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Raman study on defect structure of high-temperature protonic conducting ceramics
Solid State Ionics 177 (2006) 2443 – 2445 www.elsevier.com/locate/ssi
Raman study on defect structure of high-temperature protonic conducting ceramics Atsushi Mineshige ⁎, Sachio Okada, Masafumi Kobune, Tetsuo Yazawa Department of Materials Science and Chemistry, Graduate School of Engineering, University of Hyogo, Himeji, Hyogo 671-2201, Japan Received 1 September 2005; received in revised form 11 April 2006; accepted 3 August 2006
Abstract Various acceptor-doped strontium cerates, Sr(Ce,M)O3−δ (M; dopant ions), were synthesized and treated under dry atmosphere. Each Raman spectrum was measured just after the dry treatment and compared with that of the as-sintered sample, focusing on the defect Raman band at 630 cm− 1. Relative intensity of the defect band, which increases monotonously with an increase in δ, increased by the dry treatment due to removal of water from lattice and resulting oxygen vacancy formation. Effect of dopant species on water solubility was discussed from relative intensity and peak center of the defect band. © 2006 Elsevier B.V. All rights reserved. Keywords: Proton conductor; Water solubility; Raman spectroscopy
1. Introduction Acceptor-doped strontium cerates, SrCe1−xMxO3−δ, are well known as high-temperature protonic conductors [1] and as successful candidates for electrolyte materials in hydrogen separation membranes and electrochemical hydrogen pumps [2]. In these systems, lattice defects such as oxygen vacancies, introduced as charge compensation for replacement of Ce4+ site with trivalent metal ions, M, and interstitial protons, taken from water vapor or hydrogen in ambient gas, play an important role in electrical conduction. We are interested in proton formation mechanism and water solubility in these systems. Conductive protons are introduced mainly by dissolution of water vapor in ambient gas, accompanied by disappearance of some oxygen vacancies. Therefore, one important way to investigate them is to obtain information of oxygen vacancies.
⁎ Corresponding author. 2167 Shosha, Himeji, Hyogo 671-2201, Japan. Tel./ fax: +81 792 67 4944. E-mail address: [email protected] (A. Mineshige). 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.08.004
Raman spectroscopy is a powerful tool to detect oxygen vacancies in some oxides. In our previous studies, oxygen nonstoichiometry in doped CeO2−δ [3] and Sr(Ce,Yb)O3−δ [4] systems were studied. Furthermore, the oxygen chemical potential in doped CeO2−δ system was evaluated by employing this technique [5]. The aim of the present work is to observe a change in the Raman spectra of Sr(Ce,M)O3−δ by a dry treatment and to investigate solubility of water vapor in each oxide. 2. Experimental Samples of SrCe1−xYbxO3−δ (0.0 ≤ x ≤ 0.1) and SrCe0.95M0.05O3−δ (M = Sc, Yb, Y, Gd and Sm) were prepared by conventional solid-state reaction method using SrCO3 (N 99.9% purity), CeO 2 (N 99.99% purity) and M 2 O 3 (M = Sc3+ , Yb3+ , Y3+ , Gd3+ and Sm3+ , N 99.9% purity). Raman spectra were obtained using Raman spectrometer (model T64000, Jobin Yvon) equipped with Ar+ -ion laser of the 514.5 nm line. In addition to the as-sintered sample, a proton-free sample was prepared by a dry treatment. For this purpose the specimen was heated at 1423 K and dry gas mixture of Ar and O2 (Ar/O2 = 4/1, dew point ≤ 238 K) was
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A. Mineshige et al. / Solid State Ionics 177 (2006) 2443–2445
Fig. 1. Raman spectra of SrCe1−xYbxO3−δ as-sintered and dry treated samples. Inset shows defect bands' region for x = 0.03 and x = 0.1.
introduced. After annealing for 7 h under this condition, the specimen was quenched into room temperature and its Raman spectrum was observed as quickly as possible. 3. Results and discussion 3.1. Effect of dopant concentration on Raman spectra Fig. 1 shows Raman spectra for as-sintered and dry-annealing samples of SrCe1−xYbxO3−δ (0.0 ≤ x ≤ 0.1). Each spectrum was normalized to the main band intensity at 345 cm− 1. The bands around 315, 345, and 375 cm− 1 were attributed to the stretching vibrational modes of CeO6 octahedra [6]. For all of Raman spectra, the weak bands at around 520 and 630 cm− 1 were observed except for SrCeO3. The intensities of these bands increased with oxygen vacancy formation by acceptor doping, or low P(H2O) annealing. In our previous study [4], it was found that the intensity of the 630 cm− 1 band relative to that of the main band at ca. 345 cm− 1 was proportional to the dopant concentration, i.e., oxygen vacancy concentration. Therefore, in this study, the relative intensity of the 630 cm− 1 band was
Fig. 2. Raman spectra of SrCe0.95M0.05O3−δ as-sintered and dry treated samples.
