Tailoring the critical temperature of Ca/K-1144 superconductors: the effect of aliovalent substitution on tetragonality

Abstract

Abstract Among the iron-based superconductors, the so-called 1144 family has, in recent years, attracted significant interest due to its stoichiometric nature, with materials robust towards chemical inhomogeneities and characterized by a well-defined critical temperature. The most studied 1144 compounds are characterized by the A1AE1Fe4As4 chemical composition, where A and AE constitute an appropriate combination of alkaline and alkaline-earth metals, respectively. The 1144 structure is in fact formed only when the A and AE elements respect specific requirements in terms of relative size and parent compound structure. The stoichiometric aspect, one of their strong points, has represented, however, up to today a restriction, limiting the conceptualization of 1144 structures to quaternary compounds. In this work, we demonstrate that to obtain the 1144 crystalline phase it may be sufficient to maintain a 1:1 ratio between ions of different size that intercalate the Fe-As planes, and that in selected conditions an opportunely tailored cation substitution is possible. Using a simple mechanochemically assisted synthesis route 1144 compounds where Ca is substituted by Na, K by Ba, and both simultaneously, are obtained. We demonstrate that the critical temperature of doped compounds is not simply related to the substitution amount or to the resulting Fe valence. We show that the superconducting transition is in fact linked to the structural distortion induced by the chemical composition variation: by tailoring the chemical composition we obtain doubly substituted samples—with substitution levels up to 20%—characterized by a tetragonality ratio c/a similar to the pristine compound and critical temperatures of approximately 34 K.