APPLIED PHYSICS LETTERS 92, 161911 共2008兲

Single-crystal epitaxial thin films of SrFeO2 with FeO2 “infinite layers”
Satoru Inoue,1 Masanori Kawai,1 Yuichi Shimakawa,1,a兲 Masaichiro Mizumaki,2Naomi Kawamura,2 Takashi Watanabe,3 Yoshihiro Tsujimoto,3 Hiroshi Kageyama,3 andKazuyoshi Yoshimura3
1Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan
Japan Synchrotron Radiation Research Institute/SPring-8, Sayo, Hyogo 679-5198, Japan
3Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502, Japan2共Received 22 February 2008; accepted 2 April 2008; published online 25 April 2008兲
Single-crystal thin films of SrFeO2, which is an oxygen-deficient perovskite with “infinite layers” ofFe2+O2, were prepared by using CaH2 for low-temperature reduction of epitaxial SrFeO2.5
single-crystal films deposited on KTaO3 substrates. This reduction process, removing oxygen ionsfrom the perovskite structure framework and causing rearrangements of oxygen ions, topotacticallytransforms the brownmillerite SrFeO2.5 to the c-axis oriented SrFeO2. © 2008 American Institute ofPhysics. 关DOI: 10.1063/1.2913164兴
Strontium and iron form a number of oxygen-deficientperovskite structure oxides: SrFeO3−␦ with ␦ ranging from 0to 0.5.1 SrFeO3 共␦ = 0兲 with Fe4+O6 octahedra has been synthesized in bulk form at 400– 550 ° C under an oxygen partial pressure of 105 Torr 共Ref. 2兲 and has also been preparedin a thin film form by pulsed laser deposition 共PLD兲 following in situ oxidization with flowing O3.3,4 This strongly oxidizing atmosphere stabilized the unusually high ionic state ofFe4+. SrFeO2.5 共␦ = 0.5兲, on the other hand, has been preparedat ambient pressure and its brownmillerite structure consistsof alternate layers of Fe3+O6 octahedra and Fe3+O4 tetrahedra关Fig. 1共a兲兴. However, it was not possible to produce SrFeO2
共␦ = 1.0兲, a perovskite with Fe2+ ions, by using H2 / Ar gas orelectrochemical reduction techniques5 because the perovskite
structure was not conserved under those reducing conditionsin thermodynamically controlled solid-state reaction
processes.6SrFeO2 was recently synthesized by reducing SrFeO2.875
共Sr8Fe8O23兲 with CaH2 at 280 ° C,7 a temperature too low forstructural rearrangement but high enough that the oxygenions could change their positions. SrFeO2 is an oxygendeficient perovskite and is isostructural with the “infinitelayer” cupric oxides used to make high-TC superconductors.8
Its crystal structure 关Fig. 1共b兲兴 consists of FeO2 planes withcorner-sharing square-planar coordinated FeO4 and at room
temperature, its tetragonal 共P4 / mmm兲 lattice constants area = 3.991 and c = 3.474 Å. The lack of apical oxygen in
SrFeO2 makes its crystal structure highly anisotropic andmakes its magnetism two dimensional, and the high mobilityof the oxygen ions in SrFeO2 would make it a useful material
for the electrodes in solid-oxide fuel cells and batteries.9
Square-planar oxygen coordination around Fe2+ is extremely rare because iron ions are favorably tetrahedrally oroctahedrally coordinated by oxygen irons. To understand thecrystallographic, electronic, and magnetic characteristics ofSrFeO2, we need to investigate them in studies with singlecrystal samples. In this letter, we report the crystal structure
and the ionic state of Fe in single-crystal epitaxial thin filmsof SrFeO2 prepared by ex situ reduction of epitaxially grown
brownmillerite SrFeO2.5 films.
a兲Electronic mail: shimak@scl.kyoto-u.ac.jp.

The SrFeO2.5 precursor ceramic target was first preparedin a solid-state reaction under an oxygen partial pressure of10−4 Torr. The SrFeO2.5 precursor was deposited on aKTaO3 substrate by PLD using a KrF excimer laser pulse
with an energy density of 0.4 J / cm2. KTaO3 single-crystal
substrates were used in this study because the cubic lattice
constant of KTaO3, 3.988 Å, is quite close to the a lattice
constant of SrFeO2. The obtained SrFeO2.5 film samples
were treated in a way similar to that in which the bulk
samples described in Ref. 7 were treated. The bulk samples
were treated with CaH2 in a mixed compressed disk, whereas
each of the present 5 mm square SrFeO2.5 film samples wasembedded with about 0.2 g of CaH2 in an evacuated Pyrex
tube in an Ar-filled glove box. The tube was sealed invacuum and then kept at 280 ° C for 24 h. The residual products, including CaH2, on the film surface were removed bywashing with 2-butanone. The crystal structure of the resultant material was examined by 2␪-␪ x-ray diffraction 共XRD兲
measurement with Cu K␣ radiation. The XRD intensity mapin 共H , L兲 reciprocal lattice space was also measured with aFIG. 1. 共Color online兲 共a兲 SrFeO2.5 brownmillerite structure consisting ofalternate layers of FeO6 octahedra and FeO4 tetrahedra. 共b兲 SrFeO2 infinitelayer structure consisting of corner-sharing planar FeO4 and Sr. Note that theb axis of SrFeO2.5 and the c axis of SrFeO2 are perpendicular to the film
plane.

