Vol 450 | 13 December 2007 | doi:10.1038/nature06382

LETTERSInfinite-layer iron oxide with a square-planar
coordination
Y. Tsujimoto1, C. Tassel1,2, N. Hayashi3, T. Watanabe1, H. Kageyama1, K. Yoshimura1, M. Takano4,5, M. Ceretti2,
C. Ritter6 & W. Paulus2

Conventional high-temperature reactions limit the control of
coordination polyhedra in transition-metal oxides to thoseobtainable within the bounds of known coordination geometries
for a given transition metal1. For example, iron atoms are almost
exclusively coordinated by three-dimensional polyhedra such as
tetrahedra and octahedra. However, recent works have shown thatbinary metal hydrides act as reducing agents at low temperatures,
allowing access to unprecedented structures2–4. Here we show thereaction of a perovskite SrFeO3 with CaH2 to yield SrFeO2, a newcompound bearing a square-planar oxygen coordination aroundFe21. SrFeO2 is isostructural with ‘infinite layer’ cupric oxides5–8,
and exhibits a magnetic order far above room temperature inspite of the two-dimensional structure, indicating strong in-layer
magnetic interactions due to strong Fe d to O p hybridization.
Surprisingly, SrFeO2 remains free from the structural instability
that might well be expected at low temperatures owing to twofold
orbital degeneracy in the Fe21 ground state with D4h point
symmetry. The reduction and the oxidation between SrFeO2
and SrFeO3 proceed via the brownmillerite-type intermediate
SrFeO2.5, and start at the relatively low temperature of 400 K,
making the material appealing for a variety of applications, including oxygen ion conduction, oxygen gas absorption and catalysis.
The coordination number in ionically bonded structures isgoverned by the relative size of oppositely charged ions, which to afirst approximation may be regarded as charged rigid spheres(Pauling’s first rule)1. Each ion ‘tries’ to surround itself symmetricallywith the largest possible number of oppositely charged neighbours.
Transition metal ions normally have, owing to their relatively small
ionic radii, a preference for tetrahedral and octahedral coordination,
as found typically in perovskites and spinels.
For an octahedral coordination there is a pronounced effect on thestereochemistry, or a substantial deviation from ideal geometry, for
the Jahn–Teller ions with (eg)1 or (eg)3 configuration, such as Cr21,
Mn31(d 4) and Cu21, Ni1(d 9), in which the lengthening of a pairof bonds perpendicular to the equatorial plane gives a tetragonal
distortion (c/a . 1) and results ultimately in a square-planar coordination1. Among many copper oxides with square planar geometry,the series ACuO2 (where A 5 Sr, Ca)5,6 are known as the ‘‘infinitelayer compounds’’, or the mother structure of high-transitiontemperature (Tc) superconductors7,8, which consist of a sequenceof infinitely repeating stacking of CuO2 square lattices. The isostructural phase of LaNiO2 is formed naturally because monovalent nickel
is isoelectronic with divalent copper9. Unusual coordination beyond
the constraint of the ionic model may occur in association withcovalent bonding, which is highly directional, as realized, forexample, in many silicates and sulphides10. A successful approachfor exploring unusual coordinations in molecular compounds isthe use of organic ligands with specific steric constraints11,12. Thiscannot be applied, however, to nonmolecular inorganic solids.
Iron, one of the richest elements in the Earth, forms an uncountable number of oxides, some of which have been widely used inindustry as low-cost ferrite magnets and pigments. In almost all ofthem, the iron ions are tetrahedrally or octahedrally coordinated. To
the best of our knowledge, the only example of square-planar coordination is represented by the mineral gillespite BaFeSi4O10 (ref. 13).
However, the iron atoms in this oxide are dispersed separately within
the building blocks made of four-membered rings of SiO442 tetrahedra, rendering the electromagnetic properties of minor interest.
Ordered perovskites CaFe3Ti4O12 and CaFeTi2O6 have also been
discussed as synthetic examples of the square planar geometry14,
but they actually have four short (,2.0 Å) and four long (,2.8 Å)
Fe–O bonds, resulting in a three-dimensional environment.
Here we show a low-temperature route for the topotactic synthesisof a new infinite-layer iron oxide, SrFeO2 (Fig. 1b), using an easy-toprepare, slightly oxygen-deficient perovskite SrFeO3 2 y (y < 0.125)
as a precursor (Fig. 1a). The formation of the strongly anisotropic
framework of SrFeO2 is remarkable in that (1) the Jahn–Teller effectshould not be of essential importance for divalent iron ions, (2) suchan unusual geometry is obtained for a simple ionic compound, and(3) unlike BaFeSi4O10, the FeO2 layers form the primary building
blocks, making it a quasi-two-dimensional magnet. Furthermore,
the oxygen content in AFeO3 2 y (A 5 Sr, Ca) has been taken for
granted to be in the range of only 0 # y # 0.5, and thus the brownmillerite phase SrFeO2.5 (y 5 0.5) (see Supplementary Fig. 1),
consisting of alternative layers of FeO6 octahedra and FeO4 tehrahedra, was historically assumed to represent the lower limit of oxygenstoichiometry. This is in spite of extensive experimental efforts tocontrol oxygen content that include synthesis at high temperature
with oxygen partial pressures ranging from 1029 to 100 MPa (refs 15,16), electrochemical reactions in aqueous solution at room temperature17, and the fabrication of epitaxial films under different
atmospheres18.
Our approach follows the recent success in the use of hydrides ofelectropositive metals as powerful reducing agents2–4,19–22 such as
NaH and CaH2. For example, using NaH yields LaSrCoO3.38 from
LaSrCoO4 (ref. 2) and YSr2Mn2O5.5 from YSr2Mn2O7 (ref. 19), andusing CaH2 yields LaSrCoO3H0.7 from LaSrCoO4 (ref. 3),
Yb2Ti2O6.43 from Yb2Ti2O7 (ref. 4), and La3Ni2O6 from La3Ni2O7(ref. 20). These metal hydrides, routinely used as drying agents inorganic synthesis, are now regarded as promising reducing agentsfor nonmolecular compounds to yield unusual frameworks andcoordinations in nonmolecular solids, because they are active at

