Structure And Property Of Several Layered Chalcogenides

In this dissertation,we focus on the syntheses and physical properties of several layered chalcogenides.One is the intercalated transition metal chalcogenides,and the effects of intercalation on their structure and superconductivity were studied.The other is the exploration of new main group metal layered chalcogenides to obtain potential superconducting parent compounds.The main results are summarized as follows.Firstly,we demonstrate that a series of well crystallized superconductors in pure phase can be obtained by intercalating alkali metal Na and organic molecules(C₂N₂H₈,1,2-C₃N₂H₁₀,1,3-C₃N₂H₁₀,or C₄N₂H₁₂)in between FeSe layers,through a novel sonochemical method.Four superconductors with different crystallographic symmetry have similar superconducting transition temperature?T_c⁴6 K?,which means that intercalating organic molecule is not closely related to the T_c of iron-selenide superconductors.The structure of Na_0.39?C₂N₂H₈?_0.77Fe₂Se₂ determined by neutron powder diffraction?NPD?reveals the compressed FeSe layers and orientational disorder of the intercalated molecules.Unlike the cases in ammonia-metal intercalated FeSe,in-situ X-ray diffraction and nuclear magnetic resonance?NMR?results reveal that the alkali metals deintercalate easily in Na_0.39?C₂N₂H₈?_0.77Fe₂Se₂,leading to a dopant free FeSe intercalation compound,?C₂N₂H₈?Fe₂Se₂,accompanying the disappearance of superconductivity.Our results promote the study of structural stability,doping levels,and temperature driven structural transitions in FeSe-based superconductors,and offer an effective synthetic route for the exploration of new superconductors as well.Secondly,we find that Na_0.35(C₃N₂H₁₀)_0.426Fe₂Se₂ exists a large orthorhombic distortion and no magnetic ordering at room temperature,and the"nematic"ordered1,3-C₃N₂H₁₀ molecules in between the FeSe layers make this nematic phase stable.Its T_c is 46.5 K even though the orthorhombic distortion is much larger than that of?-FeSe and most of other iron-pnictide superconductors.First principles calculations suggest that this orthorhombic distortion behaves a selective effect on the splitting of Fe 3d_xzz and 3d_yz orbitals at the?and M points.Our results strongly support that the nematic phase is not relevant with the superconductivity at least in high T_c FeSe-based superconductors.Thirdly,we report the crystal structures and properties of Rb_xNbSe₂ with 0?x?0.5 by solid state reaction.With Rb intercalation,Rb_xNbSe₂ evolves from 2H phase for0?x?0.025,to 6R phase for x⁰.2,finally to 2H phase for 0.375?x?0.5.Moreover,6R phase transforms to a rare 6H structure by annealing at 750 K,and both 6R and 6H phases are new polymorphs in intercalated NbSe₂.A phase diagram of Rb_x NbSe₂ is established accordingly.For 0?x?0.025,T_c of Rb_xNbSe₂ decreases from 7.2 K for x=0 to 4.2 K for x=0.025 in an L-shaped way.The suppression rate of T_c is similar to that of Fe intercalation,which is greater than the rate of Cu,Ga,and Li intercalation.A superconducting phase diagram of Rb_xNbSe₂?0?x?0.025?was established and compared with the superconductivities of Li_x NbSe₂,Cu_xNbSe₂,Fe_xNbSe₂,and Ga_x NbSe₂.It has been found that the suppression of superconductivity is related to the ionic radius,valence,and magnetic properties of the intercalation elements in NbSe₂system.Rb_xNbSe₂?0.2?x?0.5?exhibits paramagnetism and metal-like transport behavior,and no superconductivity is observed down to 2 K.Finally,we design and synthesize two new BiS-based layered compounds:Bi₅O₄S₃Cl and Bi₃O₂S₂Cl.Bi₅O₄S₃Cl has a rare BiS₃ unit with 3 layers,and Bi and S are six-coordinated.The resistivity of Bi₅O₄S₃Cl and Bi₃O₂S₂Cl both significantly drops at low temperature,which indicates that both of them are potential superconducting parent compounds.First principles calculations show that the band gap of Bi₅O₄S₃Cl is 0.344 eV.We induce some S vacancies,making Bi₅O₄S₃Cl transform from semimetal to metal.Pb doping makes both T_c and superconducting volume fractions?SVFs?improved obviously.Bi₅O₄S₃Cl and Bi₃O₂S₂Cl provide good platforms to explore new BiCh-based layered superconductors.