title: CaFeO2: a new type of layered structure with iron in a distorted square planar coordination.
abstract: 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.
authors.affiliation: Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan.
authors.forename: Cédric
authors.initials: C
authors.lastname: Tassel
authors.forename: José Miguel
authors.initials: JM
authors.lastname: Pruneda
authors.forename: Naoaki
authors.initials: N
authors.lastname: Hayashi
authors.forename: Takashi
authors.initials: T
authors.lastname: Watanabe
authors.forename: Atsushi
authors.initials: A
authors.lastname: Kitada
authors.forename: Yoshihiro
authors.initials: Y
authors.lastname: Tsujimoto
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Kageyama
authors.forename: Kazuyoshi
authors.initials: K
authors.lastname: Yoshimura
authors.forename: Mikio
authors.initials: M
authors.lastname: Takano
authors.forename: Masakazu
authors.initials: M
authors.lastname: Nishi
authors.forename: Kenji
authors.initials: K
authors.lastname: Ohoyama
authors.forename: Masaichiro
authors.initials: M
authors.lastname: Mizumaki
authors.forename: Naomi
authors.initials: N
authors.lastname: Kawamura
authors.forename: Jorge
authors.initials: J
authors.lastname: Iñiguez
authors.forename: Enric
authors.initials: E
authors.lastname: Canadell
mesh_terms: 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
chemical_list: D005296:Ferrous Compounds; D010100:Oxygen; D002118:Calcium