Invited article: A round robin test of the uncertainty on the measurement of the thermoelectric dimensionless figure of merit of Co0.97Ni0.03Sb3

Engineering of Thermoelectric Composites Based on Silver Selenide in Aqueous Solution and Ambient Temperature

The direct, solid state, and reversible conversion between heat and electricity using thermoelectric devices finds numerous potential uses, especially around room temperature. However, the relatively high material processing cost limits their real applications. Silver selenide (Ag2Se) is one of the very few n-type thermoelectric (TE) materials for room-temperature applications. Herein, we report a room temperature, fast, and aqueous-phase synthesis approach to produce Ag2Se, which can be extended to other metal chalcogenides. These materials reach TE figures of merit (zT) of up to 0.76 at 380 K. To improve these values, bismuth sulfide (Bi2S3) particles also prepared in an aqueous solution are incorporated into the Ag2Se matrix. In this way, a series of Ag2Se/Bi2S3 composites with Bi2S3 wt % of 0.5, 1.0, and 1.5 are prepared by solution blending and hot-press sintering. The presence of Bi2S3 significantly improves the Seebeck coefficient and power factor while at the same time decreasing the thermal conductivity with no apparent drop in electrical conductivity. Thus, a maximum zT value of 0.96 is achieved in the composites with 1.0 wt % Bi2S3 at 370 K. Furthermore, a high average zT value (zTave) of 0.93 in the 300-390 K range is demonstrated.

Record-High Thermoelectric Performance in Al-Doped ZnO via Anderson Localization of Band Edge States

Oxides are of interest for thermoelectrics due to their high thermal stability, chemical inertness, low cost, and eco-friendly constituting elements. Here, adopting a unique synthesis route via chemical co-precipitation at strongly alkaline conditions, one of the highest thermoelectric performances for ZnO ceramics (P F max = $PF_{\text{max}} =$  21.5 µW cm-1 K-2 andz T max = $zT_{\text{max}} =$  0.5 at 1100 K inZn 0.96 Al 0.04 O ${\rm Zn}_{0.96} {\rm Al}_{0.04}{\rm O}$ ) is achieved. These results are linked to a distinct modification of the electronic structure: charge carriers become trapped at the edge of the conduction band due to Anderson localization, evidenced by an anomalously low carrier mobility, and characteristic temperature and doping dependencies of charge transport. The bi-dimensional optimization of doping and carrier localization enable a simultaneous improvement of the Seebeck coefficient and electrical conductivity, opening a novel pathway to advance ZnO thermoelectrics.

Boosting Thermoelectric Performance in Nanocrystalline Ternary Skutterudite Thin Films through Metallic CoTe2 Integration

Metal-semiconductor nanocomposites have emerged as a viable strategy for concurrently tailoring both thermal and electronic transport properties of established thermoelectric materials, ultimately achieving synergistic performance. In this investigation, a series of nanocomposite thin films were synthesized, embedding metallic cobalt telluride (CoTe2) nanophase within the nanocrystalline ternary skutterudite (Co(Ge1.22Sb0.22)Te1.58 or CGST) matrix. Our approach harnessed composition fluctuation-induced phase separation and in situ growth during thermal annealing to seamlessly integrate the metallic phase. The distinctive band structures of both materials have developed an ohmic-type contact characteristic at the interface, which raised carrier density considerably yet negligibly affected the mobility counterpart, leading to a substantial improvement in electrical conductivity. The intricate balance in transport properties is further influenced by the metallic CoTe2 phase's role in diminishing lattice thermal conductivity. The presence of the metallic phase instigates enhanced phonon scattering at the interface boundaries. Consequently, a 2-fold enhancement in the thermoelectric figure of merit (zT ∼ 1.30) is attained with CGST-7 wt. % CoTe2 nanocomposite film at 655 K compared to that of pristine CGST.

