Understanding the Interplay of Dopants, Interfaces, and AnionicConductivity in Doped Ceria/Zirconia Heteroepitaxial StructuresJose J. Plata, Antonio M. Marquez,* and Javier Fdez. Sanz

Departamento de Química Física, Facultad de Química, Universidad de Sevilla, 41012 Sevilla, Spain

ABSTRACT: Using density functional theory calculations, a comparative study at amicroscopic level of the different factors that influence the anionic conductivity in YSZ/doped-CeO2 heterointerfaces is presented. First oxygen migration paths and barriersare examined, either at the YSZ phase or at the interface as a function of the differentcations that may be present. Oxygen migration barriers and, simultaneously, vacancyformation energies are found to be significantly lower at the interface. While these twofactors should result in a larger ionic conductivity at the interface, the presence of Ce3+

cations, associated with O vacancies, is found to increase the oxygen hopping barrier.This drawback can be avoided by introducing aliovalents M3+ cations in the ceria phase.Indeed, a systematic examination of the effects in the oxygen hopping barrier showsthat the presence of trivalent dopants that present an ionic radius lower than that of theCe3+ decreases the oxygen migration barrier and increases the interfacial conductivity.However, some of these cations result in the formation of vacancy−dopant atom pairs,a situation that should be avoided as the radii of those cations are higher than the Ce4+ cation radius. Dopants that prefer a next-nearest configuration should be selected and an adequate balance between the reduction of the oxygen hopping barrier and thepreferred configuration (next or next nearest) should be established when selecting the most appropriate dopant.

■ INTRODUCTIONSolid oxide fuel cells (SOFC) are one of the most promising,sustainable, and environmentally friendly technologies forpower generation. Their main disadvantage is the high workingtemperature (800−1000 °C), which introduces chemical andstructural instabilities in the constituent materials. A high effortis, thus, being directed toward the development of newnanostructured materials that allow reducing the workingtemperature at intermediate ranges (500−700 °C). To this end,enhancement of the ionic conductivity of the solid electrolyte isa key point and one of the main current research issues.1,2

During the past decade, a new generation of high ionconductivity solid electrolyte based on epitaxial oxideheterostructures has emerged as a subject of intense andgrowing interest (see ref 3 for a recent review on the topic).There are two main groups of structures based on coupling

either an ionic conductor with an insulator (e.g., LiI/Al2O3,AgCl/Al2O3, or LiClO4/Al2O3)

4−7 or two ion-conductingoxides (e.g., yttria-stabilized zirconia (YSZ) to Sm-dopedCeO2 (SDC) or YSZ/Gd2Zr2O7).

8,9 In this regard, Garcia-Barriocanal et al.10 have recently reported colossal ionicconductivity at YSZ/SrTiO3 (STO) interface heterostructuresin which YSZ thickness is a few nanometers (1−30 nm).Conductivity measurements as high as 40 S cm−1 at 550 K wereobtained, 8 orders of magnitude higher than the conductivity inbulk YSZ. The conductivity was found to be almostindependent of the YSZ layer thickness but to increase withthe number of interfaces. This was initially interpreted as aproof of the interfacial nature of the process. However, someauthors have challenged this analysis and attributed thisphenomenon to different factors. The electronic conductivity

at the STO layers has been nominated as a factor to take intoaccount, as STO is not a good insulator at high temperatures.11

However, Garcia-Barriocanal et al. claimed that any electroniccontribution to their results is at least three orders of magnitudelower than the ionic contribution.12

Concerning the mechanism of ionic conductivity at theseoxides some authors have attributed this behavior to the spacecharge region at the interface that originates from theredistribution of ionic and electronic defects.13 However,heterostructures made of doped oxides build up very narrowspace charge regions, so the accumulation of charge carriers atthis region should not significantly affect the conductivity.3

Based on this argument other authors have explained the highionic conductivity in these heterostructures by means of thestrain produced in the interface due to the different latticeparameters of the two oxides.14−16 Depending on the mismatchdegree between the two adjacent phases, the interface can beclassified as coherent, semicoherent, or incoherent. Misfitdislocations are created at incoherent and semicoherentinterfaces in order to reduce the interfacial strain caused bythe mismatch, and these lower packing density regions can actas a diffusion pathway for ionic conduction.17 On the contrary,coherent interfaces do not produce misfit dislocations and themismatch is compensated by the elastic strain which causes theionic mobility.18 Strain lattice is difficult to evaluate, and only ina few examples has it been possible to correlate lattice elasticstrain with interfacial conductivity.19 Interfacial strain can also

