We have investigated the local structure of the iron-based CaKFe4As4 superconductor featuring distinct aliovalent substitutions at the Ca and K sites, that is CaKFe4As4, CaK0.9Sr0.1Fe4As4, CaK0.9Ba0.1Fe4As4 and Ca0.9Na0.1K0.9Ba0.1Fe4As4. Temperature-dependent Fe K-edge extended x-ray absorption fine structure (EXAFS) measurements are used to determine the near-neighbors bondlengths and their stiffness. The EXAFS analysis reveals that the Fe-As bondlength undergoes negligible changes by substitution, however, the Fe-Fe bondlength and the As height are affected by the Sr substitution. The superconducting transition temperatures of CaK0.9Sr0.1Fe4As4 and CaK0.9Ba0.1Fe4As4 are very similar even if the mean As heights are significantly different suggesting that the anion height may not be a unique parameter to describe the superconductivity in CaKFe4As4. The mean As heights show a peculiar temperature dependence characteristic of CaKFe4As4 system. Furthermore, the temperature-dependent mean square relative displacements reveal similar Fe-Fe bond stiffness in all samples, instead the Fe-As bond is substantially stiffer in case of CaK0.9Sr0.1Fe4As4. The local structure results are discussed in relation to the differing transport properties of aliovalent substituted 1144 superconductor.

1144 is a family of Iron Based Superconducting Compounds that attracts interest due to its regular structure, composed by an alternance of alkaline and alkaline-earth planes that intercalate Fe-As layers. This rigidity grants to 1144 compounds a stoichiometric nature, and thus a robust critical temperature not affected by chemical inhomogeneities, and a peculiar pinning landscape enhancing critical currents at high fields. Critical currents can be however further enhanced by introducing further defects by means of irradiation and chemical doping on the Fe site. In our works, as a different approach, we evaluate the effect of the alkaline and alkaline-earth aliovalent substitutions. For this scope, CaKFe4As4 polycrystalline powders have been successfully doped by partially replacing Ca with Na and K with Ba by means of a mechanochemically assisted thermal synthesis. Such substituted compounds crystallize in the P4/mmm structure typical of 1144 compounds. X-ray diffraction analysis revealed that depending on the level of substitution the reflections characteristic of the P4/mmm tend to vanish. In particular, peaks analysis has shown that doping introduces increasing disorder in selected crystalline planes. Barium and sodium, as dopant, do not act the same way, the latter being less invasive, so that 1144 structure is preserved up to 40% substitution. The contemporary replacement of potassium and calcium with barium and sodium exhibits an intermediate behavior. Analysis of X-ray diffraction profiles collected as a function of temperature down to 100 K allowed to calculate the thermal expansion coefficients for the synthesized compounds, with CTE values similar for pristine and substituted compounds close to 1 x 10(-6)K(-1) and 3 x 10(-5)K(-1) respectively along the a-axis and c-axis.

The 1144 phase (Ae1A1Fe4As4) shows a strong advantage of engineering fabrication among Fe (Iron)-based superconductor (FBS) family due to the robustness of its superconducting properties with respect to chemical inhomogeneities, granted by its uniform crystalline-layered structure. This regularity is furthermore associated to crystalline defects capable of acting as efficient pinning centers, from which high critical currents can achieved at high fields. Like other FBS phases, its lossless current-carrying capability can be remarkably degraded by distractions at grain boundaries (GBs). GB oxidation is an issue of upmost importance to the realization of the practical FBS application for high field (> 20T) magnet. In this study, we explore oxidized grain boundary and intrinsic grain structural properties of 1144 polycrystalline samples by applying analytical electron microscopy such as atomic resolution scanning transmission electron microscopy and atom probe tomography. These structural properties of samples produced by a mechanochemically assisted synthesis are evaluated following the degradation of superconducting properties due to oxidation. We observe a strong correlation between the contamination at grain boundaries and the decrease of transport properties of the bulk sample, while the crystallin structure seems to be not affected by the oxidation.

Among Iron Based superconductors (IBSC), the compounds belonging to the 1144 family are characterized by the A1AE1Fe4As4 chemical composition (A=Alkaline, AE=Alkaline-Earth) and considered stoichiometric.In recent experiments, we obtained 1144 samples characterized by different levels of alkali and alkaline earth metals substitutions and demonstrated how the lattice distortion induced by such variation in the chemical composition plays a fundamental role on critical temperature values.In this work, we investigate the possibility to obtain Ca/K-1144 compounds substituted with Rare-Earth (RE) elements.We produced polycrystalline samples in which RE ions partially replace Ca ions in the 1144 phase.The lattice structure is consistently affected by the substituent element inducing a contraction of the c-axis.Multiple substitution of RE and A or AE elements was also attempted, showing that appropriate combinations allow simultaneous inclusion of different elements in the structure.For compounds that include RE elements, the critical temperature of the samples appears to follow to the same trend with respect to the distortion of the crystal lattice induced by A and AE substitutions.

