Evidence of the isoelectronic character of F doping in SmFeAsO1-xFx: a first-principles investigation

We study the electronic structure of the SmFeAsO1-xFx 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 SmFeAsO1-xFx. For energies within a narrow energy range accross E-F, 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 SmO1-xFx 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 SmFe1-xCoxAsO system shows that such charge compensation by the Sm ion is not shared by donors substituting at the Fe site.