We report a remarkably high and stable thermoelectric figure of merit zT close to 2 by manipulating the electronic bands in Ga–Sb codoped GeTe, which has been processed by hybrid flash-spark plasma sintering. According to the experimental results and first-principles calculations, the vast enhancement achieved in the thermopower due to codoping of Ga (2 mol%) and Sb (8 mol%) in GeTe is attributed to a concoction of reasons: (i) suppression of hole concentration; (ii) improved band convergence by decreasing the energy separation between the two valence band maxima to 0.026 eV; (iii) Ga predominantly contributing to the top of the valence band in Ga–Sb codoped GeTe, despite the Ga-induced resonance state not being located at a favorable position near the Fermi level; (iv) active participation of several bands increasing the hole carrier effective mass; (v) facilitating band degeneracy by reducing the R3m → Fmm structural transition temperature from 700 K to 580 K. The synergy between these complementary and beneficial effects, in addition to the reduced thermal conductivity, enabled the flash sintered Ge_0.90Ga_0.02Sb_0.08Te composition to not only exhibit a peak of zT of ∼1.95 at 723 K, but also to maintain/stabilize its high performance over a broad temperature range (600–775 K), thus making it a serious candidate for mid-temperature range energy harvesting devices.

Most of the best known SnTe‐based materials exhibit an attractive thermoelectric figure of merit (zT) only at the high‐temperature regime, but their performance at the low‐mid temperature ranges is quite uninspiring, and this discordance necessitates a large temperature gradient (∆T ≥ 550 K) to effectuate a reasonable efficiency, η. Here, the transition elements, Ti and Zr, that have not been used in the past are tried as dopants for SnTe and an enhanced device/average zT and/or η are reported with a lower ∆T ≈ 400 K and without the requisite for a stupendous peak/maximum zT. This notable performance emanates from—i) improved weighted mobility by optimally balancing between effective mass, carrier concentration, and mobility, ii) coupling of charge carriers with magnetic entropy, and the paramount factor being the iii) weakening of the chemical bonds (lattice softening). The thermal damping caused by lattice softening affects the phonon group velocity and the elastic properties, and the resultant increase in the degree of anharmonicity and the high density of internal strain‐fields, along with the phonon scattering effects, play an active role in tuning the overall thermoelectric performance. This work also excavates/opens up the discussion of applying the Heikes’ equation to qualitatively compare the trend of charge carriers for a given thermoelectric material system.

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.

This work presents a novel, simpler and faster bottom-up approach to produce relatively high performance thermoelectric Al-doped ZnO ceramics from nanopowders produced by interfacial reaction followed by consolidation with Spark Plasma Sintering.