1
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502, Japan. 2University of Rennes 1, Sciences Chimiques de Rennes UMR CNRS 6226,
Campus de Beaulieu Bâtiment 10B, Rennes cedex 35042, France. 3Graduate School of Human and Environmental Studies, Kyoto University, Sakyo, Kyoto 606-8501, Japan. 4Institutefor Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan. 5Research Institute for Production Development, 15 Morimoto, Shimogamo, Sakyo, Kyoto 606-0805, Japan.
6
Institute Laue Langevin, BP 156, 38042, Grenoble, France.

1062©2007 Nature Publishing GroupLETTERS

NATURE | Vol 450 | 13 December 2007

Fe, which are in excellent agreement with the expected valences of 12
for both. The elemental analysis by energy dispersive spectroscopy(EDS) before and after the reduction gave the same molar ratio of
Sr:Fe 5 1:1.
The neutron powder diffraction (NPD) patterns of SrFeO2 at
293 K (Fig. 2a) confirmed the above structure with an excellent convergence (Rwp 5 4.70% and x2 5 3.18). They also excluded a possible
incorporation of hydrogen into the structure, and revealed the presence of the (p, p, p) antiferromagnetic order, where the magnetic
moments are perpendicular to the c axis (Fig. 2b). This is the samespin structure as that of the undoped, antiferromagnetically orderedmother phase of high-Tc copper oxide superconductors. The magnetic moment has been found to be 3.1mB per Fe atom at 293 K and
3.6mB at 10 K (Supplementary Fig. 3). The magnitude itself and its
minor variation over the wide temperature range strongly suggestthat the ferrous ions are in the high-spin state of (dxz, dyz)3(dxy)1(dz 2 )1(dx 2 {y 2 )1 with S 5 2 (where S is the magnitude of spin) andthat the antiferromagnetic transition temperature is considerably
higher than room temperature. The magnetic order at ambient temperature was also confirmed by 57Fe Mössbauer spectroscopy at
285 K (Fig. 3b) showing six well-defined peaks with a fairly large
hyperfine field of 40.1 T.
600

