Institute of Crystallography - CNR

Relation Between Composition and Crystalline Structure in Substituted CaKFe4As4

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, CaKFe 4 As 4 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 × 10 -6 K -1 and 3 × 10 -5 K -1 respectively along the a -axis and c -axis.

IEEE transactions on applied superconductivity (Online)
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A. Duchenko, A. Masi, A. Augieri, L. Barba, G. Campi, G. Celentano, L. Gigli, J. R. Plaisier, N. Pompeo, F. Rizzo, F. Varsano
Authors IC CNR