Communications
DOI: 10.1002/anie.200801146

Spin Ladders

Spin-Ladder Iron Oxide: Sr3Fe2O5**
Hiroshi Kageyama,* Takashi Watanabe, Yoshihiro Tsujimoto, Atsushi Kitada,
Yuji Sumida, Kazuyoshi Kanamori, Kazuyoshi Yoshimura, Naoaki Hayashi,
Shigetoshi Muranaka, Mikio Takano, Monica Ceretti, Werner Paulus,
Clemens Ritter, and Gilles Andr$

Angewandte

Chemie5740

 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Angew. Chem. Int. Ed. 2008, 47, 5740 –5745

Angewandte

ChemieSpin ladders, which conceptually are unidirectional sectionsof an antiferromagnetic (AF) two-dimensional (2D) squarelattice, as schematically illustrated in Figure 1, have attracted
considerable attention in the last two decades. Theories
predict that the ground state of an S = 1/2 ladder is a gapped
singlet spin liquid state when the leg number n is even, but is agapless singlet spin liquid state when n is odd.[1] Moreover,short-range AF spin correlation should lead to superconductivity when modest carriers are doped in even-legged ladders.
Experimental counterparts are the observation of the gappedand gapless ground states of SrCu2O3 (n = 2) and Sr2Cu3O5(n = 3), respectively,[2] and the appearance of superconductivity in (Sr,Ca)14Cu24O41+x (n = 2) under high pressure.[3]Another S = 1/2 two-legged Cu2+ ladder, namely,
La1xSrxCuO2.5, shows anomalous behavior associated with
its nearly critical ground state.[4]The motivation of theoretical investigations has been to
test how the one-dimensional (1D) S = 1/2 AF chain system(n = 1), which is rigorously solved even when doped withcarriers, can be related to the 2D square lattice (n = 1), which
presents various theoretical difficulties and is far from being
understood. Experimentally, Srn1CunO2n1 is the only example of a generalized spin-ladder system,[5] though the composition and structure of a two-legged ladder has been extended
to CaV2O5, Cu2(C5H12N2)Cl4, (C5H12N)2CuBr4, [Ph(NH3)([18]crown-6)][Ni(dmit)2] (dmit = dithiolene) and [(DTTTF)2][Au(mnt)2] (DT-TTF = dithiophene tetrathiafulva-[*] Prof. Dr. H. Kageyama, T. Watanabe, Y. Tsujimoto, A. Kitada,
Y. Sumida, Dr. K. Kanamori, Prof. Dr. K. Yoshimura
Department of Chemistry, Graduate School of Science
Kyoto University, Kyoto 606-8502 (Japan)
Fax: (+ 81) 75-753-3991
E-mail: kage@kuchem.kyoto-u.ac.jp
Homepage: http://kuchem.kyoto-u.ac.jp/kinso/kage/index_eng.html
Dr. N. Hayashi, Prof. Dr. S. Muranaka
Graduate School of Human and Environmental Studies
Kyoto University (Japan)
Prof. Dr. M. Takano
Institute for Chemical Research, Kyoto University (Japan)
andInstitute for Integrated Cell-Materials Sciences andResearch Institute for Production Development, Kyoto (Japan)
Dr. M. Ceretti, Prof. Dr. W. Paulus
UniversitF de Rennes 1, UMR CNRS 6226 (France)
Dr. C. Ritter
Institute Laue Langevin, Grenoble (France)
Dr. G. AndrF
Laboratoire LFon Brillouin, CEA-CNRS, Gif-sur-Yvette (France)[**] We thank Dr. K. Kato for his precious help with the measurements at
SPring-8, E. Fukui and K. Nakanishi for their help with the TG
measurements, and M. Nishi and N. Nagaosa for helpful discussions. The synchrotron radiation experiments were performed at
Spring-8 with the approval of the Japan Synchrotron Radiation
Research Institute. 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, and by
GCOE program, Kyoto University.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200801146.
Angew. Chem. Int. Ed. 2008, 47, 5740 –5745

