Highly Converged Valence Bands and Ultralow Lattice Thermal Conductivity for High‐Performance SnTe Thermoelectrics

A two‐step optimization strategy is used to improve the thermoelectric performance of SnTe via modulating the electronic structure and phonon transport. The electrical transport of self‐compensated SnTe (that is, Sn_1.03Te) was first optimized by Ag doping, which resulted in an optimized carrier concentration. Subsequently, Mn doping in Sn_1.03?xAg_xTe resulted in highly converged valence bands, which improved the Seebeck coefficient. The energy gap between the light and heavy hole bands, i.e. ΔE_v decreases to 0.10 eV in Sn_0.83Ag_0.03Mn_0.17Te compared to the value of 0.35 eV in pristine SnTe. As a result, a high power factor of ca. 24.8 μW cm^?1 K^?2 at 816 K in Sn_0.83Ag_0.03Mn_0.17Te was attained. The lattice thermal conductivity of Sn_0.83Ag_0.03Mn_0.17Te reached to an ultralow value (ca. 0.3 W m^?1 K^?1) at 865 K, owing to the formation of Ag₇Te₄ nanoprecipitates in SnTe matrix. A high thermoelectric figure of merit (z T≈1.45 at 865 K) was obtained in Sn_0.83Ag_0.03Mn_0.17Te.     

Electric and adhesion properties of an interface between Sn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis> </Subscript>Mn<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>Te single crystals and Bi–Sn alloys

The adhesion and electric properties of an interface between Sn_{1 ? x}Mn_xTe single crystals and a 57 wt % Bi and 43 wt % Sn alloy in a temperature range of ～77–300 K are studied. It is shown that the Bi–Sn alloy and the above single crystals form an ohmic contact that exhibits fairly high work of adhesion and strength of adhesion, along with low contact resistance. The deposition of the Bi–Sn alloy on the end faces of the crystals results in the formation of such intermediate phases as Bi₂Te₃ and SnTe at the interface, the doping of the near-contact region of the crystal, and the filling of vacancies in the tin sublattice in this region with diffusing atoms of the contact alloy components.     

Lattice temperature and hyperfine interactions of Pb1−xSnxTe (0.21 ≤ x ≤ 0.75)

Thin (<15 μm) samples of lead tin telluride, Pb_1?xSn_xTe (x = 0.21, 0.25, 0.55, and 0.75) have been studied by temperature dependent Mössbauer spectroscopy using the 23.8 keV gamma radiation of ^119mSn. The tin atom occupies a lattice site having cubic symmetry (QS = 0 ± 0.020 mm sec^?1) over the temperature range 78 ≤ T ≤ 240 K, and there is no evidence for a rhombic (low temperature) to cubic (high temperature) phase transition such as that reported for SnTe in this temperature interval. The lattice temperature as probed by the Sn atom is independent of the compositional parameter x and is similar to that reported for SnTe from Mössbauer studies and for Pb_0.63Sn_0.37Te from X-ray powder diffraction data. Radiation damage produced by 2-MeV proton irradiation to a total fluence of ～10¹⁷ cm^?2 at liquid nitrogen temperature does not have any effect on the Mössbauer parameters, possibly because the major damage is annealed at temperatures below 150 K.     

Tailoring of Electronic Structure and Thermoelectric Properties of a Topological Crystalline Insulator by Chemical Doping

Topological crystalline insulators (TCIs) are a new quantum state of matter in which linearly dispersed metallic surface states are protected by crystal mirror symmetry. Owing to its vanishingly small bulk band gap, a TCI like Pb_0.6Sn_0.4Te has poor thermoelectric properties. Breaking of crystal symmetry can widen the band gap of TCI. While breaking of mirror symmetry in a TCI has been mostly explored by various physical perturbation techniques, chemical doping, which may also alter the electronic structure of TCI by perturbing the local mirror symmetry, has not yet been explored. Herein, we demonstrate that Na doping in Pb_0.6Sn_0.4Te locally breaks the crystal symmetry and opens up a bulk electronic band gap, which is confirmed by direct electronic absorption spectroscopy and electronic structure calculations. Na doping in Pb_0.6Sn_0.4Te increases p‐type carrier concentration and suppresses the bipolar conduction (by widening the band gap), which collectively gives rise to a promising zT of 1 at 856 K for Pb_0.58Sn_0.40Na_0.02Te. Breaking of crystal symmetry by chemical doping widens the bulk band gap in TCI, which uncovers a route to improve TCI for thermoelectric applications.     

