Research On Thermoelectric Transport Properties Of Semiconductor Materials With Strong High-Order Lattice Anharmonicity

Thermoelectric materials offer the potential to alleviate the problems of energy scarcity and environmental pollution by enabling the direct conversion of heat energy into electricity without producing additional pollutants.Hence,we systematically investigate the thermoelectric transport properties of four thermoelectric semiconductors with strong lattice anharmonicity,namely,A₂Se₂（A=K,Rb）,natural superlattice materials Ba XYF（X=Cu,Ag;Y=Se,Te）,Full Heusler compounds Na₂KSb,X₂Cs Sb（X=K,Rb）and Na₂Tl Sb,by utilizing first-principles calculations that combine the electronic-phonon coupling and the Boltzmann transport equation.Firstly,this paper evaluates thermoelectric properties of A₂Se₂（A=K,Rb）.The calculation results reveal that two materials have ultralow lattice thermal conductivities,e.g.,0.30～0.34 Wm^-1K^-1at room temperature,which are only one third of quartz glass.Further research indicate that the coexistence of small phonon lifetime and phonon group velocity is the reason for the ultralow lattice thermal conductivities in these two materials.Additionally,due to the anisotropic electronic structure,that is,the coexistence of high dispersion and flat band edges,these two materials capture a high thermoelectric power factor along the c axis.As a resut,the anomalous high thermoelectric figure of merit of2.95 and 2.17 at 500 K along the c-axis direction are captured in n-type K₂Se₂and Rb₂Se₂.Next,this paper investigates the microscopic mechanism of lattice thermal transport in natural superlattice materials Ba XYF（X=Cu,Ag;Y=Se,Te）.The lattice thermal conductivities of the four materials exhibits significant anisotropy due to different bonding types along the a（b）and c axes.The calculations indicate that the four materials have low lattice thermal conductivities,e.g.,0.85～1.61 Wm^-1K^-1at 300 K.Full-Heusler compounds have become a research hotspot in the field of thermoelectricity,because they have the remarkable electronic properties.Hence,this paper investigates the mechanical,transport,and thermoelectric properties in Na₂KSb and X₂Cs Sb（X=K,Rb）.Due to the strong three phonon scattering combined with small phonon group velocity,we obtain the ultralow lattice thermal conductivities.An evaluation of their mechanical properties reveals that they are all brittle compounds,and Rb₂Cs Sb has the strongest elastic anisotropy.Furthermore,we consider the effect of multiple scattering mechanisms on the electron relaxation time.Finally,the thermoelectric figure of merit of the three compounds were prediected.For strongly anharmonic materials,four-phonon scattering cannot be ignored for accurate evaluation of lattice thermal conductivity.Hence,this paper investigates the thermoelectric properties of Full-Heusler compound Na₂Tl Sb including full quartic anharmonicity.The findings exhibit that the strong quartic anharmonicity and temperature dependence of the Tl atom with rattling behavior plays an important role in the lattice stability of Na₂Tl Sb.The study finds that soft Tl-Sb bonding and resonant bonding in the pseudocage composed of the Na and Sb atoms interaction is responsible for ultralow lattice thermal conductivity.Meanwhile,the multi-valley band structure increases the band degeneracy,results in a high power factor in p-type Na₂Tl Sb.Hence,Na₂Tl Sb is a potential candidate for thermoelectric applications.