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As a typical liquid-like material, the argyrodite-type Ag9GaSe6 has attracted considerable attention in the past decade due to its liquid-like characteristics, complex crystal structures, and high carrier mobility. However, the carrier concentration of pristine Ag9GaSe6 is still not optimal while the thermal conductivity remains to be further reduced. Herein, we simultaneously optimize the electrical and thermal properties of Ag9GaSe6via alloying Sn at the Ga sites and Te at the Se sites. It is found that the crystal symmetry is well maintained, while the phase transition temperature is reduced and local chemical bonding is altered after alloying. Due to the nominally reduced Ag content and the change of chemical bonding, the carrier concentration is reduced by two orders of magnitude, giving rise to a largely improved Seebeck coefficient. Meanwhile, the lattice thermal conductivity κL of Ag9−x(Ga1−xSnx)(Se1−xTex)6 is significantly suppressed by the strong phonon scattering from point defects. An ultralow κL of ∼0.25 W m−1 K−1, which approaches the theoretical minimum κL, is attained at room temperature in the alloyed samples. Consequently, the zT value is boosted to 1.4 for the x = 0.1 sample, which represents an improvement of 75% over that of pristine Ag9GaSe6. [ABSTRACT FROM AUTHOR]

Mg3Sb2-based thermoelectric materials have recently received heightened attention due to their diverse merits of high band degeneracy, ultralow lattice thermal conductivity and high carrier mobility. However, the inherently low carrier concentration of pristine Mg3Sb2 seriously hinders the advancement of this material toward high thermoelectric performance. Therefore, searching for proper dopants to optimize the carrier concentration is one of the primary avenues to realize superior thermoelectric performance in Mg3Sb2-based materials. Herein, by considering the electronegativity difference Δχ and mass difference ΔM between the dopant and host elements, we theoretically and experimentally demonstrate lanthanide Gd as an effective dopant to tune the carrier concentration of Mg3Sb1.3Bi0.7 alloys. Owing to its high doping efficiency, a large carrier concentration up to 8.9 × 1019 cm−3 is realized through Gd doping, which is close to the optimal value. Moreover, the coarse grain size commendably mitigates the grain boundary effects and thus ensures high carrier mobility. Combining the greatly suppressed lattice thermal conductivity by point defect scattering, a maximum zT of 1.55 is achieved at 700 K in Mg3.065Sb1.3Bi0.7Gd0.015. [ABSTRACT FROM AUTHOR]

Cu–Sn–S ternary compounds have attracted great attention in the thermoelectric community because they are composed of low-cost, earth-abundant, and non-toxic elements. Among them, Cu7Sn3S10 shows promising thermoelectric performance in the middle temperature range. In this work, through doping Br in Cu7Sn3S10, a peak dimensionless figure of merit (zT) around unity at 750 K has been achieved. Chemical composition analysis indicates that Br has a higher solubility in Cu7Sn3S10 than Cl. On one hand, the Br dopant introduces additional electrons and tunes the initial overhigh carrier concentration to the optimal range. On the other hand, the Br dopant creates strong strain field fluctuation to significantly suppress the lattice thermal conductivity to as low as 0.4 W m−1 K−1 at 750 K. Its synthetic effects advance the realization of high zT around unity in Br-doped Cu7Sn3S10. [ABSTRACT FROM AUTHOR]

An tetraphenylethylene-based MOF (HNU-49) has been synthesized, which exhibits interesting piezofluorochromic behavior. Additionally, rhodamine B (RhB) can be successful encapsulated in HNU-49, to emit the characteristic dual-emission of RhB and the framework. The combination of host–guest interaction and piezofluorochromism can precisely modulate the luminescence to achieve near white-light. [ABSTRACT FROM AUTHOR]

