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.

中文翻译：

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定制 Ca/K-1144 超导体的临界温度：异价取代对四方性的影响

在铁基超导体中，所谓的 1144 系列近年来因其化学计量性质而引起了人们的极大兴趣，其材料对化学不均匀性具有很强的鲁棒性，并且具有明确的临界温度。研究最多的 1144 种化合物的特征是 A ₁ AE ₁ Fe ₄ As ₄化学成分，其中 A 和 AE 分别构成碱金属和碱土金属的适当组合。1144 结构实际上只有当 A 和 AE 元素在相对大小和母体化合物结构方面满足特定要求时才会形成。然而，化学计量方面是他们的强项之一，但直到今天仍然是一种限制，将 1144 种结构的概念化限制为四元化合物。在这项工作中，我们证明，要获得 1144 晶相，可能足以在插入 Fe-As 平面的不同大小的离子之间保持 1:1 的比例，并且在选定的条件下，适当定制的阳离子取代是可能的。使用简单的机械化学辅助合成路线 1144 种化合物，其中 Ca 被 Na 取代，K 被 Ba 取代，并且两者同时获得。我们证明掺杂化合物的临界温度不仅仅与取代量或所产生的 Fe 价有关。我们表明，超导转变实际上与化学成分变化引起的结构变形有关：通过调整化学成分，我们获得了双取代样品——取代水平高达 20%——以四方比为特征C/A类似于原始化合物和大约 34 K 的临界温度。