ARTICLESPUBLISHED ONLINE: 7 FEBRUARY 2010 | DOI: 10.1038/NCHEM.547

Anisotropic oxygen diffusion at low temperaturein perovskite-structure iron oxides
Satoru Inoue1, Masanori Kawai1, Noriya Ichikawa1, Hiroshi Kageyama2, Werner Paulus3
and Yuichi Shimakawa1 *Oxygen-ion conduction in transition-metal oxides is exploited in, for example, electrolytes in solid-oxide fuel cells andoxygen-separation membranes, which currently work at high temperatures. Conduction at low temperature is a key todeveloping further utilization, and an understanding of the structures that enable conduction is also important to gain
insight into oxygen-diffusion pathways. Here we report the structural changes observed when single-crystalline, epitaxial
CaFeO2.5 thin ﬁlms were changed into CaFeO2 by low-temperature reductions with CaH2. During the reduction processfrom the brownmillerite CaFeO2.5 into the inﬁnite-layer structure of CaFeO2 , some of the oxygen atoms are released fromand others are rearranged within the perovskite-structure framework. We evaluated these changes and the reaction timethey required, and found two oxygen diffusion pathways and the related kinetics at low temperature. The results
demonstrate that oxygen diffusion in the brownmillerite is highly anisotropic, signiﬁcantly higher along the lateral
direction of the tetrahedral and octahedral layers.

T

he iron oxides SrFeO2 and CaFeO2 crystallize in the inﬁnitelayer structure that consists of FeO2 planes with cornersharing, square-planar oxygen coordination of divalent Fe
ions1–4. The square-planar oxygen coordination of Fe2þ in thisstructure is uncommon because Fe ions in oxides are usually coordinated tetrahedrally or octahedrally by oxygen ions. The lack ofapical oxygen atoms makes the crystal structure of SrFeO2 highly
anisotropic and causes this material to show two-dimensional magnetism with an unusually high antiferromagnetic Néel temperature1.
The compound is obtained by reducing SrFeO2.875 or SrFeO2.5 at
280 8C with CaH2 (refs 5–7). Although the brownmillerite structure
SrFeO2.5 with Fe3þ had been assumed to represent the lower limit ofoxygen non-stoichiometry in the SrFeO32d perovskites8–11, lowtemperature reduction with CaH2 enabled us to go below that presumed limit. As for SrFeO2.5 , the brownmillerite CaFeO2.5 can be
reduced to CaFeO2 with an inﬁnite-layer structure by the same
method2. The coordination of the high-spin Fe2þ in CaFeO2 isslightly distorted from the perfect square-planar coordination of
SrFeO2 , a distortion that occurs because Ca2þ is smaller than
Sr2þ (ref. 4).
During the reduction of the fully oxygenated perovskite AFeO3(A ¼ Ca or Sr) to the inﬁnite-layer structure AFeO2 , oxygen ionsare removed from the perovskite-structure framework and several
distinct phases with ordered arrangements of oxygen vacancies
appear. Such vacancies produce both oxygen-ion conduction andelectronic conduction, and the former could be exploited in electrolytes in solid-oxide fuel cells and in oxygen-separation membranes.
The oxygen-ion conduction in solids is ascribed generally to
hopping by way of vacant atomic sites12,13, and so an understandingof the topochemical aspects of the reduction process in AFeO32d
will give us further insight into the oxygen-diffusion pathways ofsolid oxygen-ion conductors. In addition, currently most materials
that conduct oxygen ions are used above 700 8C (refs 13–16), andthus oxygen diffusion below 300 8C is of particular interest in developing low-temperature, oxygen-ion conductor devices. Althoughoxygen conduction at high temperatures (usually above 700 8C) isa thermally activated hopping process, other mechanisms should
be considered for conduction at moderate temperatures, that is
below 300 8C to room temperature. Recent oxygen-isotope
exchange experiments on AFeO32d revealed the importance oflattice dynamics in terms of structural instability to enhance theoxygen mobility at low temperatures17. Oxygen-ion conduction in
La12xBa1þxGaO42x/2 has been found to proceed by an unusual
cooperative process, akin to that of a cog wheel, in Ga2O7 units18.
Atomic reconstruction at the coherent interface in epitaxial
ZrO2:Y2O3/SrTiO3 heterostructures increases carrier concentrationand decreases activation energy, which gives rise to increased
oxygen mobility at room temperature19. These results suggest thatspeciﬁc structural features have important roles in oxygen-ion conduction in solids, especially at low temperatures.
It is difﬁcult to examine oxygen migration paths with polycrystalline samples16, but we expect to see these paths on the atomic scalein single-crystalline materials. As the crystal structures of theoxygen-deﬁcient iron perovskite AFeO32d , where d ¼ 1/n (n ¼ 1,
2, 4, 8 and 1) (refs 1,9), become more anisotropic with deceasingn, we should be able to clarify anisotropic oxygen-diffusion behaviours when we examine oriented single-crystalline materials,
especially with n ¼ 1 and 2. Using epitaxially grown single-crystalline CaFeO2.5 thin ﬁlms with different orientations, we focusedon the reaction mechanism, in terms of the oxygen-diffusion
pathways, of the reduction from the brownmillerite CaFeO2.5 to
the inﬁnite-layer structure CaFeO2.

