ARTICLESPUBLISHED ONLINE: 20 JULY 2009 | DOI: 10.1038/NCHEM.289

Spin transition in a four-coordinate iron oxide
T. Kawakami1, Y. Tsujimoto2, H. Kageyama2 *, Xing-Qiu Chen3, C. L. Fu3, C. Tassel2, A. Kitada2,
S. Suto4, K. Hirama4, Y. Sekiya4, Y. Makino4, T. Okada5, T. Yagi5, N. Hayashi6, K. Yoshimura2, S. Nasu7,
R. Podloucky8 and M. Takano9
Spin transition has attracted the interest of researchers in various ﬁelds since the early 1930s, with thousands ofexamples now recognized, including those in minerals and biomolecules. However, so far the metal centres in which it has
been found to occur are almost always octahedral six-coordinate 3d 4 to 3d 7 metals, such as Fe(II). A ﬁve-coordinate centre
is only rarely seen. Here we report that under pressure SrFe(II)O2 , which features a four-fold square-planar coordination,
exhibits a transition from high spin (S 5 2) to intermediate spin (S 5 1). This is accompanied by a transition from anantiferromagnetic insulating state to a ferromagnetic so-called half-metallic state: only half of the spin-down (dxz ,dyz)
states are ﬁlled. These results highlight the square-planar coordinated iron oxides as a new class of magnetic andelectric materials.

S

pin transition, or spin crossover, generally occurs in compounds of octahedrally coordinated 3d transition metal ionswith d4, d5, d6 and d7 electronic conﬁgurations, and they are
driven by the competition between the intra-atomic exchange
energy and the crystal ﬁeld energy. The former stabilizes a highspin state, a spin state with a maximum spin multiplicity, as forthe free ion, while the latter stabilizes a low-spin state in which
the electrons occupy low-energy orbitals only at the expense ofincreasing the exchange energy1. These transitions, which can be
induced by external perturbations, such as heat2,3, pressure4–8,
light9, magnetic ﬁelds10, chemical substitution11 and gas adsorption12, are found in a wide range of materials, such as oxides(FeBO3 , CaFeO3) (refs 4,5,7,10), metal–organic complexes1, porous
materials12 and supramolecular systems3, as well as in human haemoglobin13. Some of these materials have found promising applications
as sensors, or as displaying and recording devices. Recent discoveriesof the spin-state transition in magnesiowüstite (Mg,Fe)O (refs 4,5)
and silicate perovskites (Mg,Fe)(Si,Al)O3 (ref. 7) have provided
further understanding of the seismic-wave heterogeneity in the
Earth’s lower mantle. Among the ions that exhibit spin-state transition phenomena, the largest number have been found for the d6 electronic conﬁguration, of which divalent iron(II) represents the
majority. The transition has been observed mostly between thehigh-spin (S ¼ 2) and the low-spin (S ¼ 0) states.
The crystal ﬁeld energy depends on the particular geometricalarrangement of both metal ion and ligands, as well as on themetal–ligand distance. It also depends on properties such as substantial deviations from octahedral symmetry, packing effects incrystal lattices, the thermal volume changes inherent to crystalline
solids and cooperative interactions. For example, it is known thata transition involving an intermediate spin state (S ¼ 1 for d 6 andS ¼ 3/2 for d5) is feasible for ﬁve-fold coordinated compounds,
such as TbBaCo2O5 , with a pyramidal coordination14 andthe (Fe(P4)Br)BPh4.CH2Cl2 complex (P4 ¼ hexaphenyl-1,4,7,10-

