Rapid One-Step Synthesis and Compaction of High-Performance n-Type Mg3Sb2 Thermoelectrics

n-type Mg₃Sb₂-based compounds have emerged as a promising class of low-cost thermoelectric materials due to their extraordinary performance at low and intermediate temperatures. However, so far, high thermoelectric performance has merely been reported in n-type Mg₃Sb₂-Mg₃Bi₂ alloys with a large amount of Bi. Moreover, current synthesis methods of n-type Mg₃Sb₂ bulk thermoelectrics involve multi-step processes that are time- and energy-consuming. Herein, we report a fast and straightforward approach to fabricate n-type Mg₃Sb₂ thermoelectrics using spark plasma sintering, which combines the synthesis and compaction in one step. Using this method, we achieve a high thermoelectric figure of merit zT of about 0.4–1.5 at 300–725 K in n-type (Sc, Te)-co-doped Mg₃Sb₂ without alloying with Mg₃Bi₂. In comparison with the currently reported synthesis methods, the complexity, process time, and cost of our method are significantly reduced. This work demonstrates a simple, low-cost route for the potential large-scale production of n-type Mg₃Sb₂ thermoelectrics.     

Few‐Layer Nanosheets of n‐Type SnSe2

Layered p‐block metal chalcogenides are renowned for thermoelectric energy conversion due to their low thermal conductivity caused by bonding asymmetry and anharmonicity. Recently, single crystalline layered SnSe has created sensation in thermoelectrics due to its ultralow thermal conductivity and high thermoelectric figure of merit. Tin diselenide (SnSe₂), an additional layered compound belonging to the Sn‐Se phase diagram, possesses a CdI₂‐type structure. However, synthesis of pure‐phase bulk SnSe₂ by a conventional solid‐state route is still remains challenging. A simple solution‐based low‐temperature synthesis is presented of ultrathin (3–5 nm) few layers (4–6 layers) nanosheets of Cl‐doped SnSe₂, which possess n‐type carrier concentration of 2×10¹⁸ cm^?3 with carrier mobility of about 30 cm² V^?1 s^?1 at room temperature. SnSe₂ has a band gap of about 1.6 eV and semiconducting electronic transport in the 300–630 K range. An ultralow thermal conductivity of about 0.67 Wm^?1 K^?1 was achieved at room temperature in a hot‐pressed dense pellet of Cl‐doped SnSe₂ nanosheets due to the anisotropic layered structure, which gives rise to effective phonon scattering.     

Manipulating Band Structure through Reconstruction of Binary Metal Sulfide for High‐Performance Thermoelectrics in Solution‐Synthesized Nanostructured Bi13S18I2

Reconstructing canonical binary compounds by inserting a third agent can significantly modify their electronic and phonon structures. Therefore, it has inspired the semiconductor communities in various fields. Introducing this paradigm will potentially revolutionize thermoelectrics as well. Using a solution synthesis, Bi₂S₃ was rebuilt by adding disordered Bi and weakly bonded I. These new structural motifs and the altered crystal symmetry induce prominent changes in electrical and thermal transport, resulting in a great enhancement of the figure of merit. The as‐obtained nanostructured Bi₁₃S₁₈I₂ is the first non‐toxic, cost‐efficient, and solution‐processable n‐type material with z T=1.0.     

A New Strategy for Realizing the Conversion of “Homo–Hetero–Homo” Heteroepitaxial Growth in Bi2Te3 and the Thermoelectric Performance

Uncovering the reason for structure‐dependent thermoelectric performance still remains a big challenge. A low‐temperature and easily scalable strategy for synthesizing Bi₂Te₃ nanostring hierarchical structures through solution‐phase reactions, during which there is the conversion of “homo–hetero–homo” in Bi₂Te₃ heteroepitaxial growth, is reported. Bi₂Te₃ nanostrings are obtained through the transformation from pure Bi₂Te₃ hexagonal nanosheets followed by Te?Bi₂Te₃ “nanotop” heterostructures to Bi₂Te₃ nanostrings. The growth of Bi₂Te₃ nanostrings appears to be a self‐assembly process through a wavy competition process generated from Te and Bi³⁺. The conversion of homo–hetero–homo opens up new platforms to investigate the wet chemistry of Bi₂Te₃ nanomaterials. Furthermore, to study the effect of morphologies and hetero/homo structures, especially with the same origin and uniform conditions on their thermoelectric properties, the thermoelectric properties of Bi₂Te₃ nanostrings and Te?Bi₂Te₃ heterostructured pellets fabricated by spark plasma sintering have been investigated separately.     