Figure 1. Spin ladders with n legs; solid and open circles denotemagnetic ions and oxide ions, respectively. a) One-legged spin ladder(i.e., the 1D chain). b) Two-legged spin ladder realized in the present
work. c) Three-legged spin ladder. d) 1-legged spin ladder (i.e., the2D square lattice) realized in SrFeO2.[9]lene, mnt = maleonitrile dithiolate).[6] It is highly desirablefrom both theoretical and experimental viewpoints to extend
these ladder systems with respect to n and S, and also withrespect to possible skews such as mixing of ferromagnetic and
AF interactions. An example of skew is the dramatic switching of the large-gapped (> 400 K) singlet spin liquid of
SrCu2O3 to an AF ordered state by the nonmagnetic
substituent Zn2+, even at Zn/Cu  1 %.[7]The most pronounced structural characteristic ofd9 copper(II) oxides is the favored formation of squareplanar CuO4 units, which are stabilized due to the Jahn–Teller
effect.[8] This 2D coordination geometry provides strong CuO-Cu superexchange interactions within a ladder along thelegs and rungs, while Cu···Cu interactions normal to thesquare plane (k z) are negligibly small because the copper(II)ions have a magnetically inert, filled (dz2›ﬂ) configuration
along this direction and also because an intervening oxygen
atom is lacking.
Unlike the case of cuprates, the coordination geometries
in iron oxides have been almost exclusively restricted to 3D
polyhedra such as octahedra and tetrahedra. However, thisrestriction was recently overcome[9] by using calcium hydride
at low temperatures as a reductant, as initiated and developed
by Hayward, Rosseinsky and co-workers.[10, 11] Low-temperature reaction of cubic perovskite SrFeIVO3 with CaH2 led tostable SrFeIIO2 with a square-planar oxygen coordination
environment around the high-spin Fe2+ ion. The structure isisostructural with the “infinite”-layer cupric oxides.[12] Herein
we report the synthesis of novel spin-ladder iron oxide
Sr3FeII2O5 through reaction of double-layered perovskite
Sr3FeIV2O7 with CaH2. Together with the synthesis of
SrFeO2, this opens up new avenues for the solid-statechemistry of iron(II) oxides with square-planar coordination,
which potentially includes the serial ladder system
Srn+1FenO2n+1, and for the solid-state physics of multiplespin ladders.
The slightly oxygen-deficient phase Sr3Fe2O7y (y  0.4),
which can be prepared easily by a conventional, high-temperature solid-state reaction, was used as precursor. It is known
that all oxygen vacancies are located at the apical O(1) sites
shared by the double FeO2 sheets (Figure 2 a and b) and thatthe structure keeps the I4/mmm space group over the entire
range from Sr3Fe2O7 (y = 0) to Sr3Fe2O6 (y = 1).[13] The lattice
parameters of the as-obtained precursor determined bypowder X-ray diffraction (XRD) were a = 3.872 and c = 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.angewandte.org

5741

Communications

Figure 2. Structural transformation from Sr3Fe2O7 to Sr3Fe2O5 via Sr3Fe2O6, where the blue, red, andyellow spheres represent Sr, O, and Fe atoms, respectively. a) Crystal structure of a stoichiometric(fully oxidized) phase Sr3Fe2O7 (y = 0). b) Crystal structure of reduced phase Sr3Fe2O6 (y = 1), once
thought to be the lower limit of the oxygen content. c) Left: Crystal structure of the new phase
Sr3Fe2O5 (y = 2), described herein. Right: A spin ladder in Sr3Fe2O5 viewed along the a axis.
d) Transformation from the octahedron (left) in Sr3Fe2O7 to the pyramid (middle) in Sr3Fe2O6 and to
the square plane (right) in Sr3Fe2O5, where the white spheres represent oxygen vacancies.