High Seebeck Coefficient from Screen-Printed Colloidal PbSe Nanocrystals Thin Film

Thin-film thermoelectrics (TEs) with a thickness of a few microns present an attractive opportunity to power the internet of things (IoT). Here, we propose screen printing as an industry-relevant technology to fabricate TE thin films from colloidal PbSe quantum dots (QDs). Monodisperse 13 nm-sized PbSe QDs with spherical morphology were synthesized through a straightforward heating-up method. The cubic-phase PbSe QDs with homogeneous chemical composition allowed the formulation of a novel ink to fabricate 2 μm-thick thin films through robust screen printing followed by rapid annealing. A maximum Seebeck coefficient of 561 μV K^-1 was obtained at 143 °C and the highest electrical conductivity of 123 S m^-1 was reached at 197 °C. Power factor calculations resulted in a maximum value of 2.47 × 10^-5 W m^-1 K^-2 at 143 °C. To the best of our knowledge, the observed Seebeck coefficient value is the highest reported for TE thin films fabricated by screen printing. Thus, this study highlights that increased Seebeck coefficients can be obtained by using QD building blocks owing to quantum confinement.

Facile Fabrication of N-Type Flexible CoSb3-xTex Skutterudite/PEDOT:PSS Hybrid Thermoelectric Films

Alongiside the growing demand for wearable and implantable electronics, the development of flexible thermoelectric (FTE) materials holds great promise and has recently become a highly necessitated and efficient method for converting heat to electricity. Conductive polymers were widely used in previous research; however, n-type polymers suffer from instability compared to the p-type polymers, which results in a deficiency in the n-type TE leg for FTE devices. The development of the n-type FTE is still at a relatively early stage with limited applicable materials, insufficient conversion efficiency, and issues such as an undesirably high cost or toxic element consumption. In this work, as a prototype, a flexible n-type rare-earth free skutterudite (CoSb₃)/poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS) binary thermoelectric film was fabricated based on ball-milled skutterudite via a facile top-down method, which is promising to be widely applicable to the hybridization of conventional bulk TE materials. The polymers bridge the separated thermoelectric particles and provide a conducting pathway for carriers, leading to an enhancement in electrical conductivity and a competitive Seebeck coefficient. The current work proposes a rational design towards FTE devices and provides a perspective for the exploration of conventional thermoelectric materials for wearable electronics.

Enhanced Thermoelectric Performance in Ternary Skutterudite Co(Ge0.5Te0.5)3 via Band Engineering

We report the phase evolution and thermoelectric properties of a series of Co(Ge_0.5Te_0.5)_3-xSb_x (x = 0-0.20) compositions synthesized by mechanical alloying. Pristine ternary Co(Ge_0.5Te_0.5)₃ skutterudite crystallizes in the rhombohedral symmetry (R3̅), and Sb doping induces a structural transition to the cubic phase (ideal skutterudite, Im3̅). The Sb substitution increases the carrier concentration while maintaining a high thermopower even at higher doping levels owing to an increased effective mass. The exceptional electronic properties exhibited by Co(Ge_0.5Te_0.5)₃ upon doping are attributed to the carrier transport from both the primary and secondary conduction bands, as shown by theoretical calculations. The enhanced electrical conductivity and high thermopower increase the power factor by more than 20 times. Because the dominant phonon propagation modes in binary skutterudites are associated with the vibrations of pnictogen rings, twisting the latter through the isoelectronic replacement of Sb₄ rings with Ge₂Te₂ ones, as done in this study, can effectively reduce the thermal conductivity. This leads to an increase in the dimensionless figure-of-merit (zT) by a factor of 30, reaching 0.65 at 723 K for Co(Ge_0.5Te_0.5)_2.9Sb_0.1.

