Impact Of High Pressure On The Physical Properties Of Three Transition Metal Chalcogenides

In material chemistry,compounds with layered structure constitute a huge system,and their rich physical properties and potential application value make them become a hot spot of research.Research on these materials mainly focuses on transition metal disulfides,graphene,transition metal oxides and oxychlorides.With the development of new layered materials in recent years,some materials with strong intercalation ability are gradually favored,including transition metals such as phosphorus thiophosphates(MPX₃)(M=transition metal element in the first row;X=S,Se),layered lanthanide oxychlorides and new transition metal oxysulfide(V₂O₄S).Among these new material systems,MPX₃has attracted more attention due to its very unique structural characteristics and interesting physical properties and potential applications.One of the research directions is to substitute phosphorus element with V group elements with larger diameter(As,Sb,Bi)to become MTX₃compounds,so as to obtain new structures.Therefore,MCrX₃(M=non-transition metal element)system compounds as a subset of the MCrX₃system came into the field of view.Its current interest lies in its rich structure and low-dimensional ferromagnetic semiconductor ground state.Pressure as a basic physical quantity,it can be for material elastic chemical and electronic magnetic structure and other properties of regulation,and promote material across the insulator and the superconductor amorphous and crystalline ionic and covalent compounds strongly between reactive and inert chemicals such as conventional barriers,make it show the nature of the unprecedented.So this thesis,through the high-pressure electrical transport,high-pressure Raman,high-pressure synchrotron radiation technology to 1T-VSe₂has carried on the detailed research,and part of high pressure researches on MnPSe₃and CrSbSe₃.This thesis mainly divided into the following five chapters:Chapter 1:First,it briefly introduces the meaning of high-pressure physics,the development,principles and steps of various high-pressure technologies,including diamond anvil(DAC),high-pressure Raman(HP-Raman),high-pressure synchrotron X-ray diffraction technology(HP-XRD),and high-pressure electrical transport.Then,it first summarized some properties of transition metal chalcogen compounds,and then detailed the properties and research status of transition metal chalcogen compounds MX₂,MPX₃and MCrX₃systems.Chapter 2:Through electrical transport,synchrotron x-ray diffraction(XRD)and Raman scattering measurements,from which two critical pressure points are unraveled.The CDW transition is found to be enhanced under compression at rate of?16.5 K/GPa up to the first critical pressure P_C1?12 GPa,at which a structural transition from hexagonal P-3m1 to monoclinic C2/m phase takes place.The second critical pressure P_C2?33 GPa corresponds to another structural transition from monoclinic C2/m to P21/m phase.These findings may help to understand pressure tuning of structures in transition metal dichalcogenides.Chapter 3:This chapter mainly introduces the synthesis of MnPSe₃single crystal and some of its characterization under ambient pressure and high-pressure Raman test.From the X-ray diffraction test results,it can be seen that it grows along the direction of(00l),which has obvious preferred orientation.According to the magnetic test results,the Neel transition temperature T_N=72 K and the disappearance of Raman peak when 23.68 GPa was observed in the high-pressure Raman proved the correctness of the sample,which provided the basis for the high-pressure transport in the subsequent sequence.Chapter 4:It mainly includes the synthesis of CrSbSe₃ single crystal sample,some characterization under ambient pressure and high-pressure Raman test.In the test results of polycrystalline X-ray diffraction,we can perfectly fit the space group PNMA according to the standard card,and no trace of impurities,which proves the purity of our sample.The ferromagnetic transition temperature Tc=72K was obtained from the magnetic test results,and new Raman peaks were found near 9 GPa and 23.5 GPa by high pressure Raman.This signal indicates that there may be structural changes or magnetic transformations near these two critical pressure points.Chapter 5:Summary and prospect.