Thermoelectric materials generate electric energy from waste heat, with conversion efficiency governed by the dimensionless figure of merit, ZT. Single-crystal tin selenide (SnSe) was discovered to exhibit a high ZT of roughly 2.2–2.6 at 913 K, but more practical and deployable polycrystal versions of the same compound suffer from much poorer overall ZT, thereby thwarting prospects for cost-effective lead-free thermoelectrics. The poor polycrystal bulk performance is attributed to traces of tin oxides covering the surface of SnSe powders, which increases thermal conductivity, reduces electrical conductivity and thereby reduces ZT. Here, we report that hole-doped SnSe polycrystalline samples with reagents carefully purified and tin oxides removed exhibit an ZT of roughly 3.1 at 783 K. Its lattice thermal conductivity is ultralow at roughly 0.07 W m–1 K–1 at 783 K, lower than the single crystals. The path to ultrahigh thermoelectric performance in polycrystalline samples is the proper removal of the deleterious thermally conductive oxides from the surface of SnSe grains. These results could open an era of high-performance practical thermoelectrics from this high-performance material.

Thermoelectric materials have attracted significant research interest in recent decades due to their promising application potential in interconverting heat and electricity. Unfortunately, the strong coupling between the material parameters that determine thermoelectric efficiency, i.e., the Seebeck coefficient, electrical conductivity, and thermal conductivity, complicates the optimization of thermoelectric energy converters. Main‐group chalcogenides provide a rich playground to alleviate the interdependence of these parameters. Interestingly, only a subgroup of octahedrally coordinated chalcogenides possesses good thermoelectric properties. This subgroup is also characterized by other outstanding characteristics suggestive of an exceptional bonding mechanism, which has been coined metavalent bonding. This conclusion is further supported by a map that separates different bonding mechanisms. In this map, all octahedrally coordinated chalcogenides with good performance as thermoelectrics are located in a well‐defined region, implying that the map can be utilized to identify novel thermoelectrics. To unravel the correlation between chemical bonding mechanism and good thermoelectric properties, the consequences of this unusual bonding mechanism on the band structure are analyzed. It is shown that features such as band degeneracy and band anisotropy are typical for this bonding mechanism, as is the low lattice thermal conductivity. This fundamental understanding, in turn, guides the rational materials design for improved thermoelectric performance by tailoring the chemical bonding mechanism.

Laser-assisted field evaporation is studied in a large number of compounds, including amorphous and crystalline phase change materials employing atom probe tomography. This study reveals significant differences in field evaporation between amorphous and crystalline phase change materials. High probabilities for multiple events with more than a single ion detected per laser pulse are only found for crystalline phase change materials. The specifics of this unusual field evaporation are unlike any other mechanism shown previously to lead to high probabilities of multiple events. On the contrary, amorphous phase change materials as well as other covalently bonded compounds and metals possess much lower probabilities for multiple events. Hence, laser-assisted field evaporation in amorphous and crystalline phase change materials reveals striking differences in bond rupture. This is indicative for pronounced differences in bonding. These findings imply that the bonding mechanism in crystalline phase change materials differs substantially from conventional bonding mechanisms such as metallic, ionic, and covalent bonding. Instead, the data reported here confirm a recently developed conjecture, namely that metavalent bonding is a novel bonding mechanism besides those mentioned previously.

Phase change memory has been developed into a mature technology capable of storing information in a fast and non-volatile way, with potential for neuromorphic computing applications. However, its future impact in electronics depends crucially on how the materials at the core of this technology adapt to the requirements arising from continued scaling towards higher device densities. A common strategy to fine-tune the properties of phase change memory materials, reaching reasonable thermal stability in optical data storage, relies on mixing precise amounts of different dopants, resulting often in quaternary or even more complicated compounds. Here we show how the simplest material imaginable, a single element (in this case, antimony), can become a valid alternative when confined in extremely small volumes. This compositional simplification eliminates problems related to unwanted deviations from the optimized stoichiometry in the switching volume, which become increasingly pressing when devices are aggressively miniaturized. Removing compositional optimization issues may allow one to capitalize on nanosize effects in information storage.

