Halide perovskites are a rapidly developing class of medium-bandgap semiconductors which, to date, have been popularized on account of their remarkable success in solid-state heterojunction solar cells raising the photovoltaic efficiency to 20% within the last 5 years. As the physical properties of the materials are being explored, it is becoming apparent that the photovoltaic performance of the halide perovskites is just but one aspect of the wealth of opportunities that these compounds offer as high-performance semiconductors. From unique optical and electrical properties stemming from their characteristic electronic structure to highly efficient real-life technological applications, halide perovskites constitute a brand new class of materials with exotic properties awaiting discovery. The nature of halide perovskites from the materials' viewpoint is discussed here, enlisting the most important classes of the compounds and describing their most exciting properties. The topics covered focus on the optical and electrical properties highlighting some of the milestone achievements reported to date but also addressing controversies in the vastly expanding halide perovskite literature.

Effiziente thermoelektrische Materialien werden durch ein Zusammenspiel von stark voneinander abhängenden Transporteigenschaften erreicht. Um diese wissentlich zu beeinflussen, wird ein tieferes Verständnis der Chemie und Physik in Festkörpern benötigt. Auf der Grundlage von Molekülorbitaldiagrammen, der “Tight-Binding”-Methode und dem klassischem Verständnis von Bindungsstärken betrachten wir das gezielte Design thermoelektrischer Materialien. Hierbei werden Parameter wie Elektronegativität, Bandbreite, Orbitalüberlappung sowie Bindungsenergie und -länge herangezogen, um Trends der elektronischen Eigenschaften wie Größe und Temperaturabhängigkeit von Bandlücken, effektive Masse der Ladungsträger sowie Bandentartung und Bandkonvergenz zu erklären. Gitterwärmeleitfähigkeiten werden in Bezug auf die Kristallstruktur und Bindungsstärke behandelt, um den Einfluss von Bindungslängen zu verdeutlichen. Wir zeigen, wie Symmetrie und Stärke von Bindungen den Transport von Elektronen und Phononen beeinflussen und wie gezielte Strategien zu Veränderungen und zur Verbesserung thermoelektrischer Effizienz führen können.

The coupled transport properties required to create an efficient thermoelectric material necessitates a thorough understanding of the relationship between the chemistry and physics in a solid. We approach thermoelectric material design using the chemical intuition provided by molecular orbital diagrams, tight binding theory, and a classic understanding of bond strength. Concepts such as electronegativity, band width, orbital overlap, bond energy, and bond length are used to explain trends in electronic properties such as the magnitude and temperature dependence of band gap, carrier effective mass, and band degeneracy and convergence. The lattice thermal conductivity is discussed in relation to the crystal structure and bond strength, with emphasis on the importance of bond length. We provide an overview of how symmetry and bonding strength affect electron and phonon transport in solids, and how altering these properties may be used in strategies to improve thermoelectric performance.

The heavy metal selenophosphate, Pb₂P₂Se₆, is a promising new material for cost-effective X-ray/γ-ray detection. Crystal boules of Pb₂P₂Se₆ up to 25 mm in length and 15 mm in diameter are grown by a vertical Bridgman method. They are cut and processed into size-appropriate wafers for physical, photo-transport property studies, as well as γ-ray detector testing. The material is a semiconductor with an indirect bandgap of 1.88 eV and has electrical resistivity in the range of 1 × 10¹⁰ Ω cm. Pb₂P₂Se₆ single crystal samples display a significant photoconductivity response to optical, X-ray, and γ-ray radiation. When tested with a ⁵⁷Co γ-ray source, Pb₂P₂Se₆ crystals show spectroscopic response and several generated pulse height spectra resolving the 122.1 and 136.5 keV ⁵⁷Co radiation. The mobility–lifetime product of Pb₂P₂Se₆ is estimated to be ≈3.5 × 10⁻⁵ cm² V⁻¹ for electron carriers. The Pb₂P₂Se₆ compound melts congruently at 812 °C and has robust chemical/physical properties that promise low cost bulk production and detector development.