International Round Robin Test of Thermoelectric Generator Modules

The status of metrology for the characterization of thermoelectric generator modules (TEM) is investigated in this work by an international round robin (RR) test including twelve laboratories from nine countries on three continents. Measurements have been performed with three samples of a Bi₂Te₃-based commercial TEM type, which has prevailed over three competing types during previous tests on the short- and long-term stability. A comparison of temperature-dependent results is provided up to 200 °C hot side temperature for the maximum power output P_max, the incident heat flow Q˙In (at maximum efficiency conditions), and the maximum efficiency η_max. Data evaluation from all RR participants reveals maximum standard deviations for these measurands of 27.2% (P_max), 59.2% (Q˙In), and 25.9% (η_max). A comparison between RR data sets and reference data from manufacturer specifications shows high deviations of up to 46%, too. These deviations reflect the absence of measurement guidelines and reference samples and confirm the need for improvements in the standardization of TEM metrology. Accordingly, the results of the RR are presented against the background of our own investigations on the uncertainty budgets for the determination of the abovementioned TEM properties using inhouse-developed characterization facilities, which comprise reference and absolute measurement techniques for the determination of heat flow.

A Tunable Structural Family with Ultralow Thermal Conductivity: Copper-Deficient Cu1-x□xPb1-xBi1+xS3

Understanding the mechanism that connects heat transport with crystal structures and order/disorder phenomena is crucial to develop materials with ultralow thermal conductivity (κ), for thermoelectric and thermal barrier applications, and requires the study of highly pure materials. We synthesized the n-type sulfide CuPbBi₅S₉ with an ultralow κ value of 0.6-0.4 W m^-1 K^-1 in the temperature range 300-700 K. In contrast to prior studies, we show that this synthetic sulfide does not exhibit the ordered gladite mineral structure but instead forms a copper-deficient disordered aikinite structure with partial Pb replacement by Bi, according to the chemical formula Cu_1/3□_2/3Pb_1/3Bi_5/3S₃. By combining experiments and lattice dynamics calculations, we elucidated that the ultralow κ value of this compound is due to very low energy optical modes associated with Pb and Bi ions and, to a smaller extent, Cu. This vibrational complexity at low energy hints at substantial anharmonic effects that contribute to enhance phonon scattering. Importantly, we show that this aikinite-type sulfide, despite being a poor semiconductor, is a potential matrix for designing novel, efficient n-type thermoelectric compounds with ultralow κ values. A drastic improvement in the carrier concentration and thermoelectric figure of merit have been obtained upon Cl for S and Bi for Pb substitution. The Cu_1-x□_xPb_1-xBi_1+xS₃ series provides a new, interesting structural prototype for engineering n-type thermoelectric sulfides by controlling disorder and optimizing doping.

Synergistic Optimization of the Electronic and Phonon Transports of N-Type Argyrodite Ag8Sn1-xGaxSe6 (x = 0-0.6) through Entropy Engineering

The argyrodite compound, Ag₈SnSe₆ (ATS), which is one of the promising thermoelectric (TE) candidates, is receiving growing attention in thermoelectrics recently. However, its TE performance is still low and phases are unstable as the temperature varies. In this work, inspired by entropy engineering, we eliminate the β/γ phase transformation at ∼355 K via alloying Ga, thus extending its high-temperature cubic phase from 320 to 730 K. In the meantime, the power factor (PF) enhances by 10% and lattice thermal conductivity (κ_L) reduces by 40% at 723 K. As a result, the ZT value is boosted to ∼1.15 for Ag₈Sn_0.5Ga_0.5Se₆, which stands high among the ATS systems. This proves that the entropy engineering is an effective approach to extend the high-temperature range for the cubic γ-phase and improve its TE performance simultaneously.

