Phase transformation and reaction kinetics of BaK122 precursor synthesized via a two-step method

122-Type iron-based superconductors have gained considerable interest due to their potential for high-field applications. A two-step method for preparing BaK122 precursor powders is shown to enhance the superconducting performance of wires and tapes. However, impurities pose formidable challenges to enhancing critical current density, while their underlying causes remain an open question. In this work, the phase formation process of the two-step precursors was investigated. After sintering at 550 °C, iron arsenides and KFe2As2 are generated in large quantities. The superconducting properties of the sample improve significantly when the sintering temperature is increased to 700 °C. However, the distribution of K and Ba elements in the 122-phase remains inhomogeneous. For the samples sintered at 850 °C, impurities are almost eliminated, resulting in the formation of high-purity Ba0.6K0.4Fe2As2. The reaction mechanisms for each exothermic process follow the Avrami-Erofeev equation and are expressed as G(α) = [− ln(1-α)]1/n, which corresponds to the nucleation and crystal growth model. Based on the above results, we summarize the phase formation process of the two-step BaK122 precursor and propose a formation mechanism for the impurities. This study provides valuable insights into optimizing the sintering process and improving the purity of BaK122 precursors. Graphic Abstract