Low Lattice Thermal Conductivity Research Articles

Cu1.5SeyTe1-y (y = 0.2-0.7): A Series of Narrow Band Gap Semiconductors with Low Thermal Conductivity at Ambient Temperature.

Coinage metal chalcogenides offer ideal prerequisites for high thermoelectric performance and sensor applications, with their usually low lattice thermal and high electrical conductivity, as well as small band gaps. In the solid solution Cu1.5SeyTe1-y we synthesized phase pure materials with y = 0.2-0.7 and characterized them concerning selected physical properties. X-ray crystal structure determination was performed for two representatives of the solid solution, Cu1.5Se0.3Te0.7 and Cu1.5Se0.5Te0.5. The entire series crystallizes cubically, in space group Pm3̅n. No structural changes are observed between room temperature and the synthesis temperature of 723 K. The conductivity measurements and Seebeck coefficients of Cu1.5Se0.3Te0.7 and Cu1.5Se0.5Te0.5 indicate that the two representatives are narrow band gap semiconductors (Eg 0.06-0.08 eV). Both compounds show positive Seebeck coefficients and reasonably low thermal conductivities at moderate temperatures. Cu1.5Se0.5Te0.5 is characterized by a bulk modulus of 40.9 GPa.

Enhancing thermoelectric performance in P-type Mg3Sb2-based Zintls through optimization of band gap structure and nanostructuring

P-type Mg3Sb2-based Zintls have attracted considerable interest in the thermoelectric (TE) field due to their environmental friendliness and low cost. However, compared to their n-type counterparts, they show relatively low TE performance, limiting their application in TE devices. In this work, we simultaneously introduce Bi alloying at Sb sites and Ag doping at Mg sites into the Mg3Sb2 to cooperatively optimize the electrical and thermal properties for the first time, acquiring the highest ZT value of ∼0.85 at 723 K and a high average ZT of 0.39 in the temperature range of 323–723 K in sample Mg2.94Ag0.06Sb1.9Bi0.1. The first-principle calculations show that the co-doping of Ag and Bi can shift the Fermi level into the valence band and narrow the band gap, resulting in the increased carrier concentration from 3.50 × 1017 cm−3 in the reference Mg3Sb0.9Bi0.1 to ∼7.88 × 1019 cm−3 in sample Mg2.94Ag0.06Sb0.9Bi0.1. As a result, a remarkable power factor of ∼778.9 µW m−1 K−2 at 723 K is achieved in sample Mg2.94Ag0.06Sb0.9Bi0.1. Meanwhile, a low lattice thermal conductivity of ∼0.48 W m−1 K−1 at 723 K is also obtained with the help of phonon scattering at the distorted lattice, point defects, and nano-precipitates in sample Mg2.94Ag0.06Sb0.9Bi0.1. The synergistic effect of using the multi-element co-doping/-alloying to optimize electrical properties in Mg3Sb2 holds promise for further improving the TE performance of Zintl phase materials or even others.

AgCl Addition to Chalcopyrite Compound for Ultra-Low Thermal Conductivity in Realizing High ZT Thermoelectric Materials.

Optimizing the performance of thermoelectric materials by reducing its thermal conductivity is crucial to enhance its thermoelectric efficiency. Novel thermoelectric materials like the CuGaTe2 compound are hindered by high intrinsic thermal conductivity, which negatively impacts its thermoelectric performance. In this paper, we report that the introduction of AgCl by the solid-phase melting method will influence the thermal conductivity of CuGaTe2. The generated multiple scattering mechanisms are expected to reduce the lattice thermal conductivity while maintaining sufficient good electrical properties. The experimental results were supported by first-principles calculations confirming that the doping of the Ag will decrease the elastic constants, bulk modulus, and shear modulus of CuGaTe2, which makes the mean sound velocity and Debye temperature of Ag-doped samples lower than those of CuGaTe2, indicating the lower lattice thermal conductivity. In addition, the Cl elements within the CuGaTe2 matrix escaping during the sintering process will create holes of various sizes within the sample. These combined effects of holes and impurities will induce phonon scattering, which further reduces the lattice thermal conductivity. Based on our findings, we conclude that the introduction of AgCl into CuGaTe2 has shown a lower thermal conductivity without compromising the electrical performance, resulting in an ultra-high ZT value of 1.4 in the (CuGaTe2)0.96(AgCl)0.04 sample at 823 K.

