Relationship between crystal structure and superconductivity in iron-based superconductors

We studied the relationship between crystal structure and superconductivity in iron-based superconductors. The superconducting phase diagram of LnFeAsO1−y (Ln = rare-earth) is established as a function of the oxygen deficient content which is determined by Rietveld refinement. The relationship between T c and the Fe–(Pn,Ch)–Fe bond angle (Pn = pnictogen, Ch = chalcogen) indicates that T c attains a maximum value when Fe(Pn,Ch)4-tetrahedrons form a regular shape. A higher T c value could be attained with samples containing regular Fe(Pn,Ch)-tetrahedrons, an Fe–(Pn,Ch) bond length that is longer than the Fe–As bond, and an intermediate (Fe plane)–(Fe plane) distance of 9 Å–15 Å.