The role of chemical composition in the synthesis of Ca/K-1144 iron based superconductors

The Iron Based Superconductors are a class of materials that attracts significant interest both for theoretical studies and practical applications due to their high magnetic critical fields and superconducting critical current densities. In this work, we describe a novel synthesis route to produce the CaKFe4As4 compound that consists of a mechanochemical treatment coupled to a mild thermal process. The high energy ball milling step is found to promote the reaction among the pure elements, with the nucleation of a 1144 nanostructured phase observed on the milled compound. The activation of the powders promoted by the mechanochemical step is effective to significantly lower the synthesis temperature with respect to previous literature (T ~ 900 °C), with the 1144 phase obtained in the 500–700 °C temperature range. Furthermore, the effect of an alkaline/alkaline-earth depletion or an iron enrichment on the starting chemical composition is studied: chemical variations are reflected in the formation of secondary phases, without however significantly influencing the superconducting properties of the samples. The superconducting critical current, in particular, seems limited by other extrinsic factors such as porosity and oxygen contamination at the boundaries of dense aggregates produced during the milling step. The reported results suggest that the mechanochemically assisted low temperature method constitutes a promising synthesis route for Ca/K 1144 compounds, highlighting at the same time the need to focus on purification and densification methodologies in view of the development of wires through the powder in tube method.