Crystal chemistry and structural design of iron-based superconductors

The second class of high-temperature superconductors (HTSCs), iron-based pnictides and chalcogenides, necessarily contain Fe₂X₂ ("X" refers to a pnictogen or a chalcogen element) layers, just like the first class of HTSCs which possess the essential CuO₂ sheets. So far, dozens of iron-based HTSCs, classified into nine groups, have been discovered. In this article, the crystal-chemistry aspects of the known iron-based superconductors are reviewed and summarized by employing "hard and soft acids and bases (HSAB)" concept. Based on these understandings, we propose an alternative route to exploring new iron-based superconductors via rational structural design.