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 Ca-10(Pt4As8)(Fe2-xPtxAs2)(5), the divalent As-2 produces As-2 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 As-2 molecules between the adjacent FeAs layers. This transition can be viewed as a valence transition between As3- and As2-. These properties of arsenic that produces various chemical bonds can be used to create new superconducting materials.