Progress and perspectives in materials chemistry of iron-based superconductors

My talk reviews discoveries and recent developments of superconducting iron based compounds from the view of materials chemistry. Initiated by the first observations of superconductivity in the pnictide-oxides LaFePO (2006) and LaFeAsO_1-xF_x (2008), tremendous activities have meanwhile spawned an impressive family of iron-based superconducting materials, which is not inferior to the cuprates. Superconductivity in iron-based compounds emerges in Fe₂X₂ layers (X = As, P, Se, S) as the common structural motif. It is intriguing how combinations of these active layers with a variety of separating blocking layers yielded many new superconductors with partly novel crystal structures, for example (CaFeAs)₁₀Pt₄As₈ with T_c up to 40 K. Remarkable combinations of separating layers occur in the recently discovered self-doped compounds RbFe₂As₂[GdOFeAs]₂ and KFe₂As₂[CaFFeAs]₂. Subtle manipulations of the Fe₂X₂ or the blocking layers by doping control the superconducting properties. Especially the 122-type compounds based on BaFe₂As₂ proved to be extremely adaptive to doping. Superconductivity in iron arsenides emerges during destabilization of antiferromagnetism by doping or pressure, while the intertwining with magnetism is less clear in iron selenides where nematic magnetism plays a role. The exciting discovery of superconductivity up to T_c = 100 K in FeSe single layers on STO raised the question if wide separation of FeSe layers by intercalation may also produce such high T_c. For example [(Li_1-xFe_x)OH]FeSe exhibits T_c = 42 K and shows the rare coexistence of ferromagnetism and superconductivity. In spite of this rich crystal chemistry, the highest T_c in iron based bulk materials is currently 55 K. However, since the true factors for high T_c are not yet understood, only explorative synthesis of further iron based or related systems can pave the way to higher T_c above the important 77 K threshold.