Local-Disorder-Induced Low Thermal Conductivity in Degenerate Semiconductor Cu22Sn10S32

S-based semiconductors are attracting attention as environmentally friendly materials for energy-conversion applications because of their structural complexity and chemical flexibility. Here, we show that the delicate interplay between the chemical composition and cationic order/disorder allows one to stabilize a new sphalerite derivative phase of cubic symmetry in the Cu-Sn-S diagram: Cu₂₂Sn₁₀S₃₂. Interestingly, its crystal structure is characterized by a semiordered cationic distribution, with the Cu-Sn disorder being localized on one crystallographic site in a long-range-ordered matrix. The origin of the partial disorder and its influence on the electronic and thermal transport properties are addressed in detail using a combination of synchrotron X-ray diffraction, Mössbauer spectroscopy, transmission electron microscopy, theoretical modeling, and transport property measurements. These measurements evidence that this compound behaves as a pseudogap, degenerate p-type material with very low lattice thermal conductivity (0.5 W m^-1 K^-1 at 700 K). We show that localized disorder is very effective in lowering κ_L without compromising the integrity of the conductive framework. Substituting pentavalent Sb for tetravalent Sn is exploited to lower the hole concentration and doubles the thermoelectric figure of merit ZT to 0.55 at 700 K with respect to the pristine compound. The discovery of this semiordered cubic sphalerite derivative Cu₂₂Sn₁₀S₃₂ furthers the understanding of the structure-property relationships in the Cu-Sn-S system and more generally in ternary and quaternary Cu-based systems.

Bismuth Doping in Nanostructured Tetrahedrite: Scalable Synthesis and Thermoelectric Performance

In this study, we demonstrate the feasibility of Bi-doped tetrahedrite Cu₁₂Sb_4-xBi_xS₁₃ (x = 0.02-0.20) synthesis in an industrial eccentric vibratory mill using Cu, Sb, Bi and S elemental precursors. High-energy milling was followed by spark plasma sintering. In all the samples, the prevailing content of tetrahedrite Cu₁₂Sb₄S₁₃ (71-87%) and famatinite Cu₃SbS₄ (13-21%), together with small amounts of skinnerite Cu₃SbS₃, have been detected. The occurrence of the individual Cu-Sb-S phases and oxidation states of bismuth identified as Bi⁰ and Bi³⁺ are correlated. The most prominent effect of the simultaneous milling and doping on the thermoelectric properties is a decrease in the total thermal conductivity (κ) with increasing Bi content, in relation with the increasing amount of famatinite and skinnerite contents. The lowest value of κ was achieved for x = 0.2 (1.1 W m^-1 K^-1 at 675 K). However, this sample also manifests the lowest electrical conductivity σ, combined with relatively unchanged values for the Seebeck coefficient (S) compared with the un-doped sample. Overall, the lowered electrical performances outweigh the benefits from the decrease in thermal conductivity and the resulting figure-of-merit values illustrate a degradation effect of Bi doping on the thermoelectric properties of tetrahedrite in these synthesis conditions.

Van der Pauw device used to investigate the thermoelectric power factor

In this paper, we describe a measuring system based on the Van der Pauw principle with four calibrated type S thermocouples. By means of this system, we conducted traceable measurements of the absolute Seebeck coefficients and the electrical conductivity of thermoelectric bulk materials to establish a precise determination of the power factor. The results of a comparative investigation of metallic (ISOTAN® and Nickel) and semiconducting (SiGe) materials in the temperature range of 300 K-1100 K are presented. The good agreement of the Seebeck coefficients and electrical conductivities measured using the system and the data reported from the literature and values of these transport properties premeasured using another measuring system forms the basis for the usage of the system for the further certification of thermoelectric reference materials for the power factor up to 1100 K.

Drastic power factor improvement by Te doping of rare earth-free CoSb3-skutterudite thin films

In the present study, we have focused on the elaboration of control of Te-doped CoSb3 thin films by RF magnetron sputtering which is an attractive technique for industrial development of thermoelectric (TE) thin films. We have successfully synthesized sputtering targets with a reliable approach in order to obtain high-quality films with controlled stoichiometry. TE properties were then probed and revealed a reliable n-type behavior characterized by poor electrical transport properties. Tellurium substitution was realized by co-sputtering deposition and allowed obtaining a significant enhancement of the power factor with promising values of PF ≈ 0.21 mW m-1 K-2 near room temperature. It is related to the Te doping effect which leads to an increase of the Seebeck coefficient and the electrical conductivity simultaneously. However, despite this large improvement, the properties remained far from the bulk material and further developments are necessary to improve the carrier mobility reduced by the thin film formatting.

