Enhancement Of ZT Research Articles

Anisotropic thermoelectric transport in textured Sb1.5Bi0.5Te3 nanomaterial synthesized by facile bottom-up physical process

High anisotropy in thermal conductivity (κ) coupled with anisotropy in Seebeck coefficient (S) has resulted in peak zT value of 0.85 at 150 °C in the textured Sb1.5Bi0.5Te3 nanomaterial in perpendicular to the preferential ab-plane direction. Further, strong and opposite temperature dependences in electrical conductivity (σ) and thermal conductivity, in the ranges of 1.6–1.8 and 1.4–1.8 respectively, have resulted in better average zT value of 0.72 in the 50°C–250 °C temperature range along this direction, where zT = (S2σ/κ)T. The high anisotropy in Seebeck coefficient in the range of 0.82–0.84 is peculiar, which may be attributed to differential scattering for holes and electrons by oxide interface on the ab-planes. Sample temperature and laser power dependent Raman spectroscopy have revealed that Eg(2) mode is dominant phonon transfer mode in this material and therefore, scattering of Eg(2) mode phonons may be critical in obtaining thermal conductivity reduction. The nanomaterial has been synthesized by a facile bottom-up physical synthesis process and consolidated by direct current hot pressing. Our synthesis process requires controlled melting of ingredient metals at just above their melting points-rocking and air quenching. The synthesized nanomaterial hardly experiences its melting point in this process. This energy efficient process does not require any kind of milling and produces single phase nanomaterial in unique plate-like morphology, which easily results in high texturing. This texturing has been studied by XRD analysis as well as SEM images and is also correlated with texture factor in the thermoelectric measurements. HRTEM image has shown high grain boundary density within the plates, but these have not been able to scatter phonons for thermal conductivity reduction within ab-plane. This is possibly due to smaller than optimum size of these randomly oriented grains. Such features also offer possibility of zT enhancement along preferential ab-plane direction in the textured specimen. Further, the study of anisotropy in power output density and thereby, engineered power factor under practical temperature gradients is also presented. The process also offers simplicity in obtaining further thermal conductivity reduction in perpendicular to the preferential ab-plane direction by use of tellurium or semiconductor interface layer or by use of metallic cluster in the ab-plane direction.

Melt-Spun SiGe Nano-Alloys: Microstructural Engineering Towards High Thermoelectric Efficiency

Silicon-germanium (SiGe) alloys are prominent high-temperature thermoelectric (TE) materials used as a powering source for deep space applications. In this work, we employed rapid cooling rates for solidification by melt-spinning and rapid heating rates for bulk consolidation employing spark plasma sintering to synthesize high-performance p-type SiGe nano-alloys. The current methodology exhibited a TE figure-of-merit (ZT) ≈ 0.94 at 1123 K for a higher cooling rate of ∼3.0 × 107 K/s. This corresponds to ≈ 88% enhancement in ZT when compared with currently used radioisotope thermoelectric generators (RTGs) in space flight missions, ≈ 45% higher than pressure-sintered p-type alloys, which results in a higher output power density, and TE conversion efficiency (η) ≈ 8% of synthesized SiGe nano-alloys estimated using a cumulative temperature dependence (CTD) model. The ZT enhancement is driven by selective scattering of phonons rather than of charge carriers by the high density of grain boundaries with random orientations and induced lattice-scale defects, resulting in a substantial reduction of lattice thermal conductivity and high power factor. The TE characteristics of synthesized alloys presented using the constant property model (CPM) and CTD model display their high TE performance in high-temperature regimes along with wide suitability of segmentation with different mid-temperature TE materials.

