Thermoelectric (TE) technology has attracted great attention recently because it can directly convert waste heat into electricity by using semiconductor materials. Hence it could play an important role in tackling the energy and environment issues arising in this technology oriented world. TE performance is determined by a dimensionless figure of merit, ZT=S2σT/κ, where S, σ, κ, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively. This expression indicates that high TE performance could be achieved in the materials, which have large Seebeck coefficient, high electrical conductivity, and low thermal conductivity.

Cu-based diamond-like chalcogenides have attracted considerable interests in TE applications because of their extremely low thermal conductivity. Thermal conductivity is related to the isometric heat capacity, group velocity of phonon, and phonon lifetimes. Compared with the elemental semiconductor Ge, Cu2GeSe3 has similar crystal structure, average mass, and isometric heat capacity, but much lower thermal conductivity. Previous studies presumed that the low thermal conductivity of Cu2GeSe3 was ascribed to its high anharmonicity. Recently, Prof. Jiang’s group, Shao and Liu et al. at Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences have investigated the lattice dynamics and thermodynamic properties of Cu2GeSe3 by first-principles calculations. The obtained phonon frequencies agree well with the measurements of Raman scattering. The thermodynamic properties are calculated within quasi-harmonic approximation, and the measured lattice thermal conductivity is well reproduced. The calculated Grüneisen parameter is found to be much smaller than previous prediction, indicating that the bonding anharmonicity is insufficient to explain the low thermal conductivity in Cu2GeSe3. Furthermore, the calculations for the density of states and electronic charge density of Cu2GeSe3 show that the overlap between Cu-d and Se-p state is rather slight, indicating a weak covalent p-d bonding. Therefore, the weak covalent Cu-Se bonding leads to the low Debye temperature, and then results in the low lattice thermal conductivity. These results have been published recently in EPL (DOI: 10.1209/0295-5075/109/47004).

This work was supported by the National Natural Science Foundation of China (No. 11234012，11404348，11404350), China Postdoctoral Science Foundation (No. 2014M561796), Zhejiang Province Preferential Postdoctoral Funded Project (Grant No. BSH1402080), Ningbo Municipal Natural Science Foundation (No. 2014A61008) and Ningbo Science and Technology Innovation Team (No. 2014B82004).

Dr Hezhu Shao: hzshao@nimte.ac.cn

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