Design, Synthesis And Physical Properties Of MM'X₂(M=Mn,Fe;M'=Ag,Cu;X=S,Se) Based Compounds

Since the last century,transition metal chalcogenides have been widely used in memory,lithium ion battery,photoelectricity,thermoelectricity,superconductors and other fields with wide application value because of their unique physical properties and tunability.They have gained the attention and interest of lots of scientific researchers.Layered compounds have simple crystal structures and are bound by van der Waals forces.Mechanical cleavage is easy to occur,which opens a convenient door for the study of the mechanism and origin of peculiar physical phenomena in related compounds.The layered chalcogenide family is very large.When chalcogen elements are combined with non-homologous transition metal elements,the unique lattice structure with different space groups can be formed,thus showing the performance applied in different fields.The main research work of this thesis is to design and synthesize several layered sulfur compounds containing transition metals Mn and Cu,and carry out measurements on their detailed structure and related physical properties.The layered structure also provides a lot of space for the accommodation and migration of lithium ions.while easily forms conductive filaments.Therefore,the obtained layered compounds are further used as resistive switching layer materials or electrode materials,the resistive switching effect and electrochemical properties in the corresponding samples are studied,and the mechanism behind the physical properties is analyzed.Our results help to reveal the relationship between the key factors influencing the properties of materials and the layered structure,and provide a new idea for the improvement of the performance and exploration of new transition metal layered chalcogenides,and lay a foundation for the potential application prospects.The main research results of this thesis are as follows:(1)Two solid electrolytes with layered structure,KMnAg Se₂and KCuZnS₂,were prepared by high temperature solid-state method,and the structure and physical properties of the samples were analyzed by combining various characterization and testing techniques.Both materials have tetragonal Th Cr₂Si₂structure(I4/mmm space group),and their surface topography clearly shows a laminated microstructure similar to"tile".For the first time,mechanical stripping technology was used to select high-quality wafers as the resistive switching layer materials,and then based on this,semiconductor devices with resistive switching effect based on Au/KMnAg Se₂/Ag and Ag/KCuZnS₂/Ag structures were prepared.A series of electrical tests were carried out to analyze and compare the resistive switching performance of the two sandwich structure devices,and it was found that their resistive switching capability not only showed bipolar characteristics,but also showed strong repeatability and data retention ability.Combined with the similar layered structures of KMnAg Se₂and KCuZnS₂and the different choices of electrode materials at both ends of the assembly process,we use the conductive filament model to explain the origin of the resistive switching behavior in these two types of devices.The resistive switching properties of the devices based on the above two layered compounds are discussed,which provides a theoretical basis for the selection of new resistive switching materials and the improvement of the performance of existing resistive switching materials.(2)Two new layered Na OH intercalation compounds(Na_0.5OH)Fe_0.5Cu_0.5X(X=Se,S)were successfully synthesized by solid state reaction and hydrothermal ion exchange method.X-ray diffraction shows that they have the(Li_1-xFe_xOH)Fe Se type structure,the space group is P4/nmm,and the(Na_0.5OH)layer and Fe_0.5Cu_0.5X layer are arranged alternately along the c axis.Magnetic and electrical measurements show that they exhibit spin-glass and semiconducting properties.The disorder of Fe plane may be one of the main reasons for the absence of superconductivity in the two compounds.Considering that(Na_0.5OH)Fe_0.5Cu_0.5X can be regarded as layered Fe_0.5Cu_0.5X preintercalated compounds with large layer spacing and various valence states,they may be suitable electrode materials.As a case study,we tested the performance of(Na_0.5OH)Fe_0.5Cu_0.5S as an electrode material for lithium-ion batteries.Electrochemical measurements indicate that it can provide a discharge capacity of589 m Ah g^-1at 0.2 A g^-1.Furthermore,by the incorporation of appropriate carbon nanotubes,its capacity is increased to 745 m Ah g^-1,with the rate capacity and cycle stability greatly improved.