CaFeO₂: A new type of layered structure with iron in a distorted square planar coordination

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, Mössbauer 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 Néel temperature with respect to SrFeO_x. 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 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.