High thermoelectric figure of merit, zT, of ~1.85 at 725 K along with significant cyclable temperature stability was achieved in Pb-free p-type Ge 1-x Sb x Te samples through simultaneous enhancement in Seebeck coefficient and reduction of thermal conductivity. Sb doping in GeTe decreases the carrier concentration due to the donor dopant nature of Sb and enhances the valence band degeneracy by increasing the cubic nature of the sample, which collectively boost Seebeck coefficient in the temperature range of 300-773 K.Significant thermal conductivity reduction was achieved due to collective phonon scattering from various meso-structured domain variants, twin and inversion boundaries, nanostructured defect layers, and solid solution point defects. The high performance Ge 0.9 Sb 0.1 Te sample shows mechanical stability (Vickers microhardness) of ~206 H v , which is significantly higher compared to other popular thermoelectric materials such as Bi 2 Te 3 , PbTe, PbSe, Cu 2 Se and TAGS.

The spontaneous formation of nanodomains of the Sb-rich layered intergrowth SnmSb2nTe3n+m compounds in a SnTe matrix resulted in ultralow lattice thermal conductivity.

GeTe and its derivatives have recently attracted wide attention as promising thermoelectric materials. The principle challenge in optimizing the thermoelectric figure of merit, zT, is the low Seebeck coefficient (S) and high thermal conductivity of GeTe. Here, we report a high zT of $2.1 at 723 K in In and Bi codoped GeTe along with an extremely high TE conversion efficiency of $12.3% in a single-leg thermoelectric generator for the temperature difference of 445 K. In and Bi play a distinct but complementary role. In doping significantly enhances the S through the formation of resonance level, which is confirmed with first-principles density functional theory calculations and Pisarenko plot considering two valance band model. However, Bi doping markedly reduces the lattice thermal conductivity due to the formation of extensive solid solution point defects and domain variants. Moreover, a high value of Vickers microhardness ($200 H v , H v = kgf/mm 2 ) reveals excellent mechanical stability.

This review summarizes the past and recent developments in the thermoelectric properties, nano/microstructure modulations, and mechanical and thermal stability of GeTe based materials and thermoelectric devices based on GeTe.

Nano/meso-structuring reduces the thermal conductivity in Ge1−xBixTe samples, which results in a thermoelectric figure of merit, zT, of 1.3 in Ge1−xBixTe.

Complementary and beneficial effects of Sb and Bi codoping in GeTe are shown to generate high thermoelectric figure of merit, zT, of 1.8 at 725 K in Ge1‑x‑y Bi x Sb y Te samples. Sb and Bi codoping in GeTe facilitates the valence band convergence enhancing the Seebeck coefficient as supported by density functional theoretical (DFT) calculations. Further, Sb and Bi codoping in GeTe releases the rhombohedral strain and increases its tendency to be cubic in structure, which ultimately enhances the valence band degeneracy. At the same time, Bi forms nanoprecipitates of size ∼5–20 nm in GeTe matrix and Sb doping increases solid solution point defects greatly, which altogether scatter low-to mid wavelength phonons and result in reduced lattice thermal conductivity down to 0.5 W/mK in the 300–750 K range.

High-performance thermoelectric materials are desirable in the lower-medium temperature range (450–650 K) for low-grade waste heat recovery. We report a thermoelectric figure of merit (zT) of 1.9 at 585 K in p-type AgSb1–x Zn x Te2, which is the highest value measured among the p-type materials in the 450–650 K range. A high average thermoelectric figure of merit (zTavg) of 1.3 is achieved in AgSb0.96Zn0.04Te2. Moreover, the AgSb1–x Zn x Te2 sample exhibits a hardness value of ∼6.3 GPa (nanoindentation), which is significantly higher than that of the pristine AgSbTe2. Substitution of Zn in AgSbTe2 suppresses the formation of intrinsic Ag2Te impurity phases, which indeed increases the thermal and mechanical stability. The lattice thermal conductivity for AgSb1–x Zn x Te2 samples is reasonably reduced compared to that of the pristine AgSbTe2 because of the significant solid solution point defect phonon scattering. Aliovalent Zn2+ doping in Sb3+ sites in AgSbTe2 increases the p-type carrier concentration, which boosts the electrical conductivity of AgSb1–x Zn x Te2.

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 ) of materials over the entire working temperature along with a high peak thermoelectric figure of merit (ZT ). Herein an ultrahigh ZT of 1.4 for (GeTe) (AgSbSe ) [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 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 ), which mainly arises due to extensive phonon scattering by hierarchical nano/meso-structures in the TAGSSe-x matrix. Addition of AgSbSe in GeTe results in κ of ≈0.4 W mK in the 300-700 K range, approaching to the theoretical minimum limit of lattice thermal conductivity (κ ) of GeTe. Additionally, (GeTe) (AgSbSe ) exhibits a higher Vickers microhardness (mechanical stability) value of ≈209 kgf mm compared to the other state-of-the-art metal chalcogenides, making it an important material for thermoelectrics.