Results and discussion
The bulk crystal of the brownmillerite CaFeO2.5 has a distorted orthorhombic unit cell, a ¼ 5.427 Å, b ¼ 14.763 Å and c ¼ 5.597 Å (spacegroup Pnma)20, which is related to the average perovskite structurewith ap (bp)  d101 ¼ 3.896 Å and cp  d040 ¼ 3.691 Å. The large
anisotropy of ap/cp means the orientation of an epitaxially grown
CaFeO2.5 thin ﬁlm depends on the substrates21. In this study we

1Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan, 2 Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto
615-8510, Japan, 3 Université de Rennes 1, Sciences Chimiques de Rennes – UMR 6226, Campus de Beaulieu, Bât 10B, F-35042 Rennes, France.
* e-mail: shimak@scl.kyoto-u.ac.jp
NATURE CHEMISTRY | VOL 2 | MARCH 2010 | www.nature.com/naturechemistry© 2010 Macmillan Publishers Limited. All rights reserved.

213

ARTICLES

NATURE CHEMISTRY DOI: 10.1038/NCHEM.547

a

STO
Log intensity (a.u.)(0 2 0)(0 3/2 0)(0 1 0)bcp = b/4
ap = a/÷2

LSATc
ap= a/÷2

20

LSAO

30

cp = b/4(2 0 0)(1 0 0)

LAO

40

50

2θ (degree)

0.88

0.88

0.86

0.86

0.86

0.86

0.84

0.84

0.84

0.84

0.82

CaFeO2.5(1 0 3)0.82(1 0 3)

0.82

0.80

0.80

0.80

0.78

0.78

0.78

0.76

STO (1 0 3)

0.76

LSAT (1 0 3)

Q001 (r.l.u.)

0.88

Q001 (r.l.u.)

0.88

Q001 (r.l.u.)

Q001 (r.l.u.)

Figure 1 | X-ray diffraction patterns of CaFeO2.5 thin ﬁlms and schematic crystal structures of the ﬁlms on the substrates. a, X-ray diffraction patterns of
as-deposited brownmillerite CaFeO2.5 thin ﬁlms grown on SrTiO3 (STO), (La0.3Sr0.7)(Al0.65Ta0.35)O3 (LSAT), LaAlO3 (LAO) and LaSrAlO4 (LSAO). The peaks
from brownmillerite CaFeO2.5 are shown in blue. b, Schematic crystal structure of cp(b)-axis-oriented brownmillerite CaFeO2.5 grown on a substrate. Thisorientation structure model represents the growths of the ﬁlms on the STO and LSAT substrates. c, Schematic crystal structure of CaFeO2.5 oriented on the
ap-axis. The model represents the growths of the ﬁlms on the LAO and LSAO substrates.

0.76

LAO (1 0 3)

LSAO
(1 0 11)

0.82

0.80

0.78

(1 0 3)0.76(1 0 3)0.25 0.26 0.27

0.25 0.26 0.27

0.25 0.26 0.27

0.25 0.26 0.27

Q100 (r.l.u.)

