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

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

Journal of the American Chemical Society2008Vol. 131(1), pp. 221–229

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Cédric Tassel, Miguel Pruneda, Naoaki Hayashi, Takashi Watanabe, Atsushi Kitada, Yoshihiro Tsujimoto, Hiroshi Kageyama, Kazuyoshi Yoshimura, Mikio Takano, Masakazu Nishi, Kenji Ohoyama, Masaichiro Mizumaki, Naomi Kawamura, Jorge Íñiguez, Enric Cañadell

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.

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Abstract

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