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Influence of In content on the electrical conduction behavior of Sm- and In-co-doped proton conductor BaCe0.80-xSm0.20InxO3-δ
Solid State Ionics 206 (2012) 17–21
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Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Influence of In content on the electrical conduction behavior of Sm- and In-co-doped proton conductor BaCe0.80-xSm0.20InxO3-δ Cuijuan Zhang a, Hailei Zhao a, b,⁎ a b
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Beijing Key Lab of New Energy Materials and Technology, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 15 January 2011 Received in revised form 29 August 2011 Accepted 25 October 2011 Available online 21 November 2011 Keywords: Proton conductor Barium cerate Co-doping Electrical conductivity
a b s t r a c t The influence of indium content on the electrical conduction behavior of Sm- and In-co-doped BaCe0.80-xSm0.20InxO3-δ proton conductor was investigated by impedance spectroscopy. The structural characteristics were studied by means of X-ray diffraction, Raman spectra, high resolution transmission electron microscopy, and thermogravimetric analysis. Both long and short range structural symmetry increased with In content, from orthorhombic (x= 0–0.50) to cubic structure (x= 0.60–0.80). No oxygen vacancy ordering or microdomains were detected. The proton concentration of the hydrated samples BaCe0.80-xSm0.20InxO3-δ (x= 0–0.20) decreased with In level. The electrical conductivity of the co-doped samples decreased with In content in wet 5% H2/Ar and wet Ar atmospheres. The reasons for the decreased electrical conductivity were discussed. The variation of electrical conductivity of BaCe0.80-xSm0.20InxO3-δ with In concentration can be applicable to other RE- and In-co-doped BaCeO3 materials. © 2011 Elsevier B.V. All rights reserved.
1. Introduction High temperature proton conductors (HTPCs) have been attracting growing attention in recent years for their promising applications in solid oxide fuel cells, hydrogen sensor, hydrogen separation, ammonia synthesis at atmospheric pressure, etc. [1]. The protonic defect can be introduced into the lattice through the hydration reaction according to Eq. (1).
· H 2 O þ V ·· O þ OO ¼ 2OH O
ð1Þ
It is obvious that the proton concentration increased with the oxygen vacancy concentration. Increasing the oxygen vacancy concentration is desirable for the improvement of the proton conductivity. BaCeO3-based materials are the most investigated among the HTPC family owing to their higher protonic conductivity [2]. However, these materials are susceptible to chemical degradation in carbon dioxide and water vapor, forming carbonate and hydroxide [2]. The chemical stability can be enhanced by partial replacement of Ce with other elements including In [3]. Although the structural characteristics, electrical conduction behavior and chemical stability of BaCeO3 doped with various cations have been widely investigated [4–8], the reports about the influence of co-dopants at the Ce-site on these properties were relatively limited [9,10]. ⁎ Corresponding author at: School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China. Tel./fax: + 86 10 82376837. E-mail address: [email protected] (H. Zhao). 0167-2738/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2011.10.026
The larger Sm 3 + has been suggested to be a good dopant for BaCeO3 but with somewhat low solubility (≤30%) [11]; in contrast, the smaller In 3 + exhibited a high solubility in BaCeO3, up to 75% [12]. Furthermore, based on recent results of the unusually high protonic conductivity reported in Ba2In2O5[13,14], it was thought that doping BaCeO3 with In might be of interest. Therefore, Sm and In were selected as the co-dopants to modify the properties of BaCeO3. The influence of In content on the electrical conduction behavior of BaCe0.80-xSm0.20InxO3-δ was investigated. The present results demonstrate that, unexpectedly, the electrical conductivity of the Sm- and In-co-doped BaCe0.80-xSm0.20InxO3-δ decreased with In content. Similar results were observed in other RE- and In-co-doped BaCeO3 compounds. X-ray diffraction (XRD), Raman spectroscopy, selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM), and thermogravimetric analysis (TGA) were employed to explore the reasons for the decreased electrical conductivity. 2. Experimental 2.1. Sample preparation The BaCe0.80-xSm0.20InxO3-δ (x= 0–0.80) powders were synthesized by the citric-nitrate method [11]. The raw materials were Ba(NO3)2 (>99.5%), Ce(NO3)3·6H2O (>99.0%, Guangdong Xilong Chemical Co., Ltd), and Sm2O3 (99.95%) and In2O3 (99.99%, Grirem Advanced Materials Co., Ltd). The latter two oxides were annealed at 1000 °C for 10 h to remove the possibly absorbed water and then dissolved in diluted nitric acid before use. The final sintering step was carried out
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C. Zhang, H. Zhao / Solid State Ionics 206 (2012) 17–21
at 1500 and 1400 °C for 5 h in air for x = 0 and 0.05–0.80, respectively. BaCe1-yInyO3-δ (y= 0.10, 0.20, and 0.30) was also prepared with the same procedures for the co-doped samples for comparison. Distinct sintering temperatures were adopted considering the enhanced sinterability due to In doping [3]. The samples with x = 0.30–0.60 cracked into pieces after sintering at 1400 °C. The sintering parameters, including the sintering temperature (1200–1500 °C), heating rate and holding time, have been optimized to try to obtain integrate samples, but unsuccessfully. Accordingly, the subsequent electrical conductivity measurement was not performed on these samples. The relative density of the sintered pellets with x = 0–0.20 and y = 0.10–0.30 was around 95%, which was estimated from the weight and dimensions measurement.
2.2. Characterization
Fig. 1. (a) XRD patterns of sintered BaCe0.80-xSm0.20InxO3-δ; (b) magnification of the 2θ range of 30–40°.