20.157 G, corresponding to a phase with Sr3Fe2O6.6 stoichiometry. Note that Sr3FeIII2O6 (y = 1) is thought to represent
the lower end of the oxygen stoichiometry, in spite ofintensive studies owing to their interesting properties suchas metal–insulator transition, charge disproportionation, andpossible applications as oxygen-separation membranes andsolid-oxide fuel cells.[14]Reduction of Sr3Fe2O6.6 with CaH2 was performed in an
evacuated glass tube at 623 K for 3 d. As shown in Figure 3,
the room-temperature synchrotron powder XRD pattern of
the final product was readily indexed with an I-centeredorthorhombic unit cell with a = 3.51485(2), b = 3.95271(2),and c = 20.91251(10) G, except for a very minor unknown
impurity (see Figure S1 in the Supporting Information).
Compared to Sr3Fe2O6.6, the b axis remained nearly
unchanged, while the a axis (c axis) substantially decreased(increased). The a axis is of similar length to the c axis of
SrFeO2 (a = b = 3.99 G, c = 3.47 G).[6] These observations
strongly suggested that the FeO2 in-plane oxygen atoms
O(2) along the a axis were all removed, while the apicaloxygen sites O(1) were filled up, resulting in the formation of
Sr3Fe2O5 crystallizing in space group Immm (Figure 2 c). The
Rietveld refinement based on this model immediately converged to Rwp = 0.0524 and c2 = 5.28 along with reasonable
individual isotropic displacement factors for all atoms. The
bond valence sum calculation gave + 1.88, + 1.86, and + 1.95
for Sr(1), Sr(2), and Fe, consistent with the expected valencesof + 2 for these elements. Neutron powder diffraction (NPD)
analysis at 293 K confirmed the above structure model, withan excellent convergence of Rwp = 0.0376 and c2 = 2.78 (see5742

www.angewandte.org

Figure S2 in the Supporting Information). We tried to refine theoxygen occupancies and found that
all O(1), O(2), and O(3) sites are
fully occupied within 1 % error,
and no extra oxygen atoms could
be found. It also excluded possiblehydrogen incorporation into thelattice, as found in LaSrCoO3H0.7.[11] The three types ofFeO bonds (dFeO(1) = 2.013 (onebond), dFeO(2) = 1.976 (two bonds),
dFeO(3) = 2.039 G (one bond)) forma nearly square FeO4 plane.
Furthermore, MIssbauer measurements (Figure 4 a) indicated not
only that all the Fe atoms areelectronically and crystallographically equivalent, but also that they
are most likely square-planar coordinated in high-spin state (S = 2),
since the obtained isomer shift (IS)of 0.46 mm s1, quadrupole splitting DE of 1.28 mm s1 at 297 K,
and the magnetic hyperfine field of
Hhf = 43.7 T at 4 K are close to
those of SrFeO2.[9] These observations are consistent with the structural analysis. ThermogravimetricFigure 3. Structural characterization of Sr3Fe2O5 by Rietveld refinementof synchrotron XRD data at 293 K. The overlying crosses, solid lines,
and ticks represent the observed and calculated intensities and theposition of the calculated Bragg reflections, respectively. The difference
between the calculated and observed profiles is plotted at the bottom.
Sr3Fe2O5 adopts the Immm space group (No. 71), a = 3.514825(2),b = 3.95271(2), c = 20.91252(10) J, Z = 2. Sr(1) on 2c (0.5,0.5,0), Sr(2)on 4i (0,0,0.31234(2)), Fe on 4i (0,0,0.09636(2)), O(1) on 2a (0,0,0),O(2) on 4j (0.5,0,0.59574), O(3) on 4i (0,0,0.19358(2)), with 100 %occupancy, Biso(Sr(1)) = 0.359(22) J2, Biso(Sr(2)) = 0.213(13) J2,
Biso(Fe) = 0.089(16) J2, Biso(O(1)) = 0.12(11) J2, Biso(O(2)) = 0.15(6) J2,
Biso(O(3)) = 0.59(8) J2, Rp = 0.0401, Rwp = 0.0524,and c2 = 5.28. Refining
the occupation factors at the oxygen sites and the vacant site did not
improve the result.

