GeTe, which undergoes phase transition from a low symmetry rhombohedral to a high symmetry cubic phase between 550 and 700 K (depending on the dopant and composition), and its alloys have widely been considered as promising candidates for mid-temperature range thermoelectrics. A variety of dopants have been tried in the past to further improve the thermoelectric performance of GeTe. In this work, an extensive experimental- and theoretical-based screening of transition (Y, Zr, Hf, V, Nb, Mo, and Ru) and rare-earth elements (La and Pr) that were not used in the past is performed to explore whether they can be of any potential use as dopants in thermoelectric GeTe. Out of these studied dopants, Ru, Nb, Pr, V, and Mo were found to be detrimental, Y and Hf to be reasonable, and Zr and La to be more promising. The transition or rare-earth dopant dependent variation of the transport properties and thermoelectric performance of GeTe is corroborated with a concoction of factors ranging from modifications of the band gap, energy difference between the two valence band maxima, magnetic character, nature of the dopant or impurity state and their position with regard to the Fermi level, secondary cubic GeTe phase, etc. In contrast to classical approaches where intrinsic Ge vacancies (that are inherently formed due to the thermodynamic nature of GeTe) are suppressed to improve its thermoelectric performance, an opposite approach is adopted in this work. Here by intentionally creating more electrically dormant Ge vacancies and modulating/balancing them with Zr-doping, an improved figure of merit, zT ∼ 1.3, at 673 K is obtained for the Ge-deficient Ge_0.98Zr_0.005Te compound, thanks to the suppression of the lattice contribution to the thermal conductivity arising due to the large density of planar defects in these vacancy-induced compounds. Co-doping of these optimized Ge-deficient Zr-doped materials with Sb results in effective convergence of electronic band valleys by tuning the crystal field effect and reduced thermal transport, thus yielding a high zT ∼ 1.8 at 723 K. This work not only screens a directory of dopant elements to enrich the current state of knowledge on GeTe-based compounds, but also indicates that the strategy of creation and synchronization of Ge vacancies with a certain dopant is an alternative and effective route for enhancing zT.

Thermoelectric materials seamlessly convert thermal into electrical energy, making them promising for power generation and cooling applications. Although historically the thermoelectric effect was first discovered in metals, state-of-the-art research focuses on semiconductors. Here, we discover unprecedented thermoelectric performance in metals and realize ultrahigh power factors up to 34 mW m ⁻¹ K ⁻² in binary Ni _x Au _{1– x} alloys, more than twice larger than in any bulk material above room temperature, reaching zT _max ∼ 0.5. In metallic Ni _x Au _{1– x} alloys, large Seebeck coefficients originate from electron-hole selective scattering of Au s electrons into more localized Ni d states. This intrinsic energy filtering effect owing to the unique band structure yields a strongly energy-dependent carrier mobility. While the metastable nature of the Ni-Au system as well as the high cost of Au pose some constraints for practical applications, our work challenges the common belief that good metals are bad thermoelectrics and presents an auspicious route toward high thermoelectric performance exploiting interband scattering.

We describe here a new, easy and scalable route for synthesising colusites by using mechanical-alloying and reactive spark plasma sintering, together with the thermoelectric behaviour of zinc-substituted derivatives, Cu_26−xZn_xV₂Sn₆S₃₂ (0 ≤ x ≤ 2). X-ray diffraction analysis coupled with transmission electron microscopy evidences the high crystallinity of the as-synthesized samples. In the pristine compound, an intrinsic exsolution phenomenon leads to the formation of two distinguishable colusite phases. Additional neutron powder diffraction results support the substitution of Zn in the tetrahedral Cu sites and ¹¹⁹Sn Mössbauer spectroscopy analyses prove the presence of only Sn⁴⁺ in colusite irrespective of the Zn content (x ≤ 2). The mechanical properties denote a wide homogeneity of the samples despite a significant impact of the exsolution on the microstructure. The Zn for Cu substitution provides electron doping, decreasing the holes concentration. High temperature thermoelectric properties in the p-type series Cu_26−xZn_xV₂Sn₆S₃₂ are reported. The highest power factor of 0.92 mW m⁻¹ K⁻² at 700 K is found for x = 1, with a corresponding ZT value of 0.4.