Powder XRD was recorded on Rigaku D/MAX-RB X-ray diffractometer with Ni filtered Cu Kα radiation with λ = 1.54056 Å, by a 0.02° step at room temperature. To investigate the short range structural characteristics, Raman spectra were collected for BaCe0.80-xSm0.20InxO3-δ (x= 0, 0.05, 0.10, 0.20, 0.50, and 0.80) and BaCe1-yInyO3-δ (y= 0.10, 0.20, and 0.30) on a Raman microscope (HORIBA JOBIN YVON LabRam HR800) at room temperature. An Ar laser with a wavelength of 514.5 nm was used for excitation. The diameter of the incident laser beam on the fine powder was 0.697 μm and the power under the microscope objective was 2 mW to avoid overheating. The samples were scanned three times to obtain a better signal to noise ratio and the summed spectra were reported here. Great homogeneity was observed for all samples. All the finely ground powders were light yellow green and thus any absorption related to color change [15] was ruled out. SAED patterns and HRTEM images of BaCe0.60Sm0.20In0.20O3-δ were acquired with a TECNAI-F20 electron microscope operating at 200 keV. The sample was polished with sand paper and ion-milled to achieve electron transparency, and then coated with carbon before observation. The electrical conductivity of the as-sintered samples BaCe0.80-xSm0.20InxO3-δ (x= 0–0.20, 0.70, and 0.80) and BaCe1-yInyO3-δ (y= 0.10, 0.20, and 0.30) was measured by AC impedance spectroscopy (Solartron 1260 FRA coupled with 1287 interface) in the frequency range from 1 MHz to 0.1 Hz with a signal amplitude of 5 mV. The data were analyzed with Zview 2.0 software with appropriate equivalent circuits. The well polished pellets (~ diameter 11 × thickness 1 mm) were painted with silver paste on both sides and baked at 600 °C for 0.5 h. The conductivity was measured in wet 5% H2/Ar (p(H2O)= 0.074 atm, 20 ml min− 1) from 300 to 750 °C in 50 °C steps. After reaching the desired temperature, 5 min elapsed before the impedance spectra were recorded. To further study the electrical conduction behavior, the samples BaCe0.80-xSm0.20InxO3-δ (x = 0–0.2) and BaCe1-yInyO3-δ (y = 0.20 and 0.30) were pre-hydrated at 300 °C for 72 h in wet Ar (p(H2O) = 0.074 atm) before the electrical conductivity measurement in
H2O–Ar (p(H2O) = 0.074 atm). The same samples were then annealed at 800 °C for 2 h in dry Ar and exposed to D2O–Ar (p(D2O) = 0.074 atm) at 300 °C for 72 h. Subsequently, the electrical conductivity was measured in D2O–Ar (p(D2O) = 0.074 atm). The duration time at each temperature was 30 min. The H2O–Ar treated samples BaCe0.80-xSm0.20InxO3-δ (x= 0, 0.10, 0.20) and BaCe1-yInyO3-δ (y = 0.20 and 0.30) were ground and then subjected to TGA measurement to determine the proton content. The TGA was performed with a NETSCH STA 449C at a rate of 5 °C min− 1 in a N2 stream (≤5 ppm, 20 ml min− 1), from room temperature to 1000 °C and then down to room temperature with a duration at 1000 °C for 30 min.
3. Results and discussion 3.1. XRD patterns The XRD patterns of the sintered samples BaCe0.80-xSm0.20InxO3-δ (x = 0–0.80) at room temperature are displayed in Fig. 1(a). It has been reported that In exhibited a high solubility in BaCeO3 due to its lower hardness [12], which was maintained in the co-doped samples. The co-doped samples exhibited the pure perovskite phase up to x = 0.80. All diffraction peaks shifted to higher angle with increasing In content, in agreement with the smaller In3 + (R(In3 +) VI = 0.80 Å, R(Ce4 +)VI = 0.87 Å) [16]. Furthermore, the peak at 2θ ~33.7° (Fig. 1(b)) gradually disappeared with increasing In content, inferring decreasing structural distortion [8]. In fact, the diffraction patterns can be well indexed with orthorhombic structure for x = 0–0.30 while cubic for x = 0.60–0.80. The calculated lattice parameters are listed in Table 1. Those of BaCe1-yInyO3-δ are also included. The values of a/√2:b, a/√2:c, and b:c became closer to unity with In doping level for both the Sm- and In-co-doped and In singly doped samples, indicating an increased long range structural symmetry. The phase
Table 1 Lattice parameters for BaCe0.80-xSm0.20InxO3 − δ and BaCe1-yInyO3 − δ. Composition
a (Å)
x=0 x = 0.05 x = 0.10 x = 0.15 x = 0.20 x = 0.30 x = 0.60 x = 0.70 x = 0.80 y = 0.10 y = 0.20 y = 0.30
8.788 8.789 8.781 8.763 8.751 8.719 4.310 4.292 4.284 8.731 8.721 8.661
b (Å) (1) (1) (1) (1) (1) (1) (1) (0) (1) (2) (5) (4)
6.234 6.237 6.211 6.201 6.195 6.165 – – – 6.182 6.153 6.150
c (Å) (1) (0) (1) (0) (0) (0)
(1) (2) (4)
6.259 6.220 6.210 6.196 6.188 6.162 – – – 6.202 6.178 6.144
(0) (0) (1) (1) (0) (4)
(1) (3) (3)
V (Å3)
a/√2:b
a/√2:c
b:c
342.9(1) 340.9 (1) 338.7 (1) 336.7 (1) 335.5 (1) 331.2 (1) 80.0 (1) 79.1 (1) 78.6 (1) 334.7 (2) 331.6 (2) 327.3 (3)
0.997 0.997 0.999 0.999 0.999 1.000 – – – 0.999 1.002 0.996
0.993 0.999 1.000 1.000 1.000 1.000 – – – 0.996 0.998 0.997
0.996 1.003 1.000 1.001 1.001 1.000 – – – 0.997 0.996 1.001
C. Zhang, H. Zhao / Solid State Ionics 206 (2012) 17–21
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assignment of BaCe0.80-xSm0.20InxO3-δ (x= 0.60–0.80), which can be viewed as Ba2(CezSm0.20In0.80-z)2O5 + δ (z= 0–0.20), was in accordance with the literature, where the cubic Ba2In2O5 (>1075 °C) was stabilized to room temperature by doping at Ba- and/or In-site [13,14].
3.2. Raman spectra The XRD analysis gives only the long range structural characteristics, while Raman spectroscopy, sensitive to the structural symmetry, can detect any possible short range structural variation (20–100 Å) [17]. Accordingly, the Raman spectra of BaCe0.80-xSm0.20InxO3-δ and BaCe1-yInyO3-δ at room temperature were recorded and the results are shown in Fig. 2. Based on the factor group analysis [18], the bands at 80–150, 345–365 and 630 cm − 1 were assigned to the Ba-[BO6] cation stretching, O\B\O bending and B\O bond stretching modes (B = Ce, Sm, and/or In), respectively. A small band near 500 cm − 1 for samples with x = 0.20–0.80 and y = 0.10–0.30 was attributed to the torsional mode of the B\O bond [19]. The vibrations below 500 cm − 1 successively weakened with increasing In concentration, both for the codoped and singly doped samples. In addition, the number of bands at 345–365 cm − 1 decreased, from three for x = 0 to two for x = 0.05–0.20, then one for x = 0.50, finally disappeared for x = 0.80. This indicated that the short range structural symmetry around Ba and B-site ions increased. This result was similar with the In singly doped samples, where In was assumed to drive the surrounding atoms recreate a cubic environment around itself based on EXAFS analysis [12].
