First Principle Study Of Electronic Topological Transition In Bi₂Te₃ And Sb₂Te₃ Under Pressure

As traditional thermoelectric materials,Sb₂Te₃ and Bi₂Te₃ are hot research topics.Recently,it has been reported that an electronic topological transition?ETT?has been found in Bi₂Te₃ under pressure,which leads to a new research upsurge of these materials.The ETT can greatly change the macroscopic properties of the semiconductor materials.Finding the identification and searching the origin of ETT as well as realizing the relationship between ETT and macroscopic properties are very important to understand the phenomenon,modulate the electronic characteristics and develop new functional materials.At present,researchers have studied the pressure induced ETT of these materials with various methods?such as isostructural phase transition,transport properties,thermoelectric properties,etc.?However,to date there is still no reliable microscopic explanation for how to identify ETT or how ETT changes the macroscopic properties,which obstruct people from understanding this phenomenon.In order to solve the problems above,this dissertation simulated the bond lengths,bond angles,effective charges and band structures of these semiconductors using the first-principles method.This dissertation has found an effective method to identify ETT,revealed the relationship between ETT and macroscopic properties,explored the microscopic mechanism of macroscopic properties and provided theoretical support for further research of ETT in the Sb₂Te₃ and Bi₂Te₃.First,the lattice parameters,bond lengths,bond angles,Bader charge,and band structures of Sb₂Te₃ under hydrostatic pressures from ambient pressure to 6 GPa were simulated.The anomalies of lattice parameters and Bader charge were found at 2.5 GPa.The relationship between lattice distortion and interatomic interactions was discussed through the Bader charge.The position of valence band edge was found to displace around 2.5 GPa.The changes of band structures under different pressure confirmed an ETT which occurred at low doping concentration in Sb₂Te₃.The strong redistribution of density of states due to the ETT was revealed by using Bader charges analysis.Thus,a new method of identifying the ETT using Bader charge analysis is proposed.Second,the lattice parameters,neighboring interlayer distance,bond lengths,bond angles,charge density and Bader charge of Bi₂Te₃ were simulated under hydrostatic pressures from ambient pressure to 6 GPa.The lattice parameters a and c both had relationship with the ETT.By the study of charge density and Bader charge,the redistribution of charge density was revealed and the reason of lattice distortion was explained by the critical changes of interactions among atoms.The changes of band structure and effective mass caused by ETT were analyzed.Seebeck coefficient,conductivity,Hall coefficient,electronic thermal conductivity under pressure were simulated.Though analyses of the band structure and effective masses of band edge it was revealed that the anomalies of transport properties origin from the abnormal behaviors of effective mass related valence band edges,due to the ETT.Finally,the lattice parameters,bond lengths,bond angles,Bader charge and the band structures of Bi₂Te₃ single quintuple layer under tensile to compressive strain were simulated.The relative displace between neighboring atom layers under single axial strain was found.The impacts of the stress on lattice and interatomic interaction were discussed.The shifts of valence band edges and the indirect to direct band gap semiconductor transition under strain were revealed by the band structure.The ETT under stress is predicted by the abnormal behaviors of band edges and Bader charge analysis.In summary,this paper demonstrated that the Bader charge analysis as an effective mean to identify ETT;gave a graph of redistribution in charge density caused by ETT of Bi₂Te₃-like materials,explained the relationship between macroscopic properties and the transition of indirect to direct band gap semiconductor in Bi₂Te₃ single quintuple layer under the effect of stress,which is a great potential functional material.