were synthesized by a molten flux method. The black needles of compound I were formed at 600°C and crystallized in the monoclinic P2 1 /m space group (No. 11) with a ) 17.492(3) Å, b ) 4.205(1) Å, c ) 18.461(4) Å, ) 90.49(2)°. The final R/R w ) 6.7/5.7%. Compound II is isostructural to I. Both I and II are isostructural with K 2 Bi 8 S 13 which is composed of NaCl-, Bi 2 Te 3 -, and CdI 2 -type units connecting to form K + -filled channels. The thin black needles of III and IV obtained at 530°C crystallize in the same space group P2 1 /m with a ) 17.534 (4) Å, b ) 4.206(1) Å, c ) 21.387(5) Å, ) 109.65(2)°and a ) 17.265(3) Å, b ) 4.0801(9) Å, c ) 21.280(3) Å, ) 109.31 (1)°, respectively. The final R/R w ) 6.3/8.3% and 5.1/3.6%. Compounds III and IV are isostructural and potassium and bismuth/antimony atoms are disordered over two crystallographic sites. The structure type is very closely related to that of I. Electrical conductivity and thermopower measurements show semiconductor behavior with ∼250 S/cm and ∼-200 µV/K for a single crystal of I and ∼150 S/cm and ∼-100 µV/K for a polycrystalline ingot of III at room temperature. The effect of vaccum annealing on these compounds is explored. The optical bandgaps of all compounds were determined to be 0.59, 0.78, 0.56, and 0.82 eV, respectively. The thermal conductivities of melt-grown polycrystalline ingots of I and III are reported.

Defect chemistry is critical to designing high performance thermoelectric materials. In SnTe, the naturally large density of cation vacancies results in excessive hole doping and frustrates the ability to control the thermoelectric properties. Yet, recent work also associates the vacancies with suppressed sound velocities and low lattice thermal conductivity, underscoring the need to understand the interplay between alloying, vacancies, and the transport properties of SnTe. Here, we report solid solutions of SnTe with NaSbTe2 and NaBiTe2 (NaSn m SbTe m+2 and NaSn m BiTe m+2, respectively) and focus on the impact of the ternary alloys on the cation vacancies and thermoelectric properties. We find introduction of NaSbTe2, but not NaBiTe2, into SnTe nearly doubles the natural concentration of Sn vacancies. Furthermore, DFT calculations suggest that both NaSbTe2 and NaBiTe2 facilitate valence band convergence and simultaneously narrow the band gap. These effects improve the power factors but also make the alloys more prone to detrimental bipolar diffusion. Indeed, the performance of NaSn m BiTe m+2 is limited by strong bipolar transport and only exhibits modest maximum ZTs ≈ 0.85 at 900 K. In NaSn m SbTe m+2 however, the doubled vacancy concentration raises the charge carrier density and suppresses bipolar diffusion, resulting in superior power factors than those of the Bi-containing analogues. Lastly, NaSbTe2 incorporation lowers the sound velocity of SnTe to give glasslike lattice thermal conductivities. Facilitated by the favorable impacts of band convergence, vacancy-augmented hole concentration, and lattice softening, NaSn m SbTe m+2 reaches high ZT ≈ 1.2 at 800–900 K and a competitive average ZTavg of 0.7 over 300–873 K. The difference in ZT between two chemically similar compounds underscores the importance of intrinsic defects in engineering high-performance thermoelectrics.

Spintronics holds great potential for next-generation high-speed and low–power consumption information technology. Recently, lead halide perovskites (LHPs), which have gained great success in optoelectronics, also show interesting magnetic properties. However, the spin-related properties in LHPs originate from the spin-orbit coupling of Pb, limiting further development of these materials in spintronics. Here, we demonstrate a new generation of halide perovskites, by alloying magnetic elements into optoelectronic double perovskites, which provide rich chemical and structural diversities to host different magnetic elements. In our iron-alloyed double perovskite, Cs2Ag(Bi:Fe)Br6, Fe3+ replaces Bi3+ and forms FeBr6 clusters that homogenously distribute throughout the double perovskite crystals. We observe a strong temperature-dependent magnetic response at temperatures below 30 K, which is tentatively attributed to a weak ferromagnetic or antiferromagnetic response from localized regions. We anticipate that this work will stimulate future efforts in exploring this simple yet efficient approach to develop new spintronic materials based on lead-free double perovskites.

The detection of γ-rays at room temperature with highenergy resolution using semiconductors is one of the most challenging applications. The presence of even the smallest amount of defects is sufficient to kill the signal generated from γ-rays which makes the availability of semiconductors detectors a rarity. Lead halide perovskite semiconductors exhibit unusually high defect tolerance leading to outstanding and unique optoelectronic properties and are poised to strongly impact applications in photoelectric conversion/detection. Here we demonstrate for the first time that large size single crystals of the all-inorganic perovskite CsPbCl 3 semiconductor can function as a high-performance detector for γ-ray nuclear radiation at room temperature. CsPbCl 3 is a wide-gap semiconductor with a bandgap of 3.03 eV and possesses a high effective atomic number of 69.8. We identified the two distinct phase transitions in CsPbCl 3 , from cubic (Pm-3m) to tetragonal (P4/mbm) at 325 K and finally to orthorhombic (Pbnm) at 316 K. Despite crystal twinning induced by phase transitions, CsPbCl 3 crystals in detector grade can be obtained with high electrical resistivity of ∼1.7 × 10 9 Ω•cm. The crystals were grown from the melt with volume over several cubic centimeters and have a low thermal conductivity of 0.6 W m −1 K −1 . The mobilities for electron and hole carriers were determined to ∼30 cm 2 /(V s). Using photoemission yield spectroscopy in air (PYSA), we determined the valence band maximum at 5.66 ± 0.05 eV. Under γ-ray exposure, our Schottky-type planar CsPbCl 3 detector achieved an excellent energy resolution (∼16% at 122 keV) accompanied by a high figure-of-merit hole mobility-lifetime product (3.2 × 10 −4 cm 2 /V) and a long hole lifetime (16 μs). The results demonstrate considerable defect tolerance of CsPbCl 3 and suggest its strong potential for γ-radiation and X-ray detection at room temperature and above.

