Integrated experimental and modeling study of the ionic conductivity of samaria-doped ceria thin films

Solid State Ionics 204-205 (2011) 13–19

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Integrated experimental and modeling study of the ionic conductivity of samaria-doped ceria thin films R. Sanghavi a,b,c, R. Devanathan d,⁎, M.I. Nandasiri b,e, S. Kuchibhatla b, L. Kovarik b, S. Thevuthasan b, S. Prasad a,f a

Department of Electrical, Computer and Energy Engineering, P.O. Box 875706, Arizona State University, Tempe, AZ 85287, USA EMSL, Pacific Northwest National Laboratory, Richland, WA 99352, USA Battelle Science and Technology India, Pune, MH 411057, India d Chemical & Materials Sciences Division, Pacific Northwest National Laboratory, Richland, WA 99352, USA e Department of Physics, Western Michigan University, Kalamazoo, MI 49008, USA f Department of Bioengineering , University of Texas at Dallas, Richardson, TX 75080, USA b c

a r t i c l e

i n f o

Article history: Received 1 December 2010 Received in revised form 29 August 2011 Accepted 5 October 2011 Available online 9 November 2011 Keywords: Samaria-doped ceria Conductivity Molecular dynamics simulation Single crystal thin films

a b s t r a c t Oxygen diffusion and ionic conductivity of samaria-doped ceria (SDC) thin films have been studied as a function of composition using experiment and atomistic simulation. SDC thin films were grown on Al2O3 (0001) substrates by oxygen plasma-assisted molecular beam epitaxy (OPA-MBE) technique. The experimental results show a peak in electrical conductivity of SDC at 15 mol% SmO1.5. The ionic conductivity obtained from molecular dynamics simulation of the same system shows a peak at about 13 mol% SmO1.5. The activation energy for oxygen diffusion was found to be in the range from 0.8 to 1 eV by simulations depending on the SmO1.5 content, which compares well with the range from 0.6 to 0.9 eV given by the experimental work. The simulations also show that oxygen vacancies prefer Sm3+ ions as first neighbors over Ce4+ ions. The present results reveal that the optimum samaria content for ionic conductivity in single crystals of SDC is less than that in polycrystals, which can be related to the preferential segregation of dopant cations to grain boundaries in polycrystals. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Solid oxide fuel cells (SOFC) based on ceramic electrolytes convert the chemical energy of fuel into electrical energy, and offer the advantages of high efficiency, fuel flexibility, and minimal pollution [1]. Successful deployment of this promising technology to ensure energy security and reduce the environmental impact of energy use requires lower cost and improved reliability. To achieve these objectives, it is desirable to operate the SOFC at intermediate temperatures of about 823 K. This offers major advantages over the current high temperature (~1223 K) operation in that some expensive ceramic components can be replaced by inexpensive and reliable metallic components, start up times can be reduced, and mechanical compatibility of components can be improved. Ceria-based electrolytes have been studied as alternatives to currently used yttria-stabilized zirconia (YSZ) electrolytes because of the higher ionic conductivity of the former at intermediate temperatures [2–7]. The ionic conductivity of SOFC electrolytes can be enhanced by choosing a suitable dopant and optimizing the dopant concentration [8]. Previous work has examined the effect of dopant concentration on the ionic conductivity of doped ceria [3,4,9–12]. Among the possible dopants for ceria, samaria is known to provide the highest conductivity enhancement ⁎ Corresponding author at: MS K2-01, P.O. Box 999, PNNL, Richland, WA 99352, USA. Tel.: + 1 509 371 6487; fax: + 1 509-371-6242. E-mail address: [email protected] (R. Devanathan). 0167-2738/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2011.10.007