Facile Synthesis and Thermoelectric Properties of Self‐assembled Bi2Te3 One‐Dimensional Nanorod Bundles

Self‐assembled Bi₂Te₃ one‐dimensional nanorod bundles have been fabricated by a low‐cost and facile solvothermal method with ethylene diamine tetraacetic acid as an additive. The phase structures and morphologies of the samples were characterized by X‐ray diffraction, scanning electron microscopy, Fourier‐transform infrared spectrometry, and transmission electron microscope measurements. The growth mechanisms have been proposed based on the experimental results. The full thermoelectric properties of the nanorod bundles have been characterized and show a large improvement in the thermal conductivity attributed to phonon scattering of the nanostructures and then enhance the thermoelectric figure of merit. This work is promising for the realization of new types of highly efficient thermoelectric semiconductors by this method.     

Screw‐Dislocation‐Driven Bidirectional Spiral Growth of Bi2Se3 Nanoplates

Bi₂Se₃ attracts intensive attention as a typical thermoelectric material and a promising topological insulator material. However, previously reported Bi₂Se₃ nanostructures are limited to nanoribbons and smooth nanoplates. Herein, we report the synthesis of spiral Bi₂Se₃ nanoplates and their screw‐dislocation‐driven (SDD) bidirectional growth process. Typical products showed a bipyramid‐like shape with two sets of centrosymmetric helical fringes on the top and bottom faces. Other evidence for the unique structure and growth mode include herringbone contours, spiral arms, and hollow cores. Through the manipulation of kinetic factors, including the precursor concentration, the pH value, and the amount of reductant, we were able to tune the supersaturation in the regime of SDD to layer‐by‐layer growth. Nanoplates with preliminary dislocations were discovered in samples with an appropriate supersaturation value and employed for investigation of the SDD growth process.     

Air‐Stable n‐Type Thermoelectric Materials Enabled by Organic Diradicaloids

Air‐stable n‐type thermoelectric materials are recognized as an important and challenging topic in organic thermoelectrics (OTEs) because conventional n‐type OTE materials prepared by chemical doping are highly volatile upon exposure to air. Besides, doping efficiency and microstructure are hard to control with the incorporation of external dopants. We report herein the design and synthesis of unconventional n‐type OTE materials based on the diradicaloids 2DQQT‐S and 2DQQT‐Se, which are proved to be neutral single‐component organic conductors that exhibit an unprecedented air stability. Without external n‐doping, a pristine film of 2DQQT‐Se shows an electrical conductivity as high as 0.29 S cm^?1 delivering a power factor of 1.4 μW m^?1 K^?2. Under ambient conditions, no decay in electrical conductivity is observed for over 260 hours. This work demonstrates that diradicaloids are promising candidates for air‐stable and high‐performance OTE materials.     

Air‐Stable,Lead‐Free Zero‐Dimensional Mixed Bismuth‐Antimony Perovskite Single Crystals with Ultra‐broadband Emission

Lead‐free zero‐dimensional (0D) organic‐inorganic metal halide perovskites have recently attracted increasing attention for their excellent photoluminescence properties and chemical stability. Here, we report the synthesis and characterization of an air‐stable 0D mixed metal halide perovskite (C₈NH₁₂)₄Bi_0.57Sb_0.43Br₇?H₂O, in which individual [BiBr₆]^3? and [SbBr₆]^3? octahedral units are completely isolated and surrounded by the large organic cation C₈H₁₂N⁺. Upon photoexcitation, the bulk crystals exhibit ultra‐broadband emission ranging from 400 to 850 nm, which originates from both free excitons and self‐trapped excitons. This is the first example of 0D perovskites with broadband emission spanning the entire visible spectrum. In addition, (C₈NH₁₂)₄Bi_0.57Sb_0.43Br₇?H₂O exhibits excellent humidity and light stability. These findings present a new direction towards the design of environmentally‐friendly, high‐performance 0D perovskite light emitters.     

