Thermoelectric Compounds Research Articles

Cu-doping boosts the thermoelectric properties of layered compound LaOBiS2 with weak anisotropy

Layered compounds like Bi2Te3 and SnSe are attracting great interest for applications in thermoelectric devices, mainly due to their low lattice thermal conductivity from large anharmonicity. Herein, we report the thermoelectric properties of the quaternary layered compound LaOBiS2, which is a n-type semiconductor constructed by stacking of fluorite-type [LaO]+ layers and rock-salt-type [BiS2]- layers. Our results reveal that the as-sintered LaOBiS2 sample features weak anisotropy due to the almost random arrangement of crystalline grains with nonpreferred-orientation growth. The pristine LaOBiS2 has high intrinsic electrical conductivity and low Seebeck coefficient as a result of a high electron concentration (n = 3.25 ×1020 cm−3, 300 K) from anionic vacancy. Meanwhile, the total thermal conductivity of LaOBiS2 is as low as 1.10 W m−1 K−1 at 873 K. Cu-doping improve the overall thermoelectric properties, which induces a decrease in both electrical conductivity and thermal conductivity. The maximum zT is 0.24 at 873 K for the sample with dopant Cu of 1 %, indicating a 33 % increase compared with pristine LaOBiS2. Our work demonstrates that constructing semiconductors by functional building blocks is an effective approach to design thermoelectric compounds.

Electronic Transport Properties of Bi<sub>2</sub>Te<sub>2.7</sub>Se<sub>0.3</sub> Fabricated by Hot Extrusion

Herein we report the optimized processing conditions of hot extrusion for fabricating an <i>n</i>-type Bi<sub>2</sub>Te<sub>2.7</sub>Se<sub>0.3</sub> thermoelectric compound, with high electronic transport properties as well as improved mechanical reliability. We fabricated a Bi<sub>2</sub>Te<sub>2.7</sub>Se<sub>0.3</sub> extrudate that was 3.8 mm in diameter and 700 mm in length by controlling the processing parameters of temperature and pressure. A 3-point bending strength of over 70 MPa, which is 7 times higher that of the commercial zone melting ingot, was obtained in the samples prepared at 460 <sup>o</sup>C temperature under 6–6.5 MPa pressure. The samples benefitted from the formation of a highly-dense microstructure (relative density > 98%). It is noted that the electronic transport properties (electrical conductivity and Seebeck coefficient) could be manipulated by controlling the applied pressure of hot extrusion at 460 <sup>o</sup>C, mainly due to the change in the characteristics of the 00<i>l</i> crystal orientation, which originated from grain rotation and rearrangement. Power factor values of ~2.9 mW/mK<sup>2</sup> at 300 K and ~2.95 mW/mK<sup>2</sup> at 320 K, similar to those of sintered bulks, were obtained in the hot extrudate fabricated under processing parameters of 460 <sup>o</sup>C and 6 MPa. Moreover, a high power factor value of 2.25 mW/mK<sup>2</sup> was observed even at the high temperature of 480 K, which is 70% higher than that of Bi<sub>2</sub>Te<sub>2.7</sub>Se<sub>0.3</sub> bulk fabricated by hot pressing.

Thermoelectric properties of ductile Ag2S0.7Te0.3 prepared via zone melting method

Recently, the discovery of ductility inorganic semiconductors Ag2S-based alloys have drawn significant interest in hetero-shaped thermoelectric applications. In this work, Ag2S0.7Te0.3 inorganic thermoelectric compound was prepared using the zone melting technique. The crystal structure of Ag2S0.7Te0.3 is similar to that of the cubic-structured Ag2S compound, in which S and Te atoms are randomly occupied at the anionic sites. The observed high hall carrier mobility of ∼410 cm2V−1s−1 and intrinsic low lattice thermal conductivity of ∼0.2 Wm−1K−1 lead to a maximum zT of 0.30 at 600 K for Ag2S0.7Te0.3. In addition, its mechanical properties including microhardness Hv and fracture toughness Kc are measured, showing great potential for thermoelectric application.