tetraphosphadecane, Ph ¼ phenyl) with a trigonal–bypyramidal
coordination1. To the authors’ knowledge, however, up to now
no spin transition has been reported for compounds with a fourcoordinate (or less) metal ion.
A recently synthesized iron(II) oxide, SrFeO2 , as prepared by the
hydride reduction of SrFeO3 at low temperatures, unprecedentedly
has a square-planar coordination of the high-spin iron(II) ion15,16.
As shown in Fig. 1d, this material is built from two-dimensional
FeO2 layers separated by strontium atoms, isomorphic with the socalled inﬁnite-layer structure, as ﬁrst found for (Ca,Sr)CuO2. In
spite of an apparent two-dimensional crystal structure, it undergoes
an antiferromagnetic order at a remarkably high temperature (Néeltemperature, TN ¼ 473 K) into a G-type spin structure, which
results from the strong hybridization between iron dx2 y2 andoxygen ps orbitals, as evidenced by a very low isomer shift in theMössbauer spectrum.
Here we present investigations (57Fe Mössbauer spectroscopy,
electrical resistance, X-ray diffraction and ﬁrst-principle calculations) on effects of pressure on the spin and electronic state in
SrFeO2. These showed that SrFeO2 is the ﬁrst material found tohave a four-fold coordinated metal ion with a spin-state transition.
The transition from high spin to intermediate spin is accompanied
by an insulator-to-metal transition, as well as by an antiferromagnetic-to-ferromagnetic transition.
Typical high-pressure 57Fe Mössbauer spectra of SrFeO2 at room
temperature are shown in Fig. 2a (see Supplementary Fig. S1 fordetails). Below 30 GPa, no signiﬁcant change was observed
within the experimental uncertainties. The hyperﬁne ﬁeld (Hhf ) at
25 GPa, for example, is 42 T, which is nearly identical with that at
ambient pressure15, and indicates that the divalent iron ion
remains in the high-spin state (S ¼ 2) and that TN remains high.
The gradual and linear decrease in the isomer shift with pressure(see Supplementary Fig. S2) is in accordance with the trend
observed in many iron compounds under high pressure1,8, and1

Institute of Quantum Science, Nihon University, Chiyoda, Tokyo 101-8308, Japan, 2 Department of Chemistry, Graduate School of Science, Kyoto University,
Sakyo, Kyoto 606-8502, Japan, 3 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA, 4 Graduate
School of Quantum Science and Technology, Nihon University, Chiyoda, Tokyo 101-8308, Japan, 5 Institute for Solid State Physics, University of Tokyo,
Kashiwa, Chiba 277-8581, Japan, 6 Graduate School of Human and Environmental Studies, Kyoto University, Sakyo, Kyoto 606-8501, Japan,
7
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan, 8 Institute of Physical Chemistry, University of Vienna, Sensengasse 8/7,
Wien, Austria, 9 Institute for Integrated Cell-Material Sciences, Kyoto University, Yoshida Ushinomita-cho, Sakyo, Kyoto 606-0801, Japan.
* e-mail: kage@kuchem.kyoto-u.ac.jp
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.289
a

b

4.0

55
3.8

a, c (Å)

Volume (Å3)

a

3.6

c

3.4

50

45
3.2

40

3.0

0

c

10

20
30
Pressure (GPa)

4.0

0

40

d

AF

10

20
30
Pressure (GPa)

40

56

F
3.8

52

a

3.4

Volume (Å3)

a, c (Å)

3.6

c

48

44

3.2

40

3.0

36

2.8

0

40

80

120

Pressure (GPa)

0

40

80

120

Pressure (GPa)Figure 1 | Lattice anomaly at the spin-state transition. a,b, Experimental lattice parameters a and c (a) and the volume V (b) as a function of pressure.
Standard deviations (error bars) were determined by least-squares ﬁt, and are within the size of the symbols. The solid lines in a are guides to the eye andthat in b is the ﬁt to the third-order Birch–Murnaghan equation of state in the low-pressure phase. c,d, Calculated cell parameters and volume for the S ¼ 2
antiferromagnetic (AF) state and the S ¼ 1 ferromagnetic (F) state. The inset is the crystal structure of SrFeO2 (d), in which blue, green and yellow spheres
represent iron, strontium and oxygen atoms, respectively.

can be interpreted in terms of a gradually increased hybridization ofiron 3d orbitals with the 2p orbitals of the oxygen neighbours.
A drastic change, however, arose in the Mössbauer spectrum at apressure of 38 GPa. Although it also consists of six well-deﬁned
peaks, which indicates the presence of magnetic order at ambient
temperature, the ﬁeld Hhf ¼ 18 T is only about half of that for the
low-pressure phase, which implies the occurrence of a spin-statetransition into an intermediate spin (S ¼ 1) state. This spectralfeature remains up to the maximum pressure of 70 GPa appliedin this study. To conﬁrm the spin-state transition, in other words
to exclude the possibility that the reduction in Hhf is merely aresult of the reduction in TN , we cooled the sample to 7 K. The corresponding spectrum has an only slightly increased Hhf value of
19 T, which provides ﬁrm evidence that the iron ions in thehigh-pressure phase are in the intermediate spin state. The welldeveloped six-line pattern in the intermediate spin state indicatesthat the magnetic-order temperature is still far above room temperature. Subsequently, Mössbauer spectroscopy of the high-pressure
phase was conducted under a magnetic ﬁeld, for which the incident
g-ray was set parallel to the applied magnetic ﬁeld. As shown in
Fig. 2 and Supplementary Fig. S3, the intensities of the second and372