Received: February 5, 2014Revised: May 22, 2014Published: May 23, 2014

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relax through the formation of defects in the interface whichmodify the conductivity of the system.The theoretical study of ceria−zirconia solid solutions was

initiated more than a decade ago by the works of Balducci etal.20−22 These works were based on classical potentials andconsidered the materials as purely ionic. Although very crude,these models allowed for a simple approximation to thecomputation of some structural parameters of these systems. Inparticular, they indicate that Ce4+/Ce3+ reduction energy ingeneral decreases with increasing dopant concentration andthat structural oxygen vacancies segregate to the oxide surface.Later on, Grau-Crespo et al. examined, using density functionalbased electronic structure calculations, the properties of ceria−zirconia solid solutions.23 They proved that theoretical modelsthat incorporate the details of the electronic structure are ofgreat utility for a systematic understanding of the properties ofthese complex systems. More recently, Kushima and Yildiz18

have studied oxygen diffusivity in strained YSZ by means ofdensity functional theory (DFT) and kinetic Monte Carlo(KMC) calculations. They observed that a moderate increase ofthe strain in the system drastically reduced the oxygenmigration barrier. However, this factor alone is not enough toexplain the colossal conductivity reported for the YSZ/STOinterface. Surprisingly, while YSZ/SDC presents an enhance-ment of ionic conductivity compared with YSZ bulk,8 YSZ/CeO2 heterostructures have a negligible effect on the transportproperties of YSZ bulk.19 DFT calculations performed toinvestigate defect formation in this interface have shown thatvacancy formation and the presence of dopants at the interfaceseem to be more favorable than in strained bulk phases.24 Fromthis overview, it is clear that the increased conductivity in YSZ/doped-CeO2 interfaces has to be ascribed to a variety of factorsand that it is mandatory to explore, at a microscopic level, howthese factors interact at the heterostructure interface.In this paper, DFT+U calculations have been performed

using a realistic model of the YSZ/doped-CeO2 interface. InYSZ, intrinsic oxygen vacancy migration is first investigated indoped models of the strained thin film. Then, the energetics ofCe3+ cation formation, that is simultaneous to vacancy creation,and oxygen migration at the interface, are evaluated. Finally,doping effects on vacancy concentration and oxygen migrationare studied in order to understand the influence of dopants inthe interfacial conductivity.

■ COMPUTATIONAL DETAILSOur model is based on a 3D fully relaxed supercell containing twooxide layers (YSZ and CeO2) with 96 atoms each as shown in Figure1. The YSZ phase model is similar to that of Kushima and Yildizwork18 and consisted of a 9% Y2O3 doped YSZ build from 26 Zr, 6 Y,and 61 O atoms. The three oxygen vacancies were distributedhomogeneously, on the YSZ phase, maximizing the distance betweenthem. This random distribution of oxygen vacancies is known to agreewith experimental diffraction studies25 and has been shown to besignificant to achieve the stability of the cubic phase.26 The Y dopantswere distributed as second neighbors to the oxygen vacancies, inagreement with known experimental27 and theoretical data28 exceptwhen we wanted to check the oxygen migration across a pathcontaining at least a Y cation. The ceria phase was built from 32 CeO2units with two Ce cations substituted by two trivalent cations when thephase was doped. The interface heterostructure was built from astacking of ceria−zirconia phases along the [001] direction.Periodic DFT+U calculations were carried out with the Vienna ab

initio simulation package (VASP).29−31 This code solves the Kohn−Sham equations for the valence electron density within a plane wavebasis set and makes use of the projector augmented wave (PAW)

method to describe the interaction between the valence electrons andthe atomic cores.32,33 The valence electron density is defined by the 12(5s25p66s25d14f1) electrons of Ce atoms, 12 (4s24p65s24d2) electronsof Zr atoms, and six (2s22p4) electrons of O atoms. In thesecalculations the energy was computed using the GGA functionalproposed by Perdew et al.34,35 (PW91) and the electronic states wereexpanded using a plane wave basis set with a cutoff of 500 eV whichensures adequate convergence with respect to the basis set size.