Study on the thermoelectric characteristics of the ternary alkali metal compound XMgSb (X = K, Rb, Cs)

We conducted a comprehensive analysis of the mechanical, transport, and thermoelectric (TE) characteristics of XMgSb (X = K, Rb, Cs) alkali metal compounds using first-principles calculations, self-consistent phonon theory, pressive sensing lattice dynamics, and the Boltzmann transport equation. Our results show that the very low lattice thermal conductivity of these materials is mainly attributed to the strong anharmonicity of the alkali metal atoms, and the lattice thermal conductivity is negatively correlated with the mass of X atoms. Furthermore, we consider five scattering mechanisms to accurately calculate transport properties and electron relaxation times. Notably, we observed that the large electronic dispersion band near the valence band maximum leads to the high conductivity of these three compounds, with a flat electronic energy band at the Γ-Z point, which generally implies a larger Seebeck coefficient. The coexistence of good electronic band dispersion and flat electronic bands leads to high conductivity and large Seebeck coefficient, resulting in high TE power. Low lattice thermal conductivity and high TE power factor jointly determine the development potential of alkali metal compound XMgSb (X = K, Rb, Cs) in the field of TE.

Electrical and Thermal Transport Properties of Ge1–xPbxCuySbyTeSe2y

AbstractBalancing the contradictory relationship between thermoelectric parameters, such as effective mass and carrier mobility, is a challenge to optimize thermoelectric performance. Herein, the exceptional thermoelectric performance is realized in GeTe through collaboratively optimizing the carrier and phonon transport via stepwise alloying Pb and CuSbSe2. The formation energy of Ge vacancy is efficiently bolstered by alloying Pb, which reduces carrier density and carrier scattering to maintain superior carrier mobility in GeTe. Additionally, CuSbSe2, acting as an n‐type dopant, further modulates carrier density and validly equilibrates carrier mobility and effective mass. Accordingly, the promising power factor of 45 µW cm−1 K−2 is achieved at 723 K. Meanwhile, point defects are found to significantly suppress phonons transport to descend lattice thermal conductivity by Pb and CuSbSe2 alloying, which barely impacts the carrier mobility. A combination with superior carrier mobility and lower lattice thermal conductivity, a maximum ZT of 2.2 is attained in Ge0.925Pb0.075Cu0.005Sb0.005TeSe0.01, which corresponds to a 100% promotion compared with that of intrinsic GeTe. This study provides a new indicator for optimizing carrier and phonon transport properties by balancing interrelated thermoelectric parameters.

Two-channel thermal transport and scattering channel of high-temperature phase SnSe using temperature-dependent effective potential

To resolve the imaginary phonon frequency of high-temperature phase SnSe (853 K) caused by ground-state theory, the temperature-dependent effective potential (TDEP) method is used to evaluate the temperature-dependent interatomic force constants (IFCs). Based on the Boltzmann transport equation, both two-channel transport and scattering channel are studied using the temperature-dependent IFCs. Considering three-phonon and four-phonon scattering, the particle-like thermal conductivity along the xx axis, yy axis, and zz axis is 0.66, 0.34, and 0.61 W/mK, respectively. For glass-like thermal conductivity, the value along the xx axis, yy axis, zz axis are 0.0777, 0.0316, and 0.0791 W/mK, respectively. For the three-phonon scattering channel, the scattering channels of X + O/A→O and X→O+O/A play a major role. For four-phonon scattering channel, the major scattering channels are X + O+A→O, X + A+A→O, X + O-O/A→A, X + A-O→A, X-O/A-O/A→O, X-O-A→A. By utilizing the crystal orbital Hamilton population (COHP) analysis, the low lattice thermal conductivity is attributed to the weak chemical bonds from anti-bonding orbitals between Se4p and Sn5s states. This work provides new insight into the physical mechanisms for thermal transport of high-temperature phase SnSe with strong anharmonic effects.

XMoSiN2 (X = S, Se, Te): A novel 2D Janus semiconductor with ultra-high carrier mobility and excellent thermoelectric performance

Two-dimensional (2D) semiconductor is a potential candidate for thermoelectric materials due to its high Seebeck coefficient. However, its high lattice thermal conductivity limits its applications in the field of thermoelectric materials. Here, we constructed an unsymmetrical 2D Janus semiconductor XMoSiN2(X = S, Se, Te) based on to significantly reduce the lattice thermal conductivity to only one-sixth that of at 300 K. We found that XMoSiN2 had an ultra-high carrier mobility up to 4640 cm2V−1s−1 leading to a metal-like electrical conductivity. Meanwhile, XMoSiN2 reserved the high Seebeck coefficient of . The lower lattice thermal conductivity and metal-like electrical conductivity resulted in excellent thermoelectric performance. possessed a record-high ZT value of 3.57 at 900 K. We believed that other materials with a similar structure to XMoSiN2 can also be potential candidates for high-performance thermoelectric materials. Our work provides valuable insights into designing novel high-performance thermoelectric materials.