Air‐Stable,Lead‐Free Zero‐Dimensional Mixed Bismuth‐Antimony Perovskite Single Crystals with Ultra‐broadband Emission

Lead‐free zero‐dimensional (0D) organic‐inorganic metal halide perovskites have recently attracted increasing attention for their excellent photoluminescence properties and chemical stability. Here, we report the synthesis and characterization of an air‐stable 0D mixed metal halide perovskite (C₈NH₁₂)₄Bi_0.57Sb_0.43Br₇?H₂O, in which individual [BiBr₆]^3? and [SbBr₆]^3? octahedral units are completely isolated and surrounded by the large organic cation C₈H₁₂N⁺. Upon photoexcitation, the bulk crystals exhibit ultra‐broadband emission ranging from 400 to 850 nm, which originates from both free excitons and self‐trapped excitons. This is the first example of 0D perovskites with broadband emission spanning the entire visible spectrum. In addition, (C₈NH₁₂)₄Bi_0.57Sb_0.43Br₇?H₂O exhibits excellent humidity and light stability. These findings present a new direction towards the design of environmentally‐friendly, high‐performance 0D perovskite light emitters.     

Tuning Optimum Temperature Range of Bi2Te3‐Based Thermoelectric Materials by Defect Engineering

Bi₂Te₃‐based solid solutions, which have been widely used as thermoelectric (TE) materials for the room temperature TE refrigeration, are also the potential candidates for the power generators with medium and low‐temperature heat sources. Therefore, depending on the applications, Bi₂Te₃‐based materials are expected to exhibit excellent TE properties in different temperature ranges. Manipulating the point defects in Bi₂Te₃‐based materials is an effective and important method to realize this purpose. In this review, we focus on how to optimize the TE properties of Bi₂Te₃‐based TE materials in different temperature ranges by defect engineering. Our calculation results of two‐band model revel that tuning the carrier concentration and band gap, which is easily realized by defects engineering, can obtain better TE properties at different temperatures. Then, the typical paradigms about optimizing the TE properties at different temperatures for n‐type and p‐type Bi₂Te₃‐based ZM ingots and polycrystals are discussed in the perspective of defects engineering. This review can provide the guidance to improve the TE properties of Bi₂Te₃‐based materials at different temperatures by defects engineering.     

Highly Porous Thermoelectric Nanocomposites with Low Thermal Conductivity and High Figure of Merit from Large‐Scale Solution‐Synthesized Bi2Te2.5Se0.5 Hollow Nanostructures

To enhance the performance of thermoelectric materials and enable access to their widespread applications, it is beneficial yet challenging to synthesize hollow nanostructures in large quantities, with high porosity, low thermal conductivity (κ ) and excellent figure of merit (z T ). Herein we report a scalable (ca. 11.0 g per batch) and low‐temperature colloidal processing route for Bi₂Te_2.5Se_0.5 hollow nanostructures. They are sintered into porous, bulk nanocomposites (phi 10 mm×h 10 mm) with low κ (0.48 W m⁻¹ K⁻¹) and the highest z T (1.18) among state‐of‐the‐art Bi₂Te_3−x Se_x materilas. Additional benefits of the unprecedented low relative density (68–77 %) are the large demand reduction of raw materials and the improved portability. This method can be adopted to fabricate other porous phase‐transition and thermoelectric chalcogenide materials and will pave the way for the implementation of hollow nanostructures in other fields.     

Conversion Reactions of Solids: From a Surprising Three‐Step Mechanism towards Directed Product Formation

Directed conversion reactions from binary to multinary compounds are discovered from the reaction of Bi₂S₃ and Bi₂Se₃ with NiCl₂ ? 6 H₂O in polyol media under basic conditions. Control of the synthesis conditions allows the preparation of NiBiSe and superconducting Ni₃Bi₂S₂ and Ni₃Bi₂Se₂. The formation of Ni₃Bi₂S₂ from Bi₂S₃ is found from an unexpected three‐step reaction path with Bi and NiBi as intermediates. In the more complex Ni/Bi/Se system, the mechanism found can be used to selectively direct the reaction between the competing ternaries and to suppress side‐product formation. Contrary to solid‐state reactions (500–900 °C) control of product formation is reached at reaction temperatures and times between 166–300 °C and 0.5–10 h, respectively. The formation of different phases is discussed from results of DFT calculations.     

