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Understanding and controlling point defects in semiconductors are essential for developing advanced electronic and optoelectronic devices. Germanium telluride (GeTe), a semiconductor with a rhombohedral-to-cubic structural phase transition and a high concentration of intrinsic vacancies on the Ge sublattice, has recently attracted much interest for thermoelectric applications. However, the role of Ge vacancies in structural change and performance optimization remains obscure. Herein, we first unraveled the importance of Ge vacancies by combining first-principles calculations and Boltzmann transport theory. It is revealed that (1) Ge vacancies are more likely to spontaneously form in cubic GeTe, addressing its defective character; (2) Ge vacancies play a vital role in stabilizing cubic GeTe; and (3) Ge vacancies produce unfavorable band structure modification, leading to a reduced power factor. The following experiment found that AgInTe2 alloying promotes a symmetry change from rhombohedral to cubic and deteriorates the thermoelectric performance, in good agreement with the abovementioned conclusions. More importantly, a single-phase cubic GeTe-based material with stable n-type conduction was first discovered based on the defect chemistry approach. Our findings shed new light on the critical role of Ge vacancies in the structure-property relationship and stimulate the strategy of point defect engineering for future thermoelectric applications. Thermoelectrics: Flawed crystals have their benefits Engineering defects into materials that convert thermal heat into electricity can improve their usefulness in applications including energy harvesting. Germanium telluride (GeTe) is a promising thermoelectric compound that contains a high number of missing-atom defects, or vacancies, within its crystal lattice. Takao Mori and co-workers from the National Institute for Materials Science in Tsukuba, Japan, have now used theoretical and experimental techniques to show how these vacancies are related to thermoelectrical performance. According to the researchers, choosing synthetic conditions that produce cubic GeTe crystals makes it more likely for vacancies to form but at the cost of diminished thermal and electrical conductivity. Fortunately, the defects provide suitable sites for addition of other atoms, such as bismuth, that can lead to the unusual and stable n-type conduction beyond that available in pure GeTe. We first shaped the critical role of Ge vacancies in GeTe on the crystal structure-thermoelectric properties relationship by combining first-principle calculations, Boltzmann transport equation, and experimental properties studies, leading to the stabilization of the metastable cubic structure, unfavorable band structure modification, and stable N-type conduction. [ABSTRACT FROM AUTHOR]

p -type TlSbTe 2 and its In- and Yb-doped variants were prepared by hot pressing of the elements in the stoichiometric ratios, and their thermoelectric properties were determined after verifying the purity of the samples by powder X-ray diffraction. Theoretically, TlSbTe 2 is an intrinsic semiconductor possessing a narrow band gap; however, the as-prepared TlSbTe 2 exhibited relatively high electrical conductivity, decreasing with ascending temperature, which provides evidence for the presence of p -type extrinsic charge carriers (holes) along with the positive Seebeck coefficient. Doping In onto the Tl site inserts more electrons, thereby decreasing the hole concentration; as a result, the electrical and thermal conductivity decreased and the Seebeck coefficient increased. The best thermoelectric performance of In-doped TlSbTe 2 was achieved at 625 K, with a figure of merit (zT) of 0.77. The overall best zT was improved to 0.85 at 620 K with the sample of nominal composition Tl 0.98 SbYb 0.02 Te 2. An anisotropy test was implemented by comparing the electrical conductivity and Seebeck coefficient measured on the prismatic bars cut parallel and vertical to the pressing direction. The results indicate basically isotropic behavior in polycrystalline TlSbTe 2 along these two orthogonal directions. • TlSbTe2 is bestowed with an extraordinarily low thermal conductivity. • Our processing conditions caused an increase in the figure-of-merit by 15%. • Doping with Yb caused a further increase by an additional 20%, for a total increase of 39%. • Doping with In led to no significant changes in performance. • Direction-dependent measurements revealed no significant anisotropies in the electrical properties. [ABSTRACT FROM AUTHOR]

In the first part of this work, thallium lanthanide tellurides with composition Tl 9 LnTe 6 (Ln = La, Ce, Pr, Nd, Sm, Gd, Tb) have been prepared and their high temperature electrical and thermal transport properties investigated. Nearly single phase samples were obtained. Rietveld refinement results showed that Tl 9 CeTe 6 had an exceptionally small unit cell volume of 1033 Å 3 . The electrical conductivity and thermal conductivity increased across the lanthanide series from La to Tb. On the other hand, the Seebeck coefficient values decreased with Ln varying from La to Tb. Tl 9 SmTe 6 or Tl 9 GdTe 6 constituted an exception, as the Seebeck coefficient of Tl 9 SmTe 6 was smaller than that of Tl 9 GdTe 6 . Tl 9 LaTe 6 has the largest figure-of-merit of this series. Thereafter, Tl 10 − x La x Te 6 samples were prepared with x = 0.90, 0.95, 1.00, 1.05, 1.10, aiming toward higher zT through composition optimization and expanding of the measurement temperature range. With the increasing La content, the unit cell volume increased, while the electrical conductivity, thermal conductivity and lattice thermal conductivity decreased, and the Seebeck coefficient increased. The highest zT = 0.57 was observed for Tl 9 LaTe 6 at 600 K. [ABSTRACT FROM AUTHOR]