The 1144 Iron-Based Superconductors (IBSC), characterized by the A 1 AE 1 Fe 4 As 4 chemical composition (A=Alkaline, AE=Alkaline-Earth), has gained significant interest in the recent years due to their crystalline structure character-ized by an intrinsic modulation of the strain along the c-axis and by the proneness in forming crystalline defects with a positive impact on flux pinning. Moreover, it has been proved that the compound is compatible with the cost-effective powder-in-tube (PIT) manufacturing process. In our recent experiments, we have showed that the A 1 AE 1 Fe 4 As 4 structure can be tailored to obtain a 1144 com-pound characterized by different (A x AE 1-x )(AE y A 1-y )Fe 4 As 4 chemical formulae without any depression in the critical temper-ature value. On the other hand, it has been recently shown that the doping with selected elements on both A and AE sites has a clear influence on the pinning and the grain boundary properties of poly-crystalline samples. In this work we report the results of the extensive magnetic characterization performed on pristine Ca 1 K 1 Fe 4 As 4 and doped (A x Ca 1-x )(AE y K 1-y )Fe4As4 with Ba as dopant on the K site and ei-ther La or Na as dopant on the Ca site. In particular, the magnet-ization hysteresis loops recorded at different temperatures M(H, T) and the critical current density dependences extracted from the M(H) using the Bean model, J c (B, T), have been analyzed in order to assess the effect of aliovalent doping on the quality of the produced samples in terms of grain boundary properties and pinning efficiency.

The 1144 phase (Ae 1 A 1 Fe 4 As 4 ) shows a strong advantage of engineering fabrication among Fe (Iron)-based superconductor (FBS) family due to the robustness of its superconducting properties with respect to chemical inhomogeneities, granted by its stoichiometric nature. This regularity is furthermore associated to defects capable of acting as efficient pinning centers with high critical currents achieved at high fields for these superconductors. Like other FBS phases, its lossless current-carrying capability can be remarkably degraded by distractions at grain boundaries (GBs). GB oxidation is an issue of upmost importance to the realization of the practical FBS application for high field (>20 T) magnet. In this study, we explore oxidized grain boundary and intrinsic grain structural properties of 1144 polycrystalline samples by applying analytical electron microscopy such as atomic resolution scanning transmission electron microscopy and atom probe tomography. These structural properties of 1144 samples are evaluated following the degradation of superconducting properties due to oxidation. We observe a strong correlation between the contamination at grain boundaries and the decrease of transport properties of the bulk sample, while the bulk crystalline structure is not affected by the oxidation. crystalline

Among IBSC superconductors, the 1144 family ( AAE Fe4As4 with A = alkali metal, AE = alkaline-earth metal), has attracted considerable interest in recent years because of their propensity to form lattice defects that positively influence the flux pinning properties. Extensive research is underway to optimize this class of materials for the low-temperature high-field regime, both in the form of single crystal and polycrystalline powder. The latter is of particular interest because the 1144 material has been shown to be suitable for wire production through the easy and well-assessed Powder In Tube (PIT) process. In our recent experiments, it has been shown that the 1144 structure can be tailored to obtain a doped compound with a different chemical formula ( A x AE 1-x )( AE y A 1-y )Fe 4 As 4 , that is characterized by unaltered critical temperature values despite the high level of substitution up to 15 at.%. In order to unveil the influence of the double substitution on the grain boundary and pinning properties of doped samples, we recently started an extensive characterization campaign whose preliminary results are presented here. Both structural and morphological properties of the compound are not affected by the simultaneous doping with 10 at.% Na and 5 at.% Ba, as revealed by XRD and SEM analyses. On the contrary, the study of magnetic properties shows that although there is no improvement in transport properties, the aliovalent substitution induces a clear change in the in-field behavior which deserves further investigations.

Ca/K-1144 compounds have been described widely in literature as stoichiometric compounds, with a Ca:K ratio equal to unity. In this work, we demonstrate that Sr ions can substitute K ions in the 1144 lattice. Polycrystalline samples were produced via a mechanochemically assisted synthesis route. Sr substitution in the Ca/K compound leads to a shortening of its c axis, while no differences are appreciable from the morphological point of view. The CaK0.9Sr0.1Fe4As4compound is characterized by a slightly reduced critical temperature of approximately 31 K, with respect to the 35 K of the pristine compound. The role of lattice distortion, carrier concentration or structural disorder that may be accounted for the reduction of critical temperature is here discussed.

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 A 1 AE 1 Fe 4 As 4 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.