Ultra‐High Thermoelectric Performance in Bulk BiSbTe/Amorphous Boron Composites with Nano‐Defect Architectures

AbstractBased on the Seebeck and Peltier effects, state‐of‐the‐art bismuth telluride‐based thermoelectric materials, which are capable of direct and reversible conversion of thermal to electrical energy, have great potential in energy harvesting and solid‐state refrigerators. However, their widespread use is limited by their low conversion efficiency, which is determined by the dimensionless figure‐of‐merit (ZT). Significant enhancement of ZT is a great challenge owing to the common interdependence of electrical and thermal conductivity. Here, it is demonstrated that by incorporating nanoamorphous boron into the p‐type Bi0.5Sb1.5Te3, a record high ZT of 1.6 at 375 K is achieved. It is shown that a high density of nanostructures and dislocations due to the incorporation of the boron inclusions, leads to a significant reduction of thermal conductivity and improved charge transport. The findings represent an important step to further promote the development of thermoelectric technology and its widespread application in solid‐state refrigeration and power generation from waste heat.

Effect of Microwave Processing and Glass Inclusions on Thermoelectric Properties of P-Type Bismuth Antimony Telluride Alloys for Wearable Applications

Depending on the application of bismuth telluride thermoelectric materials in cooling, waste heat recovery, or wearable electronics, their material properties, and geometrical dimensions should be designed to optimize their performance. Recently, thermoelectric materials have gained a lot of interest in wearable electronic devices for body heat harvesting and cooling purposes. For efficient wearable electronic devices, thermoelectric materials with optimum properties, i.e., low thermal conductivity, high Seebeck coefficient, and high thermoelectric figure-of-merit (zT) at room temperature, are demanded. In this paper, we investigate the effect of glass inclusion, microwave processing, and annealing on the synthesis of high-performance p-type (BixSb1−x)2Te3 nanocomposites, optimized specially for body heat harvesting and body cooling applications. Our results show that glass inclusion could enhance the room temperature Seebeck coefficient by more than 10% while maintaining zT the same. Moreover, the combination of microwave radiation and post-annealing enables a 25% enhancement of zT at room temperature. A thermoelectric generator wristband, made of the developed materials, generates 300 μW power and 323 mV voltage when connected to the human body. Consequently, MW processing provides a new and effective way of synthesizing p-type (BixSb1−x)2Te3 alloys with optimum transport properties.

Cu2Se thermoelectrics: property, methodology, and device

In the last decade, high-performance Cu2Se thermoelectric materials and devices are attracting increasing research interest. In this review, we firstly summarize fundamentals of Cu2Se, including crystal structure, band structure, phase transition, and intrinsic thermoelectric performance. Then, we compare and overview the effectiveness of enhancing electrical transports and reducing lattice thermal conductivity on boosting figure of merit, zT of Cu2Se and found that zT enhancement of Cu2Se should be mainly attributed to reduced lattice thermal conductivity. After that, we highlight the chemical stability and mechanical properties of Cu2Se and its thermoelectric modules. In the end, we point out future research directions in the field of Cu2Se thermoelectrics. This review fills the gap of overview on the progress and challenge of Cu2Se thermoelectrics, and provide a new perspective to achieve Cu2Se-based thermoelectric materials with high thermoelectric performance.

Bismuth Telluride Thermoelectrics with 8% Module Efficiency for Waste Heat Recovery Application.

SummaryThermoelectric generators (TEGs) offer cost-effective and sustainable solid-state energy conversion mechanism from wasted heat into useful electrical power. Thermoelectric (TE) materials based upon bismuth telluride (BiTe) systems are widely utilized in applications ranging from energy generation to sensing to cooling. There is demand for BiTe materials with high figure of merit (zT) and TEG modules with high conversion efficiency over intermediate temperatures (25°C–250°C). Here we provide fundamental breakthrough in design of BiTe-based TE materials and utilize them to demonstrate modules with outstanding conversion efficiency of 8%, which is 40% higher compared with state-of-the-art commercial modules. The average zT of 1.08 for p-type and 0.84 for n-type bismuth telluride alloys is obtained between 25 and 250°C. The significant enhancement in zT is achieved through compositional and defect engineering in both p- and n-type materials. The high conversion efficiency accelerates the transition of TEGs for waste heat recovery.