Copper sulfides have attracted great attention recently in the thermoelectric community due to the liquid-like behavior of Cu ions. Among the numerous copper sulfides, digenite Cu1.80S has a poorer thermoelectric performance but better stability than the state-of-the-art binary copper sulfide Cu1.97S. In this study, good stability and high thermoelectric performance were simultaneously obtained in Fe-doped Cu1.80S. Because Fe ions will not form a concentration gradient under an external field to change the critical voltage, Fe-doped Cu1.80S samples inherit the good stability of the pristine Cu1.80S. The critical voltage for Cu1.80Fe0.064S is 0.16 V at 750 K, which has been the largest value reported so far. Likewise, the Fe dopants can significantly improve the thermoelectric performance by suppressing the too high electrical conductivity of Cu1.80S. The peak dimensionless figure of merit (zT) for Cu1.80Fe0.064S is around 0.8 at 750 K, about four times that of Cu1.80S. The average zT for Cu1.80Fe0.064S is 0.40 in 300–750 K, which is amongst the highest values in reported thermoelectric sulfides. Combining the good stability and high thermoelectric performance, the present Cu1.80Fe0.064S has great potential to be used in the application of waste heat harvesting in the middle temperature range. [ABSTRACT FROM AUTHOR]

β-FeSi2, a semiconductor material made of two of the most earth-abundant elements, has important applications in thermoelectrics, photovoltaics and optoelectronics owing to its attractive properties such as suitable band gap and air stability over a wide temperature range. While point defects always play a vital role in semiconductor materials, only sporadic studies have been dedicated to the defects in β-FeSi2. Here, using first-principles calculations we systematically investigate the intrinsic point defects in β-FeSi2. Our results reveal that the formation energies of the intrinsic defects in β-FeSi2 are high enough to prevent them from forming in a significant concentration under thermal equilibrium growth conditions. As a possible kinetic process generating intrinsic defects, we study the α-to-β phase transition of FeSi2. We find that the phase transition is a slow process occurring on the time scale of an hour. Incomplete phase transition may lead to kinetically formed intrinsic defects. We further calculate the activation energies of the intrinsic defects and show that the experimentally observed conductivity of pure β-FeSi2 should be a result of unintentional doping. Possible extrinsic impurities that may lead to n-type and p-type conductivity and their activation energies are calculated, which are in good agreement with available experiments. Our results provide guidance for optimizing the doping strategy of β-FeSi2 for device applications. [ABSTRACT FROM AUTHOR]

Binary Cu2Se and Cu2Te have gained great attention recently because of their interesting and abnormal physical properties, such as ultralow thermal conductivity, high carrier mobility, large effective mass of carriers and excellent thermoelectric performance. In this study, we find that these two compounds are completely miscible throughout the studied composition range. The trigonal structure of Cu2Se is maintained when the Te content x is 0.2, but a new trigonal structure is formed when the Te content x is between 0.3 and 0.7. The carrier concentration is greatly improved when increasing the Te content in Cu2Se1−xTex solid solutions, resulting in a much reduced electrical resistivity and Seebeck coefficient in the whole temperature range as compared with those of binary Cu2Se. The total thermal conductivity is inversely increased due to the contribution from enhanced carrier thermal conductivity. As a result, the overall thermoelectric performance of Cu2Se1−xTex solid solutions lies between Cu2Se and Cu2Te. We also find that the quality factor of Cu2Se1−xTex is higher than those of most typical thermoelectric materials. Thus the thermoelectric performance can be further improved if the intrinsically high hole carrier concentrations can be reduced in Cu2Se1−xTex. [ABSTRACT FROM AUTHOR]