Strong anharmonicity-assisted low lattice thermal conductivities and high thermoelectric performance in double-anion Mo2 AB 2 (A = S, Se, Te; B = Cl, Br, I) semiconductors

The thermoelectric properties of layered Mo2 AB 2 (A = S, Se, Te; B = Cl, Br, I) materials are systematically investigated by first-principles approach. Soft transverse acoustic modes and direct Mo d–Mo d couplings give rise to strong anharmonicities and low lattice thermal conductivities. The double anions with distinctly different electronegativities of Mo2 AB 2 monolayers can reduce the correlation between electron transport and phonon scattering, and further benefit much to their good thermoelectric properties. Thermoelectric properties of these Mo2 AB 2 monolayers exhibit obvious anisotropies due to the direction-dependent chemical bondings and transport properties. Furthermore, their thermoelectric properties strongly depend on carrier type (n-type or p-type), carrier concentration and temperature. It is found that n-type Mo2 AB 2 monolayers can be excellent thermoelectric materials with high electric conductivity, σ, and figures of merit, ZT. Choosing the types of A and B anions of Mo2 AB 2 is an effective strategy to optimize their thermoelectric performance. These results provide rigorous understanding on thermoelectric properties of double-anions compounds and important guidance for achieving high thermoelectric performance in multi-anion compounds.

Reduced lattice thermal conductivity through tailoring of the crystallization behavior of NbCoSn by V addition

Nanostructuring is a key approach for reducing lattice thermal conductivity. Amorphous precursors can be used to obtain complex nanostructures, as their crystallization behavior can be controlled by varying their composition, temperature, and annealing time. Herein, we investigated the effects of V alloying on the crystallization behavior and resulting lattice thermal conductivity of amorphous NbCoSn alloys. After heat treatment at 783 K for 2 h, the V-containing alloy showed a significantly finer grain size (100 ± 30 nm) than the alloy without V (870 ± 360 nm) owing to the different crystallization kinetics. We revealed that Sn-rich nanoprecipitates with a diameter of 10 nm acted as heterogeneous nucleation sites for crystallization, resulting in a finer grain size in the V-containing alloy. As a result, the V-containing alloy exhibited a lower lattice thermal conductivity (2.6 ± 0.2 W m−1 K−1) than the alloy without V (3.3 ± 0.2 W m−1 K−1) at room temperature owing to increased phonon scattering at the grain boundary, point defects, and nanoprecipitates.

First‐Principles Study on Electronic and Thermal Transport Properties of FeRuTiX Quaternary Heusler Compounds (X=Si, Ge, Sn)

AbstractThe structural, electronic, thermal and lattice thermal transport properties of the three hypothetical quaternary Heusler alloys FeRuTiX (X=Si, Ge, Sn) were investigated with the aid of first‐principles calculations. All compounds were found to be semiconducting with a small indirect band gap. Flat bands near the conduction band edge and degenerate multi‐bands near the valance band edge suggest that these systems should exhibit both large Seebeck coefficients and good electrical conductivity. The analysis of the calculated vibrational spectra showed that the three compounds are thermodynamically stable. The computed lattice thermal conductivity indicates that among the three compounds that of FeRuTiSn is rather low at high temperature. Indeed, a low lattice thermal conductivity (∼3.5 Wm−1 K−1 at 1000 K) together with a small electronic band gap (0.51 eV) with an appropriate electronic structure (disperse and flat bands) render FeRuTiSn a promising candidate as a high‐temperature thermoelectric material.