ﬁfth lines (Dm ¼ 0) showed a remarkable decrease with increasing
magnetic ﬁeld and disappeared at 3 T. We thus conclude thatthe high-pressure phase is ferromagnetic. The quadrupole shift(QS ¼ s1 2 s2) changed suddenly from 1.53 mm s–1 (25 GPa) to0.70 mm s–1 (38 GPa) after the transition. Both the initial and the
new spectra coexist in the pressure range 27–37 GPa because of apressure gradient within the sample. However, analysis of therelative spectrum weight and the pressure gradient gave a roughestimate of the critical pressure Pc as 33+3 GPa.
To examine the possibility of structural change on the spin transition, we carried out in situ X-ray powder diffraction experiments
in a diamond anvil cell at pressures up to 43 GPa. The diffraction
data recorded (see Supplementary Fig. S4) are of high quality witha good signal-to-noise ratio, and so allowed an accurate determinationof the lattice parameters. All the peaks of the diffraction pattern ateach pressure could be assigned to a tetragonal inﬁnite-layer structure,and no additional reﬂections were detected within the resolution of
the experiment. Although the measured range of the diffraction
angles is not sufﬁcient to conﬁrm this fully, it is likely that the transition is isostructural. At pressures below the spin transition, thepressure dependence of the volume can be ﬁtted by the third-order

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NATURE CHEMISTRY DOI: 10.1038/NCHEM.289
0.1 MPa
RT
S1

S2

Relative transmission

25 GPaRT

b 0.2

ΔE (eV)a0.1

AF (S = 2)

F (S = 1)

0.0
AF
F

− 0.1
38 GPa
RT

0

20

40

60

80

100

Pressure (GPa)
50 GPa
RT

c 4.0

AF
F

40 GPa230 K
3T

m0 (µB)

3.5
50 GPa
7K

3.0
2.5
2.0
1.5

−10

−5

0
Velocity (mm s−1)

5

10

0

20

40

60

80

100

Pressure (GPa)Figure 2 | Transition from high spin to intermediate spin in SrFeO2 under high pressure. a, Typical high-pressure 57Fe Mössbauer spectra obtained from
SrFeO2 (see Supplementary Fig. 1 for detail). The yellow spectra correspond to the S ¼ 2 state, and the blue to the S ¼ 1 state. The solid lines represent theﬁtted curves. S1 and S2 correspond, respectively, to the split between the ﬁrst and second peaks and that between the ﬁfth and sixth peaks. Velocity scale isrelative to a-Fe at room temperature (RT). b, Calculated energy difference (DE) between the S ¼ 2 antiferromagnetic (AF) state and the S ¼ 1 ferromagnetic(F) state. c, Calculated spin moment (m0) for the S ¼ 2 antiferromagnetic (AF) state and the S ¼ 1 ferromagnetic (F) state.

