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Stability of the Infinite Layer Structure with Iron Square Planar Coordination
Cdric Tassel, Takashi Watanabe, Yoshihiro Tsujimoto, Naoaki Hayashi, Atsushi Kitada, YujiSumida, Takafumi Yamamoto, Hiroshi Kageyama, Mikio Takano, and Kazuyoshi Yoshimura
J. Am. Chem. Soc., 2008, 130 (12), 3764-3765 • DOI: 10.1021/ja800415d
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Published on Web 03/04/2008

Stability of the Infinite Layer Structure with Iron Square Planar Coordination
Cédric Tassel,† Takashi Watanabe,† Yoshihiro Tsujimoto,† Naoaki Hayashi,‡ Atsushi Kitada,†Yuji Sumida,† Takafumi Yamamoto,† Hiroshi Kageyama,*,† Mikio Takano,§,| andKazuyoshi Yoshimura†
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo, Kyoto 606-8502, Japan, Graduate
School of Human and EnVironmental Studies, Kyoto UniVersity, Sakyo, Kyoto 606-8501, Japan, Institute for
Chemical Research, Kyoto UniVersity, Uji, Kyoto 611-0011, Japan, and Research Institute for Production
DeVelopment, 15 Morimoto, Shimogamo, Sakyo, Kyoto 606-0805, Japan
Received January 18, 2008; E-mail: kage@kuchem.kyoto-u.ac.jp

The iron perovskites (Sr,Ca)FeO3-y (0 e y e 0.5) have been asubject of intensive studies for many years because of a variety ofinteresting fundamental aspects, ranging from charge disproportionation (2Fe4+ f Fe3+ + Fe5+), vacancy ordered structures, screw
spin structure, giant magnetoresistance, to high-spin-to-low-spintransition.1 Furthermore, they show fast oxygen transport properties
observed at moderate temperatures, making them appealing for lowcost industrial applications as gas sensors, electrodes for solid oxide
fuel cells, membranes for oxygen separation, and electrocatalysis.2
A reduction of transition metal oxides by metal hydride at low
temperatures has become a useful tool to generate greater oxygen
deficiencies (or lower valence) than could be achieved by standard
methods, leading to unprecedented frameworks with less coordination numbers.3 Applying the new method to SrFeO3 (Figure 1a),
we have recently obtained a new compound SrFeO2 (y ) 1) bearing
an unusual square-planar coordination around iron (Figure 1c).4 It
is isostructural with the undoped high-Tc superconductor SrCuO2,the “infinite-layer” (IL) compound.5 Doubled oxygen deficit withrespect to the brownmillerite phase SrFeO2.5 (y ) 0.5) (Figure 1b),
once thought as the terminal phase, and considerably lowtemperature reoxidation (starting at ∼400 K) may further enhance
the potentials for aforementioned applications.
The question, which is of fundamental and practical importance,
is how stable the IL structure is against external parameters(temperature T, pressure P, substitution, etc.). It has been theoretically demonstrated, for example, for an isolated d6 system that asquare-planar coordination occurs only when the metal is in thetriplet (S ) 1) ground state and is bonded with π-donor ligands.6
This is in marked contrast to the case of SrFeO2 (quintet (S ) 2)
ground state and σ-donor ligands), where the tetrahedral geometry
should have been stabilized. Given that SrFeO2 is a metastable phase
only obtainable through the low-T route, one might suspect that it
would easily be destroyed upon being heated. Furthermore, the bondvalence sum (BVS) calculations giving nearly ideal values of +2for both Sr and Fe might indicate that the formation of SrFeO2
was fortunate, i.e., a perfect match in the ionic radii of Sr2+, Fe2+,and O2-.4 Here, we will report on the successful synthesis of thecomplete solid solution of Sr1-xCaxFeO2 together with their thermal
stability measured by in situ X-ray diffraction (XRD). The robust
nature of the IL structure against both x and T allows extensive
tuning of the physical properties to meet industrial demands.
The precursors Sr1-xCaxFeO3-y (x ) 0.0, 0.2, 0.4, 0.6, 0.8, and1.0) were prepared in air by using high-T ceramic reactions. In theambient atmosphere, the oxygen deficiency y increases with x and† Department of Chemistry, Graduate School of Science, Kyoto University.
‡ Graduate School of Human and Environmental Studies, Kyoto University.
§ Institute for Chemical Research, Kyoto University.
| Research Institute for Production Development.

3764 9 J. AM. CHEM. SOC. 2008, 130, 3764-3765

Figure 1. Idealized structures of AFeO3 (A ) Sr, Ca) (a), AFeO2.5 (b),AFeO2 (c). Blue, gray, and orange spheres are A, iron, and oxygen.