High-ZT Due to the Influence of Copper in Ti(Ni1-xCux)Sn.

Most high-performance thermoelectric materials require either expensive, rare, or toxic elements. By doping TiNiSn, a low-cost, abundant thermoelectric compound, with copper as an n-type donor, some optimization can be performed for such materials. Ti(Ni1-xCux)Sn was synthesized by arc melting followed by heat treatment and hot pressing. The resulting material was analyzed for its phases using XRD and SEM and its transport properties. Cu undoped and 0.05/0.1% doped samples showed no additional phases in addition to the matrix half-Heusler phase, while the 1% copper doping initiated some Ti6Sn5 and Ti5Sn3 precipitation. The transport properties showed that copper acts as an n-type donor while also lowing the lattice thermal conductivity of the materials. the sample containing 0.1% copper showed the best figure of merit, ZT, with a maximal value of 0.75 and an average value of 0.5 through 325-750 K showing a 125% improvement over the undoped sample of TiNiSn.

Phonon transport in Cu2GeSe3 : Effects of spin-orbit coupling and higher-order phonon-phonon scattering

Much attention has been focused on understanding the mechanism of low lattice thermal conductivity in Cu-based diamondlike thermoelectric compounds. For ${\mathrm{Cu}}_{2}\mathrm{Ge}{\mathrm{Se}}_{3}$, the underlying origin of low lattice thermal conductivity remains to be clarified. In this work, the first-principles calculations are employed to systematically investigate the effects of spin-orbit coupling, higher-order phonon-phonon scattering, phonon wavelike tunneling, temperature-induced renormalization of phonon and phonon-phonon interaction, and volumetric expanse on the phonon transport of ${\mathrm{Cu}}_{2}\mathrm{Ge}{\mathrm{Se}}_{3}$. We show that the spin-orbit coupling results in no detectable change on the phonon frequencies, compared with those obtained without spin-orbit coupling, while it induces slight increase in both Gr\"uneisen parameter and lattice thermal conductivity of ${\mathrm{Cu}}_{2}\mathrm{Ge}{\mathrm{Se}}_{3}$, and the underlying mechanism is thoroughly analyzed. With the fourth-order phonon scattering and temperature-induced renormalization, the calculated lattice thermal conductivities are well consistent with the experimental results. Due to the enhanced four-phonon scattering process, there are remarkable reductions for the thermal conductivity. In ${\mathrm{Cu}}_{2}\mathrm{Ge}{\mathrm{Se}}_{3}$, the coherences term ${\ensuremath{\kappa}}_{c}$ contributes increasingly to the total lattice thermal conductivity when temperature arises. And after considering the effects of temperature-induced renormalization and higher-order phonon-phonon interactions, the ${\ensuremath{\kappa}}_{c}$ would provide $\ensuremath{\sim}5%$ and above 25% of total conductivity at 300 and 800 K, respectively. Our finding clarifies the mechanism of low thermal conductivity in ${\mathrm{Cu}}_{2}\mathrm{Ge}{\mathrm{Se}}_{3}$, and benefits the design of similar Cu-based diamondlike materials in thermoelectric applications.

High-temperature oxidation mechanism of ZrCoSb-based half-Heusler thermoelectric compounds