The Hubbard-like term was introduced according to the formalismof Dudarev et al.36 which makes use of a single Ueff parameter,hereafter denoted simply as Uf and Up, to design the effective valuesused for the Ce 4f and O 2p electrons, respectively. For Ce and O wehave used Uf and Up values of 5 eV. The use of the Uf parameter isknown to be essential to reproduce the localized electronic nature ofthe Ce 4f1 states. The Up parameter applied to the O 2p states hasbeen recently shown to result in a moderately improved description ofsome critical aspects that concern structure, electronic properties, andthermochemistry of both CeO2 and reduced ceria.36 Given that the OUp parameter is applied to all oxygen atoms we have also included a Ud

parameter for the 4d levels of Zr atoms. In this case, a Ud value of 2 eVwas chosen in order to reproduce lattice parameter of cubic yttria.

The lattice parameters a = b = 5.27 Å and c = 5.31 Å were optimizedand are quite close to the average of the experimental latticeparameters of CeO2 (5.41 Å)37,38 and YSZ (5.10−5.14 Å).39,40 Thisimplies that both phases have the same lattice parameters in the x−ydirections and are homogeneously strained: the YSZ phase isstretched, and the CeO2 phase is compressed by the same amount.In the z direction the minimization of the total energy results in eitherphase having its own cell parameter, i.e., there is no strain in thedirection perpendicular to the interface. The energy was computed atthe Γ point of the Brillouin zone, a reasonable approximation bearingin mind the size of the system. The convergence of the energy withthis setup was confirmed in a previous paper.18 Forces on the ionswere calculated through the Hellmann−Feynman theorem, includingthe Harris−Foulkes correction to forces.41 In order to localize thepolaron on a specific Ce3+ cation, the positions of the oxygen atomscoordinated to this cation have been initially distorted, increasing theCe−O distance. Iterative relaxation of the atomic positions was laterstopped when the forces on all the atoms were <0.01 eV/Å. Thebarriers for vacancy−oxygen migration were located using the climbingimage version of the nudged elastic band algorithm.42

Figure 1. Fully relaxed YSZ/CeO2 heterostructure model. Atomscolors: red, O; white, Ce; blue, Zr; yellow, Y.

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■ RESULTS AND DISCUSSIONFor the sake of comparison we will start by reexamining theissue of oxygen migration in the strained YSZ phase. YSZpresents fluorite crystal structure with the yttrium and zirconiacations on a fcc cation lattice and oxygen and vacancies on asimple cubic anion lattice. Each cation is at the center of a cubeof eight anions, and each oxygen ion or vacancy is at the centerof a cation tetrahedron. In this structure, oxygen ions migrateby hopping across the edge of a tetrahedron between twocations as shown in Figure 2. The migration barrier, thus,

depends on the neighboring atoms that form the tetrahedra,particularly atoms M and N in the shared edge. To understandthe cation effects in conductive paths, oxygen migration wasanalyzed taking into account different possibilities for the twocationic sites that are involved. If we focus on the YSZ phase,three different cation combinations must be considered on thisshared edge, namely, Zr4+−Y3+, Zr4+−Zr4+, and Y3+−Y3+.Cation substitution has little effect on the M−N distances on

the YSZ phase. The optimized M−N distance increases a mere0.07 Å, from 3.98 Å (Zr4+−Zr4+) to 4.05 Å (Y3+−Y3+). Thus,the effect of cation substitution can be ascribed to the lower(larger) cation ionic radius that increases (decreases) the spanavailable at the shared edge for oxygen migration. Computedbarriers have been represented in Figure 3 as a function of thesum of the ionic radii43 of the two cations defining the sharededge (RM + RN). Different points in this graphic reflect thatdifferent arrangements of neighboring ions on the migrationpath (those marked as 1−4 in Figure 2) have some effect onthe oxygen hopping barrier. The presence of Y3+ in nearest-neighbor (NN) configuration in sites 1 and 2 stabilizes the

oxygen vacancy on the right side of the oxygen migration path(see Figure 2). This results in a higher oxygen hopping barrierin this case. On the other side, the presence of a Y3+ cation insites 3 or 4 will stabilize the vacancy on the left side of theoxygen migration path. This results in a lower oxygen hoppingbarrier in this case. On the same footing, the presence of Y3+

cations in next-nearest-neighbor (3N) sites will influence thestability of the vacancy on either site of the oxygen migrationpath, affecting the height of the hopping barrier. The lowestbarrier found is 0.19 eV for the migrations across a Zr4+−Zr4+edge while the highest barrier is observed for the Y3+−Y3+ edge(≃1.5 eV). These data agree with other values for strained YSZreported in the literature18 and are ∼0.3−0.4 eV lower than theactivation energies obtained by Krishnamurty et al.44 in bulkYSZ, evidencing the influence of the strain in the oxygenmigration barrier. Data in Figure 3 clearly show that the largerthe ionic radius of these two cations the higher the migrationbarrier. Since in the transition state (TS) the oxygen anion islocated between the two cations, the available space forhopping lowers with increasing cation size and, thus, the barrierfor oxygen anion hopping is increased. This phenomenon hasbeen previously reported in YSZ and other fluorite-likestructured solids as ceria.18,45

