Pressure-Induced Isostructural Phase Transition and Correlation of FeAs Coordination with the Superconducting Properties of 111-Type Na1–x FeAs

The effect of pressure on the crystalline structure and superconducting transition temperature (T c) of the 111-type Na1–x FeAs system using in situ high-pressure synchrotron X-ray powder diffraction and diamond anvil cell techniques is studied. A pressure-induced tetragonal to tetragonal isostructural phase transition was found. The systematic evolution of the FeAs4 tetrahedron as a function of pressure based on Rietveld refinements on the powder X-ray diffraction patterns was obtained. The nonmonotonic T c(P) behavior of Na1–x FeAs is found to correlate with the anomalies of the distance between the anion (As) and the iron layer as well as the bond angle of As–Fe–As for the two tetragonal phases. This behavior provides the key structural information in understanding the origin of the pressure dependence of T c for 111-type iron pnictide superconductors. A pressure-induced structural phase transition is also observed at 20 GPa.