Origin and elimination of excess iron particles in off-stoichiometric Ba0.6K0.5-βFe2As2+δ superconductors

Iron impurities frequently observed in BaxK1-xFe2As2 wires and tapes not only cause local suppression of superconductivity but also act as macro-obstacles to transport current. We find that the agglomerated iron particles are not introduced by external sources or caused by incomplete reactions but result from the chemical environments determined by nominal compositions. The excess K added to the raw materials for compensating the evaporated K consumes the stoichiometric arsenic and induces unreacted iron. Decreasing the K content indeed removes the redundant Fe but also deteriorates superconductivity. Instead, slightly excessive addition of As is proved to be a twin-track approach in fabricating high-performance superconductors. Through modulation of the nominal composition Ba0.6K0.5-βFe2As2+δ, we effectively eliminate the iron impurities and improve the transport critical current density of the tape to Jc(4.2 K, 10 T)=9.59×104 A/cm2. A reaction model is proposed to explain the formation mechanism of the iron particles.