Ultralow Thermal Conductivity, Enhanced Mechanical Stability, and High Thermoelectric Performance in (GeTe)1–2x(SnSe)x(SnS)x

Thermoelectric (TE) energy conversion demands high performance crystalline inorganic solids that exhibit ultralow thermal conductivity, high mechanical stability, and good TE device properties. Pb-free germanium telluride (GeTe)-based material has recently attracted significant attention in TE power generation in mid temperatures, but pristine GeTe possesses significantly higher lattice thermal conductivity (κ_latt) compared to that of its theoretical minimum (κ_min) of ∼0.3 W/mK. Herein, we have demonstrated the reduction of κ_latt of (GeTe)_1-2x(SnSe)_x(SnS)_x very near to its κ_min. The (GeTe)_1-2x(SnSe)_x(SnS)_x system behaves as a coexistence of point-defect rich solid solution and phase separation. Initially, the addition of equimolar SnSe and SnS in the GeTe reduces the κ_latt by effective phonon scattering because of the excess point defects and rich microstructures. In the second step, introduction of Sb-doping leads to additional phonon scattering centers and optimizes the p-type carrier concentration. Notably, 10 mol % Sb-doped (GeTe)_0.95(SnSe)_0.025(SnS)_0.025 exhibits ultralow κ_latt of ∼0.30 W/mK at 300 K. Subsequently, 10 mol % Sb-doped (GeTe)_0.95(SnSe)_0.025(SnS)_0.025 exhibits a high TE figure of merit (zT) of ∼1.9 at 710 K. The high-performance sample exhibits a Vickers microhardness (mechanical stability) value of ∼194 H_V that is significantly higher compared to the pristine GeTe and other state-of-the-art thermoelectric materials. Further, we have achieved a high output power, ∼150 mW for the temperature difference of 462 K, in single leg TE device based on 10 mol % Sb-doped (GeTe)_0.95(SnSe)_0.025(SnS)_0.025.