Fig. 3. (a) ½01 3− and (b) ½01 2− zone axis SAED patterns and (c) a typical HRTEM photograph showing the (211) and (020) lattice images of BaCe0.60Sm0.20In0.20O3-δ.
3.3. SAED patterns and HRTEM image Considering the high oxygen vacancy concentration in the codoped samples, there is a tendency for the oxygen vacancy to be ordered and even to form micro-domains [20]. To study this SAED patterns and HRTEM images were collected on the sample x = 0.20 and some representative results are shown in Fig. 3. All diffraction spots can be indexed based on the orthorhombic structure and the lattices were well ranked. No additional spots originating from oxygen vacancy ordering or micro-domains were observed. 3.4. TGA Figure 4 presents the TGA result in dry N2 for the hydrated samples. Mass loss was observed during the heating process but not during the cooling process. The initial mass loss at temperatures b150 °C corresponded to the evaporation of absorbed water vapor. The loss of H2O which was incorporated into the lattice according to Eq. (1)led to the weight loss in the temperature range 300–750 °C. The mass loss at high temperatures (>800 °C) was due to the loss of lattice oxygen, in agreement with the result of Kruth and Irvine [7]. The proton content obtained from the TGA results is 1.23, 0.97, 0.97, 0.48, and 0.68% for samples x = 0, 0.10, 0.20, y = 0.20, and 0.30, respectively. The experimentally determined proton concentration decreased with In content for the BaCe0.80-xSm0.20InxO3-δ
Fig. 2. Raman spectra of BaCe0.80-xSm0.20InxO3-δ and BaCe1-yInyO3-δ at room temperature.
Fig. 4. TGA results of the pre-hydrated samples BaCe0.80-xSm0.20InxO3-δ and BaCe1-yInyO3-δ in the dry N2 atmosphere during the heating and cooling processes. The sample x = 0.10 was marked to show how the proton content was determined.
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C. Zhang, H. Zhao / Solid State Ionics 206 (2012) 17–21
Fig. 5. Arrhenius plots of σb of the as-sintered BaCe0.80-xSm0.20InxO3-δ, BaCe1-yInyO3-δ samples in wet 5% H2/Ar (p(H2O) = 0.074 atm).
samples but increased for the BaCe1-yInyO3-δ samples. As the oxygen vacancy content increased with In level for the co-doped samples, the proton concentration must be also increased according to Eq. (1). However, the present results indicated that the experimental proton content decreased with In amount. The reasons are unclear at the present stage. 3.5. Electrical conductivity AC impedance spectroscopy was employed to study the electrical conductivity of the as sintered co-doped samples in wet 5% H2/Ar in the temperature range 300–750 °C. The grain boundary resistance can only be distinguished at low temperatures (e.g., ≤450 °C). What we considered here was the bulk conductivity (σb). The Arrhenius plot of σb is shown in Fig. 5. The cases of BaCe1-yInyO3-δ (y = 0.10, 0.20, and 0.30) were included for comparison. Unexpectedly, the σb of the co-doped samples BaCe0.80-xSm0.20 InxO3-δ decreased with increasing In content, completely different
Fig. 6. Arrhenius plots of σb of the hydrated BaCe0.80-xSm0.20InxO3-δ, BaCe1-yInyO3-δ samples in H2O–Ar (p(H2O) = 0.074 atm). The inset was the σb of sample with x = 0.10 in H2O–Ar and D2O–Ar, respectively.
from the In singly doped samples BaCe1-yInyO3-δ. Incorporation of a small amount of In (x = 0.05) reduced σb slightly, further doping led to a rapid decrease of σb. The samples x = 0.70 and 0.80 exhibited electrical conduction behaviors different from other co-doped samples, but similar to that of the Ba2In2O5 based materials [13,14]. The variation of electrical conductivity with In content for other RE and In co-doped BaCeO3 was similar with the present Sm- and In-co-doped ones (See the Appendix Fig. A.1). To explore the underlying reasons for the decreased conductivity of the co-doped samples, the samples BaCe0.80-xSm0.20InxO3-δ (x = 0–0.20) and BaCe1-yInyO3-δ (y = 0.20 and 0.30) were exposed to wet Ar (p(H2O) = 0.074 atm) at 300 °C for 72 h before the electrical conductivity measurement. The σb of the hydrated samples in H2O–Ar is shown in Fig. 6. A slope change was observed for all samples at different temperatures, inferring a change of the conduction mechanism [21]. The apparent activation energy Ea in different
Fig. A.1. Arrhenius plots of the as-sintered samples in wet 5% H2/Ar (p(H2O) = 0.074 atm). (a) BaCe0.80-xY0.20InxO3-δ; (b) BaCe0.80-xNd0.20InxO3-δ; (c) BaCe0.80-xGd0.20InxO3-δ; and (d) BaCe0.80-xYb0.20InxO3-δ.
C. Zhang, H. Zhao / Solid State Ionics 206 (2012) 17–21 Table 2 Apparent activation energy Ea in different temperature ranges obtained by linear fitting the Arrhenius plots in H2O–Ar atmosphere. Composition
Ea (eV)
Ea (eV)
Ea (eV)
x=0
0.586 ± 0.010 (100–275 °C) 0.585 ± 0.007 (100–275 °C) 0.596 ± 0.024 (100–275 °C) 0.637 ± 0.021 (100–275 °C) 0.604 ± 0.011 (100–275 °C) 0.534 ± 0.017 (100–275 °C) 0.585 ± 0.012 (100–275 °C)
0.350 ± 0.007 (300–450 °C) 0.351 ± 0.013 (300–450 °C) 0.476 ± 0.005 (300–600 °C) 0.530 ± 0.006 (300–600 °C) 0.482 ± 0.007 (300–600 °C) 0.530 ± 0.002 (300–750 °C) 0.505 ± 0.002 (300–750 °C)
0.534 ± 0.013 (475–750 °C) 0.478 ± 0.016 (475–750 °C) 0.291 ± 0.011 (625–750 °C) 0.361 ± 0.006 (625–750 °C) 0.226 ± 0.010 (625–750 °C) –
x = 0.05 x = 0.10 x = 0.10a x = 0.20 y = 0.20 y = 0.30 a
–
Data in D2O–Ar atmosphere.