We present the thermoelectric properties of the quaternary telluride series Tl10- x- ySn xBi yTe6 including Seebeck coefficient ( S), electrical conductivity ( σ), and thermal conductivity ( κ). Three different Tl concentrations, namely 9, 8.67, and 8.33 Tl per formula unit, were selected followed by variations of the Sn:Bi ratio to tune the properties at each Tl concentration. Additionally, crystal structure data and electronic structure calculations were used to model the data and support the findings. The Tl9(Sn, Bi)Te6 system was found to have the highest power factor ( S2 σ) displaying a value of 8.1 μW ·cm-1 ·K-2 at 587 K, realized with Tl9Sn0.2Bi0.8Te6. Tl8.67Sn0.50Bi0.83Te6 however, displays values closer to 4 μW ·cm-1 ·K-2 at comparable temperatures. Ultimately, the thermoelectric Figure-of-merit was determined to reach a competitive 0.6 at 575 K and 525 K for Tl8.33Sn1.12Bi0.55Te6 and Tl8.67Sn0.50Bi0.83Te6, respectively, thereby outperforming the ternary variants Tl10- xSn xTe6. [ABSTRACT FROM AUTHOR]

n-Type CoSbGeTe ( x = 0.125 to 0.275) compounds with different Te contents have been synthesized by a melt-quench-anneal-spark plasma sintering method, and the effects of Te content on the structure and thermoelectric properties have been investigated. The results show that all specimens exhibited n-type conduction characteristics. The solubility limit of Te in CoSbGeTe is found to be x = 0.25. The solubility of Te in CoSb is increased through charge compensation of the element Ge. The room-temperature carrier concentration N of CoSbGeTe skutterudites increases with increasing Te content, and the compounds possess high power factors. The maximum power factor of 3.89 × 10 W m K was obtained at 720 K for the CoSbGeTe compound. The thermal conductivity decreases dramatically with increasing Te content due to strong point defect scattering. The maximum value of the thermoelectric figure of merit ZT = 1.03 was obtained at 800 K for CoSbGeTe, benefiting from a lower thermal conductivity and a higher power factor. The figure of merit is competitive with values reported for single-filled skutterudites. [ABSTRACT FROM AUTHOR]

Nearly single phase samples of Tl9LnTe6 (Ln: La, Ce, Pr, Nd, Sm, Gd, Tb) and Tl10-xLaxTe6 (x = 0.9-1.1) are prepared by melting of the elements (C-coated evacuated quartz tubes, 1200 K, 24 h). [ABSTRACT FROM AUTHOR]

Thermoelectric thin films are of great interest for self-powering Internet of Things (IoT) nodes via energy harvesting. Heusler-type Fe 2 VAl attracts some attention for room-temperature thermoelectric applications. However, the large overall thermal conductivity leads to a poor performance that restricts the current investigations. Herein, we report a comprehensive study of structural, thermoelectric, and magnetic properties of Fe 2 V 0.9 Ti 0.1 Al thin films on quartz substrate, grown by magnetron sputtering. It is found that high-temperature annealing is more effective than the long-time annealing at the low temperatures to stabilize the L 2 1 type structure of Fe 2 VAl, leading to a much larger Seebeck coefficient and slightly higher thermal conductivity. More importantly, the significant off-stoichiometry phenomenon, namely Fe-rich composition, contributes to n-type conduction of thin films. Both the picosecond and nanosecond time-domain thermoreflectance techniques were employed to successfully measure the thin-film thermal conductivities, merely one-third of bulk material, demonstrating the effectiveness of utilizing nanostructuring to reduce the thermal conductivity of Fe 2 VAl. Moreover, magnetic susceptibility measurements reveal that thin films are ferromagnets, while the bulk sample is characterized by paramagnetism. Our finding sheds new light on the composition-structure-property relationship of Fe 2 VAl thin films for future related applications. Image, graphical abstract The significant off-stoichiometry of Ti doped Fe 2 VAl films in the L2 1 type structure grown on quartz substrate leads to the unexpected n-type conduction and strong ferromagetism, which are may be helpful for thermoelectric, magnetoelectronic, and also spintronic applications. [ABSTRACT FROM AUTHOR]

The synthesis of pure pristine and vanadium-doped MnSi γ (γ = 1.74) powder by a relatively fast, 'low temperature' and high yield magnesioreduction process is described. The powder obtained by this innovative route is composed of well crystallized grains with sizes ranging from 20 to 500 nm and free from any MnSi precipitates. Mesostructured densified pellets with average grain sizes as small as 550 nm are obtained by spark plasma sintering (SPS). Detailed structural and microstructural characterization of the samples were realized at every stage of the process, highlighting a high concentration of defects such as orientational or spacing anomalies of the Nowotny phase, γ variations within a single grain and dislocations. Accordingly a significant decrease of the lattice thermal conductivity is evidenced in comparison to conventionally synthesized (arc-melting/SPS) samples having similar density and (V/)Mn/Si stoichiometry. The thermoelectric properties of these materials are discussed in regard of their complex microstructure. Image 1 • Mesostructured higher manganese silicides are synthesized by magnesioreduction. • Crystal structures are refined in composite-incommensurate unit-cells. • Microstructures are examined by EBSD and TEM. • The decrease in thermal conductivity is correlated to the microstructure. [ABSTRACT FROM AUTHOR]