Effect of the distortion of FeX4 (X = P, As) tetrahedron for the electronic structure of iron-pnictide system

We have performed an ab initio band structure calculation for the new high-Tc related iron-pnictide compounds LaFeXO (X = P, As), BaFe2As2, CaFe2As2 and LiFeAs (X = P, As). We found that LaFeXO and CaFe2As2 have many similarities in their band structures, which is expected by an ionic model. We found that the degree of distortion of FeAs4 tetrahedra in LaFeAsO considerably changes the slope of the density of states near the Fermi level, and this result may explain why REFeAsO (RE = Nd, Sm, ...) have higher Tc than LaFeAsO when electrons are doped. For all the above compounds, the density of states at the Fermi level decreases when X atoms approaches to the Fe-Fe plane, which means that the hybridization between Fe and X orbitals considerably expands the Fe d-bands.