Superconductor-in-an-Hour: Spark Plasma Synthesis of Co- and Ni-Doped BaFe₂As₂

Abstract

BaFe2As2 superconductors doped with Co or Ni were synthesized by heat treatment of the reactive BaH2 precursor and elemental Fe, Co, Ni, and As in a sealed niobium container. The hydride route provides better mixing of the starting materials, resulting in faster preparation of the target phases. The hydrogen gas evolved during sintering facilitates the preparation of the oxygen-free powders. Using powder X-ray diffraction (PXRD), including low-temperature synchrotron PXRD, it was shown that this route leads to successful doping of the Co (Ni) dopant and that the actual dopant content in the produced powdered samples of BaFe2–xCoxAs2 (0 ≤ x ≤ 0.4) and BaFe2–xNixAs2 (0 ≤ x ≤ 0.18) is close to the nominal content. The optimally doped samples of BaFe2–xCoxAs2 and BaFe2–xNixAs2 exhibit bulk superconducting properties. The highest Tc for the Co-doped sample is 24 K when x = 0.15–0.175 and 21 K for the Ni-doped sample when x ∼ 0.1. The reaction mechanism between the BaH2 hydride precursor and elemental Fe and As was studied by operando high-temperature synchrotron PXRD. The formation of the target BaFe2As2 phase occurs at the relatively low temperature of 785 K with binary iron arsenide (FeAs) being an intermediate product. Finally, the in situ combined synthesis and sintering of the intimately mixed BaH2 and Fe, Co, and As was performed inside the spark plasma sintering apparatus to obtain compact pellets of the BaFe2–xCoxAs2 superconductors. A higher sintering temperature (>873 K) is needed to ensure a homogeneous Co dopant distribution and, thus, the abrupt superconducting transitions.