EFTEM兲 has shown its power.12,13 In this paper, we applied EFTEM technique to investigate the chemistry of nanosized domains in heavily doped ceria. Based on the experimental evidence, the formation mechanism of nanosized domains is discussed. Yttrium-doped ceria powders 共25 atom %兲 were synthesized by the ammonium carbonate co-precipitation method using a reaction temperature of 70°C 共details can be found in Ref. 8兲. The resultant precipitates were washed using distilled water and ethanol, and finally dried at room temperature with flowing nitrogen. They were subsequently calcined at 600–700°C for 2 h in an oxygen environment to yield oxide powders. Dense samples were prepared from these powders by sintering at 950 and 1400°C for 6 h, respectively. The resultant grain sizes characterized by scanning electron microscope 共SEM, Hitachi S-5000兲 are about 0.9 ␮m and 90 nm, respectively. The microstructures in sintered samples were investigated by

z

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high-resolution transmission electron microscopy 共HRTEM兲 and selected area electron diffraction 共SAED兲 in a JEM-2000EX TEM, and electron energy loss spectroscopy 共EELS兲 and EFTEM in a FEI Tecnai-F30 TEM equipped with a Gatan Imaging Filtering system. The TEM specimens were prepared by mechanical polishing and dimpling, finished by ion-beam thinning.

Figure 1. 共a-c兲 HRTEM images and SAED patterns of 25 atom % yttriumdoped ceria sintered at 1400°C and 共d兲 the results of the sample sintered at 950°C. The dashed lines approximately indicate the positions of nanosized domains.

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Electrochemical and Solid-State Letters, 10 共1兲 P1-P3 共2007兲

Figure 2. EELS spectrum shows Ce N4,5-edge and Y M4,5-edge. The inset illustrates that the background subtraction was carried out separately for elemental mapping of Ce and Y.

In the sample sintered at 1400°C, extensive TEM investigation showed a high density of nanosized domains with sizes ranging between several to twenty nanometers. Figure 1a is an example that shows a 关112兴 HRTEM image containing domains 共approximately marked by dashed lines兲. The lattice in the nanosized domains deviates slightly from the fluorite-structured matrix, which can be evidenced by the lattice distortion and dislocations 共marked by T兲 in/ around these domains and by the diffuse scattering superimposed upon the diffraction spots of the fluorite matrix as a result of the domain formation 共refer to the inset of Fig. 1a兲. In addition to the diffuse scattering patterns, faint extra spots 共some marked by arrows兲 that can be indexed in terms of fluorite structure were seen on both 关110兴 and 关111兴 SAED patterns 共Fig. 1b and c兲, indicating that a superlattice based on the fluorite structure could form in the domains. However, as the sintering temperature decrease to 950°C 共Fig. 1d兲, the domains were rarely observed and the diffuse scattering shown in its corresponding SAED pattern became weak. To determine the chemical composition of these nanosized domains, EELS and EFTEM were carried out. Figure 2 is an EELS spectrum in the energy range between 100 and 200 eV, from which Ce N4,5-edge 共110 eV兲 and Y M4,5-edge 共157 eV兲 can be clearly distinguished. Using these two edges, the elemental mapping was performed using EFTEM with an energy window of 10 eV for each case. The three-window power-law technique14 was used to subtract the background for both Ce and Y edges. If the sample is sufficiently thin and deviated away from the strong Bragg reflection condition, the brightness of the map is proportional to the mapping element.14 Figure 3 shows EFTEM maps of Ce and Y elements. As can be seen from both Ce and Y maps taken from the sample sintered at 1400°C 共Fig. 3a兲, there are regions 共as marked by arrows兲 showing brighter in the Y map, while darker in the Ce map. In addition, some regions showed clear faceted boundaries with the matrix 共as shown on the top left of Fig. 3a兲. All of these indicate that these regions, with sizes ranging between several to 20 nm, are rich in Y when compared with the matrix. By taking this fact and the results shown by HRTEM 共nanosized domains兲 and SAED 共the diffused scattering and the extra diffraction spots兲 into account, it is believed that these regions correspond to the nanosized domains and they have their own composition and structure. Interestingly, this phenomenon disappears in the Ce and Y maps taken from the sample sintered at 950°C 共Fig. 3b兲, agreeing with the HRTEM observation. This observation implies that the sintering temperature played a key role in the formation of nanosized domains. The fact that the nanosized domains have different structure and composition from their fluorite-structured matrix suggests that these domains are secondary phases formed in the heavily doped ceria. Through correlating the facts of the dopant segregation, the domain formation, and the effect of sintering temperature, we believe thta

Figure 3. Ce and Y maps recorded via EFTEM. The samples are sintered at 共a兲 1400 and 共b兲 950°C. The arrows in 共b兲 indicate the grain boundaries 共GB兲.

