Ultrahigh Average Thermoelectric Figure of Merit, Low Lattice Thermal Conductivity and Enhanced Microhardness in Nanostructured (GeTe)x(AgSbSe₂)100−x

Waste heat sources are generally diffused and provide a range of temperatures rather than a particular temperature. Thus, thermoelectric waste heat to electricity conversion requires a high average thermoelectric figure of merit ( ZT avg ) of materials over the entire working temperature along with a high peak thermoelectric figure of merit ( ZT max ). Herein an ultrahigh ZT avg of 1.4 for (GeTe) 80 (AgSbSe 2 ) 20 [TAGSSe‐80, T=tellurium, A=antimony, G=germanium, S=silver, Se=selenium] is reported in the temperature range of 300–700 K, which is one of the highest values measured amongst the state‐of‐the‐art Pb‐free polycrystalline thermoelectric materials. Moreover, TAGSSe‐80 exhibits a high ZT max of 1.9 at 660 K, which is reversible and reproducible with respect to several heating–cooling cycles. The high thermoelectric performance of TAGSSe‐ x is attributed to extremely low lattice thermal conductivity ( κ lat ), which mainly arises due to extensive phonon scattering by hierarchical nano/meso‐structures in the TAGSSe‐ x matrix. Addition of AgSbSe 2 in GeTe results in κ lat of ≈0.4 W mK −1 in the 300–700 K range, approaching to the theoretical minimum limit of lattice thermal conductivity ( κ min ) of GeTe. Additionally, (GeTe) 80 (AgSbSe 2 ) 20 exhibits a higher Vickers microhardness (mechanical stability) value of ≈209 kgf mm −2 compared to the other state‐of‐the‐art metal chalcogenides, making it an important material for thermoelectrics.