Evidence of the isoelectronic character of F doping in SmFeAsO 1-x F x : a first-principles investigation.

We study the electronic structure of the SmFeAsO _1-x F _x alloy by means of first-principle calculations. We find that, contrary to common believe, F-doping does not change the charge balance between electrons and holes free-carriers in SmFeAsO _1-x F _x . For energies within a narrow energy range accross [Formula: see text], the effect of F-doping on the band structure dispersion is tiny in both the paramagnetic and stripe antiferromagnetic phase. The charge balance between the conducting FeAs-layer and the SmO _1-x F _x charge reservoir layer is not influenced by the compositional change. The additional charge carried by fluorine, with respect to the oxygen, is compensated by a change in the oxidation state of the Sm ion from 3+  to 2+. A comparison with the SmFe _1-x Co _x AsO system shows that such charge compensation by the Sm ion is not shared by donors substituting at the Fe site.