Iron-based superconductors, discovered in 2008, have attracted considerable attention due to their promising superconducting properties and their potential for high-field applications.
They share a FeAs or FeSe structural building blocks and exhibit a wide variety of structural families. Among them, the 1144 phase (AAeFe₄As₄ A=Alkaline and Ae=Alkaline earth metal), discovered in 2016, has attracted particular interest due to its robust superconducting properties. Within this family, CaKFe₄As₄ is considered one of the most promising compounds. The synthesis of these materials in polycrystalline form, relevant for superconducting wire applications, strongly affects their superconducting performance. In particular, intergrain properties, secondary phases, grain-boundary chemistry, and microstructural features such as porosity and density play a crucial role.
Chemical substitutions have been explored to optimize its superconducting properties, with partial replacement of Ca and K by elements with similar ionic radius (Na, La, Pr, Sr, and Ba). Previous studies on polycrystalline samples suggested that doping induces lattice distortions affecting the superconducting critical temperature (Tc), and enhancing the critical current density (Jc), although microstructural effects could not be excluded.
To distinguish intrinsic effects from microstructural contributions, single crystals of pristine and doped CaKFe₄As₄ were grown and investigated by single-crystal X-ray diffraction, from which the corresponding CIF files were obtained. Complementary morpho-structural and superconducting measurements were also performed to correlate the crystal structure with the physical properties.
The combined analysis highlights a clear correlation between structural modifications and superconducting behavior. Furthermore, the structural analysis suggests that defects introduced by doping act as effective pinning centers, explaining the observed enhancement of Jc.
