CaFeO2: A New Type of Layered Structure with Iron in a Distorted Square Planar Coordination

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• 作者：Cdric Tassel ; Jos Miguel Pruneda ; Naoaki Hayashi ; Takashi Watanabe ; Atsushi Kitada ; Yoshihiro Tsujimoto ; Hiroshi Kageyama ; Kazuyoshi Yoshimura ; Mikio Takano ; Masakazu Nishi ; Kenji Ohoyama ; Masaichiro
• 刊名：Journal of the American Chemical Society
• 出版年：2009
• 出版时间：January 14, 2009
• 年：2009
• 卷：131
• 期：1
• 页码：221-229
• 全文大小：438K
• 年卷期：v.131,no.1(January 14, 2009)
• ISSN：1520-5126

文摘

CaFeO₂, a material exhibiting an unprecedented layered structure containing 3d⁶ 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, Mssbauer 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−xCa_x)FeO₂ 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 FeO₂ layers in which the FeO₄ 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 SrFeO₂. However, the distortion of the FeO₂ layers leads to only a slight decrease of the Nel temperature with respect to SrFeO₂. First-principles DFT calculations provide a clear rationalization of the structural and physical observations for CaFeO₂ and highlight how the nature of the cation influences the structural details of the AFeO₂ family of compounds (A = Ca, Sr, Ba). On the basis of these calculations the driving force for the distortion of the FeO₂ layers in CaFeO₂ is discussed.