Synthesis, structure, and phase diagram of (Sr1−xNax)Fe2As2 superconductors

Abstract

(Sr1−xNax)Fe2As2 is a family of the so-called ‘122 system’ of Fe-based superconductors. So far, samples with a limited composition range, i.e. polycrystalline samples for 0 ≤ x ≤ 0.42 with a maximum transition temperature (Tc) of 26 K and a single crystal with x = 0.5 and Tc = 35 K, have been synthesized. Here, we report the synthesis of single-phase polycrystalline samples of (Sr1−xNax)Fe2As2 for a wide range of x from 0 to 0.70 using a stainless-steel pipe-and-cap method. The susceptibilities in the normal and superconducting states were measured as a function of x, and an overall superconducting phase diagram was obtained. As x increased, the structural and anti-ferromagnetic phase transition temperature decreased, and superconductivity began to emerge around x = 0.35. Tc exhibited a dome-like composition dependence with a maximum onset Tc of 36.5 K for x = 0.55. The Rietveld refinement of the powder x-ray diffraction pattern at room temperature for x = 0.55 revealed structural parameters such as the lattice parameters a = 3.857(1) Å and c = 12.564(1) Å, an Fe–As bond length lFe–As = 2.394(1) Å, an As–Fe–As bond angle αAs–Fe–As = 107.3(1)°, and an As height from the Fe layer hAs = 1.419(1) Å.