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Achieving fast oxygen diffusion in perovskites by cation ordering A. A. Taskin,∗ A. N. Lavrov,† and Yoichi Ando Central Research Institute of Electric Power Industry, Komae, Tokyo 201-8511, Japan The oxygen-exchange behavior has been studied in half-doped manganese and cobalt perovskite oxides. WehavefoundthattheoxygendiffusivityinGd0.5Ba0.5MnO3−δ canbeenhancedbyorders 5 of magnitude by inducing crystallographic ordering among lanthanide and alkali-earth ions in the 0 A-sitesublattice. Transformationofasimplecubicperovskite,withrandomlyoccupiedA-sites,into 0 alayeredcrystalGdBaMn2O5+x (orisostructural GdBaCo2O5+x forcobalt oxide)withalternating 2 lanthanide and alkali-earth planes reduces the oxygen bonding strength and provides disorder-free channelsfor ion motion, pointing to an efficient way to design new ionic conductors. n a J PACSnumbers: 66.30.Hs,82.47.Ed 7 Oxygen ion conductors - solids exhibiting very fast ] i oxygen diffusion - constitute the basis for such emerg- c ing technologies as the membrane oxygen separation or s - the solid-oxide fuel cell (SOFC) power generation.1,2. l r These technologies offer enormous economical and eco- t m logicalbenefits providedhigh performance materials can be developed: The scientific challenge is to design mate- . t rialsdemonstratinghighoxygendiffusivityatlowenough a m temperature. - In general, a crystal must meet two fundamental re- d quirements to be a good oxygen-ion conductor: (i) it n must contain a lot of vacancies in the oxygen sublattice, o and (ii) the energy barrier for oxygen migration from c ) [ othnaens∼ite1teoVa.nOonthlyeramfeuwsttybpeesfaoifrolyxisdmesa,lla,ntdyppiecraolvlysklietesss units (d) 01 002 v1 ABO3 (A is a rare-earth or an alkali-earth element and rb. 0 Bistypicallyatransitionmetal)amongthem,havebeen a 7 ( (e) 4 12 ofovusnkditetso, wphoiscshespsotshseesses afehaitguhreesl.e1c,2t,r3o,4n,5i,c6,c7,o8ndDuocpteivditpyeirn- nsity 001 002 003 00 1 addition to the ionic one, are considered for using as nte x 10 x 3 0 electrodes in SOFC and as oxygen-selective membranes; I 10 20 30 40 50 05 for example, strontium-dopedlanthanum manganese ox- 2q (deg.) ide, La Sr MnO , is a standard cathode material / 1−y y 3−δ t forSOFCapplicationsoperatingattemperaturesaround ma 1000◦C.6 Recently,seriouseffortsaremadetoreducethe FioInGs.in1:thOerdAer-sinitgeosfublalanttthiacenidofehAa’l3f+-doapneddaplkearloi-vesakritthes.A”(2a+) operation temperature of SOFC, and for the operation d- at700-800◦C,strontium-dopedlanthanumcobaltoxide, AcupsaimtiponleocfuAbi-csitpeesroisvstkriatnesfAor’0m.5eAd”i0n.5tBoO(b3)waitlhayerarenddocmrysotca-l n La1−ySryCoO3−δ, is considered to be the most promis- A’A”B2O6 by doubling the unit cell, provided the difference o ing cathode material.7,8 The performance of perovskite inionicradiiofA’andA”ionsissufficientlylarge. (c)Oxygen c oxides has been already optimized as much as possible atomscanbepartiallyorcompletelyremovedfromlanthanide : v mostly by means of various ion substitutions in both A planes in A’A”B2O5+x, providing a variability of the oxygen i and B sublattices,6,7,8,9 but they still fail to operate at content,0≤x≤1. (d,e)X-rayBragg’s(00L)peaksobtained X lowenoughtemperatures∼500◦Crequiredforsuccessful forcubicGd0.5Ba0.5MnO3−δ (d)andlayeredGdBaMn2O5+x r commercialization of the fuel cell technology. (e), which demonstrate the doubling of the unit cell along a thecaxiswiththecationordering(forconvenience,thepeak In this Letter, we show that the oxygen-ion diffu- intensity is multiplied by afactor indicated near each peak). sion in a doped perovskite can be enhanced