 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Angew. Chem. Int. Ed. 2008, 47, 5740 –5745

Angewandte

ChemieFigure 4. Magnetic order in Sr3Fe2O5. a) MLssbauer spectra at 300(top) and 4 K (bottom). The circles indicate the experimental data,
while the lines denote the fits. b) Rietveld refinement of NPD data at
10 K measured at 1.91 J. The solid lines and the overlying crosses andbars indicate the calculated intensities, the observed intensities, andthe positions of the calculated chemical (top) and magnetic (bottom)
Bragg reflections. The difference between the observed and calculated
profiles is plotted at the bottom. See Tables S1 and S2 in the
Supporting Information for details.

(TG) measurements showed the oxygen content of the
product to be less than 6 (i.e., y > 1) though its precise
determination was hampered by partial reactions (oxidation,
decomposition, hydration) of Sr3Fe2O5 while it was exposed toair for the experimental setup (see Figure S3 in the Supporting Information).
When half of the O(2) sites in Sr3Fe2O7 are removed in thepresent ordered manner, the original double perovskite layers
become decoupled into ladders with two legs running along[010] and rungs along [001] (Figure 2 c). Sr3Fe2O5 and SrFeO2
are thus two- and 1-legged S = 2 ladder compounds, respectively. The sectioning of the dense FeO2 sheet in SrFeO2 into
the two-legged ladders loosely distributed in Sr3Fe2O5 should
greatly change the magnetism, at least by drastically lowering
the 3D magnetic ordering temperature. Indeed, the NPD
pattern of Sr3Fe2O5 at 293 K does not contain any magneticreflections, and the MIssbauer spectrum at 300 K just consistsof a quadruple doublet, that is, Sr3Fe2O5 is still in theparamagnetic state, in contrast to the case of SrFeO2 with TN
as high as 473 K. In the MIssbauer spectrum at 4 K
Angew. Chem. Int. Ed. 2008, 47, 5740 –5745(Figure 4 a), however, we found a sharp sextet, clearly
indicative of long-range AF order. Furthermore, the NPD
data at 10 K (Figure 4 b) revealed that the long-range AF
order is characterized by a magnetic propagation vector q =(1/2, 1/2, 0) and that the iron moments of 2.76(5) mB arealigned parallel to the c axis (see Figure 4 S in the Supporting
Information). This so-called G-type antiferromagnetic structure is frequently observed in double-layered Ruddlesden–
Popper phases such as Sr3Mn2O7.[15] The fact that the Fe
moment is reduced by more than 20 % from that of 3.6 mB for
SrFeO2 must be a consequence of a quantum magnetic
fluctuation enhanced inherently in the ladder, though future
theoretical support is needed.
Considering that Sr3Fe2O7 and SrFeO3 are the n = 2 andn = 1 members of a homologous series Srn+1FenO3n+1, wheren represents the number of perovskite blocks,[13] we canpredict a new homologous n-legged S = 2 ladder series
Srn+1FenO2n+1 (Figure 1). In this context, the reaction of
Sr2FeO4 (n = 1) and Sr4Fe3O10 (n = 3) with CaH2 should resultin the one-legged ladder (i.e., 1D chain) compound Sr2FeO3
and the three-legged ladder compound Sr4Fe3O7, respectively(see Figure S5 in the Supporting Information). This work is inprogress. Note that (LaSr3)(Fe1.5Co1.5)O7.5, obtained by hydride reduction of the n = 3 La- and Co-substituted sample(LaSr3)(Fe1.5Co1.5)O10, has a different framework composedof three-coordinate metal ions flanked by square-based
pyramidal coordination,[10d] instead of the three-legged
ladder solely with square-planar coordination.
It is also interesting to compare the copper(II) andiron(II) ladder systems Srn1CunO2n1[5] and Srn+1FenO2n+1. In
the former, the ladders, each having the composition
CunO2n1, share their OO edges to form the dense
CunO2n1 sheet. The iron(II) ladders, on the other hand, aredispersed in the I-centered orthorhombic lattice with theirlegs and rungs oriented along the b and c axes, respectively.
The linear Fe-O-Fe bonds are strong and AF, as found for
SrFeO2. As a consequence, the phase shift along the leg
direction by p/2 between the corner and center ladders should
minimize their interactions by spin frustration, as in the dense
CunO2n1 sheet. However, if the electronic configuration of
the FeO4 unit is (dyz,dzx)3(dxy)1(dz2)1(dx2y2)1 with S = 2, as
proposed for SrFeO2, the iron(II) ladders facing each other inphase along the a axis should interact more strongly in an
AF Fe(dz2›)···Fe(dz2ﬂ) manner than the corresponding
Cu····Cu interactions, because these copper(II) ions have an
inert, filled (dz2›ﬂ) configuration. Numerical calculations fora S = 1/2 two-legged ladder system showed that the gappedspin liquid state can be replaced by an ordered state at a
critical value of J’/J  0.11, where J and J’ are the intra- andinterladder exchange interactions, respectively.[4b] There areno calculations for S = 2 systems, but there is no doubt thatthe magnetic order in Sr3Fe2O5 results mainly from interactions along the a axis.
The mechanism of reduction from Sr3Fe2O7 to Sr3Fe2O5
merits detailed inspection. We tried to slow down the processby lowering the synthesis temperature to 553 K. Interruptionof the reaction after one day resulted in Sr3Fe2O6, while a oneweek reaction at the same temperature yielded Sr3Fe2O5, thatis, the process is two-staged. The first step from Sr3Fe2O7 to 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.angewandte.org

