| Layered transition metal chalcogenides（TMDs）have attracted much attention in condensed matter physics and material science because of their rich crystal structures and unique electrical and optical properties.The crystal structures of TMDs are affected by the filling states of the outermost d-orbital electrons of the transition metal atoms.The number of outermost d-orbital electrons in the Mo and W atoms is 2,which is preferred to form the thermodynamically stable 2H and 3R MX2（M=Mo,W;X=S,Se）with trigonal prism coordination.With the increasing d-orbital electrons of transition metal atoms,[MX6]tends to change from trigonal prism configurations to regular octahedron or distorted octahedron configurations,forming metastable 1T,1T’or 1T’’’phases.Metastable MX2 will be transformed into the thermodynamically stable2H MX2 at high temperature.The complex configurations of metastable phases endow them with many novel physical properties.However,the controllable preparation of metastable MX2 crystals has become a key problem of this field due to their structural instability.To solve this problem,the strategy of topochemical conversion-synthesis is proposed in this paper.Firstly,the precursor AMX2 crystals are prepared by solid state method.The d-orbital electron number of transition metal atoms can be adjusted by inserting alkali metal ions.The changed coordination configuration of transition metal atoms and the enhanced overlap degree of electron cloud would induce the formation of specific metastable phase structures.Furthermore,a series of metastable MX2 crystals are prepared by the low-temperature topochemical method.On this basis,the kinetic process of alkali metal ions during the process of deintercalation is systematically studied,and the universal law of the structural evolution is summarized.The novel physical properties of these materials are explored,such as superconductivity,nonlinear optics and magnetoresistance.The main achievements are as follows:（1）Lithium ions were inserted into the 2H Mo S2 interlayer to induce the transformation of[Mo S6]from prismatic into octahedral coordination.Li Mo S2 crystals were synthesized by high-temperature solid state reaction.Then,1T Mo S2 crystals were obtained by a low-temperature topochemical method.The crystal structure of 1T Mo S2was confirmed by single crystal X-ray diffraction analysis for the first time,which belongs to the trigonal system and the space group is P-3m1.Compared with the semiconducting 2H Mo S2,the metastable phase 1T Mo S2 shows superconducting characteristics with a superconducting transition temperature of 4 K.（2）Potassium ions with larger ionic radius than lithium ions are inserted into Mo S2to tune the process of deintercalation.K0.7Mo S2 crystals were synthesized by solid state method.[Mo S6]takes the form of twisted octahedral coordination and forms a 2a×2a superstructure.Potassium ions of K0.7Mo S2 crystals were removed by the strong oxidizing solution to obtain the metastable-phase 1T’’’Mo S2 crystals for the first time.The molybdenum atoms of 1T’’’Mo S2 form triangular clusters,which belongs to the non-central space group P31m.1T’’’Mo S2 exhibits semiconductor characteristics with a band gap of 0.65 e V,and it exhibits a strong second-order nonlinear optical effect under incident laser of 850 nm.（3）The electron cloud overlap between transition metal atoms is increased by replacing Mo atom with W atom,which keeps the stability of 1T’structure after de-intercalation of potassium ions to form the metastable 2M WS2 crystals.K0.7WS2crystals were synthesized by high-temperature solid state reaction.The space group of2M WS2 is C2/m,which is formed by single-layer 1T’WS2 shifting and stacking along a direction.W atoms are arranged in zigzag chains.At atmospheric pressure,2M WS2shows superconductivity and the superconducting transition temperature is 8.8 K.It is the highest intrinsic superconducting temperature among the layered TMDs superconductors.The first principle calculation shows that 2M WS2 has topological surface state,which makes it have the potential to become a new topological superconductor.（4）Large-size 2M WSe2 crystals can be prepared by gas intercalation and topochemical method.Under normal pressure,2M WSe2 has a giant magnetoresistance effect and the magnetoresistance value can reach 3500%at 2 K and 9 T.2M WSe2exhibits superconducting behaviour under high pressure,superconducting transition starts to appear on the resistance curve at 4.2 GPa,and the superconducting transition temperature reaches a maximum of 7.3 K at 10.3 GPa.First principles calculations show that the density of states at the Fermi surface increases rapidly with increasing pressure,which may be the main reason for inducing superconductivity. |