Effect of multisite alloying and chloride doping for realizing a high thermoelectric performance in misfit-layered chalcogenide

Abstract Misfit-layered chalcogenide has captured large attention due to its unique opportunity for optimizing figure of merit zT originating from the natural superlattice structure. Existing strategies mainly focus on single element doping at cation site, while multisite alloying, especially for the doping at anion site, has not yet properly understood. In this work, chloride doping at anion site as a novel method is utilized to create excess electrons for manipulating the carrier concentration, thus achieving an advancement on electronic performance (up to 1.0 mW K−2 m−1) by ∼35% in oriented textured (SnS)1.2(TiS2)2 alloys. Such a chloride anion doping also enables a reduction on the lattice thermal conductivity to 0.82 and 0.35 W K−1 m−1 in the in-plane and out-of-plane directions, respectively. In addition, the multisite alloying strengthen the phonon scattering for a further reduction on lattice thermal conductivity, thus leading to an enhancement of thermoelectric performance. Consequently, a record zT∼0.45 is realized in the in-plane direction, which demonstrates (SnS)1.2(TiS2)2 based material as a promising thermoelectric candidate and the multisite alloying as an effective strategy for zT enhancement.

Enhancing the figure of merit in thermoelectric materials by adding silicate aerogel

To achieve practical applications of thermoelectric materials in large scale, materials with high thermal conversion efficiency, low cost, and operating at a proper temperature range are required. Owing to the inversely relationship between electrical conductivity (σ) and Seebeck coefficient (S), it is a great challenge to improve high power factor (PF = σS2) of a thermoelectric material. Here, we show that by adding a small amount of silicate aerogel to existing promising materials, including Bi0.5Sb1.5Te3 (p-type), Cu0.01Bi2Te2.7Se0.3 (n-type), and p-type MgAgSb, significantly improves the figure of merit (zT) of the materials. For the BiTe-based materials, the primary gain in zT comes from a significant reduced thermal conductivity, while preserving high PF for the aerogel mixed composites. On the other hand, the enhancement of zT in aerogel-added p-type MgAg0.97Sb0.99 material is not due to the reduction in thermal conductivity but rather an enhancement in PF, which is important for practical applications. Furthermore, the materials we present in this work are bulk polycrystalline samples, which are relatively easy to produce without complicated synthesis processes.

A Novel Way to Enhance the Thermoelectric Efficiency of Carbon Nanotube through Cobaltocene‐decamethyl Cobaltocene Encapsulation

AbstractHerein, we report a novel way to enhance the thermoelectric figure of merit (ZT) of carbon nanotube (CNT) by a factor as large as of 103. Our non‐equilibrium Green's function based density functional theory study on a (17, 0) CNT proved that such a huge change in the ZT of CNT is possible when the symmetry of the material is broken by a combination of cobaltocene and decamethyl cobaltocene encapsulation. The calculations also indicate that pristine carbon nanotube, which is a p‐type thermoelectric material, turns into an n‐type material upon such encapsulation. The detail analyses reveal that the reason behind this remarkable enhancement of ZT is mainly attributed to the sharp rise in the electrical conductance of cobaltocene moiety encapsulated CNT with respect to pristine CNT. We have noticed that symmetry breaking has resulted in the removal of degeneracy of HOMO (highest occupied molecular orbital), HOMO‐1 and LUMO (lowest unoccupied molecular orbital), LUMO+1 which eventually leads to a significant decrease in the HOMO‐LUMO gap of the system and subsequently a 400 times increase in the overall electrical conductance of the thermoelectric device.

Radical enhancement of molecular thermoelectric efficiency.

