Table Of ContentAchieving 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 func-
tion 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 de-
termined the parameters D and K for the tempera- in GdBaCo2O5+x. (a) Temperature dependences of the dif-
fusion 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 dif-
fusion 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
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