Q100 (r.l.u.)

Q100 (r.l.u.)

Q100 (r.l.u.)

Figure 2 | Logarithmic X-ray diffraction intensity reciprocal lattice space maps of CaFeO2.5 thin ﬁlms. The maps show X-ray diffraction intensities aroundthe (1 0 3) Bragg reﬂections of brownmillerite CaFeO2.5 thin ﬁlms grown on STO, LSAT, LAO and LSAO. Each ﬁlm was grown epitaxially on the substrate. The
in-plane lattices of CaFeO2.5 grown on STO, LSAT and LSAO exactly matched the substrate lattices, but that grown on LAO was slightly different from the
LAO substrate lattice. r.l.u. ¼ reciprocal lattice unit.

used four different single-crystalline substrates: SrTiO3 (001) (STO,
cubic lattice constant a ¼ 3.905 Å), (La0.3Sr0.7)(Al0.65Ta0.35)O3 (001)(LSAT, ap ¼ 3.868 Å), LaAlO3 (001) (LAO, ap ¼ 3.793 Å) and
LaSrAlO4 (001) (LSAO, ap ¼ 3.756 Å). The brownmillerite
CaFeO2.5 thin ﬁlm was deposited on each substrate crystal by apulsed laser-ablation method. As shown in Fig. 1, X-ray diffraction
peaks from the brownmillerite CaFeO2.5 , in addition to those fromthe substrate lattice, were observed, which conﬁrms that a singlephase CaFeO2.5 ﬁlm was obtained. Interestingly, (0 k 0) and(0 k/2 0) diffraction peaks of CaFeO2.5 (indices with respect tothe average perovskite unit cell) were found for the ﬁlms grownon STO and LSAT, whereas only (h 0 0) diffraction peaks were214

found for the ﬁlms grown on LAO and LSAO, which have lattices
smaller than those of STO and LSAT. The epitaxial growth of the
CaFeO2.5 thin ﬁlm on each substrate was conﬁrmed by X-ray diffraction in a reciprocal lattice space around the (1 0 3) Bragg reﬂection of the average perovskite structure (Fig. 2).
The in-plane and out-of-plane lattice constants of the epitaxiallygrown CaFeO2.5 thin ﬁlms were obtained from the diffraction peak
positions in the reciprocal lattice-space maps and are plotted inFig. 3. The in-plane lattices of CaFeO2.5 grown on STO, LSAT andLSAO exactly matched the substrate lattices, which conﬁrms coherent growth of the ﬁlms on the substrate crystals. The in-plane lattice
constant of CaFeO2.5 grown on LAO, however, was slightly different

NATURE CHEMISTRY | VOL 2 | MARCH 2010 | www.nature.com/naturechemistry© 2010 Macmillan Publishers Limited. All rights reserved.

ARTICLES

NATURE CHEMISTRY DOI: 10.1038/NCHEM.547(ap, cp)

3.70

3.60

3.50

3.40
CaFeO2
3.30
3.70

3.80
3.90
In-plane lattice constant (Å)

Figure 3 | Lattice constants of brownmillerite CaFeO2.5 and inﬁnite-layer
structure CaFeO2 thin ﬁlms. In-plane and out-of-plane lattice constantsof as-deposited brownmillerite CaFeO2.5 (solid squares) and reduced
inﬁnite-layer structure CaFeO2 (solid circles) thin ﬁlms. Also plotted are thepseudo-tetragonal lattice constants of bulk CaFeO2.5 , which correspond toap (bp)  d101 ¼ 3.896 Å and cp  d040 ¼ 3.691 Å (open symbols).
The straight dotted line represents isovolume transformation in CaFeO2.5.
The substrate lattice constants are given at the top of the ﬁgure.

