Enhancing Thermoelectric and Mechanical Properties of <i>p</i>‑Type (Bi, Sb)₂Te₃ through Rickardite Mineral (Cu_2.9Te₂) Incorporation

Bi2Te3-based alloys are widely utilized in Peltier coolers owing to their highest thermoelectric performance at near-room-temperatures. However, their peak dimensionless thermoelectric figure of merit, zT, is limited to a narrow temperature window due to minority carrier excitation emerging upon heating at around 400 K. Here, we show how this issue can be overcome by incorporating a synthetic rickardite mineral, Cu3–xTe2, in p-type (Bi, Sb)2Te3. The significant enhancement of the electronic and thermal properties could be achieved due to small Cu incorporation into the crystal structure of (Bi, Sb)2Te3 and homogeneous precipitation of Cu3–xTe2 at the grain boundaries. This leads to a high average zT value (zTave) of 1.22 between 350 and 500 K for two compositions, Bi0.5Sb1.5Te3 (BST-5) and Bi0.3Sb1.7Te3 (BST-3), with peak zT values of 1.32 at 467 K and 1.30 at 400 K, respectively. These high zT values result in a considerably high maximum device ZT of ca. 1.15 and a theoretical efficiency of up to 7% between 325 and 525 K. Additionally, room-temperature micro-hardness is substantially improved, which is desirable for constructing reliable and durable thermoelectric modules

Controlling Defect Chemistry in InTe by Saturation Annealing

Achieving a precise control of defects in chalcogenide semiconductors is paramount to optimizing their thermoelectric properties. Recently, p-type InTe has emerged as a potential candidate for thermoelectric applications in power generation, mainly due to its extremely low lattice thermal conductivity. Here, we show that the concentration of inherent In vacancies in both single-crystalline and polycrystalline InTe samples can be successfully controlled through saturation annealing. This process, performed on both the In-rich and Te-rich sides of the solidus line at 943, 893, 843, and 943 K, respectively, results in variations in the hole concentration from 4.9 to 8.5 × 1019 cm–3 at 300 K. This narrow density range suggests that the defect chemistry in InTe plays a less critical role in determining its thermoelectric properties compared to other state-of-the-art thermoelectric chalcogenides. The increased partially degenerate character of transport with increasing annealing temperature lowers the thermoelectric performance, with a peak ZT value of 0.9 achieved at 710 K in as-synthesized InTe

Improved Thermoelectric Properties in Melt-Spun SnTe

SnTe has been the focus of numerous experimental and theoretical studies over the last years owing to its high thermoelectric performances near 800 K when appropriately doped. Here, we demonstrate that melt-spinning, an ultrafast-quenching synthesis technique, followed by spark plasma sintering results in enhanced <i>ZT</i> values in polycrystalline SnTe. To illustrate the impact of this technique, the results are contrasted with those obtained on two polycrystalline samples prepared by direct quenching of molten SnTe and without quenching. SnTe melt-spun ribbons are characterized by a peculiar columnar microstructure that contributes to lower the lattice thermal conductivity below 700 K in pressed samples. More importantly, this technique results in a significant decrease in the hole concentration, giving rise to enhanced thermopower values above 500 K. The variation in the hole concentration is likely due to a slight loss of elemental Te during the melt-spinning process. Thanks to the decreased hole concentration, the thermoelectric performances are significantly enhanced with a peak <i>ZT</i> value of 0.6 at 800 K, which represents a 40% increase over the values measured for samples prepared with and without quenching. These findings indicate that melt-spinning provides a novel strategy to improve the thermoelectric properties of SnTe that could be worthwhile extending to substituted compounds

Cu Insertion Into the Mo₁₂ Cluster Compound Cs₂Mo₁₂Se₁₄: Synthesis, Crystal and Electronic Structures, and Physical Properties

Mo-based cluster compounds are promising materials for high-temperature thermoelectric applications due to their intrinsic, extremely low thermal conductivity values. In this study, polycrystalline cluster compounds Cs₂Cu_<i>x</i>Mo₁₂Se₁₄ were prepared for a wide range of Cu contents (0 ≤ <i>x</i> ≤ 2). All samples crystallize isostructurally in the trigonal space group <i>R</i>3̅. The position of the Cu atoms in the unit cell was determined by X-ray diffraction on a single-crystalline specimen indicating that these atoms fill the empty space between the Mo–Se clusters. Density functional theory calculations predict a metallic ground state for all compositions, in good agreement with the experimental findings. Magnetization measurements indicate a rapid suppression of the superconducting state that develops in the <i>x</i> = 0.0 sample upon Cu insertion. Transport properties measurements, performed in a wide temperature range (2–630 K) on the two end-member compounds <i>x</i> = 0 and <i>x</i> = 2, revealed a multiband electrical conduction as shown by sign reversal of the thermopower as a function of temperature