temperature ranges are listed in Table 2. The protonic defect is the dominating charge carrier at low temperatures (100–275 °C), as evidenced from the TGA results (Fig. 4). The Ea of 0.53–0.60 eV is a typical value for proton conduction [2]. The activation energy for proton conduction increased with In content for both the co-doped and In singly doped samples. The inset in Fig. 6 showed that σb in D2O–Ar was smaller than that in H2O–Ar, indicating that the proton conduction mainly contributed to σb in the whole temperature range. The ratio between the σb values in H2O and D2O was 2.53–1.14, in agreement with the literature [22–24]. Moreover, the Ea value in D2O–Ar increased by 0.041, 0.054 and 0.070 eV in different temperature ranges, close to the semi-classical theoretical predication of 0.055 eV [24]. XRD and Raman spectra revealed that both long and short range structural symmetry increased with In content for the Smand In-co-doped samples without oxygen vacancy ordering or micro-domains, which was expected to facilitate the proton diffusion. In contrast, the present results showed that the activation energy for proton conduction (Table 2) increased with In doping level. Kreuer suggested that the proton mobility increased with increasing lattice volume [2]. As shown in Table 1, the lattice volume decreased with In content for the co-doped samples, by 2% from x = 0 to x = 0.20, which should be responsible for the increased Ea (100–275 °C). On the other hand, the conventional criterion of ionic radius matching is successful in explaining the conduction behavior of doped oxygen ionic conductors with aliovalent cations. However, increasing evidence [2,12,25] has been collected that the chemical matching is more important for the choice of dopants for HTPCs. The perturbation of the basicity of the oxygen ion induced by doping can have a significant impact on the proton diffusion. The In doping has been reported to modify the basicity of the O\H bond and the whole matrix lattice dynamics of BaCeO3, which biased the proton diffusion [12]. The basicity of the O\H bond in the Sm- and In-co-doped BaCeO3 may change in some way so that it was difficult to form a percolating path for proton diffusion, thus resulting in higher Ea values. This is just a hypothesis, which stays to be proven. The TGA result showed that the proton concentration decreased with increasing In level for the co-doped samples. Therefore, the decreasing proton concentration combined with the decreasing proton
21
mobility led to the decrease in proton conductivity for the Sm- and In-co-doped samples. 4. Conclusions The relationship between structural characteristics and electrical conductivity of the Sm- and In-co-doped proton conductor BaCe0.80-x Sm0.20InxO3-δ was investigated. The indium exhibited high solubility up to 80%. XRD and Raman spectra revealed that both long and short range structural symmetry increased with In content. No oxygen vacancy ordering or micro-domain was observed from SAED and HRTEM. The electrical conductivity of the co-doped samples decreased with In content in both wet 5% H2/Ar and wet Ar atmospheres. The isotope measurement indicated that the proton conduction contributed to σb in the whole temperature range. The In doping to BaCe0.80-xSm0.20InxO3-δ led to a decrease in lattice volume, which hampered the proton diffusion and thus caused an increase in Ea for proton conduction. The TGA results showed that the proton concentration decreased with In level for the co-doped samples. Both of them resulted in the decreased conductivity with increased In level for the co-doped samples. The perturbation of the oxygen basicity in the whole matrix may be another reason for the variation in conductivity of the Sm- and In-co-doped samples. Acknowledgments This work was supported by the National Nature Science Foundation of China (no. 20973021), and the Nature Science Foundation of Beijing (no. 2102031). Mr. Ronglin Wang and Shaowei Yao from Hebei United University were acknowledged for the TGA measurement. References [1] H. Iwahara, Solid State Ionics 77 (1995) 289. [2] K.D. Kreuer, Annu. Rev. Mater. Res. 33 (2003) 333. [3] L. Bi, S. Zhang, L. Zhang, Z. Tao, H. Wang, W. Liu, Int. J. Hydrogen Energy 34 (2009) 2421. [4] K.S. Knight, Solid State Ionics 145 (2001) 275. [5] H. Iwahara, H. Uchida, K. Onno, K. Ogaki, J. Electrochem. Soc. 135 (1988) 529. [6] D.A. Stevenson, N. Jiang, R.M. Buchanan, F.E.G. Henn, Solid State Ionics 62 (1993) 279. [7] A. Kruth, J.T.S. Irvine, Solid State Ionics 62 (1993) 279. [8] K.H. Ryu, S.M. Haile, Solid State Ionics 162 (163) (2003) 83. [9] L. Yang, S. Wang, K. Blinn, M. Liu, Z. Liu, M. Liu, Science 326 (2009) 126. [10] F. Zhao, Q. Liu, S. Wang, K. Brinknman, F. Chen, Int. J. Hydrogen Energy 35 (2010) 4258. [11] C. Zhang, H. Zhao, S. Zhai, Int. J. Hydrogen Energy 36 (2011) 3649. [12] F. Giannici, A. Longo, A. Balerna, K.D. Kreuer, A. Martorana, Chem. Mater. 19 (2007) 5714. [13] K. Katsuyoshi, N. Takahashi, H. Yamamura, K. Nomura, T. Atake, Solid State Ionics 168 (2004) 69. [14] J.F. Shin, D.C. Apperley, P.R. Slater, Chem. Mater. 22 (2010) 5945. [15] K.S. Knight, Solid State Comm. 112 (1999) 73. [16] R.D. Shannon, Acta Cryst. A 32 (1976) 751. [17] T. Scherban, R. Villeneuve, L. Abello, G. Lucazeau, Solid State Ionics 61 (1993) 93. [18] F. Genet, S. Loridant, G. Lucazeau, J. Raman. Spectro. 28 (1997) 255. [19] U. Balachandran, N.G. Eror, Solid State Comm. 44 (1982) 815. [20] S. Adler, S. Russek, J. Reimer, M. Fendorf, A. Stacy, Q. Huang, A. Santoro, J. Lynn, J. Baltisberger, U. Werner, Solid State Ionics 68 (1994) 193. [21] N. Bonanous, Solid State Ionics 53–56 (1992) 967. [22] T. Scherban, A.S. Nowick, Solid State Ionics 35 (1989) 189. [23] E. Fabbri, L. Bi, H. Tanaka, D. Pergolesi, E. Traversa, Adv. Fun. Mater. 21 (2011) 158. [24] A.S. Norwick, A.V. Vaysleyb, Solid State Ionics 97 (1997) 17. [25] F. Giannici, A. Longo, A. Balerna, K.D. Kreuer, A. Martorana, Chem. Mater. 21 (2009) 2641.
