First-principles Study On Thermoelectric Performance Of MTe(M=Ge,Sn,Pb) Via Crystal Structure Distortion

Thermoelectric(TE)materials allow direct conversion between heat and electricity,thus being considered as a promising clean-energy technology.TE materials are widely used in the fields of thermoelectric cooling and aerospace manufacturing,providing new ways for utilization of secondary energy.To increase the conversion efficiency of MTe-based(M=Ge,Sn,Pb)TE devices,numerous strategies including band and defect engineering are fully explored,which generally enhances the thermoelectric performance of MTe and promotes the industrialization of thermoelectrics.The clarification of thermoelectric mechanism is a necessary prerequisite for the thermoelectric optimization.Therefore,it is of great theoretical significance and application value to investigate the thermoelectric modification mechanism of the Ⅳ-Ⅵ semiconductor MTe.The distortion of the crystal structure often accompanies the thermoelectric optimization of the group Ⅳ-Ⅵ semiconductor MTe.Yet,experiments are often unable to precisely control the freedom of lattice distortion,resulting in the inability to qualitatively analyze the underlying physics of single distortion on TE properties.Herein,we manipulate lattice structure of MTe via modifying lattice parameters,interaxial angle,reciprocal displacement and vacancy,and investigate their influence on the electronic structure,phonon dispersion and thermoelectric properties using first-principles calculations.It provides theoretical guidance for the elemental doping experiments of MTe thermoelectric materials.The main research work of this thesis is as follows:(1)We find stereochemical activity of s2 lone pair can significantly manipulate the electron structure of MTe by modifying the Rashba effect and band convergence,thereby affecting carrier transport properties and thermoelectric properties.The temperature-dependent phonon dispersion shows that the atomic off-centering distortion creates dipole electrical fields and affects lattice vibrations by introducing anharmonic bonds in the rock salt MTe.It leads to the softened transverse optical phonon modes,which reduces lattice thermal conductivities.Moreover,the strong anharmonicity causes zone-center LO phonon of PbTe and SnTe to stiffen with the markedly increased frequency.And the "avoided crossing" interaction between LO and TO further blocks the heat transport.(2)We prove that the rock salt structure MTe is a potential ferroelectric Rashba material.We build a Te off-centering model to explore the hidden Rashba effect in centrosymmetric rocksalt MTe.We find that Te off-centering behavior promotes the original centrosymmetric structure to exhibit macroscopic electric dipoles,resulting in Rashba spin-dependent shifts of energy dispersions.(3)We manipulate lattice structure of GeTe via modifying lattice parameters,interaxial angle and reciprocal displacement,and investigate their influence on electronic band structure and thermoelectric properties using first-principles calculations.Our study finds that modifying lattice parameters is the most controllable way to engineer band gap,and reasonably distorting interaxial angle is a highly efficient strategy to reduce energy offset ΔELΣ and further strengthen the band convergence.Manipulating the atomic reciprocal displacement leads to the indirectdirect transition of band gap and strong Rashba effect,which further facilitates the thermoelectric performance of GeTe.(4)We establish the GeTe vacancy model to investigate the influence intrinsic vacancy on the electronic structure.We find that GeTe vacancy can increase the band effective mass.But vacancy also decreases the band convergence of the light and heavy bands.Therefore,the influence of vacancy on the thermoelectric properties of GeTe should be considered comprehensively.