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and β-As 2 Te 3 ) and GaSe are investigated. Atom probe tomography studies reveal that four of the seven sesqui-chalcogenides (Bi 2 Te 3 , Bi 2 Se 3 , Sb 2 Te 3 , and β-As 2 Te 3 ) show an unconventional bond-breaking mechanism. The same four compounds evidence a remarkable property portfolio in density functional theory calculations including large Born effective charges, high optical dielectric constants, low Debye temperatures and an almost metal-like electrical conductivity. These results are indicative for unconventional bonding leading to physical properties distinctively different from those caused by covalent, metallic, or ionic bonding. The experiments reveal that this bonding mechanism prevails in four sesqui-chalcogenides, characterized by rather short interlayer distances at the van der Waals like gaps, suggestive of significant interlayer coupling. These conclusions are further supported by a subsequent quantum-chemistry-based bonding analysis employing charge partitioning, which reveals that the four sesqui-chalcogenides with unconventional properties are characterized by modest levels of charge transfer and sharing of about one electron between adjacent atoms. Finally, the 3D maps for different properties reveal discernible property trends and enable material design.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201904316.Chalcogenides are attracting considerable attention due to their striking properties. These characteristics enable a wide range of applications ranging from phase-change materials (PCMs) [1][2][3] to thermoelectrics [4] and topological insulators [5,6] for the heavier chalcogenides. The remarkable application potential has been attributed to an unconventional property portfolio. [2,[7][8][9] Adv. Mater. 2019, 31, 1904316

From a structural and economic perspective, tellurium-free PbSe can be an attractive alternative to its more expensive isostructural analogue of PbTe for intermediate temperature power generation. Here we report that PbSe0.998Br0.002-2%Cu2Se exhibits record high peak ZT 1.8 at 723 K and average ZT 1.1 between 300 and 823 K to date for all previously reported n- and p-type PbSe-based materials as well as tellurium-free n-type polycrystalline materials. These even rival the highest reported values for n-type PbTe-based materials. Cu2Se doping not only enhance charge transport properties but also depress thermal conductivity of n-type PbSe. It flattens the edge of the conduction band of PbSe, increases the effective mass of charge carriers, and enlarges the energy band gap, which collectively improve the Seebeck coefficient markedly. This is the first example of manipulating the electronic conduction band to enhance the thermoelectric properties of n-type PbSe. Concurrently, Cu2Se increases the carrier concentration with nearly no loss in carrier mobility, even increasing the electrical conductivity above ∼423 K. The resulting power factor is ultrahigh, reaching ∼21–26 μW cm–1 K–2 over a wide range of temperature from ∼423 to 723 K. Cu2Se doping substantially reduces the lattice thermal conductivity to ∼0.4 W m–1 K–1 at 773 K, approaching its theoretical amorphous limit. According to first-principles calculations, the achieved ultralow value can be attributed to remarkable acoustic phonon softening at the low-frequency region.

Thermoelectric materials with high average power factor and thermoelectric figure of merit (ZT) has been a soughtafter goal. Here, we report new n-type thermoelectric system Cu x PbSe 0.99 Te 0.01 (x = 0.0025, 0.004, and 0.005) exhibiting recordhigh average ZT ∼ 1.3 over 400−773 K ever reported for n-type polycrystalline materials including the state-of-the-art PbTe. We concurrently alloy Te to the PbSe lattice and introduce excess Cu to its interstitial voids. Their resulting strong attraction facilitates charge transfer from Cu atoms to the crystal matrix significantly. It follows the increased carrier concentration without damaging its mobility and the consequently improved electrical conductivity. This interaction also increases effective mass of electron in the conduction band according to DFT calculations, thereby raising the magnitude of Seebeck coefficient without diminishing electrical conductivity. Resultantly, Cu 0.005 PbSe 0.99 Te 0.01 attains an exceptionally high average power factor of ∼27 μW cm −1 K −2 from 400 to 773 K with a maximum of ∼30 μW cm −1 K −2 at 300 K, the highest among all n-and p-type PbSe-based materials. Its ∼23 μW cm −1 K −2 at 773 K is even higher than ∼21 μW cm −1 K −2 of the state-of-the-art n-type PbTe. Interstitial Cu atoms induce the formation of coherent nanostructures. They are highly mobile, displacing Pb atoms from the ideal octahedral center and severely distorting the local microstructure. This significantly depresses lattice thermal conductivity to ∼0.2 Wm −1 K −1 at 773 K below the theoretical lower bound. The multiple effects of the dual incorporation of Cu and Te synergistically boosts a ZT of Cu 0.005 PbSe 0.99 Te 0.01 to ∼1.7 at 773 K.