Superconductivity and Intercalation Statein the Lithium-Hexamethylenediamine-Intercalated Superconductor Li_x(C₆H₁₆N₂)_yFe_(2-z)Se₂: Dependence on the Intercalation Temperature and Lithium Content

The superconductivity and intercalation statein the lithium-and hexamethylenediamine (HMDA)-intercalated superconductor Li$_x$(C$_6$H$_{16}$N$_2$)$_y$Fe$_{2-z}$Se$_2$ have been investigated from powder x-ray diffraction, thermogravimetric and magnetic susceptibility measurements, changing the intercalation temperature, $T_i$, and the Li content, $x$. Both Li and HMDA have been co-intercalated stably up to $x$ = 2 roughly in the molar ratio of $x : y = 2 : 1$. In the case of $T_i$ = 45$^\circ$C, it has been found that both Li and HMDA are co-intercalated locally at the edge of FeSe crystals, indicating that both Li and HMDA are hard to diffuse into the inside of FeSe crystals at 45$^\circ$C. In the case of $T_i$ = 100$^\circ$C, on the other hand, it has been found that both Li and HMDA diffuse into the inside of FeSe crystals, so that $T_c$ tends to increase with increasing $x$ from ~30 K at $x$ = 1 up to 38 K at $x$ = 2 owing to the increase of electron carriers doped from Li into the FeSe layers.