Contents lists available at SciVerse ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Influence of In content on the electrical conduction behavior of Sm- and In-co-doped proton conductor BaCe0.80-xSm0.20InxO3-δ Cuijuan Zhang a, Hailei Zhao a, b,⁎ a b
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Beijing Key Lab of New Energy Materials and Technology, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 15 January 2011 Received in revised form 29 August 2011 Accepted 25 October 2011 Available online 21 November 2011 Keywords: Proton conductor Barium cerate Co-doping Electrical conductivity
a b s t r a c t The influence of indium content on the electrical conduction behavior of Sm- and In-co-doped BaCe0.80-xSm0.20InxO3-δ proton conductor was investigated by impedance spectroscopy. The structural characteristics were studied by means of X-ray diffraction, Raman spectra, high resolution transmission electron microscopy, and thermogravimetric analysis. Both long and short range structural symmetry increased with In content, from orthorhombic (x= 0–0.50) to cubic structure (x= 0.60–0.80). No oxygen vacancy ordering or microdomains were detected. The proton concentration of the hydrated samples BaCe0.80-xSm0.20InxO3-δ (x= 0–0.20) decreased with In level. The electrical conductivity of the co-doped samples decreased with In content in wet 5% H2/Ar and wet Ar atmospheres. The reasons for the decreased electrical conductivity were discussed. The variation of electrical conductivity of BaCe0.80-xSm0.20InxO3-δ with In concentration can be applicable to other RE- and In-co-doped BaCeO3 materials. © 2011 Elsevier B.V. All rights reserved.
1. Introduction High temperature proton conductors (HTPCs) have been attracting growing attention in recent years for their promising applications in solid oxide fuel cells, hydrogen sensor, hydrogen separation, ammonia synthesis at atmospheric pressure, etc. [1]. The protonic defect can be introduced into the lattice through the hydration reaction according to Eq. (1).
· H 2 O þ V ·· O þ OO ¼ 2OH O
ð1Þ
It is obvious that the proton concentration increased with the oxygen vacancy concentration. Increasing the oxygen vacancy concentration is desirable for the improvement of the proton conductivity. BaCeO3-based materials are the most investigated among the HTPC family owing to their higher protonic conductivity [2]. However, these materials are susceptible to chemical degradation in carbon dioxide and water vapor, forming carbonate and hydroxide [2]. The chemical stability can be enhanced by partial replacement of Ce with other elements including In [3]. Although the structural characteristics, electrical conduction behavior and chemical stability of BaCeO3 doped with various cations have been widely investigated [4–8], the reports about the influence of co-dopants at the Ce-site on these properties were relatively limited [9,10]. ⁎ Corresponding author at: School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China. Tel./fax: + 86 10 82376837. E-mail address: [email protected] (H. Zhao). 0167-2738/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2011.10.026
The larger Sm 3 + has been suggested to be a good dopant for BaCeO3 but with somewhat low solubility (≤30%) [11]; in contrast, the smaller In 3 + exhibited a high solubility in BaCeO3, up to 75% [12]. Furthermore, based on recent results of the unusually high protonic conductivity reported in Ba2In2O5[13,14], it was thought that doping BaCeO3 with In might be of interest. Therefore, Sm and In were selected as the co-dopants to modify the properties of BaCeO3. The influence of In content on the electrical conduction behavior of BaCe0.80-xSm0.20InxO3-δ was investigated. The present results demonstrate that, unexpectedly, the electrical conductivity of the Sm- and In-co-doped BaCe0.80-xSm0.20InxO3-δ decreased with In content. Similar results were observed in other RE- and In-co-doped BaCeO3 compounds. X-ray diffraction (XRD), Raman spectroscopy, selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM), and thermogravimetric analysis (TGA) were employed to explore the reasons for the decreased electrical conductivity. 2. Experimental 2.1. Sample preparation The BaCe0.80-xSm0.20InxO3-δ (x= 0–0.80) powders were synthesized by the citric-nitrate method [11]. The raw materials were Ba(NO3)2 (>99.5%), Ce(NO3)3·6H2O (>99.0%, Guangdong Xilong Chemical Co., Ltd), and Sm2O3 (99.95%) and In2O3 (99.99%, Grirem Advanced Materials Co., Ltd). The latter two oxides were annealed at 1000 °C for 10 h to remove the possibly absorbed water and then dissolved in diluted nitric acid before use. The final sintering step was carried out
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C. Zhang, H. Zhao / Solid State Ionics 206 (2012) 17–21
at 1500 and 1400 °C for 5 h in air for x = 0 and 0.05–0.80, respectively. BaCe1-yInyO3-δ (y= 0.10, 0.20, and 0.30) was also prepared with the same procedures for the co-doped samples for comparison. Distinct sintering temperatures were adopted considering the enhanced sinterability due to In doping [3]. The samples with x = 0.30–0.60 cracked into pieces after sintering at 1400 °C. The sintering parameters, including the sintering temperature (1200–1500 °C), heating rate and holding time, have been optimized to try to obtain integrate samples, but unsuccessfully. Accordingly, the subsequent electrical conductivity measurement was not performed on these samples. The relative density of the sintered pellets with x = 0–0.20 and y = 0.10–0.30 was around 95%, which was estimated from the weight and dimensions measurement.
2.2. Characterization
Fig. 1. (a) XRD patterns of sintered BaCe0.80-xSm0.20InxO3-δ; (b) magnification of the 2θ range of 30–40°.