a

293 K
500
Intensity (arbitrary units)

considerably lower temperatures than when conventional techniques
are used. They also allow solution chemistry to be avoided. However,their potential for the synthesis of new nonmolecular compounds
still seems far from realization.
The powder X-ray diffraction (XRD) patterns of the precursor
phase SrFeO2.875 (that is, Sr8Fe8O23) were assigned as the nearly
cubic perovskite phase with ap < 3.86 Å, though it in fact had a tetragonal 2ap!2 3 2ap!2 3 2ap supercell (I4/mmm, a 5 10.929 Å andc 5 7.698 Å), consistent with the literature15,16. On the other hand,the synchrotron XRD patterns of the final product (Supplementary
Fig. 2) were readily indexed assuming the tetragonal unit cell to havea 5 3.99107(3) Å and c 5 3.47481(5) Å, which is completely different
from those of any reported SrFeO3 2 y (0 # y # 0.5) phases. No extra
diffraction lines were detected. Compared with the lattice parameters(ap < 3.86 Å) of the precursor, the a axis of the final product isslightly increased, but the c axis is drastically decreased. This implies
an anisotropic extraction of oxygen atoms located originally on thec axis.
Furthermore, the similarity of the lattice parameters to those of
SrCuO2 (a 5 3.926 Å and c 5 3.432 Å; ref. 6) and also the fact that no
specific extinction rules for reflections could be determined from thediffraction pattern, strongly suggest that the two phases are isostructural in the space group of P4/mmm. The Rietveld refinement fromthe synchrotron data immediately converged to Rwp 5 6.04% andx2 5 4.41 along with reasonable individual, isotropic displacement
factors for all the atoms, suggesting successful structural analysis. The
refinement of the site occupancy factor for oxygen atoms gave
0.994(8), indicating that the occupation of the oxygen (2f) positionis unity within the standard deviation. Inclusion of oxygen atoms
into the apical (1b) site did not improve the refinement. The bondvalence sum calculations gave valences of 11.92 for Sr and 11.97 for

400
300
200
100

a

0

20

40

60

80
100
2θ (degrees)

120

140

b

c

b

b

Figure 1 | Structural transformation via a topotactic route. a, The cubic
perovskite SrFeO3. b, The infinite-layer compound SrFeO2. Iron, strontium
and oxygen atoms are represented as yellow, blue and red spheres,
respectively. For the sake of simplicity, an idealized and stoichiometric cubic
phase, obtainable under high oxygen pressure, is demonstrated in a, insteadof the distorted, slightly oxygen-deficient phase SrFeO3 - y (y < 0.125) used inthis study. The iron coordinations by oxygen atoms are illustrated byoctahedra in a and squares in b.

a

Figure 2 | Structural characterization of SrFeO2 by Rietveld refinement ofhigh-resolution neutron diffraction at room temperature. a, The solid lines
and overlying crosses indicate the calculated and observed intensities. The
green lines are the positions of the calculated nuclear (top) and magnetic(bottom) Bragg reflections. The difference between the observed andcalculated profiles is plotted at the bottom in blue. SrFeO2 adopts the
P4/mmm space group, a 5 3.991(1) Å, c 5 3.474(1) Å, Sr on 1d (0.5, 0.5, 0.5),
Fe on 1a (0, 0, 0) and O on 2f (0.5, 0, 0), with 100% occupancy,
Biso(Sr) 5 0.47(5) Å2, Biso(Fe) 5 0.47(4) Å2, Biso(O) 5 0.79(5), Rp 5 3.82%,
Rwp 5 4.70%, x2 5 3.18, RBragg 5 4.19%, Rmag 5 9.19% (see Methods fordefinitions). b, The magnetic structure with a 2ap 3 2ap 3 2cp magnetic unit
cell of sides of length a, b and c, where iron sites are drawn. Arrows denote thedirection of the magnetic moment.
1063©2007 Nature Publishing GroupLETTERS

