Fe-Based Superconductors of ( Ln_{0.5- x}Na_{0.5+ x})Fe₂As₂ ( Ln = Ce, Pr)

Recently, we succeeded in synthesizing (La_{0.5- x}Na_{0.5+ x})Fe₂As₂ ((La,Na)122) with a solid solution range of 0 ≤ x ≤ 0.35. Superconductivity was induced for 0.15 ≤ x ≤ 0.35, with the highest transition temperature T_c = 27.0 K for x = 0.3. Here, we report the synthesis and physical properties of analogous compounds ( Ln_{0.5- x}Na_{0.5+ x})Fe₂As₂ (( Ln,Na)122) ( Ln = Ce, Pr). Samples were synthesized by precisely tuning the reaction temperature according to Ln and x. The solid solution ranges, 0.1 ≤ x ≤ 0.3 ( Ln = Ce) and 0.15 ≤ x ≤ 0.25 ( Ln = Pr), become narrower with increasing atomic number of Ln (which decreases the ionic radius of Ln³⁺). Bulk superconductivity emerged for 0.2 ≤ x ≤ 0.3 and 0.15 ≤ x ≤ 0.25 with the highest T_c of 25.6 K ( x = 0.3) and 24.7 K ( x = 0.25) for Ln = Ce and Pr, respectively. Crystal structures refined via the Rietveld analysis method showed that the ( Ln,Na)122 compounds ( Ln = La, Ce, Pr) with the highest T_c have almost the same As-Fe-As bond angles (∼107°) and As heights from Fe planes (∼1.43 Å). In addition to the solid solution ranges, the phases in the samples changed depending on the ionic radius of Ln³⁺. The ( Ln,Na)122 phase competes with the non-superconducting CaFe₄As₃(143)-type phase of ( Ln,Na)Fe₄As₃ for Ln = Ce and Pr, whereas only the ( Ln,Na)122 phase was stable for Ln = La. The 143-type phase alone was observed for Ln = Nd, and neither 122- nor 143-type phases were observed for Ln = Sm and Gd.