the formation of nanosized domains in our case can be modeled as follows. Before sintering, the distribution of dopant cations is homogenous in compact pellets prepared from the nano-powders of doped ceria; during the sintering process, the diffusions of both host and dopant cations take place and result in the segregation of dopant cations and promote the growth of nanosized domains. It is believed that the oxygen vacancies segregate with the dopant cations simultaneously. This is because calculations15,16 had shown that, in heavily doped ceria, the neutral clusters involving both dopant cations and oxygen vacancies are more thermodynamically favorable than the clusters containing only dopant cations or oxygen vacancies. As a consequence of the local enrichment of both dopant cations and oxygen vacancies, the nanosized domains can be formed with dopant concentration and crystal structure different from their matrix. On the other hand, the diffuse scattering and faint extra spots shown by the SAED study 共Fig. 1兲 suggest that the formation of the nanosized domains could be a symptom of a transition from the fluorite structure to some kind of superstructure.17,18 When the domains are very small and immature, they merely produce diffuse scattering. According to the phase diagram of CeO2 and Y2O3,19 the possible secondary phase in yttrium-doped ceria is the solid solution of Ce in c-type Y2O3, forming at Y concentration over the solubility 共⬎33–40 atom %兲. Therefore, we believed that the formation of the domains with higher Y concentration could be a pretransition to the c-type Y2O3, and the crystal structure of the domains could be a c-type-related superstructure. Furthermore, because the segregation of the dopant cations can lead to a deficiency of oxygen in the domains and because the cubic unit cell of c-type structure can be constructed out of eight unit cells of fluorite structure by removing 25% of the oxygen ions along four nonintersecting 具111典 diagonals,15 it is also believed that the ordering of the concentrated oxygen vacancies is involved in the superstructure and could make the existence of nanosized domains more stable. In conclusion, the intragranular segregation of dopant cations in 25 atom % yttrium-doped ceria has been clearly identified through the EFTEM study. It has been shown that there are correlations between the dopant segregation, the domain formation and the effect of the sintering temperature, based on which it is suggested that the domain formation in yttrium-doped ceria is resulted from the segregation of dopant cations and the oxygen vacancies can also segregate with the dopant cations.


Electrochemical and Solid-State Letters, 10 共1兲 P1-P3 共2007兲 Acknowledgment The financial support from a Grant-in-Aid for Scientific Research on Priority Area, Nanoionics 共439兲 by the Ministry of Education, Culture, Sports, and Technology, Japan and the Australian Research Council are acknowledged. The National Institute for Materials Science assisted in meeting the publication costs of this article.

References 1. B. C. H. Steele and A. Heinezl, Nature (London), 414, 345 共2001兲. 2. S. Kartha and P. Grimes, Phys. Today, 47, 54 共1994兲. 3. D. Y. Wang, D. S. Park, J. Griffith, and A. S. Nowick, Solid State Ionics, 2, 95 共1981兲. 4. K. Eguchi, T. Setoguchi, T. Inoue, and H. Arai, Solid State Ionics, 52, 165 共1992兲. 5. G. B. Balazs and R. S. Glass, Solid State Ionics, 76, 155 共1995兲. 6. H. Inaba and H. Tagawa, Solid State Ionics, 83, 1 共1996兲. 7. T. Mori, J. Drennan, Y. Wang, G. Auchterlonie, J.-G. Li, and A. Yago, Sci. Technol. Adv. Mater., 4, 213 共2003兲. 8. D. R. Ou, T. Mori, F. Ye, M. Takahashi, J. Zou, and J. Drennan, Acta Mater., 54, 3737 共2006兲.

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9. 共a兲 C. R. A. Catlow, in Nonstoichiometric Oxides, O. T. Sorensen, Editor, p. 61, Academic Press, New York 共1981兲; 共b兲 J. Kilnrt and B. C. H. Steele, in Nonstoichiometric Oxides, O. T. Sorensen, Editor, p. 247, Academic Press, New York 共1981兲. 10. T. Mori, J. Drennan, Y. Wang, J.-H. Lee, J.-G. Li, and T. Ikegami, J. Electrochem. Soc., 150, A665 共2003兲. 11. T. Mori, J. Drennan, J.-H. Lee, J.-G. Li, and T. Ikegami, Solid State Ionics, 154– 155, 461 共2002兲. 12. D. Teguri, K. Matsuda, T. Sakal, and S. Ikeno, Mater. Sci. Forum, 396, 911 共2002兲. 13. A. Zern, J. Mayer, N. Janakiraman, M. Weinmann, and J. Bill, M. Ruhle, J. Eur. Ceram. Soc., 22, 1621 共2002兲. 14. R. F. Egerton, Electron Energy Loss Spectroscopy in the Electron Microscope, 2nd ed., Plenum Press, New York 共1996兲. 15. N. V. Skorodumova, S. I. Simak, B. I. Lundpvist, I. A. Abrikosov, and B. Johansson, Phys. Rev. Lett., 89, 166601 共2002兲. 16. H. Hayashi, R. Sagawa, H. Inaba, and K. Kawamura, Solid State Ionics, 131, 281 共2000兲. 17. S. García-Martín, M. A. Alario-Franco, D. P. Fagg, A. J. Feighery, and J. T. S. Irvine, Chem. Mater., 12, 1729 共2000兲. 18. A. Gallardo-Lopez, J. Martínez-Fernández, A. Domínguez-Rodríguez, and F. Ernst, Philos. Mag. A, 81, 1675 共2001兲. 19. V. Longo and L. Podda, J. Mater. Sci., 16, 839 共1981兲.
