Substitution and doping in iron pnictides

For insight into the composition-dependent electronic structure of iron pnictides, we performed a systematic study of spatial structure and electronic states by x-ray diffraction and x-ray absorption. A large number of compositions in the (Ba,Sr,AM)(Fe,TM)₂(As,P)₂ family of compounds was investigated, covering the substitution of Ba by Sr or alkali metals (AM); of Fe by transition metals (TM); and of As by P. Our observations on doping effects upon such substitutions include “reluctant” doping (charge carriers are only partially transferred away from the substituent) or “site-decoupled” doping (transferred charge carriers affect either Fe sites or As sites but not both). Here, we focus on isovalent substitutions. The energy-level schemes derived from electronic and spatial structure differ from the “standard” ones in ways that may help explain the systematics in these substitution systems. Indirect, structural effects of substitution appear to be more important for magnetism and superconductivity in iron pnictides than the direct, charge-carrier doping effects.