Lattice thermal conductivity of two-dimensional CrB4 and MoB4 monolayers against Slack’s guideline

Two-dimensional (2D) CrB4 and MoB4 monolayers are typical graphene-like materials with excellent electrical transport properties and high structural stability. Yet, their thermal transport properties are unknown. By first-principles calculations and solving the Boltzmann transport equation iteratively, we report low lattice thermal conductivity (κl) of CrB4 and MoB4 monolayers. At room temperature, κl of CrB4 and MoB4 monolayers are 31.19 and 96.59 W/mK, respectively. This trend is against the well-known Slack’s guideline that the same type of materials with greater atomic mass generally have lower κl. The cause of this anomaly is analyzed from the first-principles harmonic and anharmonic parameters. It is surprising that no obvious correlation is observed between three-phonon scattering rates and mode Grüneisen parameters, despite their common determination by third-order interatomic force constants and one-to-one mode correspondence. The applicability of Grüneisen parameters to searching for novel materials with low κl is discussed from a theoretical viewpoint.

Screening outstanding mechanical properties and low lattice thermal conductivity using global attention graph neural network

Mechanical and thermal properties of materials are extremely important for various engineering and scientific fields such as energy conversion and energy storage. However, the characterization of these properties via high throughput screening at the quantum level, although highly accurate, is inefficient and very time- and resource-consuming. In contrast, prediction at the classical level is highly efficient but less accurate. We deploy scalable global attention graph neural network for accurate prediction of mechanical properties which bridge the gap between the accuracy at the quantum level and efficiency at the classical level. Using 10,158 elastic constants as training data, we trained the models on 5 mechanical properties, namely bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and hardness. With the trained model, we predicted 775,947 data in search of materials with ultrahigh hardness. We further verify the recommended ultrahigh hardness materials by high precision first principles calculations, and we finally identify 20 structures with extreme hardness close to diamond, the hardest material in nature. Among those, two super hard materials are completely new and have not been reported in literature so far. We further recommend potential materials from bulk modulus prediction to search low lattice thermal conductivity, and we verify the thermal conductivity of 338 structures with first principles. Our results demonstrate that one can find materials with extreme mechanical properties recommended by graph neural network and low thermal conductivity material from bulk modulus prediction with minimal first principles calculations of the structures (only 0.04%) in the large-scale materials pool.

Origin of low lattice thermal conductivity and mobility of lead-free halide double perovskites

Lead-free halide double perovskites of Cs2BiAgBr6 and Cs2BiAgCl6 have attracted extensive attention due to potential application in optoelectronic field. The transport behavior plays an important role in electronic devices based on semiconductors, which includes lattice thermal conductivity and carier mobility. Since impurities and defects may exist in the samples, it is difficult to investigate the intrinsic transport behavior by experimental measurements. The theoretical prediction of their transport behaviors can provide a guidance for the application of materials. In this work, we first develop machine learning interatomic potential (MLIP) to explore their lattice thermal conductivities. They exhibit extremely low lattice thermal conductivity at 300 K with the values of κx = 0.122 ± 0.009∕κz = 0.103 ± 0.004 Wm−1K−1 for Cs2BiAgBr6, and κx = 0.352 ± 0.014∕κz = 0.337 ± 0.012 Wm−1K−1 for Cs2BiAgCl6, respectively. It is revealed that the thermal conductivity is mainly controlled by low-frequency phonons. Subsequently, we apply semi-empirical models to predict their mobilities. The mobility is mainly determined by the scattering from longitudinal optical phonon with the values of μe,x= 22.0/μh,x= 18.8cm2V−1s−1 for Cs2BiAgBr6, and μe,x= 19.2/μh,x= 19.5cm2V−1s−1 for Cs2BiAgCl6, respectively. The stretching between Bi and Br/Cl atoms limits the mobility. Finally, the origin of low thermal conductivity and mobility on double perovskites is revealed.

Electronic structure, elastic and transport properties of new Palladium-based Half-Heusler materials for thermoelectric applications

Electronic structure with reference to bandgap energies play a significant role in deciding the dimensionless figure of merit (ZT), which in turn is used to identify the thermoelectric performance of a material. Slater Pauling arrangement of valence electrons and band orientations along k-space decide the effective mass of electrons/holes in the corresponding band valleys of PdMX (M= Sc, Y, and X = P, As, Sb) semiconductors. A detailed picture of the electronic structure and transport properties is rigorously established in this report. Calculations of elastic constant, phonon dispersion and formation (cohesive) energy reveal that these alloys are mechanically, dynamically, and chemically stable materials. Substantially, low lattice thermal conductivity is observed in the range 4.57 W/mK for PdScAs and 11.51 W/mK for PdYSb alloy. The relaxation time and effective mass are calculated with Deformation potential theory. The maximum calculated ZT value for PdScP, PdScAs, PdScSb, PdYP, PdYAs and PdYSb are 0.28, 0.33, 0.19, 0.43, 0.44 and 0.27 respectively.

