Optimization of superconducting properties of the stoichiometric CaKFe₄As₄

Abstract

CaKFe4As4 ( 1144) is a unique stoichiometric iron-based superconductor which harbours 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 asses their structural, magnetic and transport properties. The samples were prepared over a wide temperature range (900-1100 • 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 × 10 4 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 interand intra-granular behaviour 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.

Mechanochemically assisted low temperature synthesis route of the 1144 Ca-K Iron Based Superconductor

“…Our results demonstrate that the reaction activated by the HEBM treatment proceeds to form the 1144 phase at 600 • C. The obtainment of the 1144 phase at low temperature is in agreement with DFT calculations [12] but constitutes a complete novelty with respect to previous results [4,8,11]. We suggest that the high reactivity and uniformity of the elemental powders promoted by the HEBM treatment play a crucial role in this: the mechanochemical treatments is expected to induce the intimate mixing of the elements with high homogeneity degrees on a local scale, and this seems necessary to promote at low temperature the reaction towards the formation of the mixed 1144 phase without the production of separate 122 compounds.…”

“…It is worth to comment on the general opinion that the 1144 phase cannot be obtained at low temperatures [11]. In particular, it has been reported recently a quite narrow temperature range for the formation of the polycrystalline 1144 phase (e.g.…”

“…Several works have been performed on single crystals, grown via the FeAs flux method at temperatures higher than 900 • C [6]. Synthesis of polycrystalline samples have been carried out in similar conditions, with ultra-fast heating methods, high temperature treatments (T > 900 • C) and quench cooling procedures [4,[7][8][9][10][11]. These synthesis methods, despite producing high quality polycrystalline samples, are however not free of drawbacks.…”

“…In particular, it has been reported recently a quite narrow temperature range for the formation of the polycrystalline 1144 phase (e.g. 925 • C-975 • C range in [11]). In the absence of a well assessed phase diagram, the research regarding the actual field of stability of the 1144 phase is however still open.…”

Critical Current and Pinning Features of a CaKFe4As4 Polycrystalline Sample

“…In Figure 4 , the obtained J c (H) curves have been reported for different temperatures. The obtained J c values are in agreement with the ones reported in the literature for other polycrystalline CaKFe 4 As 4 samples [ 16 , 22 , 33 ].…”