Arsenic chemistry of iron-based superconductors and strategy for novel superconducting materials

The progress of materials discovery of iron-based superconductors is reviewed with the emphasis on the valence states and chemical bonds of arsenic. We demonstrate that monovalent As^– produces the 112-type CaFeAs₂ with arsenic zigzag chains. When co-doping of La and Sb is performed, the superconducting transition temperature rises to 47 K. In the 10-4-8-type Ca₁₀(Pt₄As₈)(Fe_2−xPt_xAs₂)₅, the divalent As^2– produces As₂ molecules, and creates an interlayer substance with PtAs₄ planar squares. The maximum superconducting transition temperature is 38 K. In the 122-type CaFe₂As₂, Rh doping induces a lattice collapse transition accompanying the formation of As₂ molecules between the adjacent FeAs layers. This transition can be viewed as a valence transition between As^3– and As^2–. These properties of arsenic that produces various chemical bonds can be used to create new superconducting materials.