Fig. 3. Relative intensity and peak center of the defect band at around 630 cm− 1 as a function of dopant ionic radius for SrCe0.95M0.05O3−δ.
studied to observe oxygen vacancies and to evaluate water solubility into each specimen. In Fig. 1, the largest difference in the 630 cm− 1 band intensity during the annealing was observed for the sample x = 0.05. Although it was hard to see a difference between the as-sintered and dry samples in case of x = 0.03 and x = 0.1, there was a subtle difference in the 630 cm− 1 band intensity as shown in the inset. Oxygen nonstoichiometry (δ) was evaluated assuming that the intensity of the 630 cm− 1 band after peak separation was proportional to δ, and the proton concentration in the dry samples was zero (δ = x/2). From a difference in δ between the treated and as-sintered samples, proton concentration due to water dissolution was evaluated. The estimated proton concentrations introduced by water dissolution were 0.7, 1.8 and 0.4 mol% for x = 0.03, 0.05 and 0.1 as-sintered samples, respectively. 3.2. Effect of dopant species on Raman spectra Fig. 2 shows normalized Raman spectra around the defect bands for SrCe0.95M0.05O3−δ (M = Sc, Yb, Y, Gd and Sm) assintered and dry samples. It was found that the relative intensity
Fig. 4. Electrical conductivity in wet Ar as a function of dopant ionic radius for SrCe0.95M0.05O3−δ [7].
A. Mineshige et al. / Solid State Ionics 177 (2006) 2443–2445
of the 630 cm− 1 band increased by dry treatment for all samples. The relative intensity of the 630 cm− 1 band evaluated after peak separation was shown in Fig. 3 as a function of dopant ionic radius, r, for as-sintered and dry treated samples. A wave number of the peak center of this band after annealing was also plotted in the same figure. In the case of M = Yb, a difference in the band intensity between as-sintered and treated samples was the highest, which showed that the water solubility was the maximum for M = Yb. The proton concentration of as-sintered samples by water dissolution was estimated based on the similar assumptions described in the previous section. They were 1.1, 1.8, 1.1, 0.9 and 0.4 mol% for M = Sc, Yb, Y, Gd and Sm, respectively. In addition, clear dependencies of the band intensity and peak center on dopant ionic radii were observed though each δ is supposed to be the same. The Yb-doped sample exhibited the highest intensity and peak center among them. This fact suggested that the defect band was related with formation of oxygen vacancies in the vicinity of the dopant ions, and an interaction between oxygen vacancies and dopant ions was the strongest in the case M = Yb. The situation probably led to the highest water solubility for the sample M = Yb. In our previous study [7], the relative band area at 345 cm− 1 of SrCe0.95M0.05O3−δ as-sintered sample was found to be also dependent of dopant ionic radii. The band area increased with increasing ionic radius at r b 0.0868 nm, while it decreased at r N 0.0868 nm, as the relative intensity and the peak center of the 630 cm− 1 band did. A similar tendency was found for the dry treated samples in this study. In the SrCe1−xYbxO3−δ system, the 345 cm− 1 band width lineally increases with increasing dopant concentration as reported by Kosacki et al. [6]. They mentioned that the broadening of the band at 345 cm− 1 was probably induced by local distortion around dopant ions. Broadening of the main band and appearance of the defect band are probably related to the occurrence of the interaction between dopant ions and oxygen vacancies. The stronger interaction seems to give the higher water solubility. It is known that Yb-doping provides the system the highest proton conductivity in SrCeO3-based oxide, because the dopant cation, Yb3+ (0.0868 nm: 6CN [8]) has closer ionic radius against the host cation, Ce4+ (0.087 nm: 6CN [8]). Electrical conductivity of specimens SrCe0.95M0.05O3−δ (M = Sc, Yb, Y, Gd and Sm) in wet Ar at 873–1273 K was studied previously [7] and plotted in Fig. 4. The sample of SrCe0.95Yb0.05O3−δ exhibited the maximum conductivity over the whole temperature range tested. In addition, conductivity decreased gradually
