Phonon Anharmonic Effect Of Transition Metal Pentatelluride

In recent years,thermoelectric materials have attracted much attention because of their unique heat-to-electricity properties.The properties of thermoelectric materials can be characterized by the dimensionless figure of merit（ZT）.The transition metal pentatelluride is of interest because of its extremely lowk_l,which is related to phonon vibration.However,the research on phonon anharmonic effect of transition metal pentatelluride is very limited.Based on this situation,this thesis focuses on the intrinsic phonon anharmonic effects of bulk ZrTe₅ and HfTe₅ by using temperature dependent Raman Spectroscopy measurements and calculate the lattice thermal conductivity by first principles.The main results of this thesis are as follows:1.Firstly,ZrTe₅ and HfTe₅ single crystals were prepared by CVT,and the microstructure of the samples was characterized by SEM and EDS.The results of SEM and EDS show that the ZrTe₅ and HfTe₅ have obvious 2D layered features and the composition of two materials is controllable.Raman spectra show the characteristic peaks,indicating that the quality of the materials can be sure.In the electric transport experiment,both ZrTe₅ and HfTe₅ single crystal have characteristic abnormal resistance peaks corresponding to temperatures of 130 and 67 K,respectively.By Arrheninus formula,we calculated the activation energy and obtained the band gap of the two materials.The estimation of k_econfirmes that the total thermal conductivity of bulk ZrTe₅ and HfTe₅ is mainly derived from k_l.Finally,the carrier concentration and mobility of the two materials are calculated by low magnetic field Hall coefficient.They are typical topological insulators with high mobility and low carrier concentration.2.The intrinsic phonon anharmonic effect of bulk ZrTe₅ and HfTe₅ is studied with the symmetric decaying model and asymmetric decaying model respectively.First of all,the laser power（1 mW）and integration time（20 s）required for the experiment were determined by Raman spectroscopy.The Raman spectra of the bulk ZrTe₅ and HfTe₅ were obtained by using the temperature dependent Raman technology.It was also found that when the temperature was higher than 420 K,the oxidation reaction of both materials would take place.So,we only focus the Raman spectra from 80 to 400 K.The peak shift and broadening of different vibration modes are analyzed by using the symmetric decaying model.The results show that the large anharmonicity of the A_g³ vibration modes leads to the ultra-low thermal conductivity along the b-axis,and the influence of the quartic scattering should be considered in the study of phonon anharmonic effect of bulk ZrTe₅and HfTe₅.By comparing the fitting coefficients of the symmetric decaying model and the asymmetric decaying model,we find that it is reasonable to use the symmetric decaying model to study the anharmonic effects of ZrTe₅ and HfTe₅ crystals and it is more convenient to use the symmetric scattering model.By analyzing the fitting parameters of multichannel scattering,it is proved that the multichannel scattering mechanism exists in phonon decay process,and the anharmonic parameters not only represent the intensity of phonon anharmonic effect,but also reflect the characteristics of a specific scattering channel.3.The band gap and vibration modes of ZrTe₅ and HfTe₅ are calculated by using CASTEP package.The band calculation results show that ZrTe₅ is of a band gap about 36 meV while HfTe₅ is of a band gap about 30 meV.Vibration modes calculation results show that both materials have 18Raman-active modes.Finally,the elastic constants of the two materials were calculated by VASP,and the average acoustic velocities of the materials were obtained through the elastic constants.The k_l of ZrTe₅ and HfTe₅ along the b axis was estimated which was consistent with the experimental values,indicating the feasibility of using elastic parameters to calculate the k_l.