Sommerfeld-Bethe analysis of ZT in inhomogeneous thermoelectrics

The development of good thermoelectric materials exhibiting high $ZT$ (=PF/κ T) requires maximizing power factor, $PF$, mainly governed by electrons, and minimizing thermal conductivity, κ, associated not only with electrons but also with phonons. In the present work, we focus on the GeTe and Mg₃Sb₂ as high $ZT$ materials with inhomogeneous structures and analyze both electrical conductivity, L₁₁, and Seebeck coefficient, S, with help of Sommerfeld-Bethe formula, resulting in understanding the temperature dependence of $PF$ and the identification of electrons contribution to thermal conductivity, κₑₗ. Comparing the obtained κₑₗ and experimentally measured κ, the temperature dependence of phonons contribution to thermal conductivity, κₚₕ=κ-κₑₗ, is inferred and analyzed based on the formula by Holland. Comparison of the GeTe and Mg₃Sb₂ with different types of crystal structures, i.e., GeTe being of a semiordered zigzag nanostructure like a disrupted herringbone structure while Mg₃Sb₂ of rather uniform amorphous structure, discloses that size effects on temperature dependence of κₚₕ is large in the former, while very small in the latter. Hence, it is concluded that not only the size of the grain but also its shape has an important influence on κₚₕ and then $ZT$.