First-principles Study Of The Structure And Properties For Transition Metal Sulfides Under High Pressure

Since the successful preparation of graphene in 2004,transition metal sulfides with typical layered structure have become a hot issue in the fields of condensed matter physics and materials science due to their excellent physical properties.The control of crystal structure and properties by high-pressure means has also attracted attention in the study of transition metal sulfides.High pressure can significantly shorten the atomic distance in materials,change the arrangement of atoms,regulate bonding patterns,induce structural phase transitions,and produce novel physical and chemical properties.In this dissertation,starting from exploring the pressure dependence of the high-pressure stable structures and physical properties of transition metal sulfides,and the low-energy stable structures and phase transition paths of Mo S₂ and WTe₂ under high pressure,as well as the pressure dependence of their electronic,optical and mechanical properties are systematically investigated using first-principles calculation methods.Firstly,the structural phase transition,electronic structure,and mechanical and optical properties of the most widely used transition metal sulfide Mo S₂ under high pressure are studied in this thesis.The enthalpies of 12 kinds of structures,including stable and metastable structures of Mo S₂,are calculated and showed that Mo S₂ is stable in the 0-31 GPa pressure range in the P3m1 structure（2R₁-Mo S₂）,with a phase transition from P3m1 structure to P6₃/mmc structure at 31 GPa,and remains as the P6₃/mmc structure（3H_b-Mo S₂）in the pressure range 31-140 GPa.The phonon dispersion and elastic constant calculations also verify the dynamic and mechanical stability of these two structures.Based on the high-pressure structural stability calculations,the electronic and optical properties of the2R₁-Mo S₂ and 3H_b-Mo S₂ in the pressure range 0-140 GPa are calculated in detail using a generalized gradient approximation plus the Hubbard parameter U（GGA+U）.The analysis shows that the 2R₁-Mo S₂ structure transforms from semiconductor to metal in the 0-31 GPa pressure range,and the metallicity of the 3H_b-Mo S₂ structure increases significantly under pressure.With the increase of pressure,the complex dielectric function,absorption coefficient,reflectivity and optical conductivity are red-shifted in the low-energy region.The optical properties of Mo S₂ risen markedly in the whole pressure range studied.The blue-shift of loss function indicates that pressure improves the energy loss of moving electrons in materials.Further mechanical property calculations show that Mo S₂ has stronger deformation resistance and toughness under high pressure.In addition,the structural phase transitions,electronic structure and optical properties of the transition metal sulfide WTe₂ under high pressure are investigated in detail using first-principles calculations.A new high-pressure structure of WTe₂,I4/mmm,is selected by structural substitution and is stable in the pressure range of 71-200 GPa,while other stable structures,Pmn2₁,P2₁/m and P6₃/mmc,are stable in the pressure ranges of 0-5.6,5.6-15 and15-71 GPa,respectively.The calculation of the elastic constants and phonon dispersion also indicate that the high-pressure structures of WTe₂ are mechanically and dynamically stable over their pressure range.Based on the stability calculations of the high-pressure structure,the electronic and optical properties of WTe₂ in the pressure range of 0-200 GPa are calculated in detail using the GGA+U.Electronic structure and bonding analyses show that under pressure the bond length of the W-Te bond decreases and the covalent bonding properties weaken,leading to an increase in the metallicity of WTe₂ under high pressure.Calculations of the optical properties show that the complex dielectric function,absorption coefficient,reflectivity,optical conductivity,and loss function all exhibit significant pressure dependence,with the peak value of the complex dielectric function increasing significantly and red-shifting under pressure,which is associated with the enhanced metallicity of WTe₂,and the absorption coefficient,reflectivity,optical conductivity,and loss function all increasing significantly under pressure.Further calculation of the mechanical properties shows that WTe₂ has stronger incompressibility and higher hardness under pressure.