from that of LAO, which suggests that the crystal growth of the
CaFeO2.5 ﬁlm is relaxed from perfect matching to the substratelattice. Nevertheless, a single-crystalline CaFeO2.5 thin ﬁlm that
kept the epitaxial relationship was obtained on the LAO substrate.
The results of X-ray diffraction experiments demonstrated that thedeposition on STO and LSAT yielded pseudo-tetragonal CaFeO2.5
oriented on the cp(b)-axis, but that on LAO and LSAO yielded
CaFeO2.5 oriented on the ap-axis. No other orientation domains
were found in any of the ﬁlms. That the orientation of the brownmillerite CaFeO2.5 was affected by the substrate lattice size is reasonable when we compare the lattice mismatches between the ﬁlms andsubstrates (Supplementary Table 1). To reduce the lattice energy at
the interface, the oriented CaFeO2.5 thin ﬁlm with the smaller latticemismatch grows easily. Also, each of the CaFeO2.5 thin ﬁlms has
almost the same unit-cell volume.
The prepared CaFeO2.5 thin ﬁlms were then reduced by embedding them with CaH2 powder in evacuated glass tubes and annealingat 240 8C (refs 3,22). A progressive transformation from the (0 k 0)
brownmillerite reﬂections to the (0 0 l ) reﬂections of the inﬁnitelayer structure CaFeO2 was observed during the reduction of
the cp(b)-axis-oriented brownmillerite CaFeO2.5 thin ﬁlms grownon the STO and LSAT substrates (Fig. 4). After reduction for
96 hours, no CaFeO2.5 diffraction peaks were evident and the ﬁlm
became the inﬁnite-layer structure CaFeO2 oriented on the c-axis.
As the positions of the diffraction peaks did not shift during thetransformation, we inferred that there were no intermediateoxygen-deﬁcient phases for CaFeO32d with 0.5 , d , 1.0. Thesechanges are quite similar to those seen when the reduction of acp(b)-axis-oriented SrFeO2.5 thin ﬁlm grown on a KTaO3 substrate
produces SrFeO2 oriented on the c-axis with an inﬁnite-layer structure3. What is surprising here is that, under the same reduction conditions, the ap-axis-oriented CaFeO2.5 thin ﬁlm grown on the LAO
or LSAO substrate also transformed into CaFeO2 oriented on thec-axis with the inﬁnite-layer structure. This transformation resultsfrom the small lattice mismatch of the in-plane lattice of CaFeO2(0 2 0)

CaFeO2.5

As-deposited
STO

Reduced for 24 h

Reduced for 48 h

2030

(0 0 2)(cp, ap)(0 1 0)
CaFeO2.5

3.80(0 0 1)CaFeO2Out-of-plane lattice constant (Å)

3.90

to the substrate lattices (Supplementary Table 1). Note also thatthe reduction of the ap-axis-oriented CaFeO2.5 thin ﬁlms wascompleted in less than 24 hours (Supplementary Fig. S1), which ismuch faster than the reduction of the cp(b)-axis-oriented
CaFeO2.5 thin ﬁlms on STO and LSAT. X-ray diffraction measurements in a reciprocal lattice space conﬁrmed that the reductionof CaFeO2.5 oriented on the ap-axis did not affect the epitaxial
character of the c-axis-oriented CaFeO2 thin ﬁlm (Supplementary
Fig. 2). The in-plane and out-of-plane lattice constants of theinﬁnite-layer CaFeO2 we obtained were, respectively, 3.87–3.93 Å
and 3.35–3.38 Å, and the volumes of the reduced ﬁlms decreased
signiﬁcantly during the reduction process (Fig. 3). We therefore
conclude that the c-axis-oriented, single-crystalline CaFeO2 thinﬁlms of inﬁnite-layer structure were obtained regardless of theorientation of the precursor CaFeO2.5 thin ﬁlms (Fig. 5).
The result raises important questions as to how the low-temperature
oxygen diffusion proceeds in oxides. When we consider the fairly
large lateral size of the ﬁlm sample 60 nm thick (5 mm  5 mm),
it is reasonable to conclude that oxygen atoms are released mainly
from the ﬁlm surface. The oxygen diffusion thus proceeds by
chain reaction perpendicular to the ﬁlm surface using regularvacancy sites in the brownmillerite structure and vacant positions
produced by the oxygen release. Two possible reaction processeson the atomic scale can be deduced. In cp(b)-axis-oriented
CaFeO2.5 thin ﬁlms on STO and LSAT, oxygen ions should go
through the alternate stacking layers of tetrahedra and octahedra
in the brownmillerite structure (Fig. 5c). In ap-axis-oriented
CaFeO2.5 ﬁlms on LAO and LSAO, in contrast, oxygen ionsdiffuse along the ap or bp axis, that is the lateral direction of the tetrahedral and octahedral layers (Fig. 5d). The oxygen rearrangementin the ap-axis-oriented CaFeO2.5 ﬁlms on LAO and LSAO is thatwhich Hayward and Rosseinsky outlined for the change from theperovskite SrFeO3 to the inﬁnite-layer SrFeO2 (ref. 23): one of theoxygen ions in the FeO4 tetrahedron moves to a vacant site in
the octahedron to form the square-planar coordinated FeO4 in theinﬁnite-layer structure SrFeO2. Our experimental results on the
cp-axis-oriented CaFeO2.5 thin ﬁlm grown on STO or LSAT

