Phonon Localization and Entropy-Driven Point Defects Lead to Ultralow Thermal Conductivity and Enhanced Thermoelectric Performance in (SnTe)1–2x(SnSe)x(SnS)x

Understanding of phase stability, chemical bonding, and phonon transport are essential to realize ultralow thermal conductivity in crystalline solids for designing high-performance thermoelectric (TE) materials. Pristine SnTe, a homologue of PbTe, exhibits poor TE performance primarily because of high lattice thermal conductivity, κlat. Herein, the amorphous limit of κlat is achieved via engineering configurational and vibrational entropies in pseudoternary (SnTe)1–2x(SnSe)x(SnS)x. Density functional theory calculations and synchrotron X-ray pair distribution function analysis reveal that S atoms are locally off-centered in global cubic SnTe, resulting in a low-energy localized optical phonon which strongly couples with heat-carrying acoustic phonons. Additionally, substitution of Se and S in SnTe increases the configurational entropy and point defects, resulting in an ultralow κlat of 0.52 W/mK. Finally, improvement of the Seebeck coefficient is achieved via the synergistic effect of resonant doping (In) and valence band convergence (Ag), which lead to a high TE figure of merit, zT, of ∼1.3 at 854 K.