Substitution and ``site-decoupled 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 ongoing 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})$_{\mathrm{2}}$(As,P)$_{\mathrm{2}}$ family of compounds, shows, for instance, that valences are rarely as expected, that doping can be ``effective'' or ``slow'' depending on substituent, and that most doping effects are completely ``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 substitutional effects may be more crucial to magnetism and superconductivity in iron pnictides than direct doping effects.