ZrCoSb-based half-Heusler (HH) compounds are among the most promising thermoelectric (TE) materials for high-temperature power generation. Oxidation resistance is one of the key issues for realizing the practical application of TE materials for long-term service in the ambient working environment. In this work, the oxidation behavior of Zr0.5Hf0.5CoSb0.8Sn0.2 (ZHCSS) half-Heusler is systematically studied in the service temperature range from 873 to 1073 K. It is revealed that three typical layers of oxidation products tend to form on the surface of HH sample, namely, the dense oxide layer (DOL) composed of (Zr,Hf)O2 and CoSb, the alternate oxide layer (AOL) composed of repeated (Zr,Hf)O2 and CoSb2O6/Co3O4, and the CoSb layer between the DOL and AOL. The mass gain during oxidation is mainly caused by the rapid growth of AOL, which is controlled by the outward diffusion of Zr/Hf and the inward diffusion of oxygen. The formation of a continuous CoSb layer and DOL is found beneficial to block the outward diffusion of Zr/Hf. Based on the analysis of the dominant factors on the outward and inward diffusions as well as the reaction activation energy, a simple approach is proposed to improve the oxidation resistance of Zr0.5Hf0.5CoSb0.8Sn0.2 by pre-oxidizing the sample in low oxygen pressure to form the dense (Zr,Hf)O2 and CoSb layers as oxidation protecting and/or diffusion blocking layers. The oxidation test shows the effectiveness of such pre-oxidation on the formation and growth of the AOL and therefore on improving the service stability of Zr0.5Hf0.5CoSb0.8Sn0.2 at high temperatures in the air.

Synergistic modulation of electrical and thermal transport toward promising n-type MgOCuSbSe2 thermoelectric performance by MO-intercalated CuSbSe2.

The layered ternary CuSbSe2 semiconductor with ultralow thermal conductivity is particularly suitable for thermoelectric applications. Nevertheless, its poor electrical conductivity greatly lowers the dimensionless figure of merit ZT and accordingly limits its thermoelectric applications. Here, we use first-principles calculations combined with semi-classical Boltzmann transport theory to evaluate the thermoelectric properties of MO-intercalated (M = Mg, Ca, Sr, and Ba) CuSbSe2. Compared with CuSbSe2, MO-intercalated CuSbSe2 semiconductors, as a new class of semiconductors, host distorted lattices with low symmetry monoclinic structures. Such a structure feature provides desired channels for electron transport between adjacent layers and accordingly enhances electrical transport properties. Meanwhile, the MO intercalation effectively softens phonons and gives rise to an ultralow lattice thermal conductivity in MOCuSbSe2. These synergistically yield a high figure of merit ZT of ∼4.17 for MgO-intercalated CuSbSe2 at 200 K with electron doping being n = 1018 cm-3. Our study provides an effective route to improve the thermoelectric performance of layered CuSbSe2 by designing new multicomponent thermoelectric compounds with alternatively stacked [CuSbSe2] (electronic conduction units) and [MO] (electronic insulation units) layers. The approach can be extended to similar chalcostibite compounds for screening and designing thermoelectric materials.

Atomic site-targeted doping in Ti2FeNiSb2 double half-Heusler alloys: zT improvement via selective band engineering and point defect scattering

Ti2FeNiSb2 is a promising double half-Heusler thermoelectric compound with intrinsically low thermal conductivity due to low phonon group velocity. Since it is introduced in 2019, many efforts have been focused on further reducing its thermal conductivity via doping. However, the effects of doping on its electronic transport properties have been neglected. Here, we investigate the effects of doping Co and Bi at the Fe- and Sb-sites, respectively, in Ti2FeNiSb2 for the first time. Changes in band parameters due to atomic site-targeted doping are estimated by the Single Parabolic Band model. Bypassing the trade-off relation between the Seebeck coefficient and electrical conductivity is observed when doping Co at Fe-sites. The physics behind the bypass is explained in terms of temperature-dependent reduced chemical potential and non-degenerate mobility. As a result, peak figure of merit zT values of ∼0.69 in Ti2Fe0.9Co0.1NiSb2 is achieved, which is approximately six times higher than that of the pristine Ti2FeNiSb2. The thermoelectric performance of Ti2FeNiSb2 can be improved by selective band engineering to bypass the Seebeck coefficient-electrical conductivity trade-off relation from atomic-site targeted doping.