Powder XRD was recorded on Rigaku D/MAX-RB X-ray diffractometer with Ni filtered Cu Kα radiation with λ = 1.54056 Å, by a 0.02° step at room temperature. To investigate the short range structural characteristics, Raman spectra were collected for BaCe0.80-xSm0.20InxO3-δ (x= 0, 0.05, 0.10, 0.20, 0.50, and 0.80) and BaCe1-yInyO3-δ (y= 0.10, 0.20, and 0.30) on a Raman microscope (HORIBA JOBIN YVON LabRam HR800) at room temperature. An Ar laser with a wavelength of 514.5 nm was used for excitation. The diameter of the incident laser beam on the fine powder was 0.697 μm and the power under the microscope objective was 2 mW to avoid overheating. The samples were scanned three times to obtain a better signal to noise ratio and the summed spectra were reported here. Great homogeneity was observed for all samples. All the finely ground powders were light yellow green and thus any absorption related to color change [15] was ruled out. SAED patterns and HRTEM images of BaCe0.60Sm0.20In0.20O3-δ were acquired with a TECNAI-F20 electron microscope operating at 200 keV. The sample was polished with sand paper and ion-milled to achieve electron transparency, and then coated with carbon before observation. The electrical conductivity of the as-sintered samples BaCe0.80-xSm0.20InxO3-δ (x= 0–0.20, 0.70, and 0.80) and BaCe1-yInyO3-δ (y= 0.10, 0.20, and 0.30) was measured by AC impedance spectroscopy (Solartron 1260 FRA coupled with 1287 interface) in the frequency range from 1 MHz to 0.1 Hz with a signal amplitude of 5 mV. The data were analyzed with Zview 2.0 software with appropriate equivalent circuits. The well polished pellets (~ diameter 11 × thickness 1 mm) were painted with silver paste on both sides and baked at 600 °C for 0.5 h. The conductivity was measured in wet 5% H2/Ar (p(H2O)= 0.074 atm, 20 ml min− 1) from 300 to 750 °C in 50 °C steps. After reaching the desired temperature, 5 min elapsed before the impedance spectra were recorded. To further study the electrical conduction behavior, the samples BaCe0.80-xSm0.20InxO3-δ (x = 0–0.2) and BaCe1-yInyO3-δ (y = 0.20 and 0.30) were pre-hydrated at 300 °C for 72 h in wet Ar (p(H2O) = 0.074 atm) before the electrical conductivity measurement in
H2O–Ar (p(H2O) = 0.074 atm). The same samples were then annealed at 800 °C for 2 h in dry Ar and exposed to D2O–Ar (p(D2O) = 0.074 atm) at 300 °C for 72 h. Subsequently, the electrical conductivity was measured in D2O–Ar (p(D2O) = 0.074 atm). The duration time at each temperature was 30 min. The H2O–Ar treated samples BaCe0.80-xSm0.20InxO3-δ (x= 0, 0.10, 0.20) and BaCe1-yInyO3-δ (y = 0.20 and 0.30) were ground and then subjected to TGA measurement to determine the proton content. The TGA was performed with a NETSCH STA 449C at a rate of 5 °C min− 1 in a N2 stream (≤5 ppm, 20 ml min− 1), from room temperature to 1000 °C and then down to room temperature with a duration at 1000 °C for 30 min.
3. Results and discussion 3.1. XRD patterns The XRD patterns of the sintered samples BaCe0.80-xSm0.20InxO3-δ (x = 0–0.80) at room temperature are displayed in Fig. 1(a). It has been reported that In exhibited a high solubility in BaCeO3 due to its lower hardness [12], which was maintained in the co-doped samples. The co-doped samples exhibited the pure perovskite phase up to x = 0.80. All diffraction peaks shifted to higher angle with increasing In content, in agreement with the smaller In3 + (R(In3 +) VI = 0.80 Å, R(Ce4 +)VI = 0.87 Å) [16]. Furthermore, the peak at 2θ ~33.7° (Fig. 1(b)) gradually disappeared with increasing In content, inferring decreasing structural distortion [8]. In fact, the diffraction patterns can be well indexed with orthorhombic structure for x = 0–0.30 while cubic for x = 0.60–0.80. The calculated lattice parameters are listed in Table 1. Those of BaCe1-yInyO3-δ are also included. The values of a/√2:b, a/√2:c, and b:c became closer to unity with In doping level for both the Sm- and In-co-doped and In singly doped samples, indicating an increased long range structural symmetry. The phase
Table 1 Lattice parameters for BaCe0.80-xSm0.20InxO3 − δ and BaCe1-yInyO3 − δ. Composition
a (Å)
x=0 x = 0.05 x = 0.10 x = 0.15 x = 0.20 x = 0.30 x = 0.60 x = 0.70 x = 0.80 y = 0.10 y = 0.20 y = 0.30
8.788 8.789 8.781 8.763 8.751 8.719 4.310 4.292 4.284 8.731 8.721 8.661
b (Å) (1) (1) (1) (1) (1) (1) (1) (0) (1) (2) (5) (4)
6.234 6.237 6.211 6.201 6.195 6.165 – – – 6.182 6.153 6.150
c (Å) (1) (0) (1) (0) (0) (0)
(1) (2) (4)
6.259 6.220 6.210 6.196 6.188 6.162 – – – 6.202 6.178 6.144
(0) (0) (1) (1) (0) (4)
(1) (3) (3)
V (Å3)
a/√2:b
a/√2:c
b:c
342.9(1) 340.9 (1) 338.7 (1) 336.7 (1) 335.5 (1) 331.2 (1) 80.0 (1) 79.1 (1) 78.6 (1) 334.7 (2) 331.6 (2) 327.3 (3)
0.997 0.997 0.999 0.999 0.999 1.000 – – – 0.999 1.002 0.996
0.993 0.999 1.000 1.000 1.000 1.000 – – – 0.996 0.998 0.997
0.996 1.003 1.000 1.001 1.001 1.000 – – – 0.997 0.996 1.001
C. Zhang, H. Zhao / Solid State Ionics 206 (2012) 17–21
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assignment of BaCe0.80-xSm0.20InxO3-δ (x= 0.60–0.80), which can be viewed as Ba2(CezSm0.20In0.80-z)2O5 + δ (z= 0–0.20), was in accordance with the literature, where the cubic Ba2In2O5 (>1075 °C) was stabilized to room temperature by doping at Ba- and/or In-site [13,14].
3.2. Raman spectra The XRD analysis gives only the long range structural characteristics, while Raman spectroscopy, sensitive to the structural symmetry, can detect any possible short range structural variation (20–100 Å) [17]. Accordingly, the Raman spectra of BaCe0.80-xSm0.20InxO3-δ and BaCe1-yInyO3-δ at room temperature were recorded and the results are shown in Fig. 2. Based on the factor group analysis [18], the bands at 80–150, 345–365 and 630 cm − 1 were assigned to the Ba-[BO6] cation stretching, O\B\O bending and B\O bond stretching modes (B = Ce, Sm, and/or In), respectively. A small band near 500 cm − 1 for samples with x = 0.20–0.80 and y = 0.10–0.30 was attributed to the torsional mode of the B\O bond [19]. The vibrations below 500 cm − 1 successively weakened with increasing In concentration, both for the codoped and singly doped samples. In addition, the number of bands at 345–365 cm − 1 decreased, from three for x = 0 to two for x = 0.05–0.20, then one for x = 0.50, finally disappeared for x = 0.80. This indicated that the short range structural symmetry around Ba and B-site ions increased. This result was similar with the In singly doped samples, where In was assumed to drive the surrounding atoms recreate a cubic environment around itself based on EXAFS analysis [12].