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either with increasing or decreasing ionic radius from r = 0.0868 nm. Although it should be noted that this is total (proton, oxide ion and p-type electron) conductivity, the predominant charge carrier might be proton especially below 1073 K. It was found that dependency of electrical conductivity on dopant ionic radius was quite similar to that of the estimated proton concentration, and the relative intensity and peak center of 630 cm− 1 band. Hence, the dependency of proton conductivity on dopant ionic radius was closely connected with the water solubility and the local structure. We concluded that the Yb dopant ion was preferable to have oxygen vacancies at its nearest neighboring site, and suitable to accept water vapors. 4. Conclusions Raman spectroscopy is useful to evaluate water solubility as well as the local structure for the high-temperature protonic conductors. The Yb3+-doped sample had the highest protonic conductivity in SrCeO3-family. It is probably due to enhancement of solubility of water vapor into oxygen vacancies in the vicinity of the dopant ions. The dopant Yb3+ is the most preferable to accept water vapors. Acknowledgement This study was supported by Grant-in-Aid for Scientific Research (No. 16750178) from the Japan Society for the Promotion of Science. References [1] H. Iwahara, T. Esaka, H. Uchida, N. Maeda, Solid State Ionics 3/4 (1981) 359. [2] H. Matsumoto, S. Hamajima, H. Iwahara, J. Electrochem. Soc. 148 (2001) D121. [3] A. Mineshige, T. Taji, Y. Muroi, M. Kobune, S. Fujii, N. Nishi, M. Inaba, Z. Ogumi, Solid State Ionics 135 (2000) 481. [4] A. Mineshige, S. Okada, K. Sakai, M. Kobune, S. Fujii, H. Matsumoto, T. Shimura, H. Iwahara, Z. Ogumi, Solid State Ionics 162–163 (2003) 41. [5] A. Mineshige, T. Yasui, N. Ohmura, M. Kobune, S. Fujii, M. Inaba, Z. Ogumi, Solid State Ionics 152–153 (2002) 493. [6] I. Kosacki, J. Schoonman, M. Balkanski, Solid State Ionics 57 (1992) 345. [7] S. Okada, A. Mineshige, M. Kobune, T. Yazawa, J. Ceram. Soc. Jpn. 112 (2004) S700. [8] R.D. Shannon, Acta Crystallogr., Sect. A 32 (1976) 751.
Raman study on defect structure of high-temperature protonic conducting ceramics Atsushi Mineshige ⁎, Sachio Okada, Masafumi Kobune, Tetsuo Yazawa Department of Materials Science and Chemistry, Graduate School of Engineering, University of Hyogo, Himeji, Hyogo 671-2201, Japan Received 1 September 2005; received in revised form 11 April 2006; accepted 3 August 2006
Abstract Various acceptor-doped strontium cerates, Sr(Ce,M)O3−δ (M; dopant ions), were synthesized and treated under dry atmosphere. Each Raman spectrum was measured just after the dry treatment and compared with that of the as-sintered sample, focusing on the defect Raman band at 630 cm− 1. Relative intensity of the defect band, which increases monotonously with an increase in δ, increased by the dry treatment due to removal of water from lattice and resulting oxygen vacancy formation. Effect of dopant species on water solubility was discussed from relative intensity and peak center of the defect band. © 2006 Elsevier B.V. All rights reserved. Keywords: Proton conductor; Water solubility; Raman spectroscopy
1. Introduction Acceptor-doped strontium cerates, SrCe1−xMxO3−δ, are well known as high-temperature protonic conductors [1] and as successful candidates for electrolyte materials in hydrogen separation membranes and electrochemical hydrogen pumps [2]. In these systems, lattice defects such as oxygen vacancies, introduced as charge compensation for replacement of Ce4+ site with trivalent metal ions, M, and interstitial protons, taken from water vapor or hydrogen in ambient gas, play an important role in electrical conduction. We are interested in proton formation mechanism and water solubility in these systems. Conductive protons are introduced mainly by dissolution of water vapor in ambient gas, accompanied by disappearance of some oxygen vacancies. Therefore, one important way to investigate them is to obtain information of oxygen vacancies.