5743

Communications
Sr3Fe2O6 is nothing but continuous randomized removal of
the O(1) ions leading to a coordination change from FeO6
octahedra to FeO5 square pyramids. This is quite common for
the n = 2 Ruddlesden–Popper phases and there is nothing
special in the present case. However, the second step, thetransformation of the FeO5 square pyramids in Sr3Fe2O6 into
the FeO4 squares in Sr3Fe2O5 must be more complicated andcooperative. Supposing one basal oxygen atom of a given
square pyramid escapes into the reducing atmosphere,
another basal oxygen atom must move up to the vacantapical site, as illustrated in Figure 2 d, and these processes
should take place cooperatively over a certain lattice volume
for crystallization. Thus, the reduction mechanism changes at
y = 1.
It is noteworthy that there are neither intermediate phases
nor nonstoichiometry between Sr3Fe2O6 and Sr3Fe2O5, as well
as between SrFeO2.5 and SrFeO2. This is remarkable given thepresence of two intermediates, SrFeO2.875 and SrFeO2.75,
between SrFeO3 and SrFeO2.5. It is also known that electrochemical formation of SrCoO3 from SrCoO2.5, which areisomorphous with SrFeO3 and SrFeO2.5, respectively, proceeds topotactically at room-temperature via the complex
vacancy-ordered intermediate phase SrCoO2.820.07.[16] The
processes occurring in the formation of SrFeO2 from
SrFeO2.5, whereby the FeO4 tetrahedra and the FeO6 octahedra linked to each other are both converted to FeO4 squares[17]must be more complicated than the case discussed above. We
now recognize that the underlying perovskite and related
lattices are flexible enough to tolerate such drastic compositional and structural events. We thus believe that oxygentransport materials working at low temperatures that featureformation of intermediate ordered structures should bepromising.
Finally, we must assume that the present synthetic strategy
involving CaH2 reduction can be further generalized for amore rational design of new magnetic lattices comprisingextended arrays of FeO4 square planes, for which formerly
only copper(II) oxides were candidates. It would also beinteresting to test other transition metal oxides.