Thus, it should be quite clear now that the strain which isproduced by the heteroepitaxial structure decreases the barrierfor oxygen migration in the YSZ layers and should increase theionic conductivity of the system. However, the observeddecrease in the oxygen hopping barrier (∼0.3−0.4) is notenough to produce the huge enhancement of the ionicconductivity reported in the experiments.8,10,18 For this reasonwe will now explore the oxygen ion diffusion at the interface.Migration barriers were computed taking into account differentcations at the shared edge of the two tetrahedra that define themigration path as in the previous section. As migration takesplace at the interface, one of the cations that are placed on theshared edge of both tetrahedra is Ce4+ while the second site isoccupied by either Zr4+ or Y3+ cations. Moreover, as oxygenvacancies are a common defect in ceria, particularly at surfacesand interfaces, the presence of Ce3+ cations at the interfacemust be taken into account. Thus, an extra vacancy has beengenerated at the interface in order to obtain Ce3+ cations.Correspondingly, Ce4+−Y3+, Ce4+−Zr4+, Ce3+−Y3+, and Ce3+−Zr4+ cations have been considered as the shared edge of theoxygen migration path. Computed oxygen migration barriers atthe interface are depicted in Figure 4 as a function of the sum ofthe ionic radii of the two cations defining the shared edge (RM

Figure 2. Path for oxygen migration to the vacancy site. M and N arethe cations at the shared edge. Colors: red, oxygen; green, vacancy. 1−4 are cation sites occupied by Zr4+, Y3+, Ce4+, or Ce3+.

Figure 3. Activation energy (Ea, in eV) for oxygen migration at theYSZ layer as a function of the radius of the two cations at the sharededge.

Figure 4. Activation energy (Ea, in eV) for oxygen migration at theinterface as a function of the radius of the two cations at the sharededge.

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+ RN). Different points in this figure corresponding to the sameRM + RN abscissa reflect different combinations of cations insites 1−4 around the oxygen migration path. When the oxygenmigration takes place at the interface, sites 2 and 1 are at theCeO2 side and sites 1 and 4 at the YSZ side. That is, 2 and 3 areeither Ce3+, Ce4+, or dopant M3+ cations and 2 and 3 are eitherY3+ or Zr4+ cations. The presence of a Ce3+ cation in a 3Nconfiguration explains the different barriers computed for thehopping through Y3+−Ce3+ or Zr4+−Ce3+ cations. Similarly, thepresence of an Y3+ cation on a NN site explains the differentbarriers obtained for the hopping through the Zr4+-Ce4+

cations. As in the YSZ phase, the M−N distance is almostconstant (4.13−4.20 Å) and the ionic radii of the two cations atthe tetrahedral shared edge determine the span available foroxygen migration. The same trend that was observed in theYSZ phase is obtained at the interface: the larger the cation size,the higher the barrier. However, two important differencesmust be pointed out. First, the smallest barrier found, ∼0.05 eV,is considerably smaller than the barriers obtained in the YSZlayer. This allows us to attribute the enhancement of theconductivity in these heterostructures to an interfacialphenomenon. Second, the presence of Ce3+ ions considerablyincreases the barrier. The average values for the barriers acrossCe4+−Zr4+ and Ce4+−Y3+ edges are 0.19 and 0.61 eVrespectively while these values increase to 0.55 and 1.69 eVwhen replacing Ce4+ by Ce3+. This fact could explain recentexperimental results in which a negligible effect on thetransport properties of YSZ/CeO2 epitaxial structures can beassigned to the interfacial conductivity.19

In order to evaluate the concentration and stabilization ofCe3+ at the interface, vacancy formation energies have beencomputed at the different layers of the heterostructure (Figure5). There are five nonequivalent oxygen layers in our model

(Figure 1), so five different configurations were explored. Allthe configurations were labeled as I±X, where the ± symbolindicates the phase in which the vacancy has been generated (+for the ceria phase and − for the YSZ phase) and X the relativeposition of the layer taking as reference the oxygen interfacelayer. The Ce3+ cations were located in adequate positionstrying to minimize the vacancy formation energy. Hence theywere placed in the same layer of the vacancy if the vacancy is inthe ceria layer or in the interface layer if the vacancy is locatedat the YSZ phase.