Fig. 3. (a) ½01 3− and (b) ½01 2− zone axis SAED patterns and (c) a typical HRTEM photograph showing the (211) and (020) lattice images of BaCe0.60Sm0.20In0.20O3-δ.
3.3. SAED patterns and HRTEM image Considering the high oxygen vacancy concentration in the codoped samples, there is a tendency for the oxygen vacancy to be ordered and even to form micro-domains [20]. To study this SAED patterns and HRTEM images were collected on the sample x = 0.20 and some representative results are shown in Fig. 3. All diffraction spots can be indexed based on the orthorhombic structure and the lattices were well ranked. No additional spots originating from oxygen vacancy ordering or micro-domains were observed. 3.4. TGA Figure 4 presents the TGA result in dry N2 for the hydrated samples. Mass loss was observed during the heating process but not during the cooling process. The initial mass loss at temperatures b150 °C corresponded to the evaporation of absorbed water vapor. The loss of H2O which was incorporated into the lattice according to Eq. (1)led to the weight loss in the temperature range 300–750 °C. The mass loss at high temperatures (>800 °C) was due to the loss of lattice oxygen, in agreement with the result of Kruth and Irvine [7]. The proton content obtained from the TGA results is 1.23, 0.97, 0.97, 0.48, and 0.68% for samples x = 0, 0.10, 0.20, y = 0.20, and 0.30, respectively. The experimentally determined proton concentration decreased with In content for the BaCe0.80-xSm0.20InxO3-δ
Fig. 2. Raman spectra of BaCe0.80-xSm0.20InxO3-δ and BaCe1-yInyO3-δ at room temperature.
Fig. 4. TGA results of the pre-hydrated samples BaCe0.80-xSm0.20InxO3-δ and BaCe1-yInyO3-δ in the dry N2 atmosphere during the heating and cooling processes. The sample x = 0.10 was marked to show how the proton content was determined.
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Fig. 5. Arrhenius plots of σb of the as-sintered BaCe0.80-xSm0.20InxO3-δ, BaCe1-yInyO3-δ samples in wet 5% H2/Ar (p(H2O) = 0.074 atm).
samples but increased for the BaCe1-yInyO3-δ samples. As the oxygen vacancy content increased with In level for the co-doped samples, the proton concentration must be also increased according to Eq. (1). However, the present results indicated that the experimental proton content decreased with In amount. The reasons are unclear at the present stage. 3.5. Electrical conductivity AC impedance spectroscopy was employed to study the electrical conductivity of the as sintered co-doped samples in wet 5% H2/Ar in the temperature range 300–750 °C. The grain boundary resistance can only be distinguished at low temperatures (e.g., ≤450 °C). What we considered here was the bulk conductivity (σb). The Arrhenius plot of σb is shown in Fig. 5. The cases of BaCe1-yInyO3-δ (y = 0.10, 0.20, and 0.30) were included for comparison. Unexpectedly, the σb of the co-doped samples BaCe0.80-xSm0.20 InxO3-δ decreased with increasing In content, completely different
Fig. 6. Arrhenius plots of σb of the hydrated BaCe0.80-xSm0.20InxO3-δ, BaCe1-yInyO3-δ samples in H2O–Ar (p(H2O) = 0.074 atm). The inset was the σb of sample with x = 0.10 in H2O–Ar and D2O–Ar, respectively.
from the In singly doped samples BaCe1-yInyO3-δ. Incorporation of a small amount of In (x = 0.05) reduced σb slightly, further doping led to a rapid decrease of σb. The samples x = 0.70 and 0.80 exhibited electrical conduction behaviors different from other co-doped samples, but similar to that of the Ba2In2O5 based materials [13,14]. The variation of electrical conductivity with In content for other RE and In co-doped BaCeO3 was similar with the present Sm- and In-co-doped ones (See the Appendix Fig. A.1). To explore the underlying reasons for the decreased conductivity of the co-doped samples, the samples BaCe0.80-xSm0.20InxO3-δ (x = 0–0.20) and BaCe1-yInyO3-δ (y = 0.20 and 0.30) were exposed to wet Ar (p(H2O) = 0.074 atm) at 300 °C for 72 h before the electrical conductivity measurement. The σb of the hydrated samples in H2O–Ar is shown in Fig. 6. A slope change was observed for all samples at different temperatures, inferring a change of the conduction mechanism [21]. The apparent activation energy Ea in different
Fig. A.1. Arrhenius plots of the as-sintered samples in wet 5% H2/Ar (p(H2O) = 0.074 atm). (a) BaCe0.80-xY0.20InxO3-δ; (b) BaCe0.80-xNd0.20InxO3-δ; (c) BaCe0.80-xGd0.20InxO3-δ; and (d) BaCe0.80-xYb0.20InxO3-δ.
C. Zhang, H. Zhao / Solid State Ionics 206 (2012) 17–21 Table 2 Apparent activation energy Ea in different temperature ranges obtained by linear fitting the Arrhenius plots in H2O–Ar atmosphere. Composition
Ea (eV)
Ea (eV)
Ea (eV)
x=0
0.586 ± 0.010 (100–275 °C) 0.585 ± 0.007 (100–275 °C) 0.596 ± 0.024 (100–275 °C) 0.637 ± 0.021 (100–275 °C) 0.604 ± 0.011 (100–275 °C) 0.534 ± 0.017 (100–275 °C) 0.585 ± 0.012 (100–275 °C)
0.350 ± 0.007 (300–450 °C) 0.351 ± 0.013 (300–450 °C) 0.476 ± 0.005 (300–600 °C) 0.530 ± 0.006 (300–600 °C) 0.482 ± 0.007 (300–600 °C) 0.530 ± 0.002 (300–750 °C) 0.505 ± 0.002 (300–750 °C)
0.534 ± 0.013 (475–750 °C) 0.478 ± 0.016 (475–750 °C) 0.291 ± 0.011 (625–750 °C) 0.361 ± 0.006 (625–750 °C) 0.226 ± 0.010 (625–750 °C) –
x = 0.05 x = 0.10 x = 0.10a x = 0.20 y = 0.20 y = 0.30 a
–
Data in D2O–Ar atmosphere.