NATURE | Vol 450 | 13 December 2007

The temperature variations of the magnetic NPD peak intensity
and the hyperfine field nicely coincide with each other (see Fig. 3aand c, and Supplementary Table 1), giving the Néel temperature
TN 5 473 K. The quadrupole interaction appearing in the magnetically split Mössbauer spectrum, which is equal to S1 2 S2 in Fig. 3b, isalmost temperature-independent (for example, 1.16 mm s21 at
285 K), and is very close to the quadrupole splitting in the paramagnetic state, DE (Fig. 3b), of 1.06 mm s21. This proximity and the factthat the electric field gradient is uniaxial along the c axis by symmetry
indicated that the magnetic hyperfine field lies in the a–b plane23,
consistent with the NPD results.
In spite of the apparent two-dimensionality in magnetism, the TN
is considerably higher than that (,200 K) of FeO having the rocksalt-type, three-dimensionally extended linear Fe–O–Fe bonding.
This shows that SrFeO2 has fairly large in-plane exchange constants

2θ

(d

eg
r

ee

s)

a

b
Relative transmission

473 K

∆E
285 K

S2

S1
–10

–5

0
Velocity (mm s–1)

5

500
400

3

300
2
200
1

Hhf (kOe)

Magnetic moment, µB

c 4

10

100

0
0

100

200
300
Temperature (K)

400

0
500

Figure 3 | Temperature evolution of the magnetic order in SrFeO2. a, Thein situ NPD profiles upon warming. The peak at 2h 5 36ucorresponds to the(1/2, 1/2, 1/2) magnetic reflection. b, Mössbauer spectra at 285 and 473 K.
The red lines indicate the spectra from SrFeO2, while the blue line at 473 K
represents the spectrum from a small amount of SrFeO2.5. The black line is
the total fit. c, Temperature dependence of the magnetic moment and thehyperfine field Hhf determined by NPD (open circles) and Mössbauer
spectroscopy (solid circles), respectively. The solid curve is the theoretical
Brillouin function for S 5 2. Error bars are s.d., determined by the Rietveld
refinement.

owing to strong in-plane Fe dx 2 {y 2 to O px,y hybridization. Anotheraspect of the strong hybridization is the isomer shift (,0.5 mm s21 atroom temperature), which is located in the extreme covalent limit fora high-spin divalent iron23. We also point out that the electronic
configuration should be a twofold orbital degenerate ground stateof (dxz, dyz)3 (ref. 1), because all iron atoms in SrFeO2 are in a highspin state with D4h point symmetry. Remarkably, SrFeO2 is free frominstabilities such as orbital ordering or Jahn–Teller distortion even at
the temperatures of 10 and 4.2 K, as derived from NPD andMössbauer spectroscopy, respectively. We believe that the orbital
instability is overcome by the extremely strong covalency that favours
directional and symmetrical Fe–O bonding. Therefore, by applying
the present synthetic approach to other iron oxides, such as conventional ferrites, we should be able to obtain novel magnetic materials
containing Fe21 ions in square-planar geometry and thus having
greater magnetic anisotropy and higher transition temperatures.
SrFeO3 2 y and related iron perovskite oxides have been intensely
studied, because they exhibit fast oxygen transport combined withhigh electron conductivity even at low temperatures17. They are thus
potential candidates for applications as electrodes for solid oxide fuelcells and batteries24, membranes for oxygen separation25 and electrocatalysis26 and gas sensors27. In addition, their unusual properties
include aspects of vacancy orderings (at y 5 0, 0.125, 0.25 and0.5)15–17, charge disproportionation28, giant magnetoresistance29,
helical antiferromagnetic spin structure30, and high-spin to low-spintransition28. Here we have shown that the reduction of SrFeO2.875
does not necessarily stop at the brownmillerite structure SrFeO2.5,
but goes beyond this stoichiometry to form a terminal phase with an
infinite number of FeO2 layers. This means that CaH2 can provide abalanced reducing potential high enough to produce SrFeO2 but notso high that it would result in over-reduction to Fe metal. Startingfrom SrFeO3 2 y, a one-day reaction in the ranges 473 K , T , 523 K,
523 K , T , 673 K and 673 K , T produces SrFeO2.5, SrFeO2 and
SrO–Fe mixture, respectively, whereas a one-week reaction at
473 K yields only SrFeO2, demonstrating ever more control of reducing power by varying synthetic temperature and time.
We note that the reduction from SrFeO2.5 to SrFeO2 is not a naive
topotactic reduction because it involves the filling of the originallyvacant sites within the tetrahedral layers of SrFeO2.5. Upon heating
in an oxygen atmosphere of 0.1 MPa, an opposite reaction backup to SrFeO2.875 via SrFeO2.5 takes place (Supplementary Fig. 4).
Surprisingly, both the reduction and the oxygen uptake proceed attemperatures as low as ,400 K. This implies not only that oxygen ishighly mobile in solids at low temperatures, but also that a given
dense framework can rearrange towards new oxygen-ordered structures, which may be useful for solid oxide fuel cells, oxygen membranes and sensor materials oriented towards the reduction ofworking temperatures. It may also be of interest to investigate holeor electron-doping into the infinite-layer FeO2 sheets—for example,
by introducing selectively apical oxygen atoms or by replacing Sr sites
with a monovalent metal like Na.
METHODS SUMMARY
The reduction of SrFeO2.875 was performed using CaH2 as a reducing agent, as
described for LaSrCoO4 (ref. 3). SrFeO2.875 and a two-molar excess of CaH2 were
finely ground in an Ar-filled glove box, sealed in an evacuated Pyrex tube, andreacted at 553 K for two days. The residual CaH2 and the CaO byproduct were
removed from the final reaction phase by washing them out with a NH4Cl/
methanol solution. Chemical analyses were based on EDS and thermogravimetry. The synchrotron powder XRD experiment was performed on the large
Debye–Scherrer camera installed at the SPring-8 beam line BL02B2 by using an
imaging plate as a detector. The wavelength of the X-ray is 0.77747 Å. Ex situ
NPD studies were carried out on the D1A diffractometer, installed at theInstitute Laue Langevin. The wavelength of l 5 1.91 Å was used. The in situ
NPD profiles upon warming in dynamic vacuum was carried out on the D1B
diffractometer installed at the Institute Laue Langevin, where l 5 2.52 Å wasused. 57Fe Mössbauer spectra were taken under dynamical vacuum using a
57
Co/Rh source and a control absorber of a-Fe.

