Structural evolution and phase diagram of the superconducting iron selenides Lix(C2H8N2)yFe2Se2(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 $(x=0\ensuremath{\sim}0.8)$. Undoped ${({\mathrm{C}}_{2}{\mathrm{H}}_{8}{\mathrm{N}}_{2})}_{y}\mathrm{F}{\mathrm{e}}_{2}\mathrm{S}{\mathrm{e}}_{2}$ 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 $\mathrm{L}{\mathrm{i}}_{0.31(3)}{({\mathrm{C}}_{2}{\mathrm{H}}_{8}{\mathrm{N}}_{2})}_{0.52(7)}\mathrm{F}{\mathrm{e}}_{2.03(2)}\mathrm{S}{\mathrm{e}}_{2}$ 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 $\mathrm{L}{\mathrm{i}}_{0.5}{({\mathrm{C}}_{2}{\mathrm{H}}_{8}{\mathrm{N}}_{2})}_{y}\mathrm{F}{\mathrm{e}}_{2}\mathrm{S}{\mathrm{e}}_{2}$ 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.