Birch–Murnaghan equation of state with the bulk modulus
K ¼ 126 GPa, which is slightly smaller than that estimated for the
fully oxidized perovskite SrFeO3 (K ¼ 146 GPa) (ref. 17). As displayedin Fig. 1a, the unit cell parameters a and c decrease smoothlywith the application of pressure, but exhibit a signiﬁcant drop at
33+1 GPa, which agrees well with Pc as estimated from theMössbauer study. Therefore, this anomaly might be associated withthe claimed spin transition. A volume reduction of 3% (Fig. 1b) iscomparable with those for other compounds that exhibit high-spin
to low-spin transitions1.
First-principle calculations based on the hybrid density functional
theory (PBE0) method18,19 were carried out to examine the pressure
dependence of the crystal and electronic structures. The antiferromagnetic S ¼ 2 state is the ground state at atmospheric pressure
and remains stable up to Pc , where the abrupt transition to theferromagnetic S ¼ 1 state occurs (Fig. 2b), in accordance with theexperimental observations. The calculated Pc of 53 GPa (see also
Fig. 1c,d) is, however, rather high in comparison with the experimental Pc of 33 GPa. As shown in Fig. 2c, the calculated spin moment
decreases from 3.5mB to 1.8mB between the antiferromagnetic and ferromagnetic states, which is consistent with the change in Hhf from42 T to 19 T. With respect to the a axis, both theory and experimentindicated a drop of 2%, but for the c axis the changes given by
theory and experiment did not correlate. However, in terms of theunit-cell volume at Pc , the discrepancy between theory and experiment is only 3 Å3 (or 6%).
To understand the nature of the spin transition, we ﬁrst focusedon the antiferromagnetic S ¼ 2 state. The local density of states(DOS) of the iron ion at atmospheric pressure is shown in Fig. 3a.
Consistent with the results of previous ‘LDA þ U’ calculations20,21,
the lone spin-down electron was found to occupy the dz2 orbital.
The double occupation of dz2 allows a considerable reduction of
the intra-atomic Coulomb repulsion20,21. The s-bond formed

between iron dx2 y2 and oxygen px,y is split into the bonding andantibonding states by 7 eV owing to a strong in-layer iron d andoxygen p hybridization. For pressures less than Pc , both thebonding and antibonding states in the spin-up channel are occupied. In the antiferromagnetic S ¼ 2 state, the d6 state has the electronic conﬁguration of ðdz2 Þ2(dxz ,dyz)2(dxy)1ðdx2 y2 Þ1.
We attribute the spin transition to the strong in-layer hybridization between iron dx2 y2 and oxygen ps bonding states,
which leads to electronic instability towards the depopulation ofdx2 y2 and oxygen ps antibonding states. Figure 3b shows the local
iron DOS for the ferromagnetic S ¼ 1 state, which demonstrates
the depopulation of the iron dx2 y2 antibonding states. The irond electrons are increased to d 6.5 in total, with the electronic conﬁguration of ðdz2 Þ1(dxz ,dyz)2(dxy)1ðdx2 y2 Þ0.25 for the spin-up channeland ðdz2 Þ1(dxz ,dyz)1(dxy)0ðdx2 y2 Þ0.25 for the spin-down channel.
Another point is that the ferromagnetic S ¼ 1 state is half metallic.
In the antiferromagnetic S ¼ 2 state, the bandgap decreases considerably with increasing pressure, as shown in Fig. 4a, and thendrops to zero at Pc.
The spin moment of the iron dx2 y2 bonding states is quenchedbecause of the enhanced in-layer bonding between iron d andoxygen p. The double occupation of dz2 is maintained in the ferromagnetic S ¼ 1 state just after the transition, but becomesenergetically unfavourable because of the compression of theinterlayer spacing. Therefore, a charge transfer from spin-down
dz2 to spin-down dxy has to occur, whereas the spin-down(dxz ,dyz) states remain half-occupied (Fig. 3c). It follows that, as
the pressure reaches 70 GPa, the spin-down states of dxy anddz2 also become half-occupied in the ferromagnetic S ¼ 1
state ðdz2 )0.5(dxz ,dyz)1(dxy)0.5 ðdx2 y2 Þ0.25 for the spin-down states,
which results in an increase of the DOS at the Fermi level.
We conducted electrical resistance measurements on nonsintered powder samples under pressure, as summarized in Fig. 4.

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ARTICLES

NATURE CHEMISTRY DOI: 10.1038/NCHEM.289
AF (S = 2)

a

b

2

F (S = 1)1

d x2−y2
0

0

2

1

0

DOS (states eV−1)

DOS (states eV−1)

d xy

2
d yz
0

0

1
0

d xz
2

1

Up

d z2

0

0
Down

2
−10 −8 −6 −4 −2

0

2

4

1
−10 −8 −6 −4 −2

6

E − E0 (eV)
F (S = 1)

c

d

4

6

2

4

6

AF (S = 1)

0

1

2

0

0

1
0

2
0

1

2

0

0

0

2

2

0

DOS (states eV−1)

DOS (states eV−1)11−10 −8 −6 −4 −20

E − EF (eV)