Log intensity (a.u.)

LSAO
LAO
LSAT
STO
(3.756 Å) (3.793 Å) (3.868 Å) (3.905 Å)

Reduced for 96 h

40
2θ (degree)

50

Figure 4 | Progressive change of X-ray diffraction patterns during the
reduction of a CaFeO2.5 thin ﬁlm grown on the SrTiO3 substrate. The
peaks from brownmillerite CaFeO2.5 are shown in blue and those frominﬁnite-layer CaFeO2 in red. A transformation from the brownmillerite
CaFeO2.5 into the inﬁnite-layer structure CaFeO2 occurred. The ﬁlm became
the c-axis-oriented, inﬁnite-layer structure CaFeO2 after reduction for 96
hours. The positions of the diffraction peaks do not shift during thetransformation, so no intermediate oxygen-deﬁcient phases for CaFeO32d
with 0.5 , d , 1.0 exist.

NATURE CHEMISTRY | VOL 2 | MARCH 2010 | www.nature.com/naturechemistry© 2010 Macmillan Publishers Limited. All rights reserved.

215

ARTICLES

NATURE CHEMISTRY DOI: 10.1038/NCHEM.547
b(0 0 2)(0 0 1)a

Log intensity (a.u.)

STO

LSAT

c

LAO
cp
LSAO

20

30

c

d

40
2θ (degree)

50
ap

c

Figure 5 | X-ray diffraction patterns of CaFeO2 thin ﬁlms and schematic crystal structure of the ﬁlms on the substrates. a, X-ray diffraction patterns of
reduced inﬁnite-layer structure CaFeO2 thin ﬁlms on STO, LSAT, LAO and LSAO. The peaks from inﬁnite-layer CaFeO2 are shown in red. The data for ﬁlms on
STO and LSAT were obtained after reduction for 96 hours and those for ﬁlms on LAO and LSAO were obtained after reduction for 24 hours. b, Schematic
crystal structure of the c-axis-oriented, inﬁnite-layer structure CaFeO2 on each substrate of STO, LSAT, LAO or LSAO. c, Schematic model of the oxygendiffusion pathway and the rearrangement from tetrahedral to square-planar coordination in FeO4. This model represents the change in ﬁlms on the STO and
LSAT substrates. d, Same as (c), but for the change in ﬁlms on the LAO and LSAO substrates. The oxygen diffusion in the brownmillerite structure is highly
anisotropic; signiﬁcantly higher in the apbp plane (d) than along the cp-axis (c).

clearly demonstrate that the oxygen rearrangement at 240 8C also
proceeds by a different route, one in which two oxygen ions in
the FeO4 tetrahedron are involved to form the square-planar, coordinated FeO4 in CaFeO2 with the inﬁnite-layer structure.
An important difference between the two diffusion pathways
concerns kinetic aspects of release and rearrangement processes inthe brownmillerite-structure framework. The complete reductionof the cp(b)-axis-oriented CaFeO2.5 thin ﬁlm takes about fourtimes longer than that of the the ﬁlm oriented on the ap-axis, so
the activation energy for the release and rearrangement of oxygenions along the cp(b)-axis is much higher than that for the release
and rearrangement of oxygen ions along the ap-axis. This implies
that oxygen diffusion in the brownmillerite structure is highly anisotropic; signiﬁcantly higher in the apbp plane than along the stacking direction of the tetrahedral and octahedral layers.

