Strong Phonon-Phonon Interactions Securing Extraordinary Thermoelectric Ge1-xSbxTe 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 E points in Ge1-xSbx Te that is found to be induced by the Ge 4S(2) lone pairs. Since Zn is free of s(2) lone pair electrons, substituting Ge with Zn atoms can consequently diminish the Ge 4s(2) 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 mu W cm(-1) K-2 in Ge0.86Sb0.1Zn0.04Te. 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.