2

4

6

E − EF (eV)2−10 −8 −6 −4 −20

E − EF (eV)Figure 3 | Calculated local density of states of iron in SrFeO2. a, High-spin (S ¼ 2) antiferromagnetic (AF) state at 0 GPa (insulating). b, Intermediate-spin(S ¼ 1) ferromagnetic (F) state at 53 GPa (metallic). c, Intermediate-spin (S ¼ 1) ferromagnetic (F) state at 70 GPa (metallic). d, Unstable intermediate-spin(S ¼ 1) antiferromagnetic (AF) state at 65 GPa. In the S ¼ 2 antiferromagnetic state (a), the lone spin-down electron occupies the spin-down dz2 orbital.
In the S ¼ 1 ferromagnetic state (b), the dx2 y2 antibonding states become unoccupied, which results in the charge transfer from dx2 y2 to dyz (or dxz ).
The dz2 orbital maintains its double occupation through the spin-state transition at 53 GPa. The energy zero is set at the middle of the gap (a) and at the
Fermi level (b,c,d).

At ambient and low pressures, SrFeO2 is a non-conductor or asemiconductor with an out-of-range resistance. The electrical resistance at room temperature became measurable above 17 GPa. Acrossthe spin-state transition, a drastic decrease of resistance occurredfrom kiloohms at 27 GPa to ohms at 37 GPa, and to centiohms
at 50 GPa. The temperature dependence of the electrical resistance
DR/DT became smaller and, ﬁnally, its sign changed from negative
to positive at 60 GPa. Although DR/DT turns negative atlower temperatures, this possibly results from Anderson localizationand the use of the non-sintered powder sample. In addition, theresistance measurements suffered from the pressure distribution
within the sample, as in the Mössbauer experiments, which
374

must, to some extent, obscure the insulator–metal transition at Pc.
It is likely that the intermediate spin state is metallic, as
theoretically suggested.
Compared with the ferromagnetic S ¼ 1 state, the antiferromagnetic S ¼ 1 state is much less stable above Pc. An examination of theelectronic structure of its antiferromagnetic S ¼ 1 state (Fig. 3d)shows that, in addition to having a dz2 electron in the spin-down
channel, the second spin-down electron actually prefers to occupy
the dxy orbital. The sudden increase in the energy from antiferromagnetic S ¼ 2 to antiferromagnetic S ¼ 1 results from the increaseof Coulomb repulsion caused by the double occupation of dxy. We
have also examined the possibility of spin transition induced by

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NATURE CHEMISTRY DOI: 10.1038/NCHEM.289
a 105
Eg (eV)

1.6

104

b 104

F
AF

1.2
0.8

27 GPa

103

0.4
0.0

0

20 40 60 80 100
Pressure (GPa)

102

101

S=2
AF insulator

S=1
F metal

31 GPa102
Resistivity (Ω cm)

Resistivity (Ω cm)

103

101

37 GPa43 GPa100

100

48 GPa10−1

10−1

67 GPa
10−2

10−2
0

10

20

30
40
Pressure (GPa)

50

60

70

0

50

100
150
200
Temperature (K)

250

300

Figure 4 | Insulator–metal transition at the spin transition. a, Room-temperature electrical resistivity of SrFeO2 , which shows an insulating state for the
low-pressure phase (P , Pc) and a metallic state for the high-pressure phase (P . Pc). The inset represents the calculated energy gap for the S ¼ 2
antiferromagnetic (AF) state and the S ¼ 1 ferromagnetic (F) state, consistent with the experimental results. b, Temperature dependence of the electricalresistivity under pressure.