⁎ Corresponding author. 2167 Shosha, Himeji, Hyogo 671-2201, Japan. Tel./ fax: +81 792 67 4944. E-mail address: [email protected] (A. Mineshige). 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.08.004
Raman spectroscopy is a powerful tool to detect oxygen vacancies in some oxides. In our previous studies, oxygen nonstoichiometry in doped CeO2−δ [3] and Sr(Ce,Yb)O3−δ [4] systems were studied. Furthermore, the oxygen chemical potential in doped CeO2−δ system was evaluated by employing this technique [5]. The aim of the present work is to observe a change in the Raman spectra of Sr(Ce,M)O3−δ by a dry treatment and to investigate solubility of water vapor in each oxide. 2. Experimental Samples of SrCe1−xYbxO3−δ (0.0 ≤ x ≤ 0.1) and SrCe0.95M0.05O3−δ (M = Sc, Yb, Y, Gd and Sm) were prepared by conventional solid-state reaction method using SrCO3 (N 99.9% purity), CeO 2 (N 99.99% purity) and M 2 O 3 (M = Sc3+ , Yb3+ , Y3+ , Gd3+ and Sm3+ , N 99.9% purity). Raman spectra were obtained using Raman spectrometer (model T64000, Jobin Yvon) equipped with Ar+ -ion laser of the 514.5 nm line. In addition to the as-sintered sample, a proton-free sample was prepared by a dry treatment. For this purpose the specimen was heated at 1423 K and dry gas mixture of Ar and O2 (Ar/O2 = 4/1, dew point ≤ 238 K) was
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A. Mineshige et al. / Solid State Ionics 177 (2006) 2443–2445
Fig. 1. Raman spectra of SrCe1−xYbxO3−δ as-sintered and dry treated samples. Inset shows defect bands' region for x = 0.03 and x = 0.1.
introduced. After annealing for 7 h under this condition, the specimen was quenched into room temperature and its Raman spectrum was observed as quickly as possible. 3. Results and discussion 3.1. Effect of dopant concentration on Raman spectra Fig. 1 shows Raman spectra for as-sintered and dry-annealing samples of SrCe1−xYbxO3−δ (0.0 ≤ x ≤ 0.1). Each spectrum was normalized to the main band intensity at 345 cm− 1. The bands around 315, 345, and 375 cm− 1 were attributed to the stretching vibrational modes of CeO6 octahedra [6]. For all of Raman spectra, the weak bands at around 520 and 630 cm− 1 were observed except for SrCeO3. The intensities of these bands increased with oxygen vacancy formation by acceptor doping, or low P(H2O) annealing. In our previous study [4], it was found that the intensity of the 630 cm− 1 band relative to that of the main band at ca. 345 cm− 1 was proportional to the dopant concentration, i.e., oxygen vacancy concentration. Therefore, in this study, the relative intensity of the 630 cm− 1 band was
Fig. 2. Raman spectra of SrCe0.95M0.05O3−δ as-sintered and dry treated samples.
Fig. 3. Relative intensity and peak center of the defect band at around 630 cm− 1 as a function of dopant ionic radius for SrCe0.95M0.05O3−δ.
studied to observe oxygen vacancies and to evaluate water solubility into each specimen. In Fig. 1, the largest difference in the 630 cm− 1 band intensity during the annealing was observed for the sample x = 0.05. Although it was hard to see a difference between the as-sintered and dry samples in case of x = 0.03 and x = 0.1, there was a subtle difference in the 630 cm− 1 band intensity as shown in the inset. Oxygen nonstoichiometry (δ) was evaluated assuming that the intensity of the 630 cm− 1 band after peak separation was proportional to δ, and the proton concentration in the dry samples was zero (δ = x/2). From a difference in δ between the treated and as-sintered samples, proton concentration due to water dissolution was evaluated. The estimated proton concentrations introduced by water dissolution were 0.7, 1.8 and 0.4 mol% for x = 0.03, 0.05 and 0.1 as-sintered samples, respectively. 3.2. Effect of dopant species on Raman spectra Fig. 2 shows normalized Raman spectra around the defect bands for SrCe0.95M0.05O3−δ (M = Sc, Yb, Y, Gd and Sm) assintered and dry samples. It was found that the relative intensity
Fig. 4. Electrical conductivity in wet Ar as a function of dopant ionic radius for SrCe0.95M0.05O3−δ [7].
