Electronic structure and chemical bonding in novel tetragonal phase   Ca10(Pt4As8)(Fe2As2)5 as a parent material for the new family of high-TC   iron-pnictide superconductors
  By means of first-principles calculations, the electronic structure and chemical bonding for the recently discovered tetragonal (s.g. P4/n; # 85) superconducting (Tc ~ 25K) phase Ca10(Pt4As8)(Fe2As2)5 have been examined in details, and the optimized structural parameters, electronic bands, densities of states, and inter-atomic bonding picture were evaluated and analyzed in comparison with related layered iron-based superconducting materials. We have shown that (i) Ca10(Pt4As8)(Fe2As2)5 is metallic-like, and the electronic bands in the window around the Fermi level are formed mainly by the Fe 3d states of (Fe2As2)5 blocks; (ii) the (Pt4As8) blocks will behave as semi-metals with very low densities of states at the Fermi level; (iii) the near-Fermi bands adopt a "mixed" character: simultaneously with quasi-flat bands, a series of high-dispersive bands which intersect the Fermi level was found; (iv) the of chemical bonding in Ca10(Pt4As8)(Fe2As2)5 is very complicated and includes an anisotropic mixture of covalent, metallic, and ionic inter-atomic and inter-block interactions. 
I.R. Shein and A.L. Ivanovskii
cond-mat.supr-con cond-mat.mtrl-sci