Structural and physical properties of Mg3−xZnxSb2 (x=0-1.34)

The Mg_3−xZn_xSb₂ phases with x=0-1.34 were prepared by direct reactions of the elements in tantalum tubes. According to the X-ray single crystal and powder diffraction, the Mg_3−xZn_xSb₂ phases crystallize in the same P3¯m1 space group as the parent Mg₃Sb₂ phase. The Mg_3−xZn_xSb₂ structure is different from the other substituted structures of Mg₃Sb₂, such as (Ca, Sr, Ba) Mg₂Sb₂ or Mg_5.23Sm_0.77Sb₄, in a way that in Mg_3−xZn_xSb₂ the Mg atoms on the tetrahedral sites are replaced, while in the other structures Mg on the octahedral sites is replaced. Thermoelectric performance for the two members of the series, Mg₃Sb₂ and Mg_2.36Zn_0.64Sb₂, was evaluated from low to room temperatures through resistivity, Seebeck coefficient and thermal conductivity measurements. In contrast to Mg₃Sb₂ which is a semiconductor, Mg_2.36Zn_0.64Sb₂ is metallic and exhibits an 18-times larger dimensionless figure-of-merit, ZT, at room temperature. However, thermoelectric performance of Mg_2.36Zn_0.64Sb₂ is still poor and it is mostly due to its large electrical resistivity.     

The Rb7Bi3−3xSb3xCl16 Family: A Fully Inorganic Solid Solution with Room‐Temperature Luminescent Members

Low‐dimensional ns²‐metal halide compounds have received immense attention for applications in solid‐state lighting, optical thermometry and thermography, and scintillation. However, these are based primarily on the combination of organic cations with toxic Pb²⁺ or unstable Sn²⁺, and a stable inorganic luminescent material has yet to be found. Here, the zero‐dimensional Rb₇Sb₃Cl₁₆ phase, comprised of isolated [SbCl₆]^3? octahedra and edge‐sharing [Sb₂Cl₁₀]^4? dimers, shows room‐temperature photoluminescence (RT PL) centered at 560 nm with a quantum yield of 3.8±0.2 % at 296 K (99.4 % at 77 K). The temperature‐dependent PL lifetime rivals that of previous low‐dimensional materials with a specific temperature sensitivity above 0.06 K^?1 at RT, making it an excellent thermometric material. Utilizing both DFT and chemical substitution with Bi³⁺ in the Rb₇Bi_3?3xSb_3xCl₁₆ (x≤1) family, we present the edge‐shared [Sb₂Cl₁₀]^4? dimer as a design principle for Sb‐based luminescent materials.     

An O3‐type Oxide with Low Sodium Content as the Phase‐Transition‐Free Anode for Sodium‐Ion Batteries

Layered transition metal oxides Na_xMO₂ (M=transition metal) with P2 or O3 structure have attracted attention in sodium‐ion batteries (NIBs). A universal law is found to distinguish structural competition between P2 and O3 types based on the ratio of interlayer distances of the alkali metal layer d_(O‐Na‐O) and transition‐metal layer d_(O‐M‐O). The ratio of about 1.62 can be used as an indicator. O3‐type Na_0.66Mg_0.34Ti_0.66O₂ oxide is prepared as a stable anode for NIBs, in which the low Na‐content (ca. 0.66) usually undergoes a P2‐type structure with respect to Na_xMO₂. This material delivers an available capacity of about 98 mAh g^?1 within a voltage range of 0.4–2.0 V and exhibits a better cycling stability (ca. 94.2 % of capacity retention after 128 cycles). In situ X‐ray diffraction reveals a single‐phase reaction in the discharge–charge process, which is different from the common phase transitions reported in O3‐type electrodes, ensuring long‐term cycling stability.     

Rigid Coplanar Polymers for Stable n‐Type Polymer Thermoelectrics

Low n‐doping efficiency and inferior stability restrict the thermoelectric performance of n‐type conjugated polymers, making their performance lag far behind of their p‐type counterparts. Reported here are two rigid coplanar poly(p‐phenylene vinylene) (PPV) derivatives, LPPV‐1 and LPPV‐2 , which show nearly torsion‐free backbones. The fused electron‐deficient rigid structures endow the derivatives with less conformational disorder and low‐lying lowest unoccupied molecular orbital (LUMO) levels, down to ?4.49 eV. After doping, two polymers exhibited high n‐doping efficiency and significantly improved air stability. LPPV‐1 exhibited a high conductivity of up to 1.1 S cm^?1 and a power factor as high as 1.96 μW m^?1 K^?2. Importantly, the power factor of the doped LPPV‐1 thick film degraded only 2 % after 7 day exposure to air. This work demonstrates a new strategy for designing conjugated polymers, with planar backbones and low LUMO levels, towards high‐performance and potentially air‐stable n‐type polymer thermoelectrics.     