[13–20]. Samaria-doped ceria (SDC) has high ionic conductivity at intermediate temperatures (673 K–973 K), which enables low temperature SOFC operation [13,21–23]. SDC is also an attractive material system for the development of oxygen sensors for pollution control and increasing the fuel efficiency of automobiles [24–26]. Previous studies have focused mainly on polycrystalline SDC [7,27–34]. This adds an additional degree of complexity to the problem, because grain boundaries in polycrystalline doped ceria may significantly influence the overall conductivity of the material system [35]. In an effort to simplify the problem and study the fundamental mechanisms governing ionic transport in SDC, the present work has examined single-crystalline material using experimental methods and computer simulation. The experimental component includes thin film growth, in situ and ex situ characterization, and electrical conductivity measurements of SDC thin films for different dopant compositions. These results have been integrated with the findings of atomistic computer simulations in order to interpret experimental results, and understand the atomiclevel mechanisms that govern the macroscopic properties and the performance of SDC. The computational work involves molecular dynamics simulations using the DL_POLY code [36] to simulate defect dopant interactions in SDC. The results provide valuable insights into the dependence of overall conductivity of thin film SDC on the samaria content. Besides exploring the conductivity of SDC, the following sections highlight our integrated approach that aims to draw concrete conclusions about defects and ionic transport in SDC, through synthesis

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and characterization of thin films, and analysis of the results with guidance from computation. 2. Experimental details The SDC films for this study were grown by oxygen plasma-assisted molecular beam epitaxy (OPA-MBE) technique in a dual-chamber ultrahigh vacuum system equipped with an electron cyclotron resonance oxygen-plasma source. Prior to film growth, the Al2O3 (0001) substrate was ultrasonically cleaned in acetone and methanol to grow high quality films, free of crystallographic and other defects. Further, the substrate surface was cleaned at 873 K with oxygen plasma in the MBE chamber. Then, thin films were grown at simultaneous growth rates of approximately 0.2 nm/s for ceria and the samaria flux was varied to obtain desired doping levels; under oxygen plasma of 2.0 × 10 −5 Torr and at a substrate temperature of 923 K. Cerium and samarium were evaporated using an electron beam source and a resistance heated effusion cell, respectively. The growth rates of the films were monitored by a pre-calibrated quartz crystal oscillator. The film growth was monitored in-situ using reflection high-energy electron diffraction (RHEED). Rutherford backscattering spectrometry (RBS) was used to verify the SDC film quality, interface characteristics and thickness. High-resolution x-ray diffraction (HRXRD) was used to determine the epitaxial orientation of the as-grown films. The depth profile and composition of the grown films were obtained using x-ray photoelectron spectroscopy (XPS). Tapping-mode atomic force microscopy (AFM) was used to study the topology and roughness of the SDC film. A test-bed with in-situ heating and oxygen pressure control was used to measure the conductivity of the SDC samples. SDC films of a particular composition were loaded on the sample holder with the sample fastened using Haynes™ alloy 214™ clips, which are oxidation resistant at high temperatures, to ensure consistent operating conditions throughout the experiments. A heater from Heatwave Labs Inc. was used to heat the sample to temperatures up to 1073 K. The electrodes were placed on the sample and were connected to a Keithley 614 electrometer to measure the current through the sensing film with a bias voltage applied along the sample surface by a solid-state power supply. Two-probe conductivity measurement method was used to measure the conductivity of the SDC thin films [37]. This method was chosen because it is appropriate for single crystal thin films, and the single crystal SDC films studied in the present work have a thickness of 100 nm. For such thin films, the contact resistance contribution to the total resistivity of the system is not considered to be significant. The voltage and current were measured between the same contacts where the voltage was applied. Silver paste was used to establish the ohmic contact between the thin film and the electrode system. The experimental conditions correspond to a purely ionic conductivity regime for ceria systems [16,20,38]. The input gas to the test chamber was regulated by medium duty, two-stage regulators. A dual-stage, rotary vane pump driven by a single-phase or three-phase electric motor was used to pump out the oxygen gas from the test chamber. For various SDC film compositions, the current was measured for different operating temperatures at 10 Torr of oxygen pressure in the test chamber.