EuIr₄In₂Ge₄ is a new intermetallic semiconductor that adopts a non-centrosymmetric structure in the tetragonal space group with unit cell parameters a=6.9016(5) Å and c=8.7153(9) Å. The compound features an indirect optical band gap E_g=0.26(2) eV, and electronic-structure calculations show that the energy gap originates primarily from hybridization of the Ir 5d orbitals, with small contributions from the Ge 4p and In 5p orbitals. The strong spin–orbit coupling arising from the Ir atoms, and the lack of inversion symmetry leads to significant spin splitting, which is described by the Dresselhaus term, at both the conduction- and valence-band edges. The magnetic Eu²⁺ ions present in the structure, which do not play a role in gap formation, order antiferromagnetically at 2.5 K.

EuIr₄In₂Ge₄ is a new intermetallic semiconductor that adopts a non-centrosymmetric structure in the tetragonal space group with unit cell parameters a=6.9016(5) Å and c=8.7153(9) Å. The compound features an indirect optical band gap E_g=0.26(2) eV, and electronic-structure calculations show that the energy gap originates primarily from hybridization of the Ir 5d orbitals, with small contributions from the Ge 4p and In 5p orbitals. The strong spin–orbit coupling arising from the Ir atoms, and the lack of inversion symmetry leads to significant spin splitting, which is described by the Dresselhaus term, at both the conduction- and valence-band edges. The magnetic Eu²⁺ ions present in the structure, which do not play a role in gap formation, order antiferromagnetically at 2.5 K.

Phase immiscibility in PbTe–based thermoelectric materials is an effective means of top-down synthesis of nanostructured composites exhibiting low lattice thermal conductivities. PbTe_1-x S_x thermoelectric materials can be synthesized as metastable solid solution alloys through rapid quenching. Subsequent post-annealing induces phase separation at the nanometer scale, producing nanostructures that increase phonon scattering and reduce lattice thermal conductivity. However, there has yet to be any study investigating in detail the local chemical structure of both the solid solution and nanostructured variants of this material system. Herein, quenched and annealed (i.e., solid solution and phase-separated) samples of PbTe–PbS are analyzed by in situ high-resolution synchrotron powder X-ray diffraction, solid-state ¹²⁵Te nuclear magnetic resonance (NMR), and infrared (IR) spectroscopy analysis. For high concentrations of PbS in PbTe, e.g., x >16%, NMR and IR analyses reveal that rapidly quenched samples exhibit incipient phase separation that is not detected by state-of-the-art synchrotron X-ray diffraction, providing an example of a PbTe thermoelectric “alloy” that is in fact phase inhomogeneous. Thermally-induced PbS phase separation in PbTe–PbS occurs close to 200 °C for all compositions studied, and the solubility of the PbS phase in PbTe at elevated temperatures >500 °C is reported. The findings of this study suggest that there may be a large number of thermoelectric alloy systems that are phase inhomogeneous or nanostructured despite adherence to Vegard's Law of alloys, highlighting the importance of careful chemical characterization to differentiate between thermoelectric alloys and composites.

The effects of Cd-doping on the thermoelectric properties of Sn_1–xPb_xTe are investigated and compared to the properties of the corresponding Sn_1–xPb_xTe solid solutions. The addition of Cd results in a reduction in the carrier concentration and changes in the physical properties, as well as in the conduction type of Sn_1–xPb_xTe. A significant increase in the power factor accompanied by a reduction in the thermal conductivity result in a higher figure of merit (ZT) for (Sn_1–xPb_x)_0.97Cd_0.03Te than that of undoped Sn_1–xPb_xTe. The maximum ZT (∼0.7) values are observed for p-type material with x = 0.36 at 560 K. Much higher values (ZT ∼ 1.2 at 560 K for x = 0.73) are obtained on n-type samples.

The thermoelectric properties of crystalline melt-grown ingots of p-type PbTe–xMgTe (x = 1–3 mol%) doped with Na₂Te (1–2 mol%) were investigated over the temperature range of 300 K to 810 K. While the powder X-ray diffraction patterns show that all samples crystallize in the NaCl-type structure with no MgTe or other phases present, transmission electron microscopy reveals ubiquitous MgTe nanoprecipitates in the PbTe. The very small amounts of MgTe in PbTe have only a small effect on the electrical transport properties of the system, while they have a large effect on thermal transport significantly reducing the lattice thermal conductivity. A ZT of 1.6 at 780 K is achieved for the PbTe containing 2% MgTe doped with 2% Na₂Te.