phonon instability at various pressures. More speciﬁcally, we considered the effects of Jahn–Teller lattice distortion and breathingphonon mode. The former could induce a spin state of S ¼ 1,
whereas the latter could result in a coexistence of the S ¼ 1 andS ¼ 2 states through a charge-disproportionation reaction.
However, we found experimentally and theoretically that SrFeO2
is stable against a cooperative oxygen displacement in either
Jahn–Teller lattice distortion or breathing phonon mode in both
antiferromagnetic and ferromagnetic states at all pressures.
Therefore, the spin transition in SrFeO2 is not caused by thesephonon instabilities.
The spin-state transitions in 3d ions are routinely interpreted as aresult of the transfer of one or two electrons between d orbitals of
opposite spin channels. The observed S ¼ 2 to S ¼ 1 transition in
SrFeO2 is distinctly different from previously reported examples
in two aspects: it involves the transfer of non-integer d electrons
and the p ! d charge transfer (see Supplementary Table 1).
A number of transition-metal oxides22–26 have been found to
exhibit half-metallic ferromagnetism – these are limited mostly tomanganese and chromium oxides (La1.3Sr0.7Mn2O7 , Tl2Mn2O7 ,CrO2 , and so on.). The double-perovskite oxide Sr2FeMoO6 isknown as an iron-based half-metallic ferromagnet, but it contains
molybdenum, which is rare in the Earth’s crust. As iron is themost abundant transition metal, the discovery of half-metallic ferromagnetism in SrFeO2 containing only iron is important for futureapplications in industry related to magnetism and spin electronics,
although Pc is still too high. Recent studies27–29 demonstrated thatthe square-planar coordination around iron in SrFeO2 is not amere fortuity, but could be widely and universally available,
and include serial spin-ladders Srnþ1FenO2nþ1 , such as Sr3Fe2O5(n ¼ 2). Also, a square-planar FeO4 unit can be distorted towards
a tetrahedron, as in CaFeO2. Although Pc in SrFeO2 is too high, it
is desirable to reduce Pc in those related iron oxides and yet keep
the transition temperature far above room temperature.
Metallization of the FeO2 sheet would provide a hope that
increased pressure could lead to superconductivity, in analogy
with the layered cuprates. It is also interesting to compare thepresent compound with the recently discovered iron oxypnictide

superconductors30, because in both cases the iron atoms form
two-dimensional square lattices and are coordinated by four
anions. Furthermore, preliminary calculations at higher pressuressuggest a transition from the S ¼ 1 spin state to a second
intermediate spin state (S ¼ 1/2) before it ﬁnally falls into
the S ¼ 0 spin state. It would be interesting to check thesepoints experimentally.

Methods
The powder sample of SrFeO2 was synthesized by the reaction with CaH2 of aslightly oxygen-deﬁcient perovskite, SrFeO2.875. The precursor SrFeO2.875 was
prepared by a high-temperature ceramic method from SrCO3 (99.99%) and Fe2O3(99.99%). SrFeO2.875 and a 2 M excess of CaH2 were mixed, ﬁnely ground in an
argon-ﬁlled dry box, sealed in an evacuated Pyrex tube and left to react at 553 K
for two days. The residual CaH2 and the CaO by-product were removed from
SrFeO2 by washing them out with a NH4Cl–methanol solution. For details, refer tothe literature15.
The high-pressure 57Fe Mössbauer measurements were performed up to 70 GPa
using a Bassett-type diamond-anvil cell31. The 57Fe-enriched SrFeO2 powder andsmall ruby chips were enclosed in the hole of a rhenium gasket. Two types ofpressure-transmitting media, a 4:1 methanol:ethanol solution and Daphne7373,
were used. A point g-ray source of 57Co in a rhodium matrix of 370 MBq andanother one of 925 MBq with active areas of 1 mm and 4 mm in diameter,
respectively, were used. The pressure was determined by means of ruby-ﬂuorescencemanometry. To estimate the pressure distribution along the sample, several rubychips were placed inside the hole at different distances from its centre. It was foundthat the pressure gradient at the sample was not more than 8 GPa at maximalpressures. The g-ray source could not be narrowed more because the high-pressureMössbauer study allowed only a limited sample, and a long exposure time, as long as
a week, was required to accumulate fairly reasonable spectral statistics. The magnetic
ﬁeld was produced by a superconducting solenoid operated in the persistent mode
up to 3 T. 57Fe Mössbauer experiments under the external magnetic ﬁeld werecarried out with the magnetic ﬁeld applied along the g-ray propagation direction.
The velocity scale of the spectrum was relative to a-Fe at room temperature.
Electrical resistance was measured with a standard d.c. four-probe method between
5 K and 300 K up to 67 GPa. Fine alumina powder and NaCl were pressed onto thegasket surface to give electrical insulation and a pressure-transmitting medium,
respectively, on which sample powder and platinum electrodes were placed together.
Applied pressure was measured by means of ﬂuorescence manometer on ruby chips
placed around the sample. In this electrical resistance measurement, the initial
sectional area and the distance between probes were about 60 mm  50 mm and
50 mm, respectively.
Powder X-ray diffraction proﬁles at high pressures up to 43 GPa were recorded
using Mo-Ka radiation from a 5.4 kW Rigaku rotating anode generator equipped