by orders of magnitude if a simple cubic crystal [schematically shown in Fig.1(a)] transforms into a layered compound groups.10,11 We have succeeded in growing high-quality withorderedlanthanideandalkali-earthions[Fig. 1(b)], single crystals of these compounds by the floating- whichreducestheoxygenbondingstrengthandprovides zone (FZ) technique.12 What makes these layeredoxides disorder-free channels for ion motion. promising for searching a high oxygen mobility is their Recently, the A-site-ordered manganese and cobalt remarkable variability of oxygen content: Oxygen atoms perovskite oxides have been synthesized by several can be partially or even completely removed from the 2 (a) (b) lanthanide planes [as shown in Fig. 1 (c)], creating a lot of vacant sites in the crystal lattice. While polycrys- q 1.0 1.0 e ctaulbliincepaeirro-svisnktietreedcrsyasmtapllesstroufctGudreBa(Mannd2Oth5u+sx sahdooupltdtbhee nt, x (O), 650oC e (O), 350oC bgexoepngr.reosTwsehndeuafnosdrGteurdns0at.5rtBoenaog0p.l5ypMorrentdOuun3c−iitnδyg),taotthmoebootsrapdihneerrebedootphfhpaluasryeeecraaernd- en cont 0.9 700oC ((DD)),, 635500ooCC 0.5Dxnorm g disordered (D) GdBaMn2O5+x and cubic Gd0.5Ba0.5MnO3−δ phases al- y ordered (O) x lows direct comparison of the oxygen behaviour in ma- O 0.8 0.0 terials with and without the A-site sublattice ordering. 10-510-410-310-210-1 1 10 0 0 200 400 Forthecobaltoxide,ontheotherhand,onlytheordered Oxygen pressure Annealing time GdBaCo O phase can be obtained even at high oxy- 2 5+x (bar) (min) gen pressure. Thehomogeneityandcationstoichiometryofourcrys- FIG. 2: Comparison of oxygen behaviour in cubic tals are confirmed by the electron-probe microanalysis Gd0.5Ba0.5MnO3−δ with disordered A-site sublattice (i.e. (EPMA)andtheinductively-coupledplasma(ICP)spec- randomly distributed Gd3+ and Ba2+ ions) and layered troscopy. Ordering of the Gd and Ba ions into con- GdBaMn2O5+x. (a) Equilibrium oxygen content xeq (for secutive (001) layers is confirmed by the X-ray diffrac- Gd0.5Ba0.5MnO3−δ, x=1-2δ) measured at 700◦C as a function measurements [Fig. 1(d) and (e)], which clearly tionoftheoxygenpartialpressure. (b)Normalizedchangein demonstrate the doubling of the unit cell along the c theoxygencontentwith timeduringannealingintheoxygen axis for GdBaMn O single crystals. The behaviour gas flow at 350◦C and 650◦C. 2 5+x of dc resistivity shows that both GdBaMn O and 2 5+x GdBaCo O undergo an insulator-to-metal transition 2 5+x uponincreasingtemperature;athightemperatures,they contentandx istheequilibriumone),measuredduring eq possess a metallic conductivity σ of the order of 102 and theannealingintheoxygengasflowat350◦Cand650◦C, 103 S/cm, respectively. These values are much larger for samples with ordered (open symbols) and disordered thananyionicconductivityavailableinsolids,indicating (filled symbols) A-site sublattice. The relaxation time that these compounds should be mixed ionic and elec- is about two orders of magnitude smaller in the layered tronic conductors (MIECs). oxide. Surprisingly, the oxygen accumulation in the lay- It is well known that in MIECs, the ionic conductiv- eredGdBaMn2O5+x evenat350◦Cturnsouttobefaster ity σi, which is typically much smaller than the elec- than that in the cubic Gd0.5Ba0.5MnO3−δ at 650◦C. tronic conductivity σe, may be determined from diffu- HavingestablishedthattheorderingoftheA-sitesub- sion measurements.13,14 In oxides with variable oxygen lattice into a layeredstructure significantlyenhances the stoichiometry,likeGdBaMn2O5+x andGdBaCo2O5+x,a oxygenrelaxationrate, we now turn to a more quantita- sharp change in conditions (oxygen pressure or temper- tive