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161911-2

Inoue et al.

Appl. Phys. Lett. 92, 161911 共2008兲

FIG. 3. 共Color online兲 Logarithmic x-ray intensity reciprocal space maps
around the 共1 0 3兲 Bragg reflection of 共a兲 SrFeO2.5 and 共b兲 the SrFeO2 thinfilms.

FIG. 2. 共Color online兲 XRD patterns of 共a兲 SrFeO2.5 target 共linear scale兲, 共b兲
SrFeO2.5 film grown on KTaO3 共logarithmic scale兲, and 共c兲 reduced SrFeO2film 共logarithmic scale兲.

four-circle diffractometer. Here, the reciprocal-space coordinates correspond to those of the cubic KTaO3 substrate, andthus, H = 1.0 and L = 1.0 correspond to 1 / a 共KTaO3兲
= 1 / 3.988 Å−1. The x-ray absorption spectra 共XAS兲 near theFe K edge were examined at the BL39XU beamline ofSPring-8 in order to see the change in Fe oxidation state.
They were recorded at room temperature with a fluorescencemode.
The SrFeO2.5 precursor ceramic was confirmed to havethe brownmillerite structure by powder XRD 关Fig. 2共a兲兴. The
deposition with the SrFeO2.5 precursor target under an oxygen pressure of 10−5 Torr at 700 ° C yielded b-axis oriented
epitaxial SrFeO2.5 thin films. X-ray reflectivity revealed thatthe films deposited were typically 170– 200 Å thick. In the2␪-␪ XRD pattern shown in Fig. 2共b兲, one sees the 共0 0 l兲reflections of the perovskite structure and the 共0 0 l / 2兲 twofold superstructure reflections due to the ordering of the oxygen defects in the SrFeO2.5 brownmillerite structure. Notehere that the indices are based on the primitive cubic perovskite cell 共a ⬃ 3.9 Å兲 and the original b axis of the brownmillerite structure 共the stacking direction of the layeredstructure兲 is converted to the c axis. An XRD logarithmicintensity map in the reciprocal lattice space around the 共1 0
3兲 Bragg reflection of the KTaO3 perovskite structure isshown in Fig. 3共a兲, where both the KTaO3 Bragg reflection
at 共H , L兲 = 共1.00, 3.00兲 and a diffraction peak from the film at
共H , L兲 ⬇ 共1.00, 3.08兲 are evident. Since the peak positions
along the H and L directions give information on the in-planeand out-of-plane lattices, respectively, the data indicate thatthe in-plane lattice constant of the SrFeO2.5 film is comparable to the KTaO3 cubic lattice and that the out-of-planelattice constant of the film is smaller than that of the KTaO3substrate. Therefore, these XRD results clearly show that thegrowth of the pseudo-tetragonal brownmillerite SrFeO2.5
film on the KTaO3 substrate was epitaxial.
A 2␪-␪ XRD pattern of the CaH2-treated film isshown in Fig. 2共c兲. None of the diffraction peaks originatingfrom SrFeO2.5 are evident but the peaks corresponding tod = 1.75 and 0.87 Å 共2␪ ⬇ 52° and 123°, respectively兲 areevident. The positions of these peaks are close to those of
the 共0 0 2兲 and 共0 0 4兲 reflections of the bulk SrFeO2.
The intensities for the infinite-layer SrFeO2 structurethat were calculated by the XRD intensity simulation
program RIETAN-2000, I002 : I003 : I004 = 10 361: 29: 1874
= 1.0: 0.0028: 0.18, agree well with the observed intensities:
I002 : I003 : I004 ⬇ 1 : 0 : 0.14. Figure 3共b兲 shows a reciprocal lattice map of the reduced thin film sample. In addition to the共1 0 3兲 Bragg reflection of the cubic KTaO3 substrate, thereis a peak at 共H , L兲 ⬇ 共1.00, 3.42兲. The lattice constants obtained from the diffraction data were a = 3.988 andc = 3.493 Å, which are consistent with those reported forpowder samples of SrFeO2.7
XAS spectra near the Fe K edge are shown in Fig. 4 for
the SrFeO2.5 precursor and CaH2-treated thin film samples.
The difference in the shape of their XAS spectra is due to thechanges in the electronic states and local structure of Feions.10 The Fe K absorption edge energy of the reduced thinfilm was about 1.5 eV lower than that of the SrFeO2.5 precursor thin film. This chemical shift indicates that the Feoxidation state in the CaH2-treated thin film sample was reduced to that close to Fe2+, and all of the XRD and XAS
results indicate that the epitaxial SrFeO2.5 thin film was reduced to one of SrFeO2. We can therefore conclude that thereduced thin film obtained is single-crystal SrFeO2 with theinfinite-layer structure. The pre-edge structures shown in theinset of Fig. 4 have never been observed in any iron oxides,and thus, they appear to originate from the characteristicsquare-planar coordination of Fe2+ ions. The details of theanalysis will be published elsewhere.11
The low-temperature reduction used in this study moved
oxygen ions in the perovskite structure framework and removed some from the framework. This process caused theb-axis oriented single-crystal SrFeO2.5 brownmillerite thin

