Optimization of superconducting properties of the stoichiometric CaKFe₄As₄

CaKFe₄As₄ (1144) is a unique stoichiometric iron-based superconductor which harbors high upper critical fields and large critical current densities. In this work, we describe a study to optimize the synthesis conditions of stoichiometric polycrystalline samples of CaKFe₄As₄ and assess their structural, magnetic and transport properties. The samples were prepared over a wide temperature range (900 °C–1100 °C) and the pure phase formation is centered around 955 °C. Outside this temperature region, impurity phases of KFe₂As₂ and CaFe₂As₂ can also form. Magnetic susceptibility and resistivity measurements establish that the critical temperature reaches ∼34 K for the optimum synthesis conditions and the critical current reaches 2 × 10⁴ A cm⁻². The post-annealing process demonstrates the stability of the 1144 phase up to 500 °C, however, under higher temperature annealing, phase degradation occurs. Our study indicates that the formation of phase-pure 1144 occurs over a much narrower window and its highly prone to multi-phase formation as compared with the 122 family. As a result, the superconducting properties are enhanced for the pure 1144 phase but they are likely to be affected by the inter- and intra-granular behavior originating from the microstructural nature of polycrystalline CaKFe₄As₄, similar to other iron-based superconductors. Based on our study, we construct the phase diagram for polycrystalline 1144 and compared it with that reported for 1144 single crystal. (paper)