Structural evolution and phase diagram of the superconducting iron selenides Li _x (C₂H₈N₂) _y Fe₂Se₂(x= 0 ~ 0.8)

Here we report on the structural and electronic phase diagram of lithium and ethylenediamine intercalated FeSe in a wide range of dopant concentration $(x=0\sim 0.8)$ . Undoped ${\left({\text{C}}_2{\text{H}}_8{\text{N}}_2\right)}_y{\text{Fe}}_2{\text{Se}}_2$ crystallizes in an orthorhombic phase. With increasing lithium doping, an orthorhombic to tetragonal phase transition occurs at $x=0.35$ , and the superconducting tetragonal phase persists until $x=0.5$ . Meanwhile, the $T_c$ is found dependent strongly on dopant concentration, raising rapidly from 30 K at $x=0.35$ to 45 K at $x=0.5$ . The crystal structures of ${\text{Li}}_{0.31\left(3\right)}{\left({\text{C}}_2{\text{H}}_8{\text{N}}_2\right)}_{0.52\left(7\right)}{\text{Fe}}_{2.03\left(2\right)}{\text{Se}}_2$ are determined by using high-resolution neutron diffraction data at 5, 60, 150, and 295 K, respectively. The distortion of the FeSe tetrahedron is enhanced significantly from 150 to 295 K, meanwhile, the normal-state Hall resistivity changes sign from negative to positive in the same temperature range. The dominant hole carrier in electron-doped ${\text{Li}}_{0.5}{\left({\text{C}}_2{\text{H}}_8{\text{N}}_2\right)}_y{\text{Fe}}_2{\text{Se}}_2$ above 230 K suggests that the temperature-induced structure distortion may lead to a reconstruction of the Fermi surface topology and the appearance of hole pockets.