CaFeO2: a new type of layered structure with iron in a distorted square planar coordination.
CaFeO(2), a material exhibiting an unprecedented layered structure containing 3d(6) iron in a high-spin distorted square-planar coordination, is reported. The new phase, obtained through a low-temperature reduction procedure using calcium hydride, has been characterized through powder neutron diffraction, synchrotron X-ray diffraction, Mossbauer spectroscopy, XAS experiments as well as first-principles DFT calculations. The XAS spectra near the Fe-K edge for the whole solid solution (Sr(1-x)Ca(x))FeO(2) supports that iron is in a square-planar coordination for 0 </= x </= 0.8 but clearly suggests a change of coordination for x = 1. The new structure contains infinite FeO(2) layers in which the FeO(4) units unprecedentedly distort from square-planar toward tetrahedra and rotate along the c-axis, in marked contrast to the well-studied and accepted concept that octahedral rotation in perovskite oxides occurs but the octahedral shape is kept almost regular. The new phase exhibits high-spin configuration and G-type antiferromagnetic ordering as in SrFeO(2). However, the distortion of the FeO(2) layers leads to only a slight decrease of the Neel temperature with respect to SrFeO(2). First-principles DFT calculations provide a clear rationalization of the structural and physical observations for CaFeO(2) and highlight how the nature of the cation influences the structural details of the AFeO(2) family of compounds (A = Ca, Sr, Ba). On the basis of these calculations the driving force for the distortion of the FeO(2) layers in CaFeO(2) is discussed.
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Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan.
Pruneda
José Miguel
JM
Hayashi
Naoaki
N
Watanabe
Takashi
T
Kitada
Atsushi
A
Tsujimoto
Yoshihiro
Y
Kageyama
Hiroshi
H
Yoshimura
Kazuyoshi
K
Takano
Mikio
M
Nishi
Masakazu
M
Ohoyama
Kenji
K
Mizumaki
Masaichiro
M
Kawamura
Naomi
N
Iñiguez
Jorge
J
Canadell
Enric
E
D002118:Calcium / Q000737:chemistry*; D005296:Ferrous Compounds / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D033363:Neutron Diffraction; D010100:Oxygen / Q000737:chemistry*; D015204:Spectroscopy, Mossbauer; D014961:X-Ray Diffraction
D005296:Ferrous Compounds; D010100:Oxygen; D002118:Calcium