Thermodynamic study of solid solutions in the SnTe–AgSbTe<Subscript>2</Subscript> system by means of EMF with solid electrolyte Ag<Subscript>4</Subscript>RbI<Subscript>5</Subscript>

The results from studying the SnTe–AgSbTe₂ system by means of EMF with the solid electrolyte Ag₄RbI₅ in the temperature range of 300–430 K are presented. The formation of a wide (≥80 mol % of AgSbTe₂) region of solid solutions based on SnTe is confirmed. Partial thermodynamic functions ΔG?, ΔH?, and ΔS? of silver in alloys are calculated from the equations for the EMF temperature dependences. Based on the literature data regarding solid-phase equilibria in the Ag₂Te–SnTe–Sb₂Te₃–Te system, potential-determining reactions are identified that allow us to calculate the standard thermodynamic formation functions and standard entropies of solid solutions (2SnTe) _x (AgSbTe₂)_1?x (х = 0.2, 0.4, 0.6, 0.8, and 0.9).     

SIMS accurate determination of matrix composition of topological crystalline insulator material Pb1 − xSnxSe

Substitutional alloy Pb_1 − xSn_xSe is a new class of electronic materials called topological crystalline insulators, which at the temperature range from 0 K to 300 K exhibit topological state at compositions in the range 0.18 < x < 0.40 (in the rock-salt structure). In this report, we present a secondary ion mass spectrometry (SIMS) analysis technique to provide accurate Pb and Sn composition based on the measurement of PbCs⁺ and SnCs⁺ cluster ions intensities. Studies of Pb_1 − xSn_xSe bulk samples with various values of x show that x/(1 − x) is linear in relation to the intensity ratio of PbCs⁺/SnCs⁺ over the range from x = 0.15 to x = 0.41. This technique allows us to obtain an accurate Sn content for multilayered heterostructures, quantum wells containing Pb_1 − xSn_xSe with different x values for each layer.     

Ionic conductivity and crystal chemistry of ramsdellite type compounds,Li2+x(LixMg1−xSn3)O8, 0 ≤ x ≤ 0.5 and Li2Mg1−xFe2xSn3−xO8, 0 ≤ x ≤ 1

Solid solution phases Li_2+x(Li_xMg_1−xSn₃)O₈: 0 ≤ x ≤ 0.5 and Li₂Mg_1−xFe_2xSn_3−xO₈: 0 ≤ x ≤ 1, both with ramsdellite type structure, have been synthesized by solid state reaction at 1773 and 1523 K, respectively. The relationship of the ramsdellite structure to the recently illustrated, tetragonal-packed structure is given. The Li_2+x(Li_xMg_1−xSn₃)O₈ solid solutions exhibit conductivities 4 × 10⁻⁶–5 × 10⁻⁴ (Ω cm)⁻¹ at 573 K and corresponding activation energies, 0.93−0.74 eV. The highest conductivity was observed for Li_2.3(Li_0.3Mg_0.7Sn₃)O₈, x = 0.3. In the solid solution series Li₂Mg_1−xFe_2xSn_3−xO₈, the highest conductivity was exhibited by Li₂Fe₂Sn₂O₈, 2 × 10⁻⁵ (Ω cm)⁻¹ at 573 K.     