First-principles calculations of thermoelectric transport properties in WSe2/SnS2 bilayer heterostructure

Recently, the experimentally fabricated van der Waals bilayer heterostructure of WSe2/SnS2 was found to possess excellent electronic and optoelectronic applications in p-n diode, photodetector and transistor (Yang et al., Nat. Commun., 2017, 8, 1906; Zhou et al., Adv. Mater., 2018, 30, 1703286). However, the thermoelectric properties have not been studied. Here, using the first-principles calculations and Boltzmann transport theory, we present a study on the thermoelectric transport properties for the WSe2/SnS2 bilayer heterostructure. The results show that the n-type power factor of WSe2/SnS2 bilayer heterostructure is greatly improved compared to that of WSe2 monolayer. The calculated phonon relaxation time and three-phonon scattering phase space indicate that the low-frequency optical branches overlapping with the acoustic modes in WSe2/SnS2 bilayer heterostructure provides more scattering channels, resulting in a lower lattice thermal conductivity, 22.69 W m−1 K−1 at room temperature, which is smaller than that of monolayer WSe2. The optimized n-type thermoelectric ZT value at 800 K for WSe2/SnS2 can reach 1.16, which indicate the bilayer heterostructure WSe2/SnS2 is predicted to be promising for thermoelectric applications.

High thermoelectric and mechanical performance in strong-textured n-type Bi2Te2.7Se0.3 by temperature gradient method

Bismuth telluride is the only commercially available thermoelectric material, but the poor n-type thermoelectric performance restricts its further development. Herein, we report the n-type Bi2Te2.7Se0.3 + x wt% PbI2 polycrystalline with both high texture degree and various microstructures by the temperature gradient method. Based on the superior carrier mobility maintained by high grain orientation, dilute PbI2 doping not only tunes the carrier concentration to an optimal level but also induces multiscale phonon scattering centers to realize a very low lattice thermal conductivity. As a result, a peak ZT of 1.26 at 350 K, a high ZTavg of 1.22 (300–400 K), and an improved Vickers hardness of 0.48 GPa are realized in Bi2Te2.7Se0.3 + 0.22 wt% PbI2 sample. This work suggests that the dilute PbI2 doping by temperature gradient method is an effective solution for preparation of n-type bismuth telluride materials with both high thermoelectric and moderate mechanical properties.

Roles of concentration-dependent Cu doping behaviors on the thermoelectric properties of n-type Mg3Sb1.5Bi0.5

Large Seebeck coefficients induced by high degeneracy of conduction band minimum, and low intrinsic lattice thermal conductivity originated from large lattice vibrational anharmonicity render Mg3Sb2 as a promising n-type thermoelectric material. Herein, we demonstrated unique concentration-dependent occupation behaviors of Cu in Mg3.4Sb1.5Bi0.49Te0.01 matrix, evidenced by structural characterization and transport property measurements. It is found that Cu atoms prefer to enter the interstitial lattice sites in Mg3Sb2 host with low doping level (Mg3.4Sb1.5Bi0.49Te0.01 + x% Cu, x < 0.3%), acting as donors for providing additional electrons without deteriorating the carrier mobility. When x is larger than 0.3%, the excessive Cu atoms are inclined to substitute Mg atoms, yielding acceptors to decrease the electron concentration. As a result, the electrical conductivity of the Mg3.4Sb1.5Bi0.49Te0.01 + 0.3% Cu sample reaches 2.3 × 104 S/m at 300 K, increasing by 300% compared with that of the pristine sample. The figure of merit zT values in the whole measured temperature range are significantly improved by the synergetic improvement of power factor and reduction of thermal conductivity. An average zT ∼1.07 from 323 K to 773 K has been achieved for the Mg3.4Sb1.5Bi0.49Te0.01 + 0.3% Cu sample, which is about 30% higher than that of the Mg3.4Sb1.5Bi0.49Te0.01 sample.

