Synthesis And Characterization Of Several Intercalated FeSe Compounds

In this thesis,the intercalated FeSe materials were studied,and their crystal structures and basic physical properties were characterized.The charge transfer in the intercalation process and its influence on the electronic structure and basic physical properties of the material were explored.We designed and synthesized several electron/hole doped FeSe-derived materials,the main results are listed as follows:First,by means of solvothermal method,we co-intercalated a small alkali metal lithium and 1,2 propylene-diamine into the FeSe layer,successfully induced the electron-doping continuously in a wide range.As electron-doping level increased from 0.06 to 0.68,the superconductivity is in the form of a dome-like shape which rised first and then fall.The theoretical calculations also show that there are two Lifshitz transitions in this process.By studying the effect of electron-doping on the crystal structures,we found that the anion height of the material increases monotonously with the doping level,which deviates from the optimal anion height?1.38??corresponding to FeAs-based materials and high-press FeSe.We found that electron-doped FeSe materials are in a new superconducting zone which is different from FeAs-based superconductors.The orbital resolution first-principles calculations based on the crystal structure of Li_x(C₃N₂H₁₀)_0.37FeSe system showed that the electron correlation intensity has strong orbital selectivity,which is similar to FeAs-based superconductor.By summarizing the relationship between the optimal superconducting transition temperature of FeAs-based superconductors,pressurized FeSe and electron-doped FeSe and the effective mass of Fe 3d t2g orbit,we propose a scheme to explore the higher superconducting transition temperature:minimizing the Fe 3d t_2g orbital differentiation coupled with intermediate electronic correlation by tuning the anion height and carrier concentration.The new phase diagram provides a unified picture of electron-doped and pressurized FeSe superconductors and iron-arsenide superconductors.Second,as we studied another molecular intercalation of FeSe superconductor Li_x?1,3-C₃H₀N₂?_yFeSe,we found that it is different from the continuous electron doping system---Li_x(1,2-C₃N₂H₁₀)_0.37FeSe.Two tetragonal superconducting phases are identified.The lithium concentration is revealed as a key parameter controlling both the crystal structures and the T_c.The determined structures demonstrate that between the FeSe layers,the orientation of 1,3-DIA molecules is coupled with Li concentration.The Hall coefficient R_H of the two phases are negative at low temperature,indicating dominant electron-type carriers due to Li doping.Meanwhile,a sign reversal of R_H at185K is observed in both phases and at three different doping levels,implying that hole pockets appears in all these superconductors at high temperature.Fitting on resistivity in terms of the Lawrence-Doniach model and the measured field dependent diamagnetic response suggests the two superconductors belong to the quasi-two dimensional?2D?system.Further DFT calculations revealed that the increase in FeSe layer distance will lift up a‘hole band'associated with d_x2-y2 character around?point toward fermi energy.The above works are focused on electron doping FeSe,but to date there has been no precedent for hole-doped FeSe or even hole-doped layered transition metal chalcogenides.Intercalations are chemical processes which enable the insertion of a charged or neutral species into a host lattice.The pairing of ions of opposite charge is a central principle of chemistry,it remains a key challenge for numerous host materials with their outmost layers been anions.In this work,we introduce a hydrothermal ion-exchange synthesis to intercalate oxidative S and Se anions between the Se layers of FeSe,which leads to single crystals of novel compounds?Se/S?_x?NH₃?_yFe₂Se₂.The charge transfer through the Se layer to S?or Se?intercalants is further confirmed by the elevated oxidation state of Feions and the dominant hole carriers in the intercalated compounds.By intercalating S,for the first time superconductivity emerged in hole-doped iron chalcogenides.The generality of this chemical approach was further demonstrated with layered FeS and NiSe.Our findings thus open an avenue to exploring diverse aspects of anionic intercalation in similar materials.