Thermoelectric properties of p-type pseudo-binary (Ag0.365Sb0.558Te)x-(Bi0.5Sb1.5Te3)1−x (x=0-1.0) alloys prepared by spark plasma sintering

In this paper, pseudo-binary (Ag_0.365Sb_0.558Te)_x-(Bi_0.5Sb_1.5Te₃)_1−x (x=0-1.0) alloys were prepared using spark plasma sintering technique, and the composition-dependent thermoelectric properties were evaluated. Electrical conductivities range from 7.9×10⁴ to 15.6×10⁴ Ω⁻¹ m⁻¹ at temperatures of 507 and 318 K, respectively, being about 3.0 and 8.5 times those of Bi_0.5Sb_1.5Te₃ alloy at the corresponding temperatures. The optimal dimensionless figure of merit (ZT) of the sample with molar fraction x=0.025 reaches 1.1 at 478 K, whereas that of the ternary Bi_0.5Sb_1.5Te₃ alloy is 0.58 near room temperature. The results also reveal that a direct introduction of Ag_0.365Sb_0.558Te in the Bi-Sb-Te system is much more effective to the property improvement than naturally precipitated Ag_0.365Sb_0.558Te in the Ag-doped Ag-Bi-Sb-Te system.     

Adhesive and electrical properties of the interface between Pb1 − x Mn x Te Crystals and In-Ag-Au alloy

The effect of annealing of Pb_{1 ? x} Mn _x Te crystals at ～690 K and structures on their basis at ～383 K on the adhesive and electric properties of the interface in the Pb_{1 ? x} Mn _x Te-(In-Ag-Au) structure was studied over the temperature range ～77–300 K. The contacts possessed high adhesive strength. The effect of annealing on contact resistance r _c was determined by a change in the specific resistance of crystals, diffusion of Ag atoms into the near-contact area of crystals, and the formation of intermediate phases of the Ag₂Te type at the interface.     

Ground-state properties of CdxSn1–xTe: The role of d-electrons

Within the framework of density functional theory (DFT ), we calculate the ground-state electronic properties of Cd_xSn_1–x Te using norm-conserving pseudopotentials in connection with the local density (LDA ) and virtual crystal approximation (VCA ). Our particular interest is in the influence of the Cd-4d and the Sn-4d electrons by comparing results obtained with pseudopotentials, which either consider the d-electrons explicitly or in the frozen core. In the mixed crystal system Cd_xSn_1–x Te, the transition from a ten- (x = 0) to an eight-electron (x = 1) system is realized, which is accompanied by a change of the crystal structure from rock salt (SnTe) to zinc blende (CdTe). By calculating the ground-state energies, we find the equilibrium lattice constant as the function of x and the bulk modulus, as well as the crossover value of x for the transition from rock-salt to zinc blende. The calculated lattice constants and bulk moduli are in much closer agreement with experimental data if the d-electrons are taken into account explicitly, whereas the crossover is rather insensitive with respect to the d-electrons. In view of the electronic charge density, we demonstrate the decrease of ionicity for increasing x. © 1994 John Wiley & Sons, Inc.     

NaCdSb: An Orthorhombic Zintl Phase with Exceptional Intrinsic Thermoelectric Performance

Many Zintl phases are promising thermoelectric materials owning to their features like narrow band gaps, multiband behaviors, ideal charge transport tunnels, and loosely bound cations. Herein we show a new Zintl phase NaCdSb with exceptional intrinsic thermoelectric performance. Pristine NaCdSb exhibits semiconductor behaviors with an experimental hole concentration of 2.9×10¹⁸ cm⁻³ and a calculated band gap of 0.5 eV. As the temperature increases, the hole concentration rises gradually and approaches its optimal one, leading to a high power factor of 11.56 μW cm⁻¹ K⁻² at 673 K. The ultralow thermal conductivity is derived from the small phonon group velocity and short phonon lifetime, ascribed to the structural anharmonicity of Cd−Sb bonds. As a consequence, a maximum zT of 1.3 at 673 K has been achieved without any doping optimization or structural modification, demonstrating that NaCdSb is a remarkable thermoelectric compound with great potential for performance improvement.     