Effects of Bi Alloying on the Thermoelectric Properties of p‐Type Mg3Sb2‐xBix Solid Solutions

AbstractBismuth is the most common alloying element for high‐performance Mg3Sb2‐based thermoelectric compounds. However, the electrical and thermal transport data is lacking for undoped Mg3(Sb, Bi)2 solid solutions. In this study, theoretical calculation results show that Bi alloying can reduce the band gap of Mg3Sb2, favoring the regulation of carrier concentration. Both Mg3Sb2 and Mg3SbBi compounds present a single band structure on the valence band maximum at Γ of the Brillouin zone. Incorporating the Bi element can slightly increase the density of states (DOS) on the valence band maximum, which is beneficial for the Seebeck coefficient. Mg3Sb2‐xBix (0≤x≤0.8) compounds were experimentally prepared via ball milling (BM) combined with the spark plasma sintering (SPS) technique. The carrier concentration, electrical conductivity, and power factor of the samples tend to increase with the increasing Bi content. Bismuth alloying significantly reduces the total thermal conductivity in the low‐temperature range, with the lowest value of 0.65 Wm−1K−1 obtained at ∼400 K for the x=0.2 sample. The peak ZT value of 0.077 at 800 K and the average ZT of 0.051 in the temperature range of 450–800 K are obtained for the x=0.2 sample, which is 93% and 219% higher than those of the unalloyed sample, respectively. This work reveals the Bi‐alloying induced transport mechanism and provides fundamental thermoelectric data regarding the undoped Mg3Sb2‐xBix (0≤x≤0.8) solid solutions.

NaCdSb: An Orthorhombic Zintl Phase with Exceptional Intrinsic Thermoelectric Performance

AbstractMany 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.

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.

Evolution of defect structures leading to high ZT in GeTe-based thermoelectric materials

GeTe is a promising mid-temperature thermoelectric compound but inevitably contains excessive Ge vacancies hindering its performance maximization. This work reveals that significant enhancement in the dimensionless figure of merit (ZT) could be realized by defect structure engineering from point defects to line and plane defects of Ge vacancies. The evolved defects including dislocations and nanodomains enhance phonon scattering to reduce lattice thermal conductivity in GeTe. The accumulation of cationic vacancies toward the formation of dislocations and planar defects weakens the scattering against electronic carriers, securing the carrier mobility and power factor. This synergistic effect on electronic and thermal transport properties remarkably increases the quality factor. As a result, a maximum ZT > 2.3 at 648 K and a record-high average ZT (300-798 K) were obtained for Bi0.07Ge0.90Te in lead-free GeTe-based compounds. This work demonstrates an important strategy for maximizing the thermoelectric performance of GeTe-based materials by engineering the defect structures, which could also be applied to other thermoelectric materials.

Realizing high-ranged thermoelectric performance in PbSnS2 crystals

Great progress has been achieved in p-type SnS thermoelectric compound recently, while the stagnation of the n-type counterpart hinders the construction of thermoelectric devices. Herein, n-type sulfide PbSnS2 with isostructural to SnS is obtained through Pb alloying and achieves a maximum ZT of ~1.2 and an average ZT of ~0.75 within 300–773 K, which originates from enhanced power factor and intrinsically ultralow thermal conductivity. Combining the optimized carrier concentration by Cl doping and enlarged Seebeck coefficient through activating multiple conduction bands evolutions with temperature, favorable power factors are maintained. Besides, the electron doping stabilizes the phase of PbSnS2 and the complex-crystal-structure induced strong anharmonicity results in ultralow lattice thermal conductivity. Moreover, a maximum power generation efficiency of ~2.7% can be acquired in a single-leg device. Our study develops a n-type sulfide PbSnS2 with high performance, which is a potential candidate to match the excellent p-type SnS.

Transport properties of (AgSbТe2)0.7(PbTe)0.3 thermoelectric compound

Transport properties, namely electrical conductivity, Seebeck and Hall coefficients, and thermal conductivity, were measured from 80 to 560 K. The phase transition at about 410 K, representing the phase transition from α-Ag2Te to β-Ag2Te, influences the electrical transport properties. The temperature dependence of Hall coefficient passes through a maximum ∼200 K and has a negative sign. It is shown that these peculiarities are due to the presence of Ag2Te phase.