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161911-3

Appl. Phys. Lett. 92, 161911 共2008兲

Inoue et al.

In conclusion, we prepared single-crystal SrFeO2 thinfilms by low-temperature reduction of single-crystal
SrFeO2.5 with CaH2. XRD and XAS results show that theobtained SrFeO2 sample is an oxygen-deficient perovskite
with infinite layers of Fe2+O2. The present results demonstrate that this low-temperature reduction process and thelattice strain of the thin film remove some oxygen ions fromthe perovskite structure and rearrange the others, leading to
the topotactic change from SrFeO2.5 to SrFeO2.
The synchrotron radiation experiments 共Proposal No.
2007A2061兲 were performed with the approval of the Japan
Synchrotron Radiation Research Institute. This work waspartly supported by the Grants-in-Aid for Scientific Research
共Grants Nos. 19GS0207, 18350097, 18655076, 19052004,
and 16076210兲 and by the Joint Project of Chemical Synthesis Core Research Institutions from the Ministry of Education, Culture, Sports, Science and Technology, 共MEXT兲 ofJapan.
1FIG. 4. 共Color online兲 X-ray absorption spectra near the Fe K edge of 共a兲
SrFeO2.5 and 共b兲 SrFeO2 thin films. The inset shows the pre-edge spectrumof SrFeO2.

film to topotactically change into the c-axis oriented SrFeO2
infinite-ayer single crystal, which has an out-of-plane lattice
constant of 9.8% smaller than that of SrFeO2.5 but has almost
the same in-plane lattice structure. In the single-crystal thinfilm sample, the crystal lattice is under strain due to thelattice mismatch to the substrate KTaO3 lattice. Indeed, thein-plane a lattice constant of the single-crystal SrFeO2
共3.988 Å兲 is slightly smaller than that of bulk SrFeO2
共3.991 Å兲. As a result, the observed out-of-plane lattice constant of the single-crystal SrFeO2 共3.493 Å兲 is a bit larger
than that of the bulk material 共3.474 Å兲. This strain shouldplay an important role in the conversion of single-crystal
SrFeO2.5 to single-crystal SrFeO2.

J. P. Hodges, S. Short, J. D. Jorgensen, X. Xiong, B. Dabrowski, S. M.
Mini, and C. W. Kimball, J. Solid State Chem. 151, 190 共2000兲.
2J. B. MacChesney, R. C. Sherwood, and J. F. Potter, J. Chem. Phys. 43,1907 共1965兲.
3
N. Hayashi, T. Terashima, and M. Takano, J. Mater. Chem. 11, 2235
共2001兲.
4
H. Yamada, M. Kawasaki, and Y. Tokura, Appl. Phys. Lett. 80, 622
共2002兲.
5
J. C. Grenier, A. Wattiaux, J. P. Doumerc, P. Dordor, L. Fournes, J. P.
Chaminade, and M. Pouchard, J. Solid State Chem. 96, 20 共1992兲.
6
M. A. Hayward and M. J. Rosseinsky, Nature 共London兲 450, 960 共2007兲.
7
Y. Tsujimoto, C. Tassel, N. Hayashi, T. Watanabe, H. Kageyama, K.
Yoshimura, M. Takano, M. Ceretti, C. Ritter, and W. Paulus, Nature共London兲 450, 1062 共2007兲.
8M. Takano, Y. Takeda, H. Okada, M. Miyamoto, and T. Kusaka, Physica C
159, 375 共1989兲.
9
A. Atkinson, S. Barnett, R. J. Gorte, J. T. S. Irvine, A. J. Mcevoy, M.
Mogensen, S. C. Singhal, and J. Vohs, Nat. Mater. 3, 17 共2004兲.
10
S. J. Roosendaal, B. van Asselen, J. W. Elsenaar, A. M. Vredenberg, and F.
H. P. M. Habraken, Surf. Sci. 442, 329 共1999兲.
11
M. Mizumaki 共unpublished兲.

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