Methods
The brownmillerite CaFeO2.5 precursor ceramic target was prepared in a solid-state
reaction of CaCO3 and Fe2O3. A mixture of the raw materials was ﬁred at 1200 8C in
air for 36 hours with intermediate grinding. The CaFeO2.5 precursor thin ﬁlms were
deposited on substrates by pulsed laser deposition using a KrF excimer laser pulse(l ¼ 248 nm) (COHERENT COMPex-Pro 205 F). The oxygen partial pressureduring the depositions was 1025 torr, and the substrate temperature was monitoredwith a pyrometer and kept at 675 8C. Single-crystal substrates of SrTiO3 (001),(La0.3Sr0.7)(Al0.65Ta0.35)O3 (001), LaAlO3 (001) and LaSrAlO4 (001) were used inthis study. Thicknesses of the deposited ﬁlms were estimated by Raue fringe in X-ray
diffraction measurements as 60 nm. The deposited CaFeO2.5 thin-ﬁlm samples
were embedded with about 0.25 g of CaH2 powder in glass tubes in an argon-ﬁlled
glove box and the tubes were sealed under vacuum conditions, after which they were
kept at 240 8C for 8–96 hours. The residual products and unreacted CaH2 on theﬁlm surface were removed by rinsing in 2-butanone. Crystal structures of the ﬁlms
were examined by 2u2v scans and reciprocal space mappings of X-ray diffraction
with Cu-Ka radiation. A four-circle diffractometer (PANalytical X’Pert MRD) wasused for the measurement, and diffraction intensities in a reciprocal space (Q100 ,Q001) area were measured.

Received 26 June 2009; accepted 4 January 2010;
published online 7 February 2010
216

References
1. Tsujimoto, Y. et al. Inﬁnite-layer iron oxide with a square-planar coordination.
Nature 450, 1062–1065 (2007).
2. Tassel, C. et al. Stability of the inﬁnite layer structure with iron square planar
coordination. J. Am. Chem. Soc. 130, 3764–3765 (2008).
3. Inoue, S. et al. Single-crystal epitaxial thin ﬁlms of SrFeO2 with FeO2 ‘inﬁnite
layers’. Appl. Phys. Lett. 92, 161911 (2008).
4. Tassel, C. et al. CaFeO2: a new type of layered structure with iron in a distorted
square planar coordination. J. Am. Chem. Soc. 131, 221–229 (2009).
5. Hayward, M. A. & Rosseinsky, M. J. Anion vacancy distribution and magnetismin the new reduced layered Co(II)/Co(I) phase LaSrCoO3.5–x. Chem. Mater. 12,
2182–2195 (2000).
6. Hayward, M. A. et al. The hydride anion in an extended transition metal oxide
array: LaSrCoO3H0.7. Science 295, 1882–1884 (2002).
7. Blundred, G. D., Bridges, A. B. & Rosseinsky, M. J. New oxidation states anddefect chemistry in the pyrochlore structure. Angew. Chem. Int. Edn 43,
3562–3565 (2004).
8. Takeda, Y. et al. Phase relation in the oxygen nonstoichiometric system SrFeOx(2.5 , x , 3). J. Solid State Chem. 63, 237–249 (1986).
9. Hodges, J. P. et al. Evolution of oxygen-vacancy ordered crystal structures in theperovskite series SrnFenO3n21 (n ¼ 2, 4, 8, and 1), and the relationship toelectronic and magnetic properties. J. Solid State Chem. 151, 190–209 (2000).
10. Grenier, J.-C. et al. Electrochemical oxygen intercalation into oxide networks.
J. Solid State Chem. 96, 20–30 (1992).
11. Hayashi, N., Terashima, T. & Takano, M. Oxygen-holes creating different
electronic phases in Fe4þ-oxides: successful growth of single crystalline ﬁlms of
SrFeO3 and related perovskites at low oxygen pressure. J. Mater. Chem. 11,
2235–2237 (2001).
12. Fisher, C. A. J. & Islam, M. S. Mixed ionic/electronic conductors Sr2Fe2O5 andSr4Fe6O13: atomic-scale studies of defects and ion migration. J. Mater. Chem. 15,
3200–3207 (2005).
13. Goodenough, J. B. Oxide-ion electrolytes. Annu. Rev. Mater. Res. 33,
91–128 (2003).
14. Steele, B. C. H. & Heinze, A. Materials for fuel-cell technologies. Nature 414,
345–352 (2001).
15. Ormerod, R. M. Solid oxide fuel cells. Chem. Soc. Rev. 32, 17–28 (2003).
16. Atkinson, A. et al. Advanced anodes for high-temperature fuel cells. Nature
Mater. 3, 17–27 (2004).
17. Paulus, W. et al. Lattice dynamics to trigger low temperature oxygen mobility insolid oxide ion conductors. J. Am. Chem. Soc. 130, 16080–16085 (2008).