Improved Thermoelectric Performance through Double Substitution in Shandite-Type Mixed-Metal Sulfides

Substitution of tin by indium in shandite-type phases, A₃Sn₂S₂ with mixed Co/Fe occupancy of the A-sites is used to tune the Fermi level within a region of the density of states in which there are sharp, narrow bands of predominantly metal d-character. Materials of general formula Co_2.5+x Fe_0.5-x Sn_2--yIn _y S₂ (x = 0, 0.167; 0.0 ≤ y ≤ 0.7) have been prepared by solid-state reaction and the products characterized by powder X-ray diffraction. Electrical-transport property data reveal that the progressive depopulation of the upper conduction band as tin is replaced by indium increases the electrical resistivity, and the weakly temperature-dependent ρ(T) becomes more semiconducting in character. Concomitant changes in the negative Seebeck coefficient, the temperature dependence of which becomes increasingly linear, suggests the more highly substituted materials are n-type degenerate semiconductors. The power factors of the substituted phases, while increased, exhibit a weak temperature dependence. The observed reductions in thermal conductivity are principally due to reductions in the charge-carrier contribution on hole doping. A maximum figure-of-merit of (ZT)_max = 0.29 is obtained for the composition Co_2.667Fe_0.333Sn_1.6In_0.4S₂ at 573 K: among the highest values for an n-type sulfide at this temperature.

Promoted crystallisation and cationic ordering in thermoelectric Cu26V2Sn6S32 colusite by eccentric vibratory ball milling

The impact of eccentric vibratory ball milling time on the crystallisation of thermoelectric Cu₂₆V₂Sn₆S₃₂ is addressed. Mössbauer spectroscopy is confirmed as a powerful technique to investigate local cationic order/disorder in ball-milled colusites.

Structural Properties and Thermoelectric Performance of the Double-Filled Skutterudite (Sm,Gd)y(FexNi1-x)4Sb12

The structural and thermoelectric properties of the filled skutterudite (Sm,Gd)_y(Fe_xNi_1-x)₄Sb₁₂ were investigated and critically compared to the ones in the Sm-containing system with the aim of unravelling the effect of double filling on filling fraction and thermal conductivity. Several samples (x = 0.50–0.90 and y = 0.15–0.48) were prepared by melting-sintering, and two of them were densified by spark plasma sintering in order to study their thermoelectric features. The crystallographic study enables the recognition of the role of the filler size in ruling the filling fraction and the compositional location of the p/n crossover: It has been found that the former lowers and the latter moves toward lower x values with the reduction of the filler ionic size, as a consequence of the progressively weaker interaction of the filler with the Sb₁₂ cavity. The analysis of thermoelectric properties indicates that, despite the Sm³⁺/Gd³⁺ small mass difference, the contemporary presence of these ions in the 2a site significantly affects the thermal conductivity of both p- and n-compositions. This occurs by reducing its value with respect to the Sm-filled compound at each temperature considered, and making the overall thermoelectric performance of the system comparable to several multi-filled (Fe, Ni)-based skutterudites described in the literature.