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α-CsPbBi3Se6 (I), β-CsPbBi3Se6 (II), RbPbBi3Se6 (III), KPbBi3Se6 (IV), CsPbBi3S6 (V), and RbPbBi3S6 (VI) were synthesized by the polychalcogenide flux method. α-CsPbBi3Se6 was obtained at 720 °C and crystallizes in the space group Pnma (no. 62) with a = 23.564(6) Å, b = 4.210(2) Å, c = 13.798(3) Å at room temperature. Final R/R w = 3.0/3.6%. In this compound, parallel NaCl-type Pb/Bi/Se columns with rectangularly shaped cross-sections are interconnected by edge sharing to build a 3-D tunnel framework with Cs atoms located inside the tunnels. The hexagonal plates of β-CsPbBi3Se6 were obtained at 400 °C and crystallize in the space group P63/mmc (no. 194) with a = 4.213(2) Å, c = 25.22(1) Å, γ = 120° at −100 °C. Final R/R w = 4.2/4.7%. APbBi3Se6 (A = Rb, K) and APbBi3S6 (A = Cs, Rb) are isostructural with β-CsPbBi3Se6 and their hexagonal cell parameters were obtained at room temperature. The structure is composed of negatively charged Bi2Te3-type bilayers separated by alkali metals, which are distributed over two different crystallographic sites. The alkali metal ions are loosely packed in the interlayer space making them mobile. Topotactic ion-exchange reactions of two compounds, β-CsPbBi3Se6 and RbPbBi3Se6, were examined with LiI and NaI in the solid state and in aqueous solution. Prolonged water contact of the hexagonal compounds leads to decomposition and leaching of alkali metal and Pb2+ ions. Electrical conductivity and thermopower measurements for single crystals of I, II, and III show n-type semiconductor behavior with 0.6, 0.3, and 0.3 S/cm and −730, −550, and −560 μV/K at room temperature, respectively. The optical band gaps of all compounds range from 0.27 to 0.89 eV. Thermal properties of the compounds are reported.

The entanglement of lattice thermal conductivity, electrical conductivity, and Seebeck coefficient complicates the process of optimizing thermoelectric performance in most thermoelectric materials. Semiconductors with ultralow lattice thermal conductivities and high power factors at the same time are scarce but fundamentally interesting and practically important for energy conversion. Herein, an intrinsic p‐type semiconductor TlCuSe that has an intrinsically ultralow thermal conductivity (0.25 W m−1 K−1), a high power factor (11.6 µW cm−1 K−2), and a high figure of merit, ZT (1.9) at 643 K is described. The weak chemical bonds, originating from the filled antibonding orbitals p‐d* within the edge‐sharing CuSe4 tetrahedra and long TlSe bonds in the PbClF‐type structure, in conjunction with the large atomic mass of Tl lead to an ultralow sound velocity. Strong anharmonicity, coming from Tl+ lone‐pair electrons, boosts phonon–phonon scattering rates and further suppresses lattice thermal conductivity. The multiband character of the valence band structure contributing to power factor enhancement benefits from the lone‐pair electrons of Tl+ as well, which modify the orbital character of the valence bands, and pushes the valence band maximum off the Γ‐point, increasing the band degeneracy. The results provide new insight on the rational design of thermoelectric materials.

Grundkonzepte für die Erforschung thermoelektrischer Materialien, der gegenwärtige Kenntnisstand und neueste Entwicklungen auf dem Gebiet sind die Themen dieses Aufsatzes. In aktuellen Forschungsarbeiten werden der Leistungsfaktor maximiert und/oder die Wärmeleitfähigkeit minimiert, um höhere ZT–Werte zu erzielen. Ansätze zur Maximierung des Leistungsfaktors sind die Entwicklung neuer oder das Optimieren existierender Materialien durch Dotieren sowie die Erforschung nanoskaliger Materialien. Die Wärmeleitfähigkeit kann minimiert werden durch das Herstellen fester Lösungen, die Verwendung von Materialien mit niedriger intrinsischer Wärmeleitfähigkeit und durch Nanostrukturierung. Dieser Aufsatz beschreibt die aussichtsreichsten thermoelektrischen Bulkmaterialien unter Berücksichtigung der während des letzten Jahrzehnts gewonnenen Erkenntnisse. Zu Beginn werden die Kristallstruktur und die chemischen und physikalischen Eigenschaften einphasiger Bulkmaterialien sowie die Optimierung ihrer thermoelektrischen Leistung diskutiert. Anschließend wird untersucht, welche neuen Möglichkeiten sich durch den Einsatz nanostrukturierter Kompositmaterialien ergeben. Den Abschluss bildet ein Ausblick in die fernere Zukunft.