between the ions were represented by short range Buckingham potentials and long range Coulombic interactions. The parameters of the short-range potentials have been published previously [39,40]. The ions were modeled with formal charges of + 4 for Ce, + 3 for Sm, and − 2 for O. Simulations for seven different dopant concentrations from 1 mol%–30 mol% SmO1.5 and for temperatures from 1111 K to 2000 K were performed to find the optimum doping concentration and study the effect of temperature on the oxygen ion diffusion. For each composition, the system was initially equilibrated at 300 K for 25 ps, and then at five different temperatures, namely, 1111 K, 1250 K, 1429 K, 1667 K, and 2000 K, for 5 ps each in Berendsen's isothermal, isobaric ensemble at zero external pressure. The energy fluctuations were less than 1 in 10 4. Subsequently, a 2 ns production run was performed in the microcanonical ensemble at each temperature and composition. The structural features were characterized using the radial distribution function g(r) and the coordination number of the cations. The oxygen ion diffusion coefficient for each configuration was determined from the slope of the variation of the mean square displacement of oxygen ions with time. The ionic conductivity was calculated from the Nernst–Einstein equation as discussed in the following section. 4. Results and discussion The crystal quality and epitaxial orientation of the thin films of SDC grown by OPA-MBE were verified using x-ray diffraction. Fig. 1 shows the 2θ scan of the as-grown 15.4 mol% SmO1.5 doped ceria thin film. The scan depicts (003), (006) and (009) reflections from the alumina substrate and (111) and (222) reflections from the SDC film. The preferred (111) orientation for ceria is confirmed, which indicates a highly oriented single crystalline film. The roughness of the film was measured using tapping mode AFM. Fig. 2 shows the AFM image of a 100 nm-thick SDC film of the above composition. The scan size was 1 μm and the data is shown with a scale of 10 nm per unit along the Z (height) axis. The mean roughness of the film was 0.6 nm, which again shows excellent crystal quality with respect to surface morphology. The Sm/Ce cation ratio was verified by x-ray photoelectron spectroscopy. Fig. 3 shows the XPS depth profile of a 15.4 mol% SmO1.5 doped ceria sample. The depth profile confirms the samarium content of the sample. The RBS spectrum from this sample and SIMNRA simulated spectrum are shown in Fig. 4. The backscattering yield shows that there is no diffusion from the substrate into the thin film, which is another indication of crystal quality. The arrows indicate the characteristic positions expected for backscattering from Sm, Ce, Al and O

3. Simulation details The DL_POLY code [36] from Daresbury Lab, UK was used to perform classical molecular dynamics (MD) simulations with a rectangular cell containing 7 × 7 × 7 unit cells in the cubic ceria fluorite structure with periodic boundary conditions. Various compositions of SDC were generated by substituting Sm 3+ for Ce 4+ at randomly chosen cation sites and removing randomly chosen anion vacancies for charge balance. MD simulation was carried out to study ionic transport in SDC as a function of temperature and composition. The interactions

Fig. 1. X-ray diffraction scan of SDC thin film for a composition of 15.4 mol% SmO1.5.

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where A is a pre-exponential factor, ΔHm is the enthalpy of oxygen migration, and kB is the Boltzmann constant. The diffusion coefficient of SDC is given by D ¼ D0 expð−ΔHm =kB TÞ;

ð2Þ

where D0 is a pre-exponential factor. One can relate σ to D using the Nernst–Einstein equation given by 2

σ ¼ Nq D=kB T;

Fig. 2. Atomic force microscopy image of a 100 nm thick SDC film for a scan size of 1 μm with a data scale of 10 nm along the height axis.

atoms. The quality of the single crystal film and the epitaxial orientation relationship between the film and the substrate was confirmed by cross-sectional TEM imaging and selected area electron diffraction as shown in Fig. 5. The interface between the film and the substrate is very sharp within the image resolution. The film substrate epitaxial relationship can be written as CeO2(111)//Al2O3 (0001) and CeO2 [110]//Al2O3  [11 20]. In order to gain a complete understanding of the crystalline quality of the films, detailed XRD and RBS analysis was carried out on these films. Although the scope of the current article does not permit a comprehensive discussion of these results, the studies essentially prove that the films are high quality single crystals with the combination of x-ray pole figure analysis and RBS analysis. Kuchibhatla et al., Nandasiri et al., and Yu et al. [41–43] have summarized the influence of growth rate, growth temperature on the orientation and crystalline quality of ceria based thin films grown on alumina substrates. The experimentally measured conductivity of SDC is shown in Fig. 6 as a function of composition (a) and as Arrhenius plot for various compositions (b). The peak in the conductivity occurs at 15.4 mol% SmO1.5. This can be taken as the optimum doping concentration to obtain the highest ionic conductivity [24,44]. The dc ionic conductivity (σ) of SDC as a function of temperature, T, is given by σ T ¼ Aexp½−ðΔHm Þ=kB T;

Fig. 3. XPS depth profile from 15.4 mol% SDC thin film on alumina substrate.