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NATURE CHEMISTRY DOI: 10.1038/NCHEM.289

with a 100 mm collimator. A powder sample of SrFeO2 was loaded into a 300 mm
hole of a pre-indented stainless-steel gasket of the diamond-anvil cell. A 4:1
methanol:ethanol mixture was used as the pressure-transmitting medium. The shiftof ruby ﬂuorescence was used to determine the pressure. To estimate the pressure
distribution along the sample, several ruby chips were placed inside the hole atdifferent distances from its centre. It was found that the pressure gradient at the
sample was not more than 0.5 GPa at maximal pressures. The diffracted X-rays werecollected with an image plate. More details of the experimental setup are reported
elsewhere32. Four diffraction peaks, 110, 011, 020 and 121, were used to calculate thecell parameters.
The ﬁrst-principle calculations were performed using the Vienna ab initio
Simulation Package (VASP)33 with the ion–electron interaction described by the
projector-augmented wave potential (PAW)34 (see Supplementary Information for
details). We employed the hybrid density functional theory (PBE0) method18,35, as
implemented in VASP19,36, to treat the localized Fe2þ states in SrFeO2. The exchange
energy in PBE0 mixes the energies of exact Hartree–Fock (HF) exchange and PBE
exchange from the local density functional approximation19,36. The long-range partof the HF-exchange interaction is included in PBE0. The weight of HF exchange inthe mixing was generally less than 25%. We found that weights between 10 and 20%
HF exchange provided the most consistent results and described the experimentalﬁndings reasonably well for SrFeO2. The results reported in this paper were obtainedusing 15% HF with 85% PBE for the exchange energy. We found that the spintransition pressure from the antiferromagnetic (S ¼ 2) state to the ferromagnetic
state is insensitive to the weights of mixing. It is the stable ranges of the S ¼ 2 and
S ¼ 1 states within the ferromagnetic ordering that show an increased sensitivity to
the weights of HF mixing in PBE0.
In our calculations, the semicore states (the 3p and 4s states of strontium and the
3p state of iron) were treated as valence states. An energy cutoff of 500 eV waschosen for the plane-wave expansion. Full relaxation of atomic positions and lattice
parameters at each pressure was achieved by minimizing the forces and stress-tensor
components. Forces were minimized down to 10–5 eV Å–1 and the difference in the
total energies between two successive electronic iterations was required to be lessthan 10–7 eV. The angular momentum projected local density of states and magnetic
moments of iron were calculated within a sphere of 1.164 Å.

Received 9 April 2009; accepted 10 June 2009;
published online 20 July 2009

References
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I–III, Vols 233–235 (Springer, 2004).
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Acknowledgements
This work was supported by Science Research on Priority Areas (Novel States of Matter
Induced by Frustration) and also partly by the Ministry of Education, Culture, Sports,
Science and Technology of Japan. Research at Oak Ridge National Laboratory wassponsored by the Division of Materials Sciences and Engineering, US Department ofEnergy, under contract with UT-Battelle. This research used resources of the National
Energy Research Computing Center, which is supported by the Ofﬁce of Science of the US
Department of Energy. This work was supported by the University of Vienna through the
University Focus Research Area Materials Science (Multi-scale Simulations of Materials
Properties and Processes in Materials).

Author contributions
H.K. designed and coordinated the overall study seeking advice from M.T. (experiment)
and C.L.F. (theory). N.H. conceived the high-pressure Mössbauer study, and X-Q.C.
conceived the theoretical studies. Y.T., with support from K.Y., synthesized the material.
T.K., S.S., K.H., Y.S. and Y.M. conducted high-pressure Mössbauer and electrical resistivity
experiments, and T.K. analysed the data (with the help of M.T. and S.N.). Y.T., C.T., A.K.,
T.O. and T.Y. performed high-pressure X-ray diffraction experiments, and Y.T. and C.T.
analysed the data. X-Q.C., C.L.F. and R.P. performed the theoretical work and analysis.
H.K. and T.K. co-wrote the experimental part and C.L.F. and X-Q.C. co-wrote thetheoretical part, with comments from R.P. All contributed to the discussion of the results.

Additional information
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addressed to H.K.

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