The Influence of Nanoscale Heterostructures on the Thermoelectric Properties of Bi‐substituted Tl5Te3

Tl_4.5Bi_0.5Te₃ crystallizes in a distorted variant of the Tl₅Te₃ structure type in the space group I4/m. The symmetry reduction compared to Tl₅Te₃ (space group I4/mcm) is a consequence of cation ordering as shown by resonant X‐ray scattering using synchrotron radiation. Tl and Bi predominantly occupy one Wyckoff site each. This ordering is accompanied by displacements of Te atoms. The influence of nanostructuring on the thermoelectric performance of Tl_4.5Bi_0.5Te₃ was investigated for the new composite model system Tl_4.5Bi_0.5Te₃ – TlInTe₂. For the nominal composition (Tl_4.5Bi_0.5Te₃)_0.6(TlInTe₂)_0.4, the thermoelectric Figure of merit ZT reaches 0.8 at 325 °C. Nanoscaled precipitates with sizes of about 100–200 nm probably have beneficial influence on the thermal conductivity at this temperature.     

Sb2SmO4Cl,eine erste Antimon‐Seltenerd‐Oxid‐Chlorid‐Verbindung

Sb₂SmO₄Cl, the First Antimony Rare‐Earth Oxide Chloride The new quaternary phase Sb₂SmO₄Cl was prepared by solid‐state reaction of a stoichiometric mixture of SbCl₃, Sb₂O₃, and Sm₂O₃. Sb₂SmO₄Cl is the first antimony rare‐earth oxide chloride. It was characterized by X‐ray powder diffraction, IR‐spectroscopy, mass spectrometry and DTA/TG measurements. The standardenthalpy of formation was derived from heats of solution. The standard entropy were calculated from a low‐temperature measurement of the specific heat capacity.     

Colloidal Synthesis and Photophysics of M3Sb2I9 (M=Cs and Rb) Nanocrystals: Lead‐Free Perovskites

Herein we report the colloidal synthesis of Cs₃Sb₂I₉ and Rb₃Sb₂I₉ perovskite nanocrystals, and explore their potential for optoelectronic applications. Different morphologies, such as nanoplatelets and nanorods of Cs₃Sb₂I₉, and spherical Rb₃Sb₂I₉ nanocrystals were prepared. All these samples show band‐edge emissions in the yellow–red region. Exciton many‐body interactions studied by femtosecond transient absorption spectroscopy of Cs₃Sb₂I₉ nanorods reveals characteristic second‐derivative‐type spectral features, suggesting red‐shifted excitons by as much as 79 meV. A high absorption cross‐section of ca. 10⁻¹⁵ cm² was estimated. The results suggest that colloidal Cs₃Sb₂I₉ and Rb₃Sb₂I₉ nanocrystals are potential candidates for optical and optoelectronic applications in the visible region, though a better control of defect chemistry is required for efficient applications.     

Bournonite PbCuSbS3: Stereochemically Active Lone‐Pair Electrons that Induce Low Thermal Conductivity

An understanding of the structural features and bonding of a particular material, and the properties these features impart on its physical characteristics, is essential in the search for new systems that are of technological interest. For several relevant applications, the design or discovery of low thermal conductivity materials is of great importance. We report on the synthesis, crystal structure, thermal conductivity, and electronic‐structure calculations of one such material, PbCuSbS₃. Our analysis is presented in terms of a comparative study with Sb₂S₃, from which PbCuSbS₃ can be derived through cation substitution. The measured low thermal conductivity of PbCuSbS₃ is explained by the distortive environment of the Pb and Sb atoms from the stereochemically active lone‐pair s² electrons and their pronounced repulsive interaction. Our investigation suggests a general approach for the design of materials for phase‐change‐memory, thermal‐barrier, thermal‐rectification and thermoelectric applications, as well as other functions for which low thermal conductivity is purposefully sought.