Crystal Growth And Physical Properties Of Layered Transition Metal Pentatellurides ZrTe₅ And HfTe₅

Layered transition metal chalcogenides have rich physical properties because of their unique electronic structure.Among them,transition metal penttelluride ZrTe₅and HfTe₅crystals have potential applications in the field of thermoelectricity due to their large Seebeck coefficient and conductivity.However,their high thermal conductivity limits the further improvement of thermoelectric performance,so their thermal conductivity needs to be further optimized.In addition,ZrTe₅and HfTe₅are topological insulators with abnormal temperature dependent resistance.They show metal-insulator transition phenomenon around 150 K and 80 K,respectively.The origin of these special transport properties have been much conjectured and debated.Therefore,in this paper,ZrTe₅and HfTe₅crystals were prepared by different growth methods and conditions,and their thermal and electrical transport properties were studied in order to optimize their thermal conductivity and analyze the physical mechanism behind the transport properties.The main research contents include:1.Study on thermal transport properties:ZrTe₅and HfTe₅single crystals with different microstructures were grown by chemical vapor transport method and Flux method respectively,and the source of the different microstructures was studied from the growth mechanism.At the same time,thermal conductivity along b-axis of crystals with different microstructure were measured using time-domain thermoreflectance.Results show that ZrTe₅and HfTe₅single crystals grown by chemical vapor transport method have very low thermal conductivity,which was one order lower than the theoretical value,and three times smaller than that of compact ZrTe₅and HfTe₅crystals grown by Flux method.Combined with phenomenological thermal conductivity calculation and Raman spectrum fitting analysis,it is found that nanostructured ZrTe₅and HfTe₅grown by chemical vapor transport method improved phonon scattering probability,resulting in the limiting mean free path of phonon,which leads to the very low thermal conductivity of crystal.2.Study on electrical transport properties:The resistivity curves and Hall resistivity of ZrTe₅crystal under different magnetic fields were analyzed.It was found that the main carrier type changed from electron to hole when metal-insulator transition occurred.Crystals with different transition temperatures were prepared by doping growth and adjusting the ratio of Flux to raw material,to study the effect of metal-insulator transition temperature.Analysis shows that theTe defect introduced in the crystal growth process may be the origin of abnormal resistance peak,and the difference of crystal transport is due to the different defect concentration.In addition,magnetoresistance test shows all ZrTe₅crystal samples exhibit extremely large magnetoresistance effect.Combined with Hall resistivity analysis,the large magnetoresistance value may be caused by the coexistence of electrons and holes and high carrier mobility.Anisotropic magnetoresistance tests show that the magnetoresistance effect of ZrTe₅and HfTe₅crystals is only related to the component of the vertical magnetic field,indicating that ZrTe₅and HfTe₅have two-dimensional transport characteristics.In this paper,ZrTe₅and HfTe₅crystals with special microstructures were prepared to regulate the low thermal conductivity,their sources were analyzed,providing a new idea for improving the properties of thermoelectric materials.At the same time,the causes of abnormal transport phenomenon in ZrTe₅and HfTe₅crystals are revealed,the anisotropic magnetoresistance effect was studied,and the two-dimensional characteristics of electron transport are found,which provides an experimental basis for further device application research and development.