A. Mineshige et al. / Solid State Ionics 177 (2006) 2443–2445
of the 630 cm− 1 band increased by dry treatment for all samples. The relative intensity of the 630 cm− 1 band evaluated after peak separation was shown in Fig. 3 as a function of dopant ionic radius, r, for as-sintered and dry treated samples. A wave number of the peak center of this band after annealing was also plotted in the same figure. In the case of M = Yb, a difference in the band intensity between as-sintered and treated samples was the highest, which showed that the water solubility was the maximum for M = Yb. The proton concentration of as-sintered samples by water dissolution was estimated based on the similar assumptions described in the previous section. They were 1.1, 1.8, 1.1, 0.9 and 0.4 mol% for M = Sc, Yb, Y, Gd and Sm, respectively. In addition, clear dependencies of the band intensity and peak center on dopant ionic radii were observed though each δ is supposed to be the same. The Yb-doped sample exhibited the highest intensity and peak center among them. This fact suggested that the defect band was related with formation of oxygen vacancies in the vicinity of the dopant ions, and an interaction between oxygen vacancies and dopant ions was the strongest in the case M = Yb. The situation probably led to the highest water solubility for the sample M = Yb. In our previous study [7], the relative band area at 345 cm− 1 of SrCe0.95M0.05O3−δ as-sintered sample was found to be also dependent of dopant ionic radii. The band area increased with increasing ionic radius at r b 0.0868 nm, while it decreased at r N 0.0868 nm, as the relative intensity and the peak center of the 630 cm− 1 band did. A similar tendency was found for the dry treated samples in this study. In the SrCe1−xYbxO3−δ system, the 345 cm− 1 band width lineally increases with increasing dopant concentration as reported by Kosacki et al. [6]. They mentioned that the broadening of the band at 345 cm− 1 was probably induced by local distortion around dopant ions. Broadening of the main band and appearance of the defect band are probably related to the occurrence of the interaction between dopant ions and oxygen vacancies. The stronger interaction seems to give the higher water solubility. It is known that Yb-doping provides the system the highest proton conductivity in SrCeO3-based oxide, because the dopant cation, Yb3+ (0.0868 nm: 6CN [8]) has closer ionic radius against the host cation, Ce4+ (0.087 nm: 6CN [8]). Electrical conductivity of specimens SrCe0.95M0.05O3−δ (M = Sc, Yb, Y, Gd and Sm) in wet Ar at 873–1273 K was studied previously [7] and plotted in Fig. 4. The sample of SrCe0.95Yb0.05O3−δ exhibited the maximum conductivity over the whole temperature range tested. In addition, conductivity decreased gradually
2445
either with increasing or decreasing ionic radius from r = 0.0868 nm. Although it should be noted that this is total (proton, oxide ion and p-type electron) conductivity, the predominant charge carrier might be proton especially below 1073 K. It was found that dependency of electrical conductivity on dopant ionic radius was quite similar to that of the estimated proton concentration, and the relative intensity and peak center of 630 cm− 1 band. Hence, the dependency of proton conductivity on dopant ionic radius was closely connected with the water solubility and the local structure. We concluded that the Yb dopant ion was preferable to have oxygen vacancies at its nearest neighboring site, and suitable to accept water vapors. 4. Conclusions Raman spectroscopy is useful to evaluate water solubility as well as the local structure for the high-temperature protonic conductors. The Yb3+-doped sample had the highest protonic conductivity in SrCeO3-family. It is probably due to enhancement of solubility of water vapor into oxygen vacancies in the vicinity of the dopant ions. The dopant Yb3+ is the most preferable to accept water vapors. Acknowledgement This study was supported by Grant-in-Aid for Scientific Research (No. 16750178) from the Japan Society for the Promotion of Science. References [1] H. Iwahara, T. Esaka, H. Uchida, N. Maeda, Solid State Ionics 3/4 (1981) 359. [2] H. Matsumoto, S. Hamajima, H. Iwahara, J. Electrochem. Soc. 148 (2001) D121. [3] A. Mineshige, T. Taji, Y. Muroi, M. Kobune, S. Fujii, N. Nishi, M. Inaba, Z. Ogumi, Solid State Ionics 135 (2000) 481. [4] A. Mineshige, S. Okada, K. Sakai, M. Kobune, S. Fujii, H. Matsumoto, T. Shimura, H. Iwahara, Z. Ogumi, Solid State Ionics 162–163 (2003) 41. [5] A. Mineshige, T. Yasui, N. Ohmura, M. Kobune, S. Fujii, M. Inaba, Z. Ogumi, Solid State Ionics 152–153 (2002) 493. [6] I. Kosacki, J. Schoonman, M. Balkanski, Solid State Ionics 57 (1992) 345. [7] S. Okada, A. Mineshige, M. Kobune, T. Yazawa, J. Ceram. Soc. Jpn. 112 (2004) S700. [8] R.D. Shannon, Acta Crystallogr., Sect. A 32 (1976) 751.