Superconductivity and intercalation state in the lithium-hexamethylenediamine-intercalated superconductor Li_x(C₆H₁₆N₂)_yFe_2-zSe₂: Dependence on the intercalation temperature and lithium content

The superconductivity and intercalation state in the lithium- and hexamethylenediamine (HMDA)-intercalated superconductor Li_x(C₆H₁₆N₂)_yFe_2-zSe₂ 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 a molar ratio of x : y = 2 : 1. In the case of T_i = 45°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°C. In the case of T_i = 100°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 in the number of electron carriers doped from Li into the FeSe layers.