GeTe is a promising mid-temperature thermoelectric compound but inevitably contains excessive Ge vacancies hindering its performance maximization. This work reveals that significant enhancement in the dimensionless figure of merit (ZT) could be realized by defect structure engineering from point defects to line and plane defects of Ge vacancies. The evolved defects including dislocations and nanodomains enhance phonon scattering to reduce lattice thermal conductivity in GeTe. The accumulation of cationic vacancies toward the formation of dislocations and planar defects weakens the scattering against electronic carriers, securing the carrier mobility and power factor. This synergistic effect on electronic and thermal transport properties remarkably increases the quality factor. As a result, a maximum ZT > 2.3 at 648 K and a record-high average ZT (300-798 K) were obtained for Bi_0.07Ge_0.90Te in lead-free GeTe-based compounds. This work demonstrates an important strategy for maximizing the thermoelectric performance of GeTe-based materials by engineering the defect structures, which could also be applied to other thermoelectric materials.

Mg₃(Sb,Bi)₂ is a promising thermoelectric material suited for electronic cooling, but there is still room to optimize its low-temperature performance. This work realizes >200% enhancement in room-temperature zT by incorporating metallic inclusions (Nb or Ta) into the Mg₃(Sb,Bi)₂-based matrix. The electrical conductivity is boosted in the range of 300–450 K, whereas the corresponding Seebeck coefficients remain unchanged, leading to an exceptionally high room-temperature power factor >30 μW cm⁻¹ K⁻²; such an unusual effect originates mainly from the modified interfacial barriers. The reduced interfacial barriers are conducive to carrier transport at low and high temperatures. Furthermore, benefiting from the reduced lattice thermal conductivity, a record-high average zT > 1.5 and a maximum zT of 2.04 at 798 K are achieved, resulting in a high thermoelectric conversion efficiency of 15%. This work demonstrates an efficient nanocomposite strategy to enhance the wide-temperature-range thermoelectric performance of n-type Mg₃(Sb,Bi)₂, broadening their potential for practical applications.

GeTe is a promising p-type material with increasingly enhanced thermoelectric properties reported in recent years, demonstrating its superiority for mid-temperature applications. In this work, the thermoelectric performance of GeTe is improved by a facile composite approach. We find that incorporating a small amount of boron particles into the Bi-doped GeTe leads to significant enhancement in power factor and simultaneous reduction in thermal conductivity, through which the synergistic modulation of electrical and thermal transport properties is realized. The thermal mismatch between the boron particles and the matrix induces high-density dislocations that effectively scatter the mid-frequency phonons, accounting for a minimum lattice thermal conductivity of 0.43 Wm⁻¹K⁻¹ at 613 K. Furthermore, the presence of boron/GeTe interfaces modifies the interfacial potential barriers, resulting in increased Seebeck coefficient and hence enhanced power factor (25.4 μWcm⁻¹K⁻² at 300 K). Consequently, we obtain a maximum figure of merit Z_max of 4.0 × 10⁻³K⁻¹ at 613 K in the GeTe-based composites, which is the record-high value in GeTe-based thermoelectric materials and also superior to most of thermoelectric systems for mid-temperature applications. This work provides an effective way to further enhance the performance of GeTe-based thermoelectrics.

Isobaric specific heat capacity (C_p) is an important parameter not only in physics but also for most materials. Its accurate measurement is particularly critical for performance evaluation of thermoelectric materials, but the experiments by differential scanning calorimetry (DSC) often lead to large uncertainties in the measurements, especially at elevated temperatures. In this study, we propose a simple method to determine C_p by measuring the sound velocity (υ) based on lattice vibration and expansion theory. The relative standard error of the υ is smaller than 1%, showing good accuracy and repeatability. The calculated C_p at elevated temperature (>300 K) increases slightly with increasing temperature due to the lattice expansion, which is more reasonable than the Dulong–Petit value. image