Experimental Section
Precursor Sr3Fe2O7y (y  0.4) was prepared by a conventional hightemperature ceramic method from predried SrCO3 (99.99 %) andFe2O3 (99.99 %) by heating a pelletized stoichiometric mixture ofthese at 1273 K in air for 24 h and again for 24 h at 1473 K after
grinding and pelletization. The reduction of Sr3Fe2O7y (y  0.4) was
performed with CaH2 as reducing agent. Sr3Fe2O7y (0.43 g) and afour-molar excess of CaH2 (0.15 g) were finely ground in an Ar-filled
glove box, sealed in an evacuated pyrex tube (V = 15 cm3) with aresidual pressure of less than 1.3 L 102 Pa, and heated at 623 K for3 d. Residual CaH2 and the CaO byproduct were removed from thefinal reaction phase by washing with 0.1m NH4Cl in dried methanol.
Sr3Fe2O5 is air-sensitive, in contrast to SrFeO2.
Thermogravimetric measurements were performed with a thermal analyzer (Rigaku Thermoplus TG8120). Measurements toanalyze reoxidation behavior of Sr3Fe2O5 were performed on asample of around 10 mg that was rapidly loaded into a platinum
crucible and then heated at 10 K min1 in air. Prior to the experiment,
the sample was dried at 373 K for about 1 h. The identity of thereoxidized product was determined by powder X-ray diffraction.

5744

www.angewandte.org

The synchrotron powder XRD experiment was performed on thelarge Debye–Scherrer camera installed at SPring-8 BL02B2 by using
an imaging plate as detector. Incident beams from a bending magnet
were monochromatized to 0.77709 G. The sample was contained in a
glass capillary tube with an inner diameter of 0.1 mm and was rotatedduring measurements. The diffraction data were collected at room
temperature in a 2 q range from 1 to 758 with a step interval of 0.018.
The NPD studies were carried out on the D1A diffractometer
installed at the Institute Laue Langevin (Grenoble, France). A 250mg sample sealed in an He-filled vanadium can was used, and awavelength of l = 1.91 G was employed.
MIssbauer spectra were taken in transmission geometry by using
a 57Co/Rh g-ray source kept at room temperature on a powderedsample of Sr3Fe2O5. The source velocity was calibrated by using purea-Fe as control material, and the isomer shift is relative to a-Fe. The
spectra were fitted by using the Lorentzian function.
The XRD and NPD patterns were analyzed by the Rietveld
method with RIETAN 2000[18] and FULLPROF software,[19] respectively. The agreement indices used were the weighted profile Rwp =[wi(yioyic)2/wi(yio)2]1/2 and the goodness of fit (GOF), c2 = (Rwp/
Rexp)2, where Rexp = [(NP)/wiyio2]1/2, yio and yic are the observed andcalculated intensities, wi is the weighting factor, N the total number ofyio data when the background is refined, and P the number of adjusted
parameters. The bond valence sum method was applied to estimate
the valence of cations by using tabulated parameters.[20]Received: March 10, 2008
Revised: May 14, 2008.