Cu Insertion Into the Mo₁₂ Cluster Compound Cs₂Mo₁₂Se₁₄: Synthesis, Crystal and Electronic Structures, and Physical Properties

Mo-based cluster compounds are promising materials for high-temperature thermoelectric applications due to their intrinsic, extremely low thermal conductivity values. In this study, polycrystalline cluster compounds Cs₂Cu_<i>x</i>Mo₁₂Se₁₄ were prepared for a wide range of Cu contents (0 ≤ <i>x</i> ≤ 2). All samples crystallize isostructurally in the trigonal space group <i>R</i>3̅. The position of the Cu atoms in the unit cell was determined by X-ray diffraction on a single-crystalline specimen indicating that these atoms fill the empty space between the Mo–Se clusters. Density functional theory calculations predict a metallic ground state for all compositions, in good agreement with the experimental findings. Magnetization measurements indicate a rapid suppression of the superconducting state that develops in the <i>x</i> = 0.0 sample upon Cu insertion. Transport properties measurements, performed in a wide temperature range (2–630 K) on the two end-member compounds <i>x</i> = 0 and <i>x</i> = 2, revealed a multiband electrical conduction as shown by sign reversal of the thermopower as a function of temperature

X‑ray Characterization, Electronic Band Structure, and Thermoelectric Properties of the Cluster Compound Ag₂Tl₂Mo₉Se₁₁

We report on a detailed investigation of the crystal and electronic band structures and of the transport and thermodynamic properties of the Mo-based cluster compound Ag₂Tl₂Mo₉Se₁₁. This novel structure type crystallizes in the trigonal space group <i>R</i>3̅<i>c</i> and is built of a three-dimensional network of interconnected Mo₉Se₁₁ units. Single-crystal X-ray diffraction indicates that the Ag and Tl atoms are distributed in the voids of the cluster framework, both of which show unusually large anisotropic thermal ellipsoids indicative of strong local disorder. First-principles calculations show a weakly dispersive band structure around the Fermi level as well as a semiconducting ground state. The former feature naturally explains the presence of both hole-like and electron-like signals observed in Hall effect. Of particular interest is the very low thermal conductivity that remains quasi-constant between 150 and 800 K at a value of approximately 0.6 W·m^–1·K^–1. The lattice thermal conductivity is close to its minimum possible value, that is, in a regime where the phonon mean free path nears the mean interatomic distance. Such extremely low values likely originate from the disorder induced by the Ag and Tl atoms giving rise to strong anharmonicity of the lattice vibrations. The strongly limited ability of this compound to transport heat is the key feature that leads to a dimensionless thermoelectric figure of merit <i>ZT</i> of 0.6 at 800 K

Large-Scale Colloidal Synthesis of Chalcogenides for Thermoelectric Applications

A simple and effective preparation of solution-processed chalcogenide thermoelectric materials is described. First, PbTe, PbSe, and SnSe were prepared by gram-scale colloidal synthesis relying on the reaction between metal acetates and diphenyl dichalcogenides in hexadecylamine solvent. The resultant phase-pure chalcogenides consist of highly crystalline and defect-free particles with distinct cubic-, tetrapod-, and rod-like morphologies. The powdered PbTe, PbSe, and SnSe products were subjected to densification by spark plasma sintering (SPS), affording dense pellets of the respective chalcogenides. Scanning electron microscopy shows that the SPS-derived pellets exhibit fine nano-/micro-structures dictated by the original morphology of the key constituting particles, while the powder X-ray diffraction and electron microscopy analyses confirm that the SPS-derived pellets are phase-pure materials, preserving the structure of the colloidal synthesis products. The resultant solution-processed PbTe, PbSe, and SnSe exhibit low thermal conductivity, which might be due to the enhanced phonon scattering developed over fine microstructures. For undoped n-type PbTe and p-type SnSe samples, an expected moderate thermoelectric performance is achieved. In contrast, an outstanding figure-of-merit of 0.73 at 673 K was achieved for undoped n-type PbSe outperforming, the majority of the optimized PbSe-based thermoelectric materials. Overall, our findings facilitate the design of efficient solution-processed chalcogenide thermoelectrics

Synthesis, Crystal and Electronic Structures, and Thermoelectric Properties of the Novel Cluster Compound Ag₃In₂Mo₁₅Se₁₉

