The Systematic Studies Of Structural And Electrical Transport Properties Of ZrTe₃ And TiTe₂

Transition metal chalcogenides have become more and more attractive and important materials studied due to their unique structure properties.In this thesis,with high pressure in-situ X-ray diffraction,high pressure Raman Spectroscopy and high pressure in-situ electrical transport properties measurement,together with the first principle calculations,we have systematically studied ZrTe3 and TiTe2 under high pressure.This study reveals the correlation between the structural evolution and electrical transport properties at high pressure,which can shed light on the study of these two materials and better understand their high pressure behaviors.The experimental investigations and theoretical calculations of the study are as follows:1?Firstly we carried out the in-situ high pressure electrical transport measurement on single crystalline TiTe2 and observed the resistance changes with temperature at different pressures.During the compression process,the superconducting phase emerged at around 8.2 GPa.The superconducting transition temperature Tc increases with increasing pressure and reaches a maximum 6.8 K at around 27,7 GPa,and then decreases monotonically with pressure.During the decompression process,the relation of the resistance versus temperature has the same trend with compression process.The superconductivity remains to 3.3 GPa after decreasing pressure,which is much lower than 8.2 GPa,quite commonly seen in many related research.In the compression procedure,ZrTe3 had a sudden resistance drop at 27.7 GPa,a significant increase in Tc up to 1.6 K was also observed.To better understand the abnormal change of the electrical transport behavior,we carried out the powder X-ray diffraction upon the ZrTe3 sample.Result showed that ZrTe3 of the P21/m monoclinic structure was quite sta'ble in the whole pressure range we studied.Further research showed that the lattice parameter also had an anomaly at around 25 GPa.By fitting with the third order Birch-Murnaghan equation of state,we find that there is a 2.5%volume collapse after structure change.The bulk modulus changes from 45 GPa to 118 GPa.We carried out the first-principle calculation upon ZrTe3 sample.Result showed that the density of state at Fermi level was increasing with increasing pressure to the maximum at 27 GPa and then decreasing with pressure.It showed the same trend as the Tc changes.The calculated lattice parameters are almost the same as our experimental results.Thus we conclude that the anomaly in resistance of the sample is well related with lattice parameter changes of the structures.As we know,BSC theory shows that the electron-phonon coupling was proportional to the DOS at Fermi level.Pressure tunes the DOS at Fermi level,resulting in the change of Tc.2?Using in-situ high pressure X-ray diffraction,electrical transport properties measurement and Raman spectroscopy,we investigated the structural and electrical properties of TiTe2 sample at high pressure.TiTe2 is trigonal phase with P-3ml space group at ambient pressure.In compression up to 15 GPa,a structure phase transition occurred from P-3ml trigonal phase to C2/m monoclinic phase.This high pressure monoclinic phase was quite stable to 43.4 GPa,the highest pressure we achieved in the study.Upon decompression,the Pmn21 phase can be quenched down to 2.4 GPa.The sample returns to its trigonal phase when decompressed to ambient pressure.High pressure electrical resistance measurement shows that the resistance of TiTe2 single crystal sample had a sudden drop at 3.7 K around 5.6 GPa,which indicates potential superconductivity.Zero resistance of the sample was observed at around 6.7 GPa.Tc increases monotonically with increasing pressure up to 15 GPa,and stay unchanged in further compression up to 36.6 GPa.To better understand the superconductivity behavior,we conducted the high pressure Raman spectroscopy experiment.TiTe2 had only two prominent Raman mode Eg and Aig at ambient pressure.A new Raman peak emerged upon a little compression.In further compression,this evolution seems completed at around 6 GPa.Above 10 GPa,all Raman peaks broaden and lost the intensity,indicative of metallization in the TiTe2 sample.The emerging of the superconducting phase may arise from a new phonon electron coupling.The calculated lattice parameters are in agreement with the experimental results,further confirming the pressure-induced structure evolution at high pressure.