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.