analysis of the oxygen diffusion. For that purpose, ature) results in a gradual transition into a new equilib- however, large single-crystal samples of GdBaMn O 2 5+x riumstatethatdiffersinoxygencontent;thismeansthat appeartobeunsuitable;dependingonx,theorthorhom- theoxygenconcentrationincrystalsampleschangeswith bicity (b−a)/a in GdBaMn O varies from 0 up to 2 5+x time, which can be measured as a weight change. The 7%,andtheenormousstrainsemerginguponthe oxygen observed relaxation time depends on two processes: the intercalationintolargesinglecrystalsresultintheforma- oxygenexchangeattheinterfacebetweenthegasandthe tionofaregulararrayofcracksonthecrystalsurface. We solid, and the oxygen bulk diffusion.13,14 In the present therefore focus our quantitative analysis on another lay- study, both the surface exchange coefficient K and the ered material GdBaCo O , which does not have such 2 5+x chemical diffusion coefficient D are obtained by examin- a problem. To separate the surface-exchange and bulk- ing the weight change of crystals as a function of time. diffusion contributions to the overall oxygen exchange The oxygen-exchange behaviour in cubic manganese rate,we study the oxygenkinetics inGdBaCo O sin- 2 5+x oxide and its layered counterpart, studied in polycrys- glecrystalsofdifferentsize: theoxygenkineticsismostly tallinesampleswithcomparablemorphology,isshownin limited by the surface exchange for small-size crystals, Fig. 2. There are two clear consequences of the A-site while for large-size crystals it is mostly limited by the ordering: First, as shown in Fig. 2(a), the change in the bulk diffusion. The change in weight with time in a equilibrium oxygen content x upon decreasing oxygen rectangular-shapesampleofagivensizehasananalytical eq pressure from 1 to 10−5 bar at 700◦C is much more pro- solution,14 which is a function of both the chemical dif- nounced in the layered oxide. Thus, it is much easier fusion coefficient D and the surface exchange coefficient to remove oxygen from the layered GdBaMn O than K. Figure 3 shows the experimental data (symbols) and 2 5+x fromcubicGd Ba MnO . Second,andmoreimpor- analytical fits (solid lines) of the change in the oxygen 0.5 0.5 3−δ tant,isahugedifferenceintherateoftheoxygenuptake. content with time for GdBaCo O single-crystal sam- 2 5+x Figure2(b)showsanormalizedchangeintheoxygencon- ples of different sizes upon annealing in the oxygen flow tent∆x =(x−x )/(x −x )(x istheinitialoxygen at 350◦C. From this set of data, the parameters D and norm 0 eq 0 0 3 not exceed ∼ 10 in the whole temperature and oxygen- pressure range studied [Fig. 4(b)]. Thus, the ionic con- 0.50 ductivity of 0.01 S/cm, which is considered as a cri- terion for SOFCs to be valuable1, can be achieved in ent, x 0.45 L (m m) x L (m m) GstidllBbaCeoa2Opo5+ssxibaillriteyadfoyraftu∼rth5e0r0◦iCm.pCrolevaermlye,ntthoerfepsrhoopuelrd- nt x y ties in ordered perovskite oxides, and the most obvious o ~30 x 30 way is a substitution of different rare earths for Gd. n c (set of crystals) ge 0.40 150 x 60 Oxy Ly Lx 510000 0x x4 06000 xeq 0.6 (b) 0.35 12100000 xx 7200000 10-3 ontent, 0 . 