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

Abstract

Among the Iron Based Superconductors (IBSC), the so-called 1144 family attracted in recent years 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 in fact 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 compounds 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.

Influence of Rare-Earth Inclusion on Structure and Properties of Ca/K-1144 IBSC

“…The points fall on the left side of the graph, corresponding to the contraction of the c-axis, as also observed when replacing K with Ba and Sr. The trend of the resulting points follows and falls within the bell-shaped pattern shown in Fig 6 and described in our previous work on substitutions with A and/or AE [7]. Furthermore, also for the reported samples, we can exclude a primary role of charge doping on the Tc value, as samples with the same iron valence value such as e.g.…”

“…Critical temperature as a function of the lattice tetragonality ratio for the different substitutions. The curve is taken from [7] and updated with points measured in this work. The series of "multiple" refers to partial double substitutions at different concentrations of Ca with Na and K with Ba i.e.…”

“…Conversely, it is known that the critical temperature of AE/A-122 phases is influenced by the different AE/A ratio [4], [5]. In our previous works [6], [7], we investigated whether it was possible to vary the AE/A ratio in 1144, while still maintaining a clean 1144 phase but influencing the superconducting properties as with the 122 family. For this purpose, we performed partial substitutions in the Ca/K-1144 compound (Ca/K=1) of Ca and K with A and AE metals and showed that such substitutions are possible for ions with similar ionic radius.…”