Thermoelectric properties of the tetrahedrite-tennantite solid solutions Cu12Sb4-xAsxS13 and Cu10Co2Sb4-yAsyS13 (0 ≤ x, y ≤ 4)

Tetrahedrites, a class of copper- and sulfur-rich minerals, exhibit inherently very low lattice thermal conductivity and adjustable electronic properties that make them interesting candidates for thermoelectric applications. Here, we investigate the influence of isovalent As substitution on the Sb site on the structural and transport properties (5-700 K) of the two solid solutions Cu12Sb4-xAsxS13 and Cu10Co2Sb4-yAsyS13 (0 ≤ x, y ≤ 4). Electronic band structure calculations predict that As has only a weak influence on the valence bands and hence, on the p-type metallic character of Cu12Sb4S13. In agreement with these predictions, all the samples of the series Cu12Sb4-xAsxS13 exhibit p-type metallic behavior with relatively low electrical resistivity and moderate thermopower values that only slightly evolve with the As content. In contrast, the substitution of Co for Cu in As-rich samples seems less favorable as suggested by a decrease in the Co concentration with increasing the As content. This trend leads to a concomitant increase in the electrical resistivity and thermopower leaving the ZT values practically unchanged with respect to purely Cu-based samples. As a result, peak ZT values ranging between 0.60 and 0.75 are achieved at 700 K for both series. The lack of significant variations in the ZT values confirms the robustness of the thermoelectric performances of tetrahedrites with respect to variations in the Sb-to-As ratio.

Laser-based setup for simultaneous measurement of the Seebeck coefficient and electrical conductivity for bulk and thin film thermoelectrics

In this paper, an original homemade system is presented in detail for the electrical and thermoelectrical characterizations of several types of materials from bulk to thin films. This setup was built using a modulated CO₂ laser beam to probe the thermoelectric properties at different depths below the surface. It allows a simultaneous measurement of the electrical conductivity (σ) and the Seebeck coefficient (S), from room temperature up to 250 °C. A commercial sample of Bi₂Te₃ was first used to validate the Seebeck coefficient measurement. Single crystalline silicon (sc-Si) was used for the uncertainty quantification during the simultaneous measurement of the Seebeck coefficient and the electrical conductivity. At the micrometer scale, thermoelectric characterization of the mesoporous Si (50 μm thickness) was achieved and results gave very promising values (S ≈ 700 μV K^-1) for micro-thermo-generator fabrication. In the case of thin film materials, metals (copper and constantan) and oxide thin films (titanium oxide) were also characterized in the in-plane configuration in order to determine the metrology limits of our thermoelectric setup. In this case, a typical sensitivity of about 2μV K^-1 was achieved.

Synthesis and transport properties of the Te-substituted homologous compounds Pb5Bi6Se14-xTex (0 ≤ x ≤ 1.0)

The crystal structure and transport properties (2-723 K) of the homologous compound Pb5Bi6Se14 with partial substitution of Te for Se are studied by means of powder X-ray diffraction, scanning electron microscopy, electrical resistivity, thermopower, thermal conductivity and Hall effect measurements. Polycrystalline samples of Pb5Bi6Se14-xTex (0 ≤ x ≤ 1.0) were prepared by a two-step synthesis method based on the pseudo-binary PbSe-Bi2Se3 phase diagram combined with Te substitution in the PbSe precursor. The successful insertion of Te into the crystal structure of Pb5Bi6Se14 was confirmed by powder X-ray diffraction and scanning electron microscopy. Transport property measurements indicate an increase in the heavily doped character of the transport with increasing the Te concentration. The extremely low lattice thermal conductivity values (0.3-0.4 W m-1 K-1 at 723 K) that approach the glassy limit at high temperatures are nearly independent of the chemical composition suggesting no influence on point-defect scattering mechanisms in the substituted compounds. Despite the inherent complexity of this system, the evolution of the electronic properties with x is well described by a simple single-parabolic band model. Because the increase in the power factor with increasing x is compensated by the concomitant increase in the electronic thermal conductivity, this substitution does not yield enhanced ZT values with respect to the pristine compound with a similar peak ZT value of 0.5 achieved at 723 K. Nevertheless, the simple synthetic method used in this study to insert a doping element opens new avenues for controlling the transport properties of the homologous series (PbSe)5(Bi2Se3)3m (m = 1, 2 and 3).

