Structure, Magnetism, and Superconductivity of the Layered Iron Arsenides Sr_1-xNa_xFe₂As₂

The synthesis and evolution of the structure, magnetic ordering, and superconductivity of the layered iron arsenides Sr 1- x Na x Fe 2 As 2 is reported. In the Sr 1- x Na x Fe 2 As 2 solid solution, the limiting Na-rich composition in samples made using conventional solid state synthesis at elevated temperatures occurs at an unusually small value ( x = 0.4) compared with other alkali-metal-doped alkaline earth iron arsenides. Above this limiting value of the sodium content, competing phases are formed: for x = 0.42, an elemental iron impurity is evident, and additional impurities appear for x > 0.42. Superconductivity is detected in the compositions approaching the phase limit ( T c = 26 K for x = 0.4) in line with analogous isoelectronic materials. However, the magnetically ordered state which competes with the superconducting state appears not to be completely suppressed even at the limiting composition. The Na doping of SrFe 2 As 2 is contrasted with the K-doping of SrFe 2 As 2 and Na-doping of BaFe 2 As 2 and other “122” iron arsenide compounds.