High-symmetry crystal structures are preferred for thermoelectrics because high structural symmetry usually yields good electron transport properties. Entropy engineering is an effective approach to improve the structural symmetry of low-symmetry materials, and thus to enhance their thermoelectric performance. In this study, via introducing Te into the argyrodite-type compound Cu7PSe6, the configurational entropy is significantly increased to successfully improve its initial low-symmetry cubic structure (P213) to the high-symmetry cubic structure (F4̅3m) at room temperature. Such improved structural symmetry leads to a high density-of-state effective mass but similar carrier mobility in the same carrier concentration range as compared with the pristine Cu7PSe6. Thus, significantly optimized electron transport properties are achieved in the Te-alloyed Cu7PSe6 samples. In particular, at room temperature, the power factor of the high-symmetry cubic Cu7PSe5.7Te0.3 sample is about 15-times higher than that of the low-symmetry Cu7PSe6 matrix. Combining the well-maintained ultralow lattice thermal conductivity, a maximum ZT of around 0.55 at 600 K is obtained in Cu7PSe5.7Te0.3. This work strongly shows that entropy engineering using multiple components is a very powerful strategy to discover or design novel high-performance TE materials starting from low-symmetry compounds. [ABSTRACT FROM AUTHOR]

Bi2Te3-based alloys are the most well-known thermoelectric materials for room-temperature applications. However, the maximum ZT values of Bi2Te3-based materials are usually achieved near room temperature. The ZT values dramatically drop above 400 K because of the intrinsic thermal excitation in Bi2Te3-based materials, which seriously restricts their applications as thermoelectric power generators. In this study, by doping a tiny amount of Ag into Bi0.5Sb1.5Te3, we successfully suppressed the intrinsic excitation in p-type Bi2Te3-based materials and shifted the ZT peak temperature to high temperatures. Eventually, due to the enhanced power factor and greatly depressed bipolar thermal conductivity at high temperature, a maximum ZT of 1.25 at 400 K was obtained in Ag0.002Bi0.5Sb1.498Te3, and an average ZT value of approximately 1.03 was achieved between 300 and 600 K. The theoretical energy conversion efficiency of the TE module fabricated with Ag0.002Bi0.5Sb1.498Te3 achieves a maximum value of 11.0% when ΔT = 300 K, which makes p-type Bi2Te3-based devices attractive for applications in TE power generation. [ABSTRACT FROM AUTHOR]

We report a ternary argyrodite-type Ag9GaSe6 compound as a promising thermoelectric material in a moderate temperature range. Due to high carrier mobility and ultralow lattice thermal conductivity, a maximum ZT of 1.1 was obtained with stoichiometric Ag9GaSe6 at 800 K. Via introducing slight Se-deficiency to optimize the carrier concentration, the maximum ZT is further enhanced to 1.3. [ABSTRACT FROM AUTHOR]

Recently, copper chalcogenides Cu2−xδ (δ = S, Se, Te) have attracted great attention due to their exceptional thermal and electrical transport properties. Besides these binary Cu2−xδ compounds, the ternary Cu2−xδ solid solutions are also expected to possess excellent thermoelectric performance. In this study, we have synthesized a series of Cu2Se1−xSx (x = 0.2, 0.3, 0.5, and 0.7) solid solutions by melting the raw elements followed by spark plasma sintering. The energy dispersive spectroscopy mapping, powder and single-crystal X-ray diffraction and X-ray powder diffraction studies suggest that Cu2Se and Cu2S can form a continuous solid solution in the entire composition range. These Cu2Se1−xSx solid solutions are polymorphic materials composed of varied phases with different proportions at room temperature, but single phase materials at elevated temperature. Increasing the sulfur content in Cu2Se1−xSx solid solutions can greatly reduce the carrier concentration, leading to much enhanced electrical resistivity and Seebeck coefficients in the whole temperature range as compared with those in binary Cu2Se. In particular, introducing sulfur at Se-sites reduces the speed of sound. Combining the strengthened point defect scattering of phonons, extremely low lattice thermal conductivities are obtained in these solid solutions. Finally, a maximum zT value of 1.65 at 950 K is achieved for Cu2Se0.8S0.2, which is greater than those of Cu2Se and Cu2S. [ABSTRACT FROM AUTHOR]