ð1Þ

where N is the concentration of anions and q is the anion charge. This is an approximation, especially at high dopant concentrations, because we have neglected correlation effects. As a result the calculated ionic conductivity is expected to reproduce the trends in the experimental data without providing good quantitative agreement. Brinkman et al. [45] observed the calculated values of the ionic conductivity of yttria-stabilized zirconia to be smaller by a factor of four than corresponding experimental values. In SDC, as the Sm content increases, the number of oxygen vacancies increases to maintain charge balance after aliovalent doping. The replacement of N Ce4+ by Sm 3+ (Sm′Ce) is compensated by N/2 oxygen vacancies (Vö). For small dopant concentrations, the increase in the number of vacant sites leads to an increase in oxygen ion transport, which increases the ionic conductivity. At higher dopant concentrations, the vacancy and dopant densities are sufficiently high to cause significant vacancy–vacancy, dopant–dopant and dopant–vacancy interactions that can decrease ionic conductivity by effectively increasing ΔHm. The opposing effects due to the increased availability of vacant sites for the migrating oxygen ion to hop and the increase in ΔHm results in the occurrence of a peak in the ionic conduction as the samaria content is increased. Thus, we observe a maximum in the ionic conductivity for 15.4 mol% samaria in thin films of single crystal SDC. This behavior has been reported by Shimojo et al. [46] and Devanathan et al. [47] for YSZ, Yu et al. for scandia-stabilized zirconia [48] and by Inaba et al. for gadolinia-doped ceria (GDC) [46,49]. The experimental results from this study cannot be directly compared with experimental studies reported in the literature, as most of the reported studies involve conductivity measurements as a function of dopant concentration in polycrystalline samples. Due to differences in the crystalline nature of the samples and the growth technique used, one can only perform a qualitative analysis to understand the difference in the conductivity values between this work and literature reports. Fu et al. [50] found that the conductivity of the SDC samples prepared by co-precipitation method increases with increase in the SmO1.5 mol% with a maximum observed at 20 mol% as 0.0223 S.cm −1 at 1073 K. The increase in conductivity with increasing SmO1.5 mol% in this case was attributed to the decrease in grain size with increasing dopant concentration [50]. Eguchi et al., Jung et al. and Yahiro et al. found out the conductivity at 973 K to be about 0.02 S.cm −1 for SDC samples [13,51,53]. On a similar note, Jung et al. and Yahiro et al. observed a maximum conductivity of 0.056 S.cm −1 and 0.0945 S.cm −1 respectively at 1073 K for 20 mol% SmO1.5 [21,52,53]. It is interesting to note that while comparing the conductivity values obtained by different groups, the temperature range under consideration makes a significant difference in the values especially going from 973 K to 1073 K. Other research groups have itemized the conductivity values as a function of the grain size and found that 20 mol% SmO1.5 is the optimal doping concentration [31,32]. Huang et al. found out that the optimum conductivity of sol–gel prepared SDC is obtained at 20 mol% SmO1.5 [33]. Fig. 7 compares the composition dependence of the conductivity of SDC from the literature and the present work. The trend of increase in the conductivity up to a particular doping level and then a decrease after a particular dopant concentration is uniform for all the reported results. The difference in the

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Fig. 4. RBS spectrum from a 15.4 mol% SDC thin film on alumina substrate.

