Understanding the Unique Structural and Electronic Properties of SrFeO2
  We report a first-principles study of SrFeO$_2$, an infinite-layer oxide with Fe atoms in a perfect square-planar coordination down to essentially 0 K. Our results reveal this striking behavior relies on the double occupation of the 3$d_{z^2}$ orbitals of high-spin Fe$^{2+}$. Such an electronic state results from the hybridization of iron's 3$d_{z^2}$ and 4$s$ orbitals, which enables a large reduction of the intra-atomic exchange splitting of the $z^2$ electrons. The generality of the phenomenon is discussed. 
J.M. Pruneda, Jorge Iniguez and Enric Canadell (ICMAB-CSIC), Hiroshi   Kageyama and Mikio Takano (Kyoto University)