Recently, liquid-like superionic thermoelectric materials have attracted great attention due to their extremely low lattice thermal conductivity and high thermoelectric figure of merit (ZT). Argyrodite-type compounds are typical superionic semiconductors with two independent structural units that can be used to separately tune electrical and thermal transport properties. In this work, we report that Cu8GeSe6 with an argyrodite structure is a new class of thermoelectric materials with extremely low lattice thermal conductivity. The presence of two independent structural units in Cu8GeSe6 provides the possibility of greatly improving its electrical transport properties while maintaining ultralow lattice thermal conductivity. By alloying Ag and Te in Cu8GeSe6, the ZT values are significantly improved to above unity at 800 K in Cu7.6Ag0.4GeSe5.1Te0.9, comparable with the best superionic liquid-like thermoelectric materials. The ultralow thermal conductivity is mainly attributed to the weak chemical bonding between Cu atoms and the rigid [GeSe6] sublattice. [ABSTRACT FROM AUTHOR]

A series of rare-earth filled skutterudites are successfully fabricated by using a non-equilibrium synthesis approach involving super-cooling and high pressure sintering. The filler filling fractions are significantly improved to realize optimum carrier concentrations and hugely suppressed lattice thermal conductivity, thereby leading to significantly enhanced zT. [ABSTRACT FROM AUTHOR]

A pseudocubic structure approach has been proposed recently to screen and design good thermoelectric materials via realizing overlapped band edges for excellent electrical transport properties. A diamond-like compound is a typical example agreeing with the concept of the pseudocubic structure by tuning its lattice distortion parameter to unity. However, besides the band structure, optimized carrier concentration and reduced lattice thermal conductivity are also required for a high thermoelectric figure of merit (zT). In this work, taking CuGaTe2 as an example, we have successfully demonstrated that Cu-deficiency can effectively tune carrier concentrations and In-alloying at Ga sites can effectively lower lattice thermal conductivity. By combining these two strategies, the electrical and thermal transports can be separately optimized in CuGaTe2-based pseudocubic diamond-like compounds, leading to much enhanced zTs, about 24% improvement for Cu0.99In0.6Ga0.4Te2 at 800 K. Furthermore, the average zTs from 300 K to 800 K are improved by 87% compared with that of the CuGaTe2 matrix. This study provides a promising way to optimize the TE performance in pseudocubic diamond-like compounds by simultaneously tuning electrical and thermal transport. [ABSTRACT FROM AUTHOR]

CuAgSe is a narrow band gap semiconducting material with superior carrier mobility and low lattice thermal conductivity, which are important and useful for high thermoelectric performance. However, its electrical and thermal transport properties are greatly affected by ionic deficiencies or compositional non-stoichiometry, which lead to a low thermoelectric figure of merit near room temperature. In this work, we systematically studied the compound defects in CuAgSe by tuning its starting chemical composition. We found that its phase purity is very sensitive to nominal chemical compositions. Only a small amount of Ag deficiency is allowed in CuAgSe to maintain phase purity, while the other non-stoichiometric compositions lead to impurity phases. Thermoelectric properties are weakly affected by these compound defects or impurity phases at 300 K, but greatly change at high temperatures. A single type carrier conduction is observed in CuAgSe, but a noticeable two-type carrier conduction is observed in the non-stoichiometric samples. This leads to an evident n to p conduction transition. Consequently, zT values in CuAgSe continuously increase to 0.6 at 450 K while the non-stoichiometric samples display considerably low values due to the contribution from both electrons and holes. The high zT value in n-type CuAgSe suggests that it is a promising thermoelectric material near room temperature. [ABSTRACT FROM AUTHOR]

Lithium intercalation and de-intercalation processes have been used to fabricate bulk Bi2Se0.3Te2.7with internal nanostructures. The doped Li content can be precisely controlled through this method. It provides a chance to directly optimize electrical properties when preparing nano-structured materials, leading to the optimum carrier concentration for improved thermoelectric figure of merit. [ABSTRACT FROM AUTHOR]

