Structural evolution and phase diagram of the superconducting iron selenides Li (CHN) FeSe(= 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 . Undoped crystallizes in an orthorhombic phase. With increasing lithium doping, an orthorhombic to tetragonal phase transition occurs at , and the superconducting tetragonal phase persists until . Meanwhile, the is found dependent strongly on dopant concentration, raising rapidly from 30 K at to 45 K at . The crystal structures of 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 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.