Thermoelectric materials based on Pb-free compositions are of considerable current interest in environmentally friendly power-generation applications derived from waste-heat sources. Here, a new study of the thermoelectric properties of the tin-based compositions with the general formula AgSn_mSbTe_m+2 (m = 2, 4, 5, 7, 10, 14, 18) is presented, where the m value is used as the tuning parameter of the thermoelectric properties. The electrical conductivity, Seebeck coefficient, and thermal conductivity are measured from 300 K to 723 K and the resulting thermoelectric figure of merit is determined as a function of the SnTe/AgSbTe₂ ratio. A thermoelectric figure of merit ZT ≈1 is obtained at 710 K for m = 4, indicating that the system AgSn_mSbTe_m+2 holds great promise as an alternative p-type, lead-free, thermoelectric material.

A facile method for the synthesis of crystalline and amorphous GeTe nanoparticles (NPs) using bis((trimethylsilyl)amido)germanium(II), Ge[N(SiMe₃)₂]₂, and elemental tellurium dispersed in tri-n-octylphosphine (TOP) is reported. As synthesized, crystalline particles exhibit narrow dispersity at smaller sizes and tend to grow into anisotropic shapes with increasing reaction time (growth). Furthermore, crystalline GeTe NPs possess rhombohedral symmetry with absorption band energies in near IR region (0.76–0.86 eV). Amorphous GeTe particles prepared at low temperatures are nearly spherical in morphology and display amorphous-to-crystalline phase transition at 209–237 °C depending on their primary particle size. Detailed investigation of the local structure of the amorphous GeTe using pair distribution function (PDF) method reveals that it is closely related to that of the pressure- and temperature-stabilized orthorhombic GeTe.

The front cover picture shows the clock tower, the “Campanile”, of Iowa State University where John Corbett did the ground-breaking research in polar intermetallics that forms the basis of his Viewpoint in this cluster issue. Superimposed on this background are structures and data to visualize the broad scope of topic. The complexity of structure is displayed by the ternary rare-earth cobalt gallides that contain interstitial atoms (top left, A. Mar et al.), a calcium-poor intermetallic phase of the Ca/Ni/Ge system (top right from the lab of T. Fässler), and a single crystal of a polymorph of thallium nickel gallide (bottom right, J. Chan et al.). The potentially general synthesis of colloidal nanoparticles – Au₃Li from the lab of R. E. Schaak – is outlined mid left. The groups of M. H. Whangbo and G. Miller devote their contributions to the theoretical aspects of bonding (depicted top centre, the plots showing Au–Au bonding and antibonding interactions in Dy₂Au₂In and mid right, the effects of ionic interactions on the structural properties of isoelectronic intermetallic compounds, respectively). Representative of the range of properties discussed is the magnetic susceptibility of Yb₅Ni₄Ge₁₀ (bottom left, M. G. Kanatzidis et al.). We thank the authors for the use of the graphics from their papers on the cover.

The new compound Yb₅Ni₄Ge₁₀ was obtained as single crystals in high yield from reactions run in liquid indium. Single-crystal X-ray diffraction data showed that it crystallizes in the Sc₅Co₄Si₁₀ structure type in the tetragonal space group P4/mbm. The structure is a three-dimensional framework of [Ni₄Ge₁₀] atoms with voids filled by Yb atoms. The Ni and Ge atoms form pentagons, hexagons, and octagons along the ab one-dimensional plane, which are interconnected along the c axis via Ni–Ge–Ni zigzag chains. Magnetic measurements reveal that Yb₅Ni₄Ge₁₀ crystals show paramagnetic behavior with a lack of long-range magnetic ordering down to 2 K. The magnetic susceptibility deviates from the Curie–Weiss law below 100 K, presumably because of crystal field effects from Yb³⁺ ions. In specific heat measurements, the compound exhibits non-Fermi-liquid-like behavior at low temperatures.