1064©2007 Nature Publishing GroupLETTERS

NATURE | Vol 450 | 13 December 2007

Full Methods and any associated references are available in the online version of
the paper at www.nature.com/nature.
Received 3 July; accepted 5 October 2007.
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Supplementary Information is linked to the online version of the paper at
www.nature.com/nature.
Acknowledgements We thank Y. Kiuchi, H. Ueda, M. Isobe and Y. Ueda for their
help in EDS and thermogravimetric measurements and K. Kato for his help in thesynchrotron X-ray experiments at SPring-8. This work was supported by Young
Scientists A (H.K.), the Grant-in-Aid for Scientific Research on Priority Areas (H.K.
and K.Y.) and Scientific Research S (M.T.) from MEXT. See the Supplementary
Notes for more details.
Author Contributions H.K. designed the study in collaboration with W.P, with
M.T.’s help; C.T. performed the initial synthesis and proposed the structural model;
Y.T. and T.W. optimized the synthetic conditions, performed chemical
characterizations, X-ray diffraction measurements and corresponding structuralrefinement; N.H. conducted the Mössbauer experiment, with M.T.’s help; M.C.,
C.R. and W.P. performed the neutron diffraction measurements and M.C. and W.P.
performed the corresponding structural refinement; All the authors discussed theresults; H.K. wrote the manuscript, with comments mainly from M.T. and W.P.
Author Information Atomic coordinates and structure factors for the crystal
structure of SrFeO2 have been deposited with the ICSD database under accession
codes 418603, 418605 and 418606. Reprints and permissions information isavailable at www.nature.com/reprints. Correspondence and requests formaterials should be addressed to H.K. (kage@kuchem.kyoto-u.ac.jp).