Figure 2. (a) XRD patterns of Sr1-xCaxFeO2 and (b) the cell constants.

becomes y ) 0.5 for x ) 1. Subsequently, the reaction with CaH2
was performed at 553 K in an evacuated Pyrex tube. A longer
reaction time is required with greater x. For example, SrFeO2 wasreadily formed by a 1-day reaction at 553 K, but CaFeO2 wasformed by a 1-week reaction at the same temperature.
As shown in Figure 2a, the powder XRD patterns of the final
products for 0.2 e x e 1.0 were very similar to those for x ) 0,where all peaks are indexable in the tetragonal lattice. Rietveld
structural refinements were performed using the synchrotron XRD
data taken at SPring-8. We assumed the IL structure of SrFeO2,P4/mmm space group with A ) (Sr1-xCax) on 1d (0.5, 0.5, 0.5), Fe
on 1a (0, 0, 0), and O on 2f (0.5, 0, 0). The fits readily converged
to give Rwp ) 4.20%, χ2 ) 0.21 for x ) 0.2, Rwp ) 3.86%, χ2 )0.24 for x ) 0.4, Rwp ) 3.35% and χ2 ) 0.48 for x ) 0.8. Siteoccupancy was confirmed as 100% for all sites. Both the a and cparameters decrease linearly with x up to x ) 0.8, in consistency
with the reduced ionic radius for Ca (vs Sr).
10.1021/ja800415d CCC: $40.75 © 2008 American Chemical Society

COMMUNICATIONSSrFeO2 would be linked to the low rate of reduction and thepresence of defects in CaFeO2. The higher ΘD for CaFeO2 in turnwould be due to the shorter c-axis, leading to reinforced interlayer
coupling. In fact, the c-axis variation for SrFeO2 as a function ofT is approximately twice that for CaFeO2 (Figure 3b and 3c).
In conclusion, we have demonstrated that the IL iron oxide isamazingly robust against substitution as well as temperature. Thesestabilities should be attributed to the extremely strong hybridization
between Fe dx2-y2 and O p, as evidenced by the surprisingly low
isomer shift in the 57Fe Mössbauer spectroscopy of SrFeO2 andCaFeO2 (see Supporting Information and ref 4). We strongly believethat this is a genuine and universal feature for structures with asquare-planar coordinated iron. Thus, using the hydride reduction
to other iron oxides, we will be able to access new materials withsquare-planar coordinated irons that have potential future applications working even at high temperatures.

Figure 3. (a) XRD patterns of SrFeO2 at high T, The SrFeO2.5 peaks areindicated with *; (b) and (c) evolution of the lattice parameters of SrFeO2(open) and CaFeO2 (solid).

However, we could not obtain a sufficient fit to the data for x )1 (Rwp ) 6.20% and χ2 ) 18.38). In addition, a deviation from thelinear dependence (the Vegard’s law) is seen for the c-axis (Figure2b), though the linear dependence for the a-axis is kept up to x )1. These results suggest the existence of some structural defects
such us corrugation of the layers and/or stacking faults in CaFeO2.
The destabilization of the IL structure with increasing x is
rationalized by calculating valences of A and Fe, both of which
deviate more from +2 (see for Supporting Information). However,
we would like to emphasize more strongly that its solubility range
is considerably wide, in fact much wider than that of the IL cuprates
Sr1-xCaxCuO2 (0.13 < x < 0.17) at ambient pressure.8 It isnoteworthy that the application of high pressure widens thesolubility range of the cuprates from x ) 0 to x ≈ 1.9
Shown in Figure 3a and Figure 6S are evolutions of the in situ
synchrotron XRD profiles of SrFeO2 and CaFeO2, where the sample
was sealed in a silica capillary tube with an inner diameter of 0.3mm. Each scan was performed for 5 min after a time of interval of
2 min between scans. The brownmillerite phase appeared and grewupon heating due to the oxidation of the IL phase. It means thatthe oxygen uptake occurs through O2 gas left in a tiny inner spaceof capillary and even through the capillary itself. Nevertheless, what
is remarkable here is the observation that the IL phase persists atleast up to the highest temperature measured in this study, 960 K
for SrFeO2 and 630 K for CaFeO2. These temperatures are muchhigher than the temperature at which the reducing reaction withCaH2 was conducted. Furthermore, the Debye temperatures ΘD
calculated from the Debye-Grüneisen model are 395 K for SrFeO2
and 733 K for CaFeO2, which are comparable with those for stablephases, e.g., 525.5 K (SrRuO3), 542 K (CaRuO3), 612 K (SrVO2.5)
and 337 K (TbFeO3),10 indicating that the IL structure, once it isformed, could be as structurally rigid as thermodynamically stablephases. The lower thermal stability of CaFeO2 with respect toAcknowledgment. We acknowledge T. Saito, M. Azuma, andK. Kato for help in the synchrotron radiation experiments atSpring-8 and M. Isobe and Y. Ueda for the XRD experiments atISSP. The authors express their thanks to the MEXT, Japan, forGrants-in-Aid Nos. 19052004, 17684018, 17105002, and 16076210
and to the GCOE program, Kyoto University.
Supporting Information Available: Experimental details andcharacterization data for the new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
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