There is a worldwide race to find materials with high thermoelectric efficiency to convert waste heat to useful energy in consumer electronics and server farms. Here, we propose a radically new method to enhance simultaneously the electrical conductance and thermopower and suppress heat transport through ultra-thin materials formed by single radical molecules. This leads to a significant enhancement of room temperature thermoelectric efficiency. The proposed strategy utilises the formation of transport resonances due to singly occupied spin orbitals in radical molecules. This enhances the electrical conductance by a couple of orders of magnitude in molecular junctions formed by nitroxide radicals compared to the non-radical counterpart. It also increases the Seebeck coefficient to high values of 200 μV K−1. Consequently, the power factor increases by more than two orders of magnitude. In addition, the asymmetry and destructive phonon interference that was induced by the stable organic radical side group significantly decreases the phonon thermal conductance. The enhanced power factor and suppressed thermal conductance in the nitroxide radical lead to the significant enhancement of room temperature ZT to values ca. 0.8. Our result confirms the great potential of stable organic radicals to form ultra-thin film thermoelectric materials with unprecedented thermoelectric efficiency.

Possible Charge Density Wave and Enhancement of Thermoelectric Properties at Mild-Temperature Range in n-Type CuI-Doped Bi2Te2.1Se0.9 Compounds.

Bi2Te3-based compounds have long been studied as thermoelectric materials in cooling applications near room temperature. Here, we investigated the thermoelectric properties of CuI-doped Bi2Te2.1Se0.9 compounds. The Cu/I codoping induces the lattice distortion partially in the matrix. We report that the charge density wave caused by the local lattice distortion affects the electrical and thermal transport properties. From the high-temperature specific heat, we found a first-order phase transitions near 490 and 575 K for CuI-doped compounds (CuI)xBi2Te2.1Se0.9 (x = 0.3 and 0.6%), respectively. It is not a structural phase transition, confirming from the high-temperature X-ray diffraction. The temperature-dependent electrical resistivity shows a typical behavior of charge density wave transition, which is consistent with the temperature-dependent Seebeck coefficient and thermal conductivity. The transmission electron microscopy and electron diffraction show a local lattice distortion, driven by the charge density wave transition. The charge density wave formation in the Bi2Te3-based compounds are exceptional because of the possibility of coexistence of charge density wave and topological surface states. From the Kubo formula and Boltzmann transport calculations, the formation of charge density wave enhances the power factor. The lattice modulation and charge density wave decrease lattice thermal conductivity, resulting in the enhancement of thermoelectric performance simultaneously in CuI-doped samples. Consequently, an enhancement of thermoelectric performance ZT over 1.0 is achieved at 448 K in the (CuI)0.003Bi2Te2.1Se0.9 sample. The enhancement of ZT at high temperature gives rise to a superior average ZTavg (1.0) value than those of previously reported ones.

Half-Heusler alloys: Enhancement of ZT after severe plastic deformation (ultra-low thermal conductivity)

Several n- and p-type Half-Heusler (HH) thermoelectric materials (Ti0.5Zr0.5NiSn-based and NbFeSb-based) have been processed by high-pressure torsion (HPT) to improve their thermoelectric performance via a drastic reduction towards ultra-low thermal conductivity. This reduction occurs due to grain refinement and a high concentration of deformation-induced defects, i.e. vacancies and dislocations as inferred by this severe plastic deformation and documented via SEM and TEM investigations. In most cases the figure of merit, ZT, and the thermo-electric conversion efficiency were enhanced up to η ∼ 10% for the thermally stable HPT-processed sample. Raman spectroscopy, backed by DFT calculations, proves that HPT induces a stiffening of the lattice and as a consequence, a blue-shift of the lattice vibrations occurs.Furthermore for all investigated specimens Vickers hardness values after HPT were significantly higher, whereas the change in the elastic moduli was less than 5% in comparison to the HP reference sample.

