New alkali-metal-And 2-phenethylamine-intercalated superconductors Ax(C8H11N)yFe1-zSe (A = Li, Na) with the largest interlayer spacings and Tc 40K

New FeSe-based intercalation superconductors, A_x(C₈H₁₁N)_yFe₁-_zSe (A = Li, Na), with T_c = 39-44K 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 A_x(C₈H₁₁N)_yFe₁-_zSe (A = Li, Na) are 19.04(6) and 18.0(1) Å, 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 T_c and d in the FeSe-based intercalation superconductors is not domic but T_c is saturated at 45 K, which is comparable to the T_c values of single-layer FeSe films, for d ≥ 9Å.