Optimization of a non-arsenic iron-based superconductor for wire fabrication

We report on the optimization of synthesis of iron selenide-based superconducting powders and the fabrication of selenide-based wire. The powders were synthesized by an ammonothermal method, whereby Ba is intercalated between FeSe layers to produce Ba_x(NH₃)_yFe₂Se₂, with tetragonal structure similar to AFe₂X₂ (X: As, Se), '122', superconductors. The optimal T_c (up to 38 K) and Meissner and shielding superconducting fractions are obtained from the shortest reaction time (t) of reactants in liquid ammonia (30 min). With the increase of t, a second crystalline 122 phase, with a smaller unit cell, emerges. A small amount of NH₃ is released from the structure above ∼200 °C, which results in loss of superconductivity. However, in the confined space of niobium/Monel tubing, results indicate there is enough pressure for some of NH₃ to remain in the crystal lattice, and thermal annealing can be performed at temperatures of up to 780 °C, increasing wire density and yielded a reasonable T_c ≈ 16 K. Here, we report of the first successful wire fabrication of non-arsenic high-T_c iron-based superconductor. Although bulk materials are estimated to carry critical current densities >100 kA cm^-2 (4 K, self-field), the current transport within wires need to be optimized (J_c ∼ 1 kA cm^-2).