Polycrystalline samples and single crystals of the new compound Ag₃In₂Mo₁₅Se₁₉ were synthesized by solid-state reaction in a sealed molybdenum crucible at 1300 °C. Its crystal structure (space group <i>R</i>3̅<i>c</i>, <i>a</i> = 9.9755(1) Å, <i>c</i> = 57.2943(9) Å, and <i>Z</i> = 6) was determined from single-crystal X-ray diffraction data and constitutes an Ag-filled variant of the In₂Mo₁₅Se₁₉ structure-type containing octahedral Mo₆ and bioctahedral Mo₉ clusters in a 1:1 ratio. The increase of the cationic charge transfer due to the Ag insertion induces a modification of the Mo–Mo distances within the Mo clusters that is discussed with regard to the electronic structure. Transport properties were measured in a broad temperature range (2–1000 K) to assess the thermoelectric potential of this compound. The transport data indicate an electrical conduction dominated by electrons below 25 K and by holes above this temperature. The metallic character of the transport properties in this material is consistent with electronic band structure calculations carried out using the linear muffin-tin orbital (LMTO) method. The complex unit cell, together with the cagelike structure of this material, results in very low thermal conductivity values (0.9 W m^–1 K^–1 at 300 K), leading to a maximum estimated thermoelectric figure of merit (<i>ZT</i>) of 0.45 at 1100 K

Effect of Isovalent Substitution on the Electronic Structure and Thermoelectric Properties of the Solid Solution α‑As₂Te_3–<i>x</i>Se_<i>x</i> (0 ≤ <i>x</i> ≤ 1.5)

We report on the influence of Se substitution on the electronic band structure and thermoelectric properties (5–523 K) of the solid solution α-As₂Te_3–<i>x</i>Se_<i>x</i> (0 ≤ <i>x</i> ≤ 1.5). All of the polycrystalline compounds α-As₂Te_3–<i>x</i>Se_<i>x</i> crystallize isostructurally in the monoclinic space group <i>C</i>2/<i>m</i> (No. 12, <i>Z</i> = 4). Regardless of the Se content, chemical analyses performed by scanning electron microscopy and electron probe microanalysis indicate a good chemical homogeneity, with only minute amounts of secondary phases for some compositions. In agreement with electronic band structure calculations, neutron powder diffraction suggests that Se does not randomly substitute for Te but exhibits a site preference. These theoretical calculations further predict a monotonic increase in the band gap energy with the Se content, which is confirmed experimentally by absorption spectroscopy measurements. Increasing <i>x</i> up to <i>x</i> = 1.5 leaves unchanged both the p-type character and semiconducting nature of α-As₂Te₃. The electrical resistivity and thermopower gradually increase with <i>x</i> as a result of the progressive increase in the band gap energy. Despite the fact that α-As₂Te₃ exhibits very low lattice thermal conductivity κ_L, the substitution of Se for Te further lowers κ_L to 0.35 W m^–1 K^–1 at 300 K. The compositional dependence of the lattice thermal conductivity closely follows classical models of phonon alloy scattering, indicating that this decrease is due to enhanced point-defect scattering

Polymorphism in Thermoelectric As₂Te₃

Metastable β-As₂Te₃ (<i>R</i>3̅<i>m</i>, <i>a</i> = 4.047 Å and <i>c</i> = 29.492 Å at 300 K) is isostructural to layered Bi₂Te₃ and is known for similarly displaying good thermoelectric properties around 400 K. Crystallizing glassy-As₂Te₃ leads to multiphase samples, while β-As₂Te₃ could indeed be synthesized with good phase purity (97%) by melt quenching. As expected, β-As₂Te₃ reconstructively transforms into stable α-As₂Te₃ (<i>C</i>2/<i>m</i>, <i>a</i> = 14.337 Å, <i>b</i> = 4.015 Å, <i>c</i> = 9.887 Å, and β = 95.06°) at 480 K. This β → α transformation can be seen as the displacement of part of the As atoms from their As₂Te₃ layers into the van der Waals bonding interspace. Upon cooling, β-As₂Te₃ displacively transforms in two steps below <i>T</i>_S1 = 205–210 K and <i>T</i>_S2 = 193–197 K into a new β′-As₂Te₃ allotrope. These reversible and first-order phase transitions give rise to anomalies in the resistance and in the calorimetry measurements. The new monoclinic β′-As₂Te₃ crystal structure (<i>P</i>2₁/<i>m</i>, <i>a</i> = 6.982 Å, <i>b</i> = 16.187 Å, <i>c</i> = 10.232 Å, β = 103.46° at 20 K) was solved from Rietveld refinements of X-ray and neutron powder patterns collected at low temperatures. These analyses showed that the distortion undergone by β-As₂Te₃ is accompanied by a 4-fold modulation along its <i>b</i> axis. In agreement with our experimental results, electronic structure calculations indicate that all three structures are semiconducting with the α-phase being the most stable one and the β′-phase being more stable than the β-phase. These calculations also confirm the occurrence of a van der Waals interspace between covalently bonded As₂Te₃ layers in all three structures