Improved Thermoelectric Performance of Cu3Sb1−x−ySnxInySe4 Permingeatites Double-Doped with Sn and In

Cu–Sb–Se ternary chalcogenide compounds composed of earth-abundant low-toxicity elements are attracting attention as economical and ecofriendly semiconductors. Among them, permingeatite (Cu3SbSe4) is a potential thermoelectric material with high Seebeck coefficient and low lattice thermal conductivity. However, it is necessary to improve its thermoelectric properties through doping, as it has low electrical conductivity due to its low intrinsic carrier concentration. In this study, samples of Cu3Sb1−x−ySnxInySe4 (0.02 ≤ &lt;i&gt;x&lt;/i&gt; ≤ 0.08 and 0.04 ≤ &lt;i&gt;y&lt;/i&gt; ≤ 0.06) double-doped with In (group 13 element) and Sn (group 14 element) at the Sb (group 15 element) sites of permingeatite were synthesized and their thermoelectric performances were evaluated. All samples exhibited a single phase of permingeatite with tetragonal structure, and high relative densities of 97.4–98.9%. The lattice constants of the a- and c-axes were 0.5651–0.5654 and 1.1249–1.1257 nm, respectively, owing to the successful substitution of Sn and In at the Sb sites. As the doping concentrations of Sn and In increased, the carrier (hole) concentration increased. Thus, the Seebeck coefficient decreased, while the electrical and thermal conductivities increased. Sn doping was found to be more effective than In doping. Because Cu3Sb0.96−xSnxIn0.04Se4 exhibits higher Seebeck coefficients than Cu3Sb0.94−xSnxIn0.06Se4, larger power factors and higher dimensionless figures of merit (ZTs) were achieved for the Cu3Sb0.96−xSnxIn0.04Se4 specimens. Cu3Sb0.92Sn0.04In0.04Se4 achieved a maximum ZT of 0.59 at 623 K, based on its Seebeck coefficient of 161 µVK&lt;sup&gt;−1&lt;/sup&gt;, electrical conductivity of 4.69 × 10&lt;sup&gt;4&lt;/sup&gt; Sm&lt;sup&gt;−1&lt;/sup&gt;, thermal conductivity of 0.77 Wm&lt;sup&gt;−1&lt;/sup&gt;K&lt;sup&gt;−1&lt;/sup&gt;, and power factor of 1.22 mWm&lt;sup&gt;−1&lt;/sup&gt;K&lt;sup&gt;−2&lt;/sup&gt;.

Anomalous high [formula omitted] above 2.2 in light element dominated compound Li[formula omitted]OTe due to low thermal conductivity

Thermoelectric materials usually contain heavy metal elements to ensure low lattice thermal conductivity κl. However, such elements like Pb, Bi and Sb have the expenses of high prices and potential risks of pollution. Herein, we propose a novel design strategy for thermoelectrics by constructing new light element dominated materials, such as quasi-antifluorite Li4OTe, with a high figure of merit surpassing 2.2. The introduction of O atom in Li4OTe crystal causes optical phonon softening and large phase space for three-phonon scattering processes, leading to much lower κl (<1 W m−1 K−1) than antifluorite Li2X (X=O, Te). This picture can be extended to other quasi-antifluorites, which will stimulate intense interests in the exploration of the light element dominated compounds, as new environmentally friendly and low-cost thermoelectric materials.

Selective Scatterings of Phonons and Electrons in Defective Half-Heusler Nb1- δ CoSb for the Figure of Merit zT > 1.

The recently developed defective 19-electron half-Heusler (HH) compounds, represented by Nb1- δ CoSb, possess massive intrinsic vacancies at the cation site and thus intrinsically low lattice thermal conductivity that is desirable for thermoelectric (TE) applications. Yet the TE performance of defective HHs with a maximum figure of merit (zT) <1.0 is still inferior to that of the conventional 18-electron ones. Here, a peak zT exceeding unity is obtained at 1123 K for both Nb0.7 Ta0.13 CoSb and Nb0.6 Ta0.23 CoSb, a benchmark value for defective 19-electron HHs. The improved zT results from the achievement of selective scatterings of phonons and electrons in defective Nb0.83 CoSb, using lanthanide contraction as a design factor to select alloying elements that can strongly impede the phonon propagation but weakly disturb the periodic potential. Despite the massive vacancies induced strong point defect scattering of phonons in Nb0.83 CoSb, Ta alloying is still found effective in suppressing lattice thermal conductivity while maintaining the carrier mobility almost unchanged. In comparison, V alloying significantly deteriorates the carrier transport and thus the TE performance. These results enlarge the category of high-performance HH TE materials beyond the conventional 18-electron ones and highlight the effectiveness of selective scatterings of phonons and electrons in developing TE materials even with massive vacancies.