optimum dopant level for ionic conductivity can be attributed to the crystalline quality and growth technique of the SDC electrolyte. Moreover, by growing films by MBE technique, a higher purity film is obtained as compared to growth methods such as solution route wherein the purity is compromised due to minor impurities from the sources. Our work shows that crystalline quality, purity, and the presence of grain boundaries can have a considerable influence on the optimum dopant concentration [54]. As compared to the findings of other groups that 20 mol% SmO1.5 is the optimum dopant concentration for polycrystalline SDC electrolytes, we achieved the highest ionic conductivity at 15.4 mol% SmO1.5 for single crystal SDC thin films. The temperature dependence of the O2− diffusion coefficient (DO) from MD simulation is shown in Fig. 8(a) as an Arrhenius plot for various SmO1.5 content. At and above 1250 K (1000/K=0.8), which is closest to the current SOFC operating temperature, as the SmO1.5 mol% increases, DO increases, reaches a peak at about 12.8–17.4 mol% and then decreases. The composition dependence of σ calculated using the Nernst–Einstein equation is shown at different temperatures in Fig. 8(b). Over the entire temperature range simulated, the lowest σ value is attained for 1 mol% followed by 3 mol% SmO1.5. At the highest temperature simulated

(2000 K), σ reaches a peak at about 12.8 mol% SmO1.5 and declines only slightly between 12.8 and 29.8 mol% SmO1.5. The width of this plateau region appears to increase with increasing temperature. The simulated σ appears to be an order of magnitude smaller than corresponding experimental values, but the trends in variation with composition are reproduced. The value of ΔHm is also reproduced well as discussed below. As mentioned previously, this level of discrepancy between experimental and simulated conductivity values is not unusual given the many simplifying assumptions included in the calculation. The temperatures in the MD simulation were much higher than the intermediate temperature desired for SOFC operation, because the migration of O2− is so slow at temperatures below 1000 K that sufficient statistics cannot be accumulated in the nanosecond time scale of MD simulations to extract a reliable value of DO. One can extend the MD simulation results at 1111 K to the 973 K experimental temperature by fitting the data in Fig. 8(b) to straight   ΔHm , where D0O is a pre-exponent, lines of the form, D ¼ DO0 exp − kB T ΔHm is the activation energy for O2− migration and kB is the Boltzmann constant. However, one cannot extend this extrapolation to much lower

Fig. 5. Cross-sectional TEM image and SAED pattern showing epitaxial orientation between the SDC film and alumina substrate. The SAED pattern was taken from an area of approximately 200 nm in diameter, covering the SDC film and the sapphire substrate.

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a)

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1 3 5 8 13 17 30

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ln (D cm2/s)

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1000/T (K-1)

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2000 K 1667 K 1429 K 1250 K 1111 K

0.0001 0

5

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Samaria mol % Fig. 8. a) Arrhenius plot of oxygen diffusion coefficient (cm2/s) in SDC for various compositions in mol%; b) The variation of the ionic conductivity (S/cm) with dopant concentration at different temperatures. The results are from molecular dynamics simulations.

Fig. 6. a) Plot of the conductivity of the SDC film as a function of samarium dopant at various temperatures. The peak indicates the optimum samaria mol percent concentration [24].; b) The same data is shown as an Arrhenius plot for various samaria concentrations.

Fig. 7. Composition dependence of the conductivity of SDC reported by different research groups.

temperatures, because the association energy will play an increasingly important role. The composition dependence of ΔHm is plotted in Fig. 9 along with a linear fit to the data. The activation energy for oxygen migration in SDC can be described by ΔHm (eV) = 0.75 + x, where x is the mole fraction of SmO1.5 (in the range from 0 to 0.3). The activation energy shows an upward trend with increasing Sm content. The activation energy of the single crystal SDC material system was found to be in the range from 0.76 eV to 1.1 eV for Sm2O3 concentrations from 0 to 29.8 mol%. Mansila et al. found out the activation energy increases with increase in Sm2O3 content in the SDC thin films prepared by ion beam assisted electron beam evaporation with 0.9 eV for 2% wt. Sm2O3 to 1.1 eV for 36 wt.% Sm2O3 [55]. Fu et al. observed that the activation energy decreases with increasing SmO1.5 mol% with a minimum obtained as 0.869 eV for 20 mol% SmO1.5 [50]. Jung et al. found the lowest activation energy of 1 eV and Perez et al. found the lowest activation energy of 0.95 eV at 20 mol% SmO1.5 [30,53]. The activation energy of SDC with optimal composition is found out to be in the range of 0.8 to 1 eV from simulations and 0.6 to 0.9 eV from experimental results, which is consistent with the activation energy calculated experimentally by several research groups [14,24,56,57]. The increase in activation energy with increasing samaria content is consistent with the explanation proposed in the present work based on defect–dopant interactions. Evidence of defect–dopant interactions can be obtained by examining radial distribution functions and coordination numbers in SDC.