Keywords: iron · layered compounds · perovskite phases ·reduction · spin ladders[1] a) E. Dagotto, T. M. Rice, Science 1996, 271, 618 – 623; b) E.
Dagotto, Rep. Prog. Phys. 1999, 62, 1525 – 1571.
[2] a) M. Azuma, Z. Hiroi, M. Takano, K. Ishida, Y. Kitaoka, Phys.
Rev. Lett. 1994, 73, 3463 – 3466; b) K. Kojima, A. Keren, G. M.
Luke, B. Nachumi, W. D. Wu, Y. J. Uemura, M. Azuma, M.
Takano, Phys. Rev. Lett. 1995, 74, 2812 – 2815.
[3] a) M. Uehara, T. Nagata, J. Akimitsu, H. Takahashi, N. Mori, K.
Kinoshita, J. Phys. Soc. Jpn. 1996, 65, 2764 – 2767; b) T. Vuletić,B. Korin-Hamzić, T. Ivek, S. Tomić, B. Gorshunov, M. Dressel, J.
Akimitsu, Phys. Rep. 2006, 428, 169 – 258.
[4] a) Z. Hiroi, M. Takano, Nature 1995, 377, 41 – 43; b) M. Troyer,
M. E. Zhitomirsky, K. Ueda, Phys. Rev. B 1997, 55, R6117 –R6120.
[5] Z. Hiroi, M. Azuma, M. Takano, Y. Bando, J. Solid State Chem.
1991, 95, 230 – 238.
[6] a) Y. Ueda, Chem. Mater. 1998, 10, 2653 – 2664; b) B. Chiari, O.
Piovesana, T. Tarantelli, P. F. Zanazzi, Inorg. Chem. 1990, 29,
1172 – 1176; c) C. Rovira, Chem. Eur. J. 2000, 6, 1723 – 1729;
d) Y. Hosokoshi, K. Katoh, Y. Nakazawa, H. Nakano, K. Inoue,
J. Am. Chem. Soc. 2001, 123, 7921 – 7922.
[7] M. Azuma, Y. Fujishiro, M. Takano, M. Nohara, H. Takagi, Phys.
Rev. B 1997, 55, R8658 – R8861.
[8] A. F. Wells, Structural Inorganic Chemistry, 3rd ed., Oxford
University Press, Oxford, UK, 1962.
[9] Y. Tsujimoto, C. Tassel, N. Hayashi, T. Watanabe, H. Kageyama,
K. Yoshimura, M. Takano, M. Ceretti, C. Ritter, W. Paulus,
Nature 2007, 450, 1062 – 1065.
[10] a) M. A. Hayward, M. A. Green, M. J. Rosseinsky, J. Sloan, J.
Am. Chem. Soc. 1999, 121, 8843 – 8854; b) G. D. Blundred, A. B.
Bridges, M. J. Rosseinsky, Angew. Chem. 2004, 116, 3646 – 3649;
Angew. Chem. Int. Ed. 2004, 43, 3562 – 3565; c) V. V. Poltavets,
K. A. Lokshin, S. Dikmen, M. Croft, T. Egami, M. Greenblatt, J.
Am. Chem. Soc. 2006, 128, 9050 – 9051; d) A. Bowman, M. Allix,

 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Angew. Chem. Int. Ed. 2008, 47, 5740 –5745

Angewandte

ChemieD. Pelloquin, M. J. Rosseinsky, J. Am. Chem. Soc. 2006, 128,
12606 – 12607.
[11] M. A. Hayward, E. J. Cussen, J. B. Claridge, M. Bieringer, M. J.
Rosseinsky, C. J. Kiely, S. J. Blundell, I. M. Marshall, F. L. Pratt,
Science 2002, 295, 1882 – 1884.
[12] a) T. Siegrist, S. M. Zahurak, D. W. Murphy, D. W. Roth, Nature
1988, 334, 231 – 232; b) M. Takano, T. Takeda, H. Okada, M.
Miyamoto, T. Kusaka, Physica C 1989, 159, 375 – 378.
[13] a) P. K. Gallagher, J. B. MacChenney, D. N. E. Buchanan, J.
Chem. Phys. 1966, 45, 2466 – 2471; b) S. E. Dann, M. T. Weller,
D. B. Currie, J. Solid State Chem. 1992, 97, 179 – 185.
[14] a) P. Adler, J. Solid State Chem. 1997, 130, 129 – 139; b) L.
Mogni, J. Fouletier, F. Prado, A. Caneiro, J. Solid State Chem.

Angew. Chem. Int. Ed. 2008, 47, 5740 –5745

2005, 178, 2715 – 2723; c) V. M. Zainullina, M. A. Korotin, V. L.
Kozhevnikov, Eur. Phys. J. B 2006, 49, 425 – 431.
[15] J. F. Mitchell, J. E. Millburn, M. Medarde, S. Short, J. D.
Jorgensen, M. T. FernQndez-DRaz, J. Solid State Chem. 1998,
141, 599 – 603.
[16] R. LeToquin, W. Paulus, A. Cousson, C. Prestipino, C. Lamberti,
J. Am. Chem. Soc. 2006, 128, 13161 – 13174.
[17] M. A. Hayward, M. J. Rosseinsky, Nature 2007, 450, 960 – 961.
[18] F. Izumi, T. Ikeda, Mater. Sci. Forum 2000, 321–324, 198 – 203.
[19] J. RodrRguez-Carvajal, J. Phys. B 1993, 192, 55 – 69.
[20] I. D. Brown, D. Altermatt, Acta Crystallogr. Sect. B 1985, 41,244 – 247.

 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.angewandte.org

5745