Structural Tunning And Properties Of Several Layered Compounds

Compounds with layered structure have attracted much attention in many fields due to their rich physical properties and potential application value.The basic structural units of layered compounds can be roughly divided into electrically neutral layers and charged layers.For compounds composed of electrically neutral layers along a certain crystallographic direction,the interlayer interaction is generally a weak van der Waals force.Mechanical exfoliation,chemical intercalation,and chemical doping can be used to control the layer spacing,interlayer interaction and carrier concentration so as to regulate carrier concentrations and physical properties.As for the compound stacked by electropositive layer and electronegative layer,the interlayer force is relatively strong coulomb force and chemical bond.The main method of structural control is to replace the chemical elements into the layer in order to control the carrier concentration and physical properties.In this thesis,for the compounds with different interlayer forces,the methods of ion intercalation,mechanical exfoliation and chemical doping are adopted respectively.The changes of crystal structures are systematically regulated and their low temperature physical properties are comprehensively characterized.Combined with the first-principles calculations,the relationship between structure and physical properties of layered compounds are revealed.The main acheivements are as follows:1. We have successfully grown the layered narrow band gap semiconductor SnSe₂ single crystal by using chemical vapor transport method.Li ion intercalation is successfully realized in ultra-thin Sn Se₂ sample with a thickness of 13 nm under electrical gating technique.A semiconductor-metal-superconductor phase transition is induced and a superconducting phase of 4.8 K is finally obtained.After the equivalent S doping of SnSe₂,the superconducting phase transition shows a dome-shaped variation with the change of S doping content.SnSe_1.8S_0.2 has the optimal superconducting temperature of 6.2 K.The low temperature Hall resistance measurement shows that the electric field gating can significantly enhance the carrier concentration to 2.58×10¹⁵cm^-2.Combined with the theoretical calculation data,we prove that LiSnSe₂ formed by Li-intercalation is thermodynamically stable.Meanwhile,Li-intercalation could cause partial phonon softening and enhance the electron-phonon coupling interaction.The superconducting transition temperature calculated theoretically is 5-8 K,indicating that the superconductor is a BCS superconductor.This work successfully provides a highly effective method for systematically studying the metallization and superconducting transition of narrow-bandgap semiconductors.2. Layered single crystal IrTe₂was synthesized by the Te self-flux method.The resistivity measurements show that the IrTe₂ bulk phase has a charge density wave phase transition?CDW?at?280 K,which shows metallic behavior at lower temperature.Thin samples with different thickness were obtained by mechanical exfoliation,and the typical thickness were 100 nm,40 nm and 20 nm.We find that no charge density wave transition in the thin layers at low temperatures,and confirm that they are superconductors with T_c of 3 K.The Hall resistance test proves that the carrier concentration of the sample decreases with decreasing thickness.Low temperature Raman measurements show that the CDW phase transition temperature of the thin-layer samples may rise above room temperature,and with the collective blue shift of the characteristic peak.This phenomenon is different from the competition between CDW and superconductor.Our experimental results provide an important insight for studying the complex electronic behavior caused by condensation and fluctuation in low-dimensional systems.3. For the layered compound CsCo₂S₂ of Th Cr₂Si₂-type,its crystal structure is Cs⁺and[Co₂S₂]^-layer stacked along the c axis,and the interlayer interaction is strong.Therefore,we adopted the method of doping of chemical elements in the layer to regulate its structure and physical properties.Three novel layered sulfides CsLiCoS₂,CsAgCoS₂ and CsLiFeS₂ were prepared by in-layer chemical doping.Structural analysis confirmed that CsLiCoS₂ is isostructural to CsCo₂S₂,being a tetragonal phase with random mixture of Li⁺and Co²⁺.CsAgCoS₂ and Cs Li Fe S₂ exhibit××1superstructure of ThCr₂Si₂-type because the monovalent Ag⁺/Li⁺ and divalent Co²⁺/Fe²⁺independently occupy inequivalent sites in[AMS₂]^- layer,respectively.By comparing and analyzing the crystal structures of related compounds,a disorder-order phase boundary is proposed and the inherent mechanism is explained,where the difference in ion radius and lattice mismatch cooperatively determines the formation of ordered phases.The systematic low-temperature physical characterization found that this method has an effective control effect on the carrier concentration,electrical and magnetic properties,and provides a new way of thinking for the further exploration of new functional layered materials containing transition metal elements.4. In the layered compound La₂M₅As₃O₂?M=Cu,Ni?,the structural and magnetic changes were studied by the method of Co element substitution.Through crystal structure analysis,it was found that the As1-As2 covalent bond length in Cu₅As₃structural unit showed an inflection point with the increase of Co doping amount,and the O-La-O bond angle gradually decreased with the increase of Co content,indicating that Co doping mainly causes deformation in the c direction.La₂(M_1-xCo_x)₅As₃O₂ compounds show metal behavior.In terms of magnetism,there is a phase transition from paramagnetism to ferromagnetism in La₂(Cu_1-xCo_x)₅As₃O₂ as Co>0.3.While La₂(Ni_1-xCo_x)₅As₃O₂ shows paramagnetism at the low temperature.Finally,we discuss the structural evolution and the absence of superconductivity of the system from the structure and overdoping effects.