Near-Infrared Emission from Tin–Lead (Sn–Pb) Alloyed Perovskite Quantum Dots by Sodium Doping

Phase-stable CsSn_xPb_1−xI₃ perovskite quantum dots (QDs) hold great promise for optoelectronic applications owing to their strong response in the near-infrared region. Unfortunately, optimal utilization of their potential is limited by the severe photoluminescence (PL) quenching, leading to extremely low quantum yields (QYs) of approximately 0.3 %. The ultra-low sodium (Na) doping presented herein is found to be effective in improving PL QYs of these alloyed QDs without alerting their favourable electronic structure. X-ray photoelectron spectroscopy (XPS) studies suggest the formation of a stronger chemical interaction between I⁻ and Sn²⁺ ions upon Na doping, which potentially helps to stabilize Sn²⁺ and suppresses the formation of I vacancy defects. The optimized PL QY of the Na-doped QDs reaches up to around 28 %, almost two orders of magnitude enhancement compared with the pristine one.     

The relationship between SnII fraction and visible light activated photocatalytic activity of SnOx·SiO2 glass studied by Mössbauer spectroscopy

The relationship between local structure and visible-light photocatalytic ability of tin silicate glass prepared by sol–gel method was investigated. ¹¹⁹Sn Mössbauer spectrum of SnO_x·SiO₂ glass prepared from SnCl₂ showed a small peak of Sn^II component besides the major amount of Sn^IV. The smallest bandgap energy of 2.5 ± 0.5 eV was estimated from Tauc plot, and the largest first order rate constant (k) of (13.8 ± 0.1) 10⁻³ min⁻¹ was recorded from the methylene blue degradation test under visible-light irradiation. It is concluded that Sn^II shows remarkable photocatalytic ability when it is incorporated into silica glass matrix.

     

Synthesis and Characterization of Ag8(Ge1−x,Snx)(S6−y,Sey) Colloidal Nanocrystals

A facile colloidal approach to synthesize Ag₈(Ge_1?x,Sn_x)(S_6?y,Se_y) nanocrystals (NCs) in a highly controlled way across the entire compositional ranges (0≤x≤1, 0≤y≤6) has been developed. The NCs exhibit a uniform size distribution, highly crystalline structure, over 1 g scalable synthesis, and tunable band gaps in the range of 0.88–1.45 eV by varying their chemical compositions. The Ag₈GeS₆ NCs with a band gap of approximately 1.45 eV were employed as a model light harvester to assess their applicability in solar cells by a full solution‐processing device, yielding an efficiency of 0.28 % under AM1.5 illumination, demonstrating their application potential in solar energy utilization.     

Chemical Bonding Tuned Lattice Anharmonicity Leads to a High Thermoelectric Performance in Cubic AgSnSbTe3

Comprehension of chemical bonding and its intertwined relation with charge carriers and heat propagation through a crystal lattice is imperative to design compounds for thermoelectric energy conversion. Here, we report the synthesis of large single crystal of new p-type cubic AgSnSbTe₃ which shows an innately ultra-low lattice thermal conductivity (κ_lat) of 0.47–0.27 Wm⁻¹ K⁻¹ and a high electrical conductivity (1238 – 800 S cm⁻¹) in the temperature range 294–723 K. We investigated the origin of the low κ_lat by analysing the nature of the chemical bonding and its crystal structure. The interaction between Sn(5 s)/Ag(4d) and Te(5p) orbitals was found to generate antibonding states just below the Fermi level in the electronic band structure, resulting in a softening of the lattice in AgSnSbTe₃. Furthermore, the compound exhibits metavalent bonding which provides highly polarizable bonds with a strong lattice anharmonicity while maintaining the superior electrical conductivity. The electronic band structure exhibits nearly degenerate valence-band maxima that help to achieve a high Seebeck coefficient throughout the measured temperature range and, as a result, the maximum thermoelectric figure of merit reaches to ≈1.2 at 661 K in pristine single crystal of AgSnSbTe₃.     

Enhanced thermoelectric performance of hydrothermally synthesized polycrystalline Te-doped SnSe

In this study,large-scale Te-doped polycrystalline SnSe nanopowders were synthesized by a facile hydrothermal approach and the effect of Te doping on the thermoelectric properties of SnSe was fully investigated.It is found that the carrier concentration increases due to the reduction of band gap by alloying with Te,which contributes to significant enhancement of electrical conductivity especially at room temperature.Combined with the moderated Seebeck coefficient,a high power factor of 4.59μW cm ~1 K ~2 is obtained at 773 K.Furthermore,the lattice the rmal conductivity is greatly reduced upon Te substitution owing to the atomic point defect scattering.Benefiting from the synergistically optimized both electrical-and thermal-transport properties by Te-doping,thermoelectric performance of polycrystalline SnSe is enhanced in the whole temperature range with a maximum ZT of-0.79 at a relatively low temperature(773 K) for SnSe_(0.85)Te_(0.15).This study provides a low-cost and simple lowtemperature method to mass production of SnSe with high thermoelectric performance for practical applications     