NATURE CHEMISTRY | VOL 2 | MARCH 2010 | www.nature.com/naturechemistry© 2010 Macmillan Publishers Limited. All rights reserved.

ARTICLES

NATURE CHEMISTRY DOI: 10.1038/NCHEM.547
18. Kendrick, E., Kendrick, J., Knight, K. S., Iskam, M. S. & Slater, P. R. Cooperative
mechanisms on fast-ion conduction in gallium-based oxides with tetrahedral
moieties. Nature Mater. 6, 871–875 (2007).
19. Garcia-Barriocanal, J. et al. Colossal ionic conductivity at interfaces of epitaxial
ZrO2:Y2O3/SrTiO3 heterostructures. Science 321, 676–680 (2008).
20. Redhammer, G. J., Tippelt, G., Roth, G. & Amthauer, G. Structural variations in
the brownmillerite series Ca2(Fe22xAlx)O5: single-crystal X-ray diffraction at
25 8C and high-temperature X-ray powder diffraction (25 8C  T  1000 8C).
Am. Mineral. 89, 405–420 (2004).
21. Rossell, M. D. et al. Structure of epitaxial Ca2Fe2O5 ﬁlms deposited on different
perovskite-type substrates. J. Appl. Phys. 95, 5145–5152 (2004).
22. Kawai, M. et al. Reversible changes of epitaxial thin ﬁlms from perovskite
LaNiO3 to inﬁnite-layer-structure LaNiO2. Appl. Phys. Lett., 94, 082102 (2009).
23. Hayward, M. A. & Rosseinsky, M. J. Cool conditions for mobile ions. Nature
450, 960–961 (2007).

Acknowledgements
We thank K. Yoshimura for his support during this work, and K. Matsumoto and A.
Sakaiguchi for discussions. This work was supported in part by Grants-in-Aid for ScientiﬁcResearch (19GS0207 and 19052004), by the Global Centers of Excellence Program‘International Center for Integrated Research and Advanced Education in Materials
Science’ and by a grant for the Joint Project of Chemical Synthesis Core ResearchInstitutions from the Ministry of Education, Culture, Sports, Science and Technology ofJapan. The work was also partly supported by a European Master programme, Master inMaterials Science Exploiting European Large Scale Facilities.

Author contributions
S.I. and Y.S. conceived and designed the study. S.I. and M.K. performed the experimentswith the help of N.I. H.K. and W.P. contributed the main discussion on oxygen diffusion iniron oxides. All of the authors discussed the results. S.I. and Y.S. wrote the manuscript withcomments from H.K. and W.P.

Additional information
The authors declare no competing ﬁnancial interests. Supplementary informationaccompanies this paper at www.nature.com/naturechemistry. Reprints and permission
information is available online at http://npg.nature.com/reprintsandpermissions/.
Correspondence and requests for materials should be addressed to Y.S.

NATURE CHEMISTRY | VOL 2 | MARCH 2010 | www.nature.com/naturechemistry© 2010 Macmillan Publishers Limited. All rights reserved.

217