Ni/SnSe2 interfacial reactions and Ni–Se–Sn phase equilibria

SnSe2 is a promising thermoelectric compound. Ni is a commonly used barrier layer material. To provide fundamental information, Ni–Se–Sn phase equilibria isothermal sections and Ni/SnSe2 interfacial reactions at 500 °C and 300 °C are determined. Significant and peculiar interfacial reaction results are observed. The reaction zone has several phase regions, and within each phase region there are various phases, and yet these regions have clear boundaries with different average compositions. In the Ni/SnSe2 couples reacted at 500 °C, there are three phase regions with average compositions equal to Ni5SnSe2, Ni3SnSe2 and NiSnSe2. In those reacted at 300 °C, there are four phase regions with average compositions of Ni5SnSe2, Ni3SnSe2, Ni2SnSe2 and NiSnSe2. The significant interfacial reactions indicate Ni is not a suitable barrier layer for SnSe2. The reaction phase regions all have planar interfaces and fine microstructures, but they are not single-phase. The experimental results show the microstructures are unaffected by the quenching rates and become coarser with longer annealing time. These results indicate the different phases within each phase region are not formed during quenching. The mechanisms of the peculiar phenomena need to be further explored. Yet, preliminary speculation is metastable ternary phases are formed with the rapid diffusion of Ni atoms into the SnSe2 substrate. The reaction phase regions grow with the formation of the metastable ternary phases, and the thin metastable ternary phases decompose into stable phases with fine microstructures.

Improved Solubility in Metavalently Bonded Solid Leads to Band Alignment, Ultralow Thermal Conductivity, and High Thermoelectric Performance in SnTe

AbstractSnTe is an emerging Pb‐free thermoelectric compound that has drawn significant attention for clean energy conversion. Chemical doping is routinely used to tailor its charge carrier concentration and electronic band structures. However, the efficacy of dopants is often limited by their small solubility. For example, only 0.5% Ag can be incorporated into the SnTe matrix. Yet, significantly more Ag (>7%) can be dissolved if SnTe is alloyed with AgSbTe2. This large enhancement of solubility can be understood from a chemical bonding perspective. Both SnTe and AgSbTe2 employ metavalent bonding as identified by an unusual bond‐rupture in atom probe tomography. Density functional theory calculations show that upon Ag doping the energy offset of the upmost two valence bands decreases significantly. This induces band alignment in SnTe, which results in an enhanced power factor over a broad temperature range. Moreover, the increased concentration of point defects and associated lattice strain lead to strong phonon scattering and softening, contributing to an extremely low κL of 0.30 Wm−1K−1. These synergistic effects contribute to a peak ZT of 1.8 at 873 K and a record‐high average ZT of ≈1.0 between 400 and 873 K in Sn0.87Mn0.08Sb0.08Te–5%AgSbTe2 alloy.

Co/SnSe2 and (Co,Ni)/SnSe2 interfacial reactions

BackgroundSnSe2 is a promising thermoelectric compound. Co, Ni and (Co,Ni) are barrier layer materials. Interfacial reactions are examined since they are very important for the reliability of the thermoelectric modules. MethodsCo/SnSe2 and (Co, Ni)/SnSe2 reaction couples are prepared and examined. Their reaction paths are illustrated with the Co-Se-Sn phase equilibria isothermal section and Co-Ni-Se-Sn phase equilibria isothermal tetrahedron, respectively. Significant FindingsAt both 500 °C and 300 °C, CoSe and SnSe phases are formed in the Co/SnSe2 couples. The reaction path is Co/CoSe/SnSe/SnSe2. In the (Co,Ni)/SnSe2 couples reacted at 500 °C, there are three reaction phase regions. The microstructures of these phase regions are very fine. Based on their average compositions, these three phase regions are regarded as Ni3Se2, Ni3SnSe2 and NiSnSe2 phases. But the Ni3SnSe2 and NiSnSe2 phase regions are not single-phase regions and are composed of (Co,Ni)Se+Ni3Sn2 and NiSe+SnSe phases, respectively. It is concluded among the four elements, Ni is the fastest diffusion species, Sn is the slowest, whilst Co and Se have similar diffusion rates. The results indicate Ni is not a suitable barrier layer for SnSe2 because the interfacial reactions are more significant with more Ni.

