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 (ZTavg ) of materials over the entire working temperature along with a high peak thermoelectric figure of merit (ZTmax ). Herein an ultrahigh ZTavg of 1.4 for (GeTe)80 (AgSbSe2 )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 ZTmax 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 (κlat ), which mainly arises due to extensive phonon scattering by hierarchical nano/meso-structures in the TAGSSe-x matrix. Addition of AgSbSe2 in GeTe results in κlat of ≈0.4 W mK-1 in the 300-700 K range, approaching to the theoretical minimum limit of lattice thermal conductivity (κmin ) of GeTe. Additionally, (GeTe)80 (AgSbSe2 )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.