The vacancy formation energy at the middle layer of the ceriaphase (I+2) is 2.42 eV, which is quite close to values reportedusing the same setup for bulk ceria.46 However, vacancyformation energy at the YSZ phase is significantly higher thanin the ceria phase for equivalent positions, 4.00 eV. Thevacancy formation energy decreases considerably for I±1positions, 2.18−2.19 eV. Nevertheless, the most stable sitefor vacancy formation is the interface, where the vacancycreation energy drastically decreases to 1.72 eV. These datasuggest that oxygen vacancies and thus Ce3+ ions are stronglystabilized at the interface and agree with the experimental andtheoretical data found for other ceria−metal oxide interface.47

Therefore, two main consequences can be extracted. First, thevacancy concentration at the interface is going to be higherthan in the separated oxides. This fact can explain the increasein conductivity at the interface of the heterostructure comparedto the separated phases. Second, Ce3+ cations will be located atthe interface, and this will result in an increment of the oxygenhopping barrier. This fact agrees with previous results reportedby Traversa et al.19 in which a negligible effect on the transportproperties of YSZ/CeO2 heterostructure is observed if thissystem is compared with other YSZ based systems. However,while YSZ/CeO2 does not exhibit an enhancement of the ionicconductivity, YSZ/doped-CeO2 systems were reported to showa sizable increase of the conductivity with increasing number ofYSZ/SDC interfaces which does not have an electronicorigin.19

Different trivalent cations have been used for doping ceria,however lanthanide elements have been by far the mostemployed group (see ref 48 and references therein). The firstYSZ/doped-CeO2 epitaxial heterostructure was reported byAzad et al.9 and used Gd3+ ions (YSZ/GDC). Some years laterthe YSZ/Sm-doped CeO2 (YSZ/SDC) epitaxial heterostruc-ture was synthesized and characterized.8 Both systems present asignificant increase in conductivity at the interface. In order tounderstand this phenomenon, we have computed the vacancyformation energy in the YSZ/SDC heterostructure. Togenerate a model for the Sm-doped system, two Ce atomswere substituted by two Sm atoms at the interface. In this waythe formation of Ce3+ cations is avoided when the oxygenvacancy is generated. The lowest vacancy formation energy forthe undoped system was found to be 1.72 eV, close to thatestimated for the ceria−titania interface or for the ceriasurface.37,49 However, this value dramatically decreases to−0.86 eV in the Sm-doped heterostructure. This figure agreespretty well with previous results for bulk doped ceria.50 Thus,vacancy formation energy is drastically reduced at the interfacein doped systems, and the concentration of vacancies anddopant ions should be considerably higher at the interface.However, ionic conductivity not only depends on the vacancyconcentration but also on the oxygen migration barrier. Torationalize the possible effects in the oxygen hopping processwe have computed this barrier at the interface for differentdoped systems.For doped-ceria systems, the least favorable situation (that

can be associated with the rate limiting step in the anionicconduction mechanism) can be associated with the combina-tion of the trivalent dopant cation (M3+) and yttrium cation(Y3+) at the tetrahedra shared edge. The data in Figure 6 showthe same trend that was observed in undoped ceria systems: thebarrier height decreases with decreasing cation size, i.e., whilefor the Ce3+−Y3+ the barrier is 1.57 eV, for the Sc3+−Y3+

combination the barrier is 1.10 eV. This is a lowering of 0.47 eV

Figure 5. Oxygen vacancy formation energy, Ev, in different oxygenlayers of the YSZ/CeO2 model system.

