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

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 points in Ge_1-xSb_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.