Phonon Scattering and Electron Doping by 2D Structural Defects in In/ZnO

In/ZnO bulk compounds have been synthesized using a simple solid-state process. In this study, both the structural features and thermoelectric properties of the Zn_1-xIn_xO series with ultralow indium content (0 ≤ x ≤ 0.02) have been studied. High-angle annular dark-field scanning transmission electron microscopy analyses highlight that indium has the ability to create multiple basal plane and pyramidal defects that produce ZnO domains with inverted polarity starting from dopant concentrations as low as 0.25 atom %. Interestingly, the formation of parallel inversion boundaries consisting of InO₆ octahedra in the ZnO₄ tetrahedra matrix is responsible for phonon scattering while increasing electrical conductivity, thereby enhancing the thermoelectric properties. This effect of multiple extended two-dimensional defects on the thermoelectric properties of ZnO is reported for the first time with such low indium doping. On the chemistry side, the present results point toward a lack of In solubility in the ZnO structure. Moreover, this study is a step forward to the synthesis of other thermoelectric compounds where dopant-induced planar defects in bulk transition metal compounds have the potential to enhance both phonon scattering and electronic conductivity.

High-Performance Thermoelectric Bulk Colusite by Process Controlled Structural Disordering

High-performance thermoelectric bulk sulfide with the colusite structure is achieved by controlling the densification process and forming short-to-medium range structural defects. A simple and powerful way to adjust carrier concentration combined with enhanced phonon scattering through point defects and disordered regions is described. By combining experiments with band structure and phonons calculations, we elucidate, for the first time, the underlying mechanism at the origin of intrinsically low thermal conductivity in colusite samples as well as the effect of S vacancies and antisite defects on the carrier concentration. Our approach provides a controlled and scalable method to engineer high power factors and remarkable figures of merit near the unity in complex bulk sulfide such as Cu₂₆V₂Sn₆S₃₂ colusites.

Crystal Structure and Transport Properties of the Homologous Compounds (PbSe)5(Bi2Se3)3m (m = 2, 3)

We report on a detailed investigation of the crystal structure and transport properties in a broad temperature range (2-723 K) of the homologous compounds (PbSe)₅(Bi₂Se₃)_3m for m = 2, 3. Single-crystal X-ray diffraction data indicate that the m = 2, 3 compounds crystallize in the monoclinic space groups C2/m (No. 12) and P2₁/m (No. 11), respectively. In agreement with diffraction data, high-resolution transmission electron microscopy analyses carried out on single crystals show that the three-dimensional crystal structures are built from alternating Pb-Se and m Bi-Se layers stacked along the a axis in both compounds. Scanning electron microcopy and electron-probe microanalyses reveal deviations from the nominal stoichiometry, suggesting a domain of existence in the pseudo binary phase diagram at 873 K. The complex atomic-scale structures of these compounds lead to very low lattice thermal conductivities κ_L that approach the glassy limit at high temperatures. A comparison of the κ_L values across this series unveiled an unexpected increase with increasing m from m = 1 to m = 3, in contrast to the expectation that increasing the structural complexity should tend to lower the thermal transport. This result points to a decisive role played by the Pb-Se/Bi-Se interfaces in limiting κ_L in this series. Both compounds behave as heavily doped n-type semiconductors with relatively low electrical resistivity and thermopower values. As a result, moderate peak ZT values of 0.25 and 0.20 at 700 K were achieved in the m = 2, 3 compounds, respectively. The inherent poor ability of these structures to conduct heat suggests that these homologous compounds may show interesting thermoelectric properties when properly optimized by extrinsic dopants.

Thermoelectric properties of double-substituted tetrahedrites Cu12−xCoxSb4−yTeyS13

This work reports on a detailed study of the thermoelectric properties of the double-substituted tetrahedrites Cu_12−xCo_xSb_4−yTe_yS₁₃ in a wide range of temperatures (5–700 K) with a peak ZT of 0.8 at 700 K.