Role of fluorine in the iron pnictides: phonon softening and effective hole-doping*

Using first principles techniques, we investigate the influence of fluorine doping on the electronic structure, lattice dynamics and electron-phonon coupling in LaFeAsO. We explicitly simulate the F-doping using a supercell model in order to explore properties not described by virtual crystal models. Our analysis reveals that local lattice relaxation accompanies the fluorine doping and modifies the lattice dynamics, in accord with recent experimental data. In addition, it is found that the charge density of the doped electrons cannot be described by simplified models of electron-doping in the two-dimensional Fe-plane.