4 400 550 10-3 0 200 400 600 800 1000 n c 0.2 700oC 10-4 e 10-4 g Annealing time (min) y x 0.0 O 10-5 10-3 10-1 101 FIG. 3: Time dependence of the oxygen content in ) 10-5 Oxygen press u 1 r e 0 -( 5b ar) s rectangular-shaped GdBaCo2O5+x single crystalsofdifferent 2/ sizes while annealing in the oxygen flow at 350◦C (symbols). m pSaorliadmleinteerssrDepr=es3en·1t0t−h7eocrmet2i/csalacnudrvKes=w2it·h10a−u6ncimqu/espfoarirthoef D (c 10-6 10-6 /s) m whole set of experimental data. c 10-7 10-7 ( K (a) K can be uniquely determined to be D =3·10−7 cm2/s andK =2·10−6 cm/s. Wenotethattheoxygenkinetics 10-8 10-8 1.2 1.4 1.6 1.8 2.0 doesnotshowanydetectabledifferenceuponvaryingthe sample size along the c axis in the range of 30-500 µm, 1000/T (K-1) implying an essentially two-dimensional character of the oxygen diffusion in this layered material. FIG. 4: Oxygen bulk diffusion and surface oxygen exchange Using the technique described above, we have determined the parameters D and K for the tempera- in GdBaCo2O5+x. (a) Temperature dependences of the diffusion coefficient D(T) and the surface exchange coefficient ture range of 250-600◦C at 1 bar oxygen pressure [Fig. K(T),obtainedfrom theoxygenkineticsexperimentssimilar 4(a)]. These data are well fitted by the activation laws tothoseshowninFig. 3. Theinset(b)showsthedependence D(cm2/s) = 0.15 · exp(−0.7eV/kBT) and K(cm/s) = oftheequilibriumoxygencontentxeq ontheoxygenpressure 15 ·exp(−0.85eV/kBT), revealing quite low activation P for several temperatures. energies for both processes. As a result, the oxygen diffusion in GdBaCo O becomes very fast already at In conclusion, we have found a remarkable en- 2 5+x ratherlowtemperatures,exceeding10−5cm2/sat600◦C. hancement of the oxygen diffusivity in ordered per- Toappreciatethetruemeritofthishighoxygendiffusiv- ovskite oxides as exemplified in GdBaMn O and 2 5+x ity in GdBaCo O , it is worth estimating the ionic GdBaCo O , which opens a possibility to develop a 2 5+x 2 5+x conductivity, which is uniquely determined by the self- new class of materials suitable for applications that re- diffusion(ortrace-diffusion)coefficientD∗.15 Oneshould quire a fast oxygen transport in the intermediate tem- keep in mind that the chemical diffusion coefficient D, perature range. The improvement of the oxygen trans- obtained in our experiments, is related to D∗ through a port properties induced by the cation ordering in half- so-called thermodynamic factor Θ [≡ (∂lnP/∂lnx)/2], doped perovskites provides a good example of how the D = Θ · D∗, reflecting the fact that the true driving formation of a layered crystal structure, with disorder- force for diffusion is a gradient in the chemical poten- free channels for ions migration and with a weakened tial but not a gradient in the oxygen concentration.15 In bonding strength of oxygen, can significantly facilitate GdBaCo O , Θ turns out to be quite low and does the oxygen motion in the crystal lattice. 2 5+x ∗ [email protected] 2 N. Q. Minh and T. Takahashi, Science and Technology of † Presentaddress: InstituteofInorganicChemistry,Novosi- Ceramic Fuel Cells (Elsevier, Amsterdam, 1995). birsk 630090, Russia 3 P. Lacorre, F. Goutenoire, O. Bohnke, R. Retoux, and Y. 1 B. C. H.Steele and A.Heinzel, Nature 414, 345 (2001). Laligant, Nature 404, 856 (2000). 4 4 J.B. Goodenough, Annu.Rev.Mater. Res. 33, 91 (2003). Raveau,J. Solid StateChem. 142, 247 (1999). 5 Z. Shao and S.M. Haile, Nature431, 170 (2004). 12 A.A.Taskin, A.N.Lavrov,and Y.Ando,Phys.Rev.Lett. 6 S.J. Skinner,Int.J. Inorg. Mater. 3, 113 (2001). 90, 227201 (2003). 7 R.A.DeSouzaandJ.A.Kilner,SolidStateIonics106,175 13 I. Yasuda and M. Hishinuma, J. Solid State Chem. 123, (1998). 382 (1996). 8 J.A. Kilner, Solid StateIonics 129, 13 (2000). 14 J.A.LaneandJ.A.Kilner,SolidStateIonics136-137,997 9 Y.L. Yang, A.J. Jacobson, C.L. Chen, G.P. Luo, K.D. (2000). Ross, and C.W. Chu,Appl.Phys. Lett., 79, 776 (2001). 15 A. Van der Ven, G. Ceder, M. Asta, and P.D. Tepesch, 10 F. Millange, V. Caignaert, B. Domenges, B. Raveau, and Phys. Rev.B 64, 184307 (2001). E. Suard, Chem. Mater. 10, 1974 (1998). 11 A. Maignan, C. Martin, D. Pelloquin, N. Nguyen, and B.