Correction for 'Thermodynamics, kinetics and electronic properties of point defects in β-FeSi2' by Jun Chai et al., Phys. Chem. Chem. Phys., 2019, 21, 10497–10504. [ABSTRACT FROM AUTHOR]

Copper sulfides have attracted great attention recently in the thermoelectric community due to the liquid-like behavior of Cu ions. Among the numerous copper sulfides, digenite Cu1.80S has a poorer thermoelectric performance but better stability than the state-of-the-art binary copper sulfide Cu1.97S. In this study, good stability and high thermoelectric performance were simultaneously obtained in Fe-doped Cu1.80S. Because Fe ions will not form a concentration gradient under an external field to change the critical voltage, Fe-doped Cu1.80S samples inherit the good stability of the pristine Cu1.80S. The critical voltage for Cu1.80Fe0.064S is 0.16 V at 750 K, which has been the largest value reported so far. Likewise, the Fe dopants can significantly improve the thermoelectric performance by suppressing the too high electrical conductivity of Cu1.80S. The peak dimensionless figure of merit (zT) for Cu1.80Fe0.064S is around 0.8 at 750 K, about four times that of Cu1.80S. The average zT for Cu1.80Fe0.064S is 0.40 in 300-750 K, which is amongst the highest values in reported thermoelectric sulfides. Combining the good stability and high thermoelectric performance, the present Cu1.80Fe0.064S has great potential to be used in the application of waste heat harvesting in the middle temperature range.

β-FeSi 2 , a semiconductor material made of two of the most earth-abundant elements, has important applications in thermoelectrics, photovoltaics and optoelectronics owing to its attractive properties such as suitable band gap and air stability over a wide temperature range. While point defects always play a vital role in semiconductor materials, only sporadic studies have been dedicated to the defects in β-FeSi 2 . Here, using first-principles calculations we systematically investigate the intrinsic point defects in β-FeSi 2 . Our results reveal that the formation energies of the intrinsic defects in β-FeSi 2 are high enough to prevent them from forming in a significant concentration under thermal equilibrium growth conditions. As a possible kinetic process generating intrinsic defects, we study the α-to-β phase transition of FeSi 2 . We find that the phase transition is a slow process occurring on the time scale of an hour. Incomplete phase transition may lead to kinetically formed intrinsic defects. We further calculate the activation energies of the intrinsic defects and show that the experimentally observed conductivity of pure β-FeSi 2 should be a result of unintentional doping. Possible extrinsic impurities that may lead to n-type and p-type conductivity and their activation energies are calculated, which are in good agreement with available experiments. Our results provide guidance for optimizing the doping strategy of β-FeSi 2 for device applications.

Copper-based chalcogenides have ultralow thermal conductivity and ultrahigh thermoelectric performance, but most of them are p-type semiconductors. It is urgent to develop n-type counterparts for high efficiency thermoelectric modules based on these copper based-chalcogenides. Cu 4 Sn 7 S 16 is an intrinsically n-type semiconductor with complex crystal structure and low thermal conductivity. However, its thermoelectric properties have not been well studied when compared to the well-known n-type CuFeS 2 . In this work, high-quality Cu 4 Sn 7 S 16 -based compounds are fabricated and their thermoelectric properties are systematically studied. Using Ag and Sb as dopants, the carrier concentration is tuned over a wide range. The electrical transport properties can be well described by the single parabolic band model with carrier acoustic phonons scattering. It is revealed that Cu 4 Sn 7 S 16 exhibits a low effective mass and relatively high mobility. The thermal conductivity is lower than 0.8 W m -1 K -1 from 300 to 700 K and shows a weak dependence on temperature. A maximum zT of 0.27 is obtained in Cu 3.97 Ag 0.03 Sn 7 S 16 at 700 K. Further enhancement of thermoelectric performance is possible when a more efficient n-type dopant is used., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)