Synthesis and post-annealing effects of alkaline-metal-ethylenediamine-intercalated superconductors Ax(C2H8N2)yFe2-zSe2 (A = Li, Na) with Tc = 45 K

Abstract

New iron-based intercalation superconductors Ax(C2H8N2)yFe2-zSe2 (A = Li, Na) with Tc = 45 K have successfully been synthesized via intercalation of dissolved alkaline metal in ethylenediamine. The c-axis lengths of Ax(C2H8N2)yFe2-zSe2 (A = Li, Na) are 20.74(7) {\AA} and 21.9(1) {\AA}, respectively, and are about 50 % larger than that of KxFe2Se2, indicating that not only alkaline metal but also ethylenediamine is intercalated between the Se-Se layers of FeSe. It seems that the high-Tc of Ax(C2H8N2)yFe2-zSe2 (A = Li, Na) is caused by the possible two-dimensional electronic structure due to the large c-axis length. Through the post-annealing in an evacuated glass tube, it has been found that Tc decreases with increasing post-annealing temperature and that deintercalation of EDA from the as-intercalated sample takes place at low temperatures below 250 {\deg}C.