The reduction of thermal conductivity, and a comprehensive understanding of the microstructural constituents that cause this reduction, represent some of the important challenges for the further development of thermoelectric materials with improved figure of merit. Model PbTe-based thermoelectric materials that exhibit very low lattice thermal conductivity have been chosen for this microstructure–thermal conductivity correlation study. The nominal PbTe_0.7S_0.3 composition spinodally decomposes into two phases: PbTe and PbS. Orderly misfit dislocations, incomplete relaxed strain, and structure-modulated contrast rather than composition-modulated contrast are observed at the boundaries between the two phases. Furthermore, the samples also contain regularly shaped nanometer-scale precipitates. The theoretical calculations of the lattice thermal conductivity of the PbTe_0.7S_0.3 material, based on transmission electron microscopy observations, closely aligns with experimental measurements of the thermal conductivity of a very low value, ∼0.8 W m⁻¹ K⁻¹ at room temperature, approximately 35% and 30% of the value of the lattice thermal conductivity of either PbTe and PbS, respectively. It is shown that phase boundaries, interfacial dislocations, and nanometer-scale precipitates play an important role in enhancing phonon scattering and, therefore, in reducing the lattice thermal conductivity.

Thermoelectric materials based on quaternary compounds Ag_1−xPb_mSbTe_2+m exhibit high dimensionless figure-of-merit values, ranging from 1.5 to 1.7 at 700 K. The primary factor contributing to the high figure of merit is a low lattice thermal conductivity, achieved through nanostructuring during melt solidification. As a consequence of nucleation and growth of a second phase, coherent nanoscale inclusions form throughout the material, which are believed to result in scattering of acoustic phonons while causing only minimal scattering of charge carriers. Here, characterization of the nanosized inclusions in Ag_0.53Pb₁₈Sb_1.2Te₂₀ that shows a strong tendency for crystallographic orientation along the {001} planes, with a high degree of lattice strain at the interface, consistent with a coherent interfacial boundary is reported. The inclusions are enriched in Ag relative to the matrix, and seem to adopt a cubic, 96 atom per unit cell Ag₂Te phase based on the Ti₂Ni type structure. In-situ high-temperature synchrotron radiation diffraction studies indicated that the inclusions remain thermally stable to at least 800 K.

The H_2xMn_xSn_3-xS₆ (x = 0.11–0.25) is a new solid acid with a layered hydrogen metal sulfide (LHMS). It derives from K_2xMn_xSn_3–xS₆ (x = 0.5–0.95) (KMS-1) upon treating it with highly acidic solutions. We demonstrate that LHMS-1 has enormous affinity for the very soft metal ions such as Hg²⁺ and Ag⁺ which occurs via a rapid ion exchange process. The tremendous affinity of LHMS-1 for Hg²⁺ is reflected in very high distribution coefficient K_d^Hg values (>10⁶ mL g⁻¹). The large affinity and selectivity of LHMS-1 for Hg²⁺ persists in a very wide pH range (from less than zero to nine) and even in the presence of highly concentrated HCl and HNO₃ acids. LHMS-1 is significantly more selective for Hg²⁺ and Ag⁺ than for the less soft cations Pb²⁺ and Cd²⁺. The Hg²⁺ ions are immobilized in octahedral sites between the sulfide layers of the materials via Hg–S bonds as suggested by pair distribution function (PDF) analysis. LHMS-1 could decrease trace concentrations of Hg²⁺ (e.g. <100 ppb) to well below the acceptable limits for the drinking water in less than two min. Hg-laden LHMS-1 shows a remarkable hydrothermal stability and resistance in 6 M HCl solutions. LHMS-1 could be regenerated by treating Hg-loaded samples with 12 M HCl and re-used without loss of its initial exchange capacity.

Herein we cover the key concepts in the field of thermoelectric materials research, present the current understanding, and show the latest developments. Current research is aimed at increasing the thermoelectric figure of merit (ZT) by maximizing the power factor and/or minimizing the thermal conductivity. Attempts at maximizing the power factor include the development of new materials, optimization of existing materials by doping, and the exploration of nanoscale materials. The minimization of the thermal conductivity can come through solid-solution alloying, use of materials with intrinsically low thermal conductivity, and nanostructuring. Herein we describe the most promising bulk materials with emphasis on results from the last decade. Single-phase bulk materials are discussed in terms of chemistry, crystal structure, physical properties, and optimization of thermoelectric performance. The new opportunities for enhanced performance bulk nanostructured composite materials are examined and a look into the not so distant future is attempted.

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.