Advances and Challenges of AgSbSe2‐based Thermoelectric Materials

AbstractThermoelectric materials have aroused wide attention because of the capability to directly convert heat into electricity. AgSbSe2 is a structural analogue of PbTe but does not contain toxic element Pb or expensive element Te. Besides, it possesses both high Seebeck coefficient and inherently low thermal conductivity, making AgSbSe2 a competitive candidate for mid‐temperature thermoelectric power generation. This review summarizes the most recent updates of AgSbSe2‐based thermoelectric compounds. It starts with a general introduction of the crystal and electronic band structures of pristine AgSbSe2 with particular emphasis on the debate about whether cations arrangement is ordered or disordered. We then discuss why AgSbSe2 displays glass‐like heat conduction despite its crystalline nature. Subsequently, the strategies of boosting the electrical properties of the titled compound are elaborated. Finally, we point out the challenges and outlooks toward the future development of AgSbSe2‐based thermoelectric materials.

Strong anharmonicity induced low lattice thermal conductivity and high thermoelectric performance in (CuInTe2)1−x(AgSbTe2)x system

The suppression of lattice thermal conductivity of thermoelectric CuInTe2 was often realized by complex micrometer-structure control. Herein, we demonstrate that simple alloying AgSbTe2 in CuInTe2 is capable of strongly strengthening lattice anharmonicity, which gives rise to an extremely low lattice thermal conductivity of ∼ 0.62 W m−1 K−1 at 823 K for (CuInTe2)0.95(AgSbTe2)0.05. In addition, the intentionally introduced copper vacancies can effectively increase the carrier concentration and dramatically improve the electrical performance of (CuInTe2)0.95(AgSbTe2)0.05. As a result, the sample with nominal composition of (Cu0.95InTe2)0.95(AgSbTe2)0.05 reaches a maximum zT value of ∼ 1.3 at 823 K. This study provides some insight into designing thermoelectric compounds in the diamond-like semiconductor family by inducing strong anharmonicity through a small amount of alloying.

Chemical trends in the high thermoelectric performance of the pyrite-type dichalcogenides ZnS2, CdS2 , and CdSe2

The thermoelectric (TE) properties of the three pyrite-type IIB-${\mathrm{VIA}}_{2}$ dichalcogenides (${\mathrm{ZnS}}_{2}, {\mathrm{CdS}}_{2}$, and ${\mathrm{CdSe}}_{2}$) are systematically investigated and compared with those of the prototype ${\mathrm{ZnSe}}_{2}$ to optimize their TE properties. Using the phonon Boltzmann transport equation, we find that they all have ultralow lattice thermal conductivities. By analyzing their vibrational properties, this is attributed to soft phonon modes derived from the loosely bound rattlinglike metal atoms and to strong anharmonicity caused by the vibrations of all atoms perpendicular to the strongly bound nonmetallic dimers. Additionally, by correlating those properties along the series, we elucidate several chemical trends. We find that the heavier atom masses, larger atomic displacement parameters, and longer bond lengths between metal and nonmetal atoms can be beneficial to the looser rattling behaviors of the metal atoms and therefore lead to the softer phonon modes and that the stronger nonmetallic dimer bonds can boost the anharmonicities, both leading to the lower thermal conductivities. Furthermore, we find that all three compounds have complex energy isosurfaces at valence and conduction band edges that simultaneously allow for large density-of-states effective masses and small conductivity effective masses for both $p$- and $n$-type carriers. Consequently, the calculated TE figures of merit ($ZT$) can reach large values for both $p$- and $n$-type doping. In this paper, we illustrate the effects of the rattlinglike metal atoms and the localized nonmetallic dimers on the thermal transport properties and the importance of different carrier effective masses to electrical transport properties in these pyrite-type dichalcogenides, which can be used to predict and optimize the TE properties of other TE compounds in the future.