temperature ranges are listed in Table 2. The protonic defect is the dominating charge carrier at low temperatures (100–275 °C), as evidenced from the TGA results (Fig. 4). The Ea of 0.53–0.60 eV is a typical value for proton conduction [2]. The activation energy for proton conduction increased with In content for both the co-doped and In singly doped samples. The inset in Fig. 6 showed that σb in D2O–Ar was smaller than that in H2O–Ar, indicating that the proton conduction mainly contributed to σb in the whole temperature range. The ratio between the σb values in H2O and D2O was 2.53–1.14, in agreement with the literature [22–24]. Moreover, the Ea value in D2O–Ar increased by 0.041, 0.054 and 0.070 eV in different temperature ranges, close to the semi-classical theoretical predication of 0.055 eV [24]. XRD and Raman spectra revealed that both long and short range structural symmetry increased with In content for the Smand In-co-doped samples without oxygen vacancy ordering or micro-domains, which was expected to facilitate the proton diffusion. In contrast, the present results showed that the activation energy for proton conduction (Table 2) increased with In doping level. Kreuer suggested that the proton mobility increased with increasing lattice volume [2]. As shown in Table 1, the lattice volume decreased with In content for the co-doped samples, by 2% from x = 0 to x = 0.20, which should be responsible for the increased Ea (100–275 °C). On the other hand, the conventional criterion of ionic radius matching is successful in explaining the conduction behavior of doped oxygen ionic conductors with aliovalent cations. However, increasing evidence [2,12,25] has been collected that the chemical matching is more important for the choice of dopants for HTPCs. The perturbation of the basicity of the oxygen ion induced by doping can have a significant impact on the proton diffusion. The In doping has been reported to modify the basicity of the O\H bond and the whole matrix lattice dynamics of BaCeO3, which biased the proton diffusion [12]. The basicity of the O\H bond in the Sm- and In-co-doped BaCeO3 may change in some way so that it was difficult to form a percolating path for proton diffusion, thus resulting in higher Ea values. This is just a hypothesis, which stays to be proven. The TGA result showed that the proton concentration decreased with increasing In level for the co-doped samples. Therefore, the decreasing proton concentration combined with the decreasing proton
21
mobility led to the decrease in proton conductivity for the Sm- and In-co-doped samples. 4. Conclusions The relationship between structural characteristics and electrical conductivity of the Sm- and In-co-doped proton conductor BaCe0.80-x Sm0.20InxO3-δ was investigated. The indium exhibited high solubility up to 80%. XRD and Raman spectra revealed that both long and short range structural symmetry increased with In content. No oxygen vacancy ordering or micro-domain was observed from SAED and HRTEM. The electrical conductivity of the co-doped samples decreased with In content in both wet 5% H2/Ar and wet Ar atmospheres. The isotope measurement indicated that the proton conduction contributed to σb in the whole temperature range. The In doping to BaCe0.80-xSm0.20InxO3-δ led to a decrease in lattice volume, which hampered the proton diffusion and thus caused an increase in Ea for proton conduction. The TGA results showed that the proton concentration decreased with In level for the co-doped samples. Both of them resulted in the decreased conductivity with increased In level for the co-doped samples. The perturbation of the oxygen basicity in the whole matrix may be another reason for the variation in conductivity of the Sm- and In-co-doped samples. Acknowledgments This work was supported by the National Nature Science Foundation of China (no. 20973021), and the Nature Science Foundation of Beijing (no. 2102031). Mr. Ronglin Wang and Shaowei Yao from Hebei United University were acknowledged for the TGA measurement. References [1] H. Iwahara, Solid State Ionics 77 (1995) 289. [2] K.D. Kreuer, Annu. Rev. Mater. Res. 33 (2003) 333. [3] L. Bi, S. Zhang, L. Zhang, Z. Tao, H. Wang, W. Liu, Int. J. Hydrogen Energy 34 (2009) 2421. [4] K.S. Knight, Solid State Ionics 145 (2001) 275. [5] H. Iwahara, H. Uchida, K. Onno, K. Ogaki, J. Electrochem. Soc. 135 (1988) 529. [6] D.A. Stevenson, N. Jiang, R.M. Buchanan, F.E.G. Henn, Solid State Ionics 62 (1993) 279. [7] A. Kruth, J.T.S. Irvine, Solid State Ionics 62 (1993) 279. [8] K.H. Ryu, S.M. Haile, Solid State Ionics 162 (163) (2003) 83. [9] L. Yang, S. Wang, K. Blinn, M. Liu, Z. Liu, M. Liu, Science 326 (2009) 126. [10] F. Zhao, Q. Liu, S. Wang, K. Brinknman, F. Chen, Int. J. Hydrogen Energy 35 (2010) 4258. [11] C. Zhang, H. Zhao, S. Zhai, Int. J. Hydrogen Energy 36 (2011) 3649. [12] F. Giannici, A. Longo, A. Balerna, K.D. Kreuer, A. Martorana, Chem. Mater. 19 (2007) 5714. [13] K. Katsuyoshi, N. Takahashi, H. Yamamura, K. Nomura, T. Atake, Solid State Ionics 168 (2004) 69. [14] J.F. Shin, D.C. Apperley, P.R. Slater, Chem. Mater. 22 (2010) 5945. [15] K.S. Knight, Solid State Comm. 112 (1999) 73. [16] R.D. Shannon, Acta Cryst. A 32 (1976) 751. [17] T. Scherban, R. Villeneuve, L. Abello, G. Lucazeau, Solid State Ionics 61 (1993) 93. [18] F. Genet, S. Loridant, G. Lucazeau, J. Raman. Spectro. 28 (1997) 255. [19] U. Balachandran, N.G. Eror, Solid State Comm. 44 (1982) 815. [20] S. Adler, S. Russek, J. Reimer, M. Fendorf, A. Stacy, Q. Huang, A. Santoro, J. Lynn, J. Baltisberger, U. Werner, Solid State Ionics 68 (1994) 193. [21] N. Bonanous, Solid State Ionics 53–56 (1992) 967. [22] T. Scherban, A.S. Nowick, Solid State Ionics 35 (1989) 189. [23] E. Fabbri, L. Bi, H. Tanaka, D. Pergolesi, E. Traversa, Adv. Fun. Mater. 21 (2011) 158. [24] A.S. Norwick, A.V. Vaysleyb, Solid State Ionics 97 (1997) 17. [25] F. Giannici, A. Longo, A. Balerna, K.D. Kreuer, A. Martorana, Chem. Mater. 21 (2009) 2641.