Strong Phonon-Phonon Interactions Securing Extraordinary Thermoelectric Ge1- xSb xTe with Zn-Alloying-Induced Band Alignment

Min Hong; Yuan Wang; Tianli Feng; Qiang Sun; Shengduo Xu; Syo Matsumura; Sokrates T Pantelides; Jin Zou; Zhi-Gang Chen

doi:10.1021/jacs.8b12624

Strong Phonon-Phonon Interactions Securing Extraordinary Thermoelectric Ge_{1- x}Sb _xTe with Zn-Alloying-Induced Band Alignment

J Am Chem Soc. 2019 Jan 30;141(4):1742-1748. doi: 10.1021/jacs.8b12624. Epub 2019 Jan 14.

Authors

Min Hong^{1

2}, Yuan Wang¹, Tianli Feng^{3

4}, Qiang Sun², Shengduo Xu², Syo Matsumura⁵, Sokrates T Pantelides^{3

4}, Jin Zou^{2

6}, Zhi-Gang Chen^{1

2}

Affiliations

¹ Centre for Future Materials , University of Southern Queensland , Springfield , Queensland 4300 , Australia.
² Materials Engineering , The University of Queensland , Brisbane , Queensland 4072 , Australia.
³ Department of Physics and Astronomy and Department of Electrical Engineering and Computer Science , Vanderbilt University , Nashville , Tennessee 37235 , United States.
⁴ Materials Science and Technology Division , Oak Ridge National Laboratory , Oak Ridge , Tennessee 37831 , United States.
⁵ Department of Applied Quantum Physics and Nuclear Engineering , Kyushu University , Motooka 744 , Nishi-ku, Fukuoka 819-0395 , Japan.
⁶ Centre for Microscopy and Microanalysis , The University of Queensland , Brisbane , Queensland 4072 , Australia.

PMID: 30592419
DOI: 10.1021/jacs.8b12624

Abstract

The ability of substitution atoms to decrease thermal conductivity is usually ascribed to the enhanced phonon-impurity scattering by assuming the original phonon dispersion relations. In this study, we find that 10% Sb_Ge alloying in GeTe modifies the phonon dispersions significantly, closes the acoustic-optical phonon band gap, increases the phonon-phonon scattering rates, and reduces the phonon group velocities. These changes, together with grain boundaries, nanoprecipitates, and planar vacancies, lead to a significant decrease in the lattice thermal conductivity. In addition, an extra 2-6% Zn alloying decreases the energy offset between valence band edges at L and Σ points in Ge_{1- x}Sb _xTe that is found to be induced by the Ge 4s² lone pairs. Since Zn is free of s² lone pair electrons, substituting Ge with Zn atoms can consequently diminish the Ge 4s² lone-pair characters and reduce the energy offset, resulting in two energetically merged valence band maxima. The refined band structures render a power factor up to 40 μW cm^-1 K^-2 in Ge_0.86Sb_0.1Zn_0.04Te. Ultimately, a superhigh zT of 2.2 is achieved. This study clarifies the impacts of high-concentration substitutional atoms on phonon band structure, phonon-phonon scattering rates, and the convergence of electron valence band edges, which could provide guidelines for developing high-performance thermoelectric materials.