An n-Type Polythiophene Derivative with Excellent Thermoelectric Performance

Typical n-type conjugated polymers are based on fused-ring electron-accepting building blocks. Herein, we report a non-fused-ring strategy to design n-type conjugated polymers, i.e. introducing electron-withdrawing imide or cyano groups to each thiophene unit of a non-fused-ring polythiophene backbone. The resulting polymer, n-PT1 , shows low LUMO/HOMO energy levels of −3.91 eV/−6.22 eV, high electron mobility of 0.39 cm² V⁻¹ s⁻¹ and high crystallinity in thin film. After n-doping, n-PT1 exhibits excellent thermoelectric performance with an electrical conductivity of 61.2 S cm⁻¹ and a power factor (PF) of 141.7 μW m⁻¹ K⁻². This PF is the highest value reported so far for n-type conjugated polymers and this is the first time for polythiophene derivatives to be used in n-type organic thermoelectrics. The excellent thermoelectric performance of n-PT1 is due to its superior tolerance to doping. This work indicates that polythiophene derivatives without fused rings are low-cost and high-performance n-type conjugated polymers.     

Potassium silver tin selenide,K2Ag2Sn2Se6

The title compound was synthesized by a reactive salt reaction at 773 K over a period of 5 d. It has a one-dimensional chain structure consisting of K⁺ cations and one-dimensional [Ag₂Sn₂Se₆]²⁻ anions. The chain is constructed by edge-sharing bitetrahedral [Sn₂Se₆] units connected in a 1:2 ratio via linear Ag⁺ ions.     

Effect of Li excess content and Ti dopants on electrochemical properties of LiFePO<Subscript>4</Subscript> prepared by thermal reduction method

Carbon coated Li_{1 + x} FePO₄ (x = 0, 0.01, 0.02, 0.03, 0.04) and doped compositions Li_1.03Fe_0.99Ti_0.01PO₄ have been synthesized by thermal reduction method in this paper. The results showed that increasing the content in Li_{1 + x} FePO₄ result in better electrochemical properties and cyclic performances until x = 0.03, which had similar change law with the particle size of samples; and the initial discharge capacity and cycle life of Li_1.03Fe_0.9Ti_0.01PO₄ was better than other samples under 1 C rate. When the Li_1.03Fe_0.99Ti_0.01PO₄/C sample cycled before 60 times, this sample exhibited a trend of increased capacity, and reached the highest discharging rate capacity of 156 mA h g⁻¹ at 60 cycles. The electrochemical performances of LiFePO₄ compositions synthesized by thermal reduction method, to some extent, can be improved by Li excess content and Ti doping.     

Room-Temperature High-Performance Thermoelectric Bi0.6Sb0.4Te: Elimination of Detrimental Band Inversion in BiTe

Room-temperature thermoelectric materials are the key to miniaturizing refrigeration equipment and have great scientific and social implications, yet their application is hindered by their extreme scarcity. BiTe exhibiting strong spin-orbit coupling peaks ZT at 600 K. Herein, we discover the synergy effect of Sb doping in BiTe that eliminates the detrimental band inversion and leads to an overlap of conduction band (CB) and valence band that significantly increases the S from 33 to 124 μV K⁻¹. In addition, this effect enhances the μ from 58 to 92 cm² V⁻¹ s⁻¹ owing to the sharp increase in the CB slope along the Γ-A in the first Brillouin zone. Furthermore, Sb doping increases the anharmonicity, shortens the phonon lifetime and lowers κ_lat. Finally, Se/Sb codoping further optimizes the ZT to 0.6 at 300 K, suggesting that Bi_0.6Sb_0.4Te_1−ySe_y is a potential room-temperature TE material.