New Alkali-Metal- and 2-Phenethylamine-Intercalated Superconductors A x (C 8 H 11 N) y Fe 1− z Se ( A = Li, Na) with the Largest Interlayer Spacings and T c ∼ 40 K

New FeSe-based intercalation superconductors, Ax(C8H11N)yFe1−zSe (A = Li, Na), with Tc = 39–44 K have been successfully synthesized via the intercalation of alkali metals and 2-phenethylamine into FeSe. The interlayer spacings, namely, the distances between neighboring Fe layers, d, of Ax(C8H11N)yFe1−zSe (A = Li, Na) are 19.04(6) and 18.0(1) A, respectively. These d values are the largest among those of the FeSe-based intercalation compounds and are understood to be due to the intercalation of two molecules of 2-phenethylamine in series perpendicular to the FeSe layers. It appears that the relationship between Tc and d in the FeSe-based intercalation superconductors is not domic but Tc is saturated at ∼45 K, which is comparable to the Tc values of single-layer FeSe films, for d ≥ 9 A.