Maximizing Thermoelectric Figures of Merit by Uniaxially Straining Indium Selenide

A majority of the electricity currently generated is regrettably lost as heat. Engineering high-efficiency thermoelectric materials which can convert waste heat back into electricity is therefore vital for reducing our energy fingerprint. ZT, a dimensionless figure of merit, acts as a beacon of promising thermoelectric materials. However, engineering materials with large ZT values is practically challenging, since maximizing ZT requires optimizing many interdependent material properties. Motivated by recent studies on bulk indium selenide that suggest it may have favorable thermoelectric properties, here we present the thermoelectric properties of monolayer indium selenide in the presence of uniaxial strain using first-principles calculations conjoined with semiclassical Boltzmann transport theory. Our calculations indicate that conduction band convergence occurs at a compressive strain of −6% along the zigzag direction and results in an enhancement of ZT for p-type indium selenide at room temperature. Fu...

Tailoring Nanoporous Structures in Bi2Te3 Thin Films for Improved Thermoelectric Performance.

Thin-film thermoelectrics (TEs) with unique advantages have triggered great interest in thermal management and energy harvesting technology for ambient temperature microscale systems. Although they have exhibited a good prospect, their unsatisfactory performances still seriously hamper their widespread application. Tailoring the porous structure has been demonstrated to be a facile strategy to significantly reduce thermal conductivity and enhance the figure of merit (ZT) of bulk TE materials; however, it is challenging for thin-film TEs. Here, the nanoporous Bi2Te3 thin films with faceted pore shapes and various porosities, pore sizes, and pore intervals are carefully designed and fabricated by evacuating the over-stoichiometry Te atoms. The dependence of the carrier mobility and lattice thermal conductivity on the pore characteristics is investigated. In the case of the pore interval longer than the electron mean free path, the porous structure greatly reduces the lattice thermal conductivity without affecting the electrical conductivity obviously. Phonon specular backscattering that is highly related to the pore characteristics is suggested to be mainly responsible for thermal conductivity reduction, resulting in ∼60% enhancement in ZT at room temperature, that is, from ∼0.42 for the dense film to ∼0.67 for the nanoporous film. The enhanced ZT value is comparable to that of commercial bulk TEs and can be further improved by optimizing the carrier concentrations. This work provides a general approach to fabricate high-performance chalcogenide TE thin-film materials.

Achieving Out-of-Plane Thermoelectric Figure of Merit ZT = 1.44 in a p-Type Bi2Te3/Bi0.5Sb1.5Te3 Superlattice Film with Low Interfacial Resistance.

Recently, low-dimensional superlattice films have attracted significant attention because of their low dimensionality and anisotropic thermoelectric (TE) properties such as the Seebeck coefficient, electrical conductivity, and thermal conductivity. For these superlattice structures, both electrons and phonons show highly anisotropic behavior and exhibit much stronger interface scattering in the out-of-plane direction of the films compared to the in-plane direction. However, no detailed information is available in the literature for the out-of-plane TE properties of the superlattice-based films. In this report, we present the out-of-plane Seebeck coefficient, thermal conductivity, and electrical properties of p-type Bi2Te3/Bi0.5Sb1.5Te3 (bismuth telluride/bismuth antimony telluride, BT/BST) superlattice films in the temperature range of 77-500 K. Because of the synergistic combination of the energy filtering effect and low interfacial resistance of the superlattice structure, an impressively high ZT of 1.44 was achieved at 400 K for the 200 nm-thick p-type BT/BST superlattice film, corresponding to a 43% ZT enhancement compared to the pristine p-BST films with the same thickness.

Dependence of thermoelectric power and electrical conductivity on structural order of PEDOT-Tos-graphene nanocomposite via charge carrier mobility