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Fig. 9. Plot of the activation energy calculated by computer simulations as a function of the Sm concentration with a linear fit to the data.

The radial distribution functions for Sm–O and Ce–O pairs in the 13 mol% SmO1.5, SDC system is shown in Fig. 10. The data were obtained as an average over 100 configurations from MD simulations performed at 1111 K. The first neighbor peaks in the Ce–O and Sm–O distributions are found at about 0.2325 and 0.2425 nm respectively. This trend is similar at other compositions and temperatures examined. The average Ce–O first neighbor distance is shorter than the corresponding Sm–O distance by 0.01 nm. The first neighbor environment is different for Ce and Sm in SDC. The second and subsequent neighbor distributions of Ce–O and Sm–O are similar. The coordination numbers (CNs) of Ce4+ and Sm 3+ in SDC at 1111 K are shown in Fig. 11. The average number of anion first neighbors for Ce4+ decreases from 7.98 to 7.53, and for Sm 3+ it decreases from 7.71 to 7.35 as the SmO1.5 content increases from 1 to 30 mol%. Due to the presence of anion vacancies, the coordination number is less than the typical value of 8 for the cubic fluorite structure. The coordination number of Ce is higher than that of Sm by 0.2 to 0.3 depending on the composition, which indicates that oxygen vacancies prefer to have Sm instead of Ce as the nearest neighbor. The larger radius of Sm 3+ leads to strong vacancy-Sm association that hampers ionic conduction by the vacancy mechanism. Lei et al. [58] have shown that oxygen vacancies and trivalent dopants segregate to the grain boundaries in fluorite structured ceramics. Given this association, it is likely that the Sm3+ dopants segregate to the grain boundaries in polycrystalline 8

Radial distribution function g(r)

SDC, thereby effectively lowering the samaria content of the grain interior through which much of the ionic conduction takes place. In this scenario, the samaria content for optimum conduction of the grain interior may be lower than the overall samaria content reported in previous work on polycrystalline samples and closer to the optimum value seen in the present single crystal thin films. 5. Conclusions The optimal doping content for ionic conductivity of thin film single crystal samples of samaria doped ceria (SDC) is found to be about 14–15 mol% SmO1.5 by two probe electrical conductivity measurements. This finding closely matches atomistic computer modeling results which show 13–17 mol% SmO1.5 as the optimal doping concentration. Simulations yield the activation energy for oxygen ion migration to be in the range from 0.8 to 1 eV depending on the Sm concentration, which is consistent with the present experimental findings and experimental literature on similar material systems. The average coordination number of Sm3+ is less than that of Ce4+, which shows that the oxygen vacancy prefers the dopant as first neighbor. Beyond the optimum doping level, the defect interactions at higher Sm concentration contribute to a decrease in the overall conductivity of SDC. The optimum samaria content for conduction in the grain interior of polycrystalline samples may be close to the optimum value determined in the present study for single crystal thin films if grain boundary segregation is considered. Acknowledgments

13 mole% SmO1.5 at 1111 K

Ce-O Sm-O

The research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. This work was supported by the ONR Grant — N00014-071-0457. RD was supported by the Materials Science and Engineering Division, Office of Basic Energy Sciences, US Department of Energy under Contract DE-AC05-76RL01830. The authors thank A. Singh, S. Gupta, C. Wang, V. Shutthanandan, W. Jiang, P. Nachimuthu, T. Varga and M. Engelhard for providing help, guidance and material characterization results for this study.

6

4

2

0 0.2

Fig. 11. Average number of O neighbors for Ce and Sm in SDC at 1111 K.

References 0.4

0.6

0.8

1.0

Distance r (nm) Fig. 10. Radial distribution function of Ce–O (dashed line) and Sm–O (solid line) pairs in 13 mol% SDC at 1111 K.

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