1065©2007 Nature Publishing Groupdoi:10.1038/nature06382

METHODSA precursor SrFeO2.875 (y < 0.125) was prepared by a conventional hightemperature ceramic method from predried SrCO3 (99.99%) and Fe2O3(99.99%). Stoichiometric amounts of SrCO3 and Fe2O3 were ground together,heated at 1,273 K in air, ground again, and heated for an additional 24 h at
1,473 K. The reduction of SrFeO2.875 was performed using CaH2 as a reducing
agent. SrFeO2.875 (0.45 g) and a two-molar excess of CaH2 (0.2 g) were finely
ground in an Ar-filled glove box, sealed in an evacuated Pyrex tube (volume
15 cm3) with a residual pressure less than 1.3 3 1028 MPa, and reacted at 553 K
for two days. The residual CaH2 and the CaO byproduct were removed from thefinal reaction phase by washing them out with 0.1 M NH4Cl in dried methanol.
The EDS experiments were carried out for the precursor and the final productusing a JEOL (JSM-5600) scanning electron microscope equipped with an EDAX
(Oxford Link ISIS) microanalytical system. Thermogravimetric measurements
were performed using a Mac Science thermal analyser (TG-DTA2000). Measurements to analyse re-oxidation behaviour of SrFeO2 were performed on a sample
of around 20 mg that was rapidly loaded into an aluminium crucible and thenheated at 10 K min21 under flowing O2 (0.1 MPa). Before the experiment, thesample was dried at 373 K for about 1 hour. The identity and oxygen stoichiometry of the re-oxidized products were determined by XRD.
The synchrotron powder diffraction experiment was performed on the large
Debye–Scherrer camera installed at SPring-8 BL02B2 by using an imaging plate
as a detector. Incident beams from a bending magnet were monochromatized to
0.77747 Å. The sample was contained in a glass capillary tube with an innerdiameter of 0.1 mm and was rotated during measurements. The synchrotron
X-ray diffraction data were collected at room temperature in a 2h range from 1u
to 75u with a step interval of 0.01u.
The ex situ neutron powder diffraction studies were carried out on the D1A
diffractometer, installed at the Institute Laue Langevin (Grenoble, France). A
200 mg sample sealed in a He-filled vanadium can was used and a wavelength ofl 5 1.91 Å was used. The in situ neutron powder diffraction experiments of
SrFeO2 upon warming in dynamic vacuum (pO2 , 1.3 3 10210 MPa) were carried out using the Institute Laue Langevin vacuum furnace on the D1B diffractometer installed at Institute Laue Langevin, where l 5 2.52 Å was used. The
temperature was varied from 270 to 500 K with a 0.5 K min21 ramp rate. The
temperature stability and average temperature were determined using a thermocouple placed at the sample position.
Mössbauer spectra of SrFeO2 were taken under a dynamical vacuum, and thedata were collected in transmission geometry by using a 57Co/Rh c-ray source at
low temperature in combination with a constant-acceleration spectrometer. The
source velocity was calibrated by using pure a-Fe as a control material. The lowtemperature measurements were carried out using a cryostat, while the hightemperature spectra were taken with a small amount of CaH2 that was mixed
with the SrFeO2 powder to minimize re-oxidation of SrFeO2 into the brownmillerite phase SrFeO2.5 upon being heated. The obtained spectra were fitted by alorentzian function. The subspectrum of SrFeO2.5 observed at high temperatures
was compared with that of ref. 31.
The X-ray and neutron diffraction patterns were analysed by the Rietveld
33, respecmethod using the RIETAN 2000 (ref. 32) and FULLPROF software
P
P
5
jy
–
yio,
tively. The agreement indices
used
were
the
profile,Rp
io yicj/
P
P
2
2 1/2
/ wi(yio) ] and the goodness of fit,weighted profile, Rwp 5 [ wi(yio – yic) P
x2 5 [Rwp/Rexp]2 where Rexp 5 [(N – P)/ wiyio2]1/2, yio and yic are the observedand calculated intensities, wi is the weighting factor, N is the total number of yio
data when the background is refined, andP
P is the number
P of adjusted parameters. RBragg and Rmag are the R factors, jIko – Ikcj/ jIkoj, for nuclear andmagnetic peaks, respectively, where Iko and Ikc are the observed and calculated
integrated intensity. Biso is the isotropic temperature factor. The bond valence
sum method was applied to estimate the valence of cations using tabulatedparameters34.
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neutron powder diffraction. J. Phys. B 192, 55–69 (1993).
34. Brown, I. D. & Altermatt, D. Bond-valence parameters obtained from a systematic
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