Recently, Zhang Yongsheng, a researcher at the Institute of Solid State Physics, Hefei Institute of Material Science, Chinese Academy of Sciences, made new progress in theoretical calculations and interpretation of the physical mechanism of a new type of thermoelectric material (Pyrite-type ZnSe2). This study solved the phonon and electron Boltzmann transport equations, and theoretical calculations found that Pyrite-type ZnSe2 has good thermoelectric properties; based on the analysis of the bonding of atoms in the material and the vibration properties of phonons, it is explained that it has a low lattice The physical mechanism of thermal conductivity. Related research results were published in Physical Review B with the topic Localized dimers drive strong anharmonicity and low lattice thermal conductivity in ZnSe2.
Understanding the physical mechanisms in high-performance thermoelectric materials is the basis for finding, designing and optimizing the thermoelectric properties of materials. Zhang Yongsheng’s research group has discovered that Pyrite-type ZnSe2 is a potential high-performance thermoelectric material through high-throughput methods, and discovered that it is a cubic crystal composed of widely available cheap elements. However, the physical mechanism has not been analyzed in detail. Using more precise theoretical calculation methods to verify the good thermoelectric properties of the material and explain the physical mechanism can help expand the types of thermoelectric materials and enrich the existing thermoelectric theories.
In the research, the research team of Zhang Yongsheng calculated the thermal and electrical transport properties of Pyrite-type ZnSe2 in detail by solving the phonon and electron Boltzmann transport equations, and obtained its thermoelectric properties at different temperatures and carrier concentrations . The study analyzed the atomic bonding and phonon vibration properties of Pyrite-type ZnSe2, and found that there is a strong covalent bonding local Se-Se dimer in the material, and the existence of this dimer can make Zn in ZnSe2 Atoms have the properties of rattling atoms, which results in the low-frequency optical phonons formed by the rotation of Zn atoms around the Se-Se dimer with very strong anharmonic effects. This strong anharmonic effect makes the material extremely Low lattice thermal conductivity. In terms of electronic structure, studies have found that both the top and bottom of the valence band of ZnSe2 have complex Fermi surfaces, which makes it have good electrical properties. ZnSe2 has extremely low lattice thermal conductivity and good electrical transport properties, so that under p-type and n-type doping, the important parameters that determine the performance of thermoelectric materials-the maximum thermoelectric figure of merit can reach 2.21 and 1.87 respectively. The research shows that it is possible to find materials containing similar dimer or trimer local structure as potential new high-performance thermoelectric materials, providing guidance and reference for future experimental and theoretical research.
The research work was funded by the National Natural Science Foundation of China.
Figure 1. The crystal structure of Pyrite-type ZnSe2 and the local charge distribution on Se-Se dimer
Figure 2. ZT value of ZnSe2 at different temperatures and carrier concentrations
Figure 3. The contribution of the rotation of the Zn atom around the Se-Se dimer to the low-frequency optical phonons
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