Insight into the intrinsic microstructures of polycrystalline SnSe based compounds.

SnSe based compounds have attracted much attention due to the ultra-low lattice thermal conductivity and excellent thermoelectric properties. The origin of the low thermal conductivity has been ascribed to the strong phonon anharmonicity. Generally, the microstructures are also effective in scattering the phonons and further reducing the lattice thermal conductivity. In this work, the microstructures of undoped SnSe and Bi-doped Sn0.97SeBi0.03have been investigated by transmission electron microscopy. A characteristic microstructure of lath-like grains has been observed in SnSe based compounds from perpendicular to the pressure direction. In addition, there exist a large quantity of low-angle grain boundaries and a high concentration of edge dislocations and stacking faults in the grains. All these microstructures result in lattice mismatch and distortion and can act as the phonon scattering centers, which broaden the understanding of the low thermal conductivity of SnSe based compounds.

Electronic Topological Transition as a Route to Improve Thermoelectric Performance in Bi0.5 Sb1.5 Te3.

The electronic structure near the Fermi surface determines the electrical properties of the materials, which can be effectively tuned by external pressure. Bi0.5 Sb1.5 Te3 is a p-type thermoelectric material which holds the record high figure of merit at room temperature. Here it is examined whether the figure of merit of this model system can be further enhanced through some external parameter. With the application of pressure, it is surprisingly found that the power factor of this material exhibits λ behavior with a high value of 4.8 mW m-1 K-2 at pressure of 1.8 GPa. Such an enhancement is found to be driven by pressure-induced electronic topological transition, which is revealed by multiple techniques. Together with a low thermal conductivity of about 0.89 W m-1 K-1 at the same pressure, a figure of merit of 1.6 is achieved at room temperature. The results and findings highlight the electronic topological transition as a new route for improving the thermoelectric properties.

Vacancy ordering induced topological electronic transition in bulk Eu2ZnSb2.

Metal-semiconductor transitions from changes in edge chirality from zigzag to armchair were observed in many nanoribbon materials, especially those based on honeycomb lattices. Here, this is generalized to bulk complex Zintl semiconductors, exemplified by Eu2ZnSb2 where the Zn vacancy ordering plays an essential role. Five Eu2ZnSb2 structural models are proposed to guide transmission electron microscopy imaging. Zigzag vacancy ordering models show clear metallicity, while the armchair models are semiconducting with indirect bandgaps that monotonously increase with the relative distances between neighboring ZnSb2 chains. Topological electronic structure changes based on cation ordering in a Zintl compound point toward tunable and possibly switchable topological behavior, since cations in these are often mobile. Thus, their orderings can often be adjusted by temperature, minor alloying, and other approaches. We explain the electronic structure of an interesting thermoelectric and point the way to previously unidentified types of topological electronic transitions in Zintl compounds.