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while the decrease in the radius is just 0.29 Å. Therefore, itcould be concluded that trivalent dopants that present a radiuslower than that of the Ce3+ cation will decrease the oxygenmigration barrier and increase the interfacial ionic conductivity.However, the presence of trivalent atoms with short ionicradius have been reported to result in the formation ofvacancy−dopant atoms pairs.35,51

In order to evaluate the preference for the formation of thosevacancy−dopant associations, the energy difference betweennearest-neighbor (NN) and next-nearest-neighbor (3N)configurations of M3+ dopant and oxygen vacancy (ΔENN/3N= ENN − E3N) has been computed for a number of dopantatoms. Taking as reference the structure depicted in Figure 2,sites 4, M, and 1 are at the YSZ interface side and sites 3, N,and 2 at the CeO2 side. The NN configuration corresponds tothe dopant being located at the N site while it is moved to the 3site at the 3N configuration. As we are computing energydifferences between those two arrangements, the particularoccupancies of sites at the YSZ side have little effect on theresults. The computed energy differences are presented inFigure 7 against the eight-coordinated M3+ cation ionic

radius.43 A recently published study by Grieshammer et al.52

that computed association energies between oxygen vacanciesand rare earth ions in a host matrix of CeO2 shows the sametrend for these energy differences. For the Sm3+ or Gd3+

dopants the difference between both arrangements is small,producing a distribution of sites where there are a significantnumber of 3N configurations. This results in a reduced barrierof the O diffusion process as the shared edge at the migrationpath barrier will consist of two Ce4+ cations of minimal radius.

While the preference for the 3N arrangement seems to favorthe La3+ dopant, its higher cationic radius will increase thebarrier in those sites where the dopant is nearest-neighbor tothe O-vacant site.Dopants with lower radius (Sc3+, Y3+) determine lower

barrier for the oxygen migration process, but their preferencefor a NN configuration results in vacancies to be stronglybound to the dopant cation in deep ionic traps. This problemhas been recently studied for Sc-doped CeO2 where vacancymigration has been shown to consist of the movement of thevacancy between a seven-coordinate Sc site and an eight-coordinate Sc site.53 Thus, dopants that prefer a next-nearest-neighbor configuration are more convenient and a balancebetween both effects should be established when selecting themost appropriate dopant.

■ CONCLUSIONSIn conclusion, the models and results exposed in the precedingparagraphs allow us to rationalize the trends observed in theenhancement of the ionic conductivity of YSZ/doped-CeO2heterostructures in terms of a combination of different factors.Oxygen migration barriers have been shown to be significantlylower at the YSZ/CeO2 interface. Simultaneously, vacancyformation energies are found to be lower at the interface layer.These two factors should result in a larger ionic conductivity atthe interface. However, the presence of Ce3+ cations greatlyincreases the oxygen hopping barrier, reducing the ionicconductivity. This shortcoming can be solved by introducingaliovalent M3+ cations in the ceria phase. The presence oftrivalent dopant cations reduces the vacancy formation energy,and the vacancy concentration should increase at the interfacewhile avoiding the formation of Ce3+ cations. Our results alsoshow that the incorporation of trivalent dopant cations thatpresent an ionic radius lower than that of the Ce3+ cationeffectively decreases the oxygen migration barrier. Conse-quently it is possible to reduce the oxygen migration barrier byselecting M3+ dopants whose ionic radius is smaller than that ofthe Ce3+ cation and prefer a next-nearest-neighbor config-uration with respect to the oxygen vacant.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: marquez@us.es.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was funded by the Ministerio de Economıa yCompetitividad (Spain, Grants MAT2012-31526 andCSD2008-0023) and European FEDER. Computationalresources were provided in part by the Barcelona Super-computing Center/Centro Nacional de Supercomputacion(Spain). The COST Action CM1104 is gratefully acknowl-edged.

■ REFERENCES(1) Boivin, J. C.; Mairese, G. Chem. Mater. 1998, 10, 2870.(2) Brett, D. J. L.; Atkinson, A.; Brandon, N. P.; Skinner, S. J. Chem.Soc. Rev. 2008, 37, 1568.(3) Fabbri, E.; Pergolesi, D.; Traversa, E. Sci. Technol. Adv. Mater.2010, 11, 054503.(4) Liang, C. C. J. Electrochem. Soc. 1973, 120, 1289.(5) Gupta, R. K.; Agrawal, R. C. Solid State Ionics 1994, 72, 314.

Figure 6. Activation energy for oxygen migration at the interface ofYSZ/doped-CeO2 heterostructures.

Figure 7. Energy difference (in eV) between nearest-neighbor andnext-nearest-neighbor configuration of dopant and oxygen vacancy atthe interface of YSZ/doped-CeO2 heterostructures. The radius of theCe4+ cation is shown as a vertical line.

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Chemistry of Materials Article

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