The realization of ultralow and glass-like lattice thermal conductivity (κL) in crystalline solids remains a significant challenge, yet it is pivotal for thermoelectrics. Polycrystalline bulk AgSbTe2 has long been recognized as a promising mid-temperature thermoelectric material due to its intrinsically low κL; however, its thermodynamic instability in its pristine form has hindered its practical deployment. Here, we report the simple and scalable synthesis of nanocrystalline, off-stoichiometric, phase-pure Ag0.8Sb1.2Te2.2 via a facile hydrothermal route, without any detectable minor secondary phases such as Ag2Te or Sb2Te3, which are commonly present in AgSbTe2 and are known to degrade its intrinsic thermoelectric performance. The solution-phase synthesis and spark plasma sintering promote the formation of cation-ordered nanodomains, yielding nanoscale superstructures (∼5-20 nm) and consequently strain in the lattice, which effectively scatter heat-carrying acoustic phonons. This results in a glass-like κL with an ultralow value of ∼0.1-0.2 W/m K in the temperature range of 30-575 K. Furthermore, the semiconducting nature of nanocrystalline Ag0.8Sb1.2Te2.2 enables an exceptionally high room-temperature Seebeck coefficient of ∼330 μV/K. Cumulatively, these attributes deliver a promising thermoelectric figure of merit (zT) of ∼1.2 at ∼575 K in nanocrystalline Ag0.8Sb1.2Te2.2, which is almost 100% higher than that of bulk Ag0.8Sb1.2Te2.2. With a high zT maintained across a broad temperature range, the average thermoelectric figure of merit (zTavg) for nanocrystalline Ag0.8Sb1.2Te2.2 is estimated to be 0.86 from room temperature to 575 K.
Das et al. (Fri,) studied this question.