Substitution and doping in iron pnictides

The composition-dependent electronic structure of iron pnictides, in particular the question if and how charge carriers are introduced to the system upon substitution -- by Sr or alkali metals (\textit{AM}) for Ba; by transition metals (\textit{TM}) for Fe; and/or by P for As -- continues to provide surprises. Our systematic study of spatial structure and electronic states by x-ray diffraction and x-ray absorption, performed on a large number of compositions in the (Ba,Sr,\textit{AM})(Fe,\textit{TM})$_{2}$(As,P)$_{2}$ family of compounds, shows, for instance, that valences are often different from expectations, that doping can be ``effective'' or not, depending on substituent, and that many doping effects are ``site decoupled'', meaning that they occur \textit{either} on the Fe site \textit{or} on the As site but not on both simultaneously. Furthermore, the energy-level schemes derived from electronic and spatial structure differ from the present ``standard'' assumptions. In all, it appears that indirect, structural effects of substitution may in many cases be more important for magnetism and superconductivity in iron pnictides than direct, charge-carrier doping effects.