Of Substitution and Doping: Spatial and Electronic Structure in Fe Pnictides

A highly intriguing aspect in iron-pnictide superconductors is the composition-dependent electronic structure, in particular the question if and how charge carriers are introduced to the system upon substitution of Ba by alkali metals or of Fe by other transition metals, TM. We report on a systematic study of spatial structure and electronic states by x-ray diffraction and x-ray absorption on a large number of compositions in the (Ba,K)(Fe,TM)^sub 2^As^sub 2^ family. The coherent combination of detailed structural information with an in-depth analysis of the electronic structure allows us to sensitively disentangle (charge-carrier) "doping" effects from "substitutional" effects. Results include a doping character that is site-decoupled, as well as TM 3d energy-level schemes that exhibit non-standard level sequences and even t^sub 2^-e level crossings. Our study indicates that doping per se seems to play a lesser role than expected for pnictide superconductivity and magnetism.