Nanocomposites of tosylate doped poly (3, 4 ethylenedioxythiophene) (PEDOT-Tos) and graphene with improved structural order were synthesized and investigated as organic thermoelectric (TE) materials. A TE figure of merit ZT > 0.05 is reported. Gradual incorporation of graphene results in an increase in the degree of structural order of the samples. A consequent increase in the figure of merit is observed, the maximum value crossing 0.05 (higher than that reported for bulk nanostructured polymer based composites) for a graphene content of 25%. The tuning of the ordered structure boosts up the charge carrier mobility, which in turn improves the thermoelectric power (S) simultaneously with the electrical conductivity (σ). Consequently there is an increase in the TE power factor. Moreover, the thermal conductivity of the nanocomposites is very low due to selective phonon scattering by the introduction of the nanointerfaces designed in the PEDOT-Tos-graphene nanocomposites, which further contributes to the enhancement in ZT. An opposite trend is observed in structure and ZT, for a graphene content of 40%; arising probably from reduced π-stacking interactions between graphene and the PEDOT units. This study proves to be an effective way and can escort the future design and synthesis of high-performance thermoelectric conjugated polymers.

Grain Boundary Interfaces Controlled by Reduced Graphene Oxide in Nonstoichiometric SrTiO3-\u03b4 Thermoelectrics

Point defect or doping in Strontium titanium oxide (STO) largely determines the thermoelectric (TE) properties. So far, insufficient knowledge exists on the impact of double Schottky barrier on the TE performance. Herein, we report a drastic effect of double Schottky barrier on the TE performance in undoped STO. It demonstrates that incorporation of Reduced Graphene Oxide (RGO) into undoped STO weakens the double Schottky barrier and thereby results in a simultaneous increase in both carrier concentration and mobility of undoped STO. The enhanced mobility exhibits single crystal-like behavior. This increase in the carrier concentration and mobility boosts the electrical conductivity and power factor of undoped STO, which is attributed to the reduction of the double Schottky barrier height and/or the band alignment of STO and RGO that allow the charge transfer through the interface at grain boundaries. Furthermore, this STO/RGO interface also enhances the phonon scattering, which results in low thermal conductivity. This strategy significantly increases the ratio of σ/κ, resulting in an enhancement in ZT as compared with pure undoped STO. This study opens a new window to optimize the TE properties of many candidate materials.

Novel n-type thermoelectric material of ZnIn2Se4

Besides enhancing thermoelectric performance of the existing thermoelectric materials, many efforts have been focused on the exploration of novel thermoelectric materials with potentially high performance typically. This is largely guided by the concept of the intrinsic low lattice thermal conductivity, which is arisen from various mechanisms, such as intrinsic vacancy, low sound velocity, complex crystal structure and lattice anharmonicity. Motivated by the inherent feature of highly disorder cations and intrinsic vacancies introducing strong lattice anharmonicity, this work focuses on a semiconducting compound, ZnIn2Se4, as a potential thermoelectric material. Pristine ZnIn2Se4 shows an n-type conduction and a low carrier concentration of 2.8 × 1018 cm−3. A lattice thermal conductivity as low as ∼0.9 W/m*K at high temperature is obtained, due to the additional phonon scattering by highly disorder cations and intrinsic vacancies. Thus, a peak zT of ∼0.23 is demonstrated in pristine ZnIn2Se4 even with the carrier concentration far deviating from the optimum. This work not only indicates this compound as a promising n-type thermoelectric material, but also provides a direction for enhancement of zT.

Quantum and Phonon Interference-Enhanced Molecular-Scale Thermoelectricity.

Simultaneous engineering of electron and phonon transport through nanoscale molecular junctions is fundamental to the development of high-performance thermoelectric materials for the conversion of waste heat into electricity and cooling. Here, we demonstrate a systematic improvement of the room-temperature thermoelectric figure of merit (ZT) of molecular junctions. This is achieved by phonon interference (PI)-suppressed thermal conductance and quantum interference-enhanced electrical conductance and Seebeck coefficient. This strategy leads to a significant enhancement of ZT from low values ca. 10–6 in oligo(phenylene-ethynylene) (OPE2) to the record values of 2.4 in dinitro-functionalized OPE2 (DOPE2). The dinitro functionalization also considerably enhances ZT of biphenyl-dithiol (BDT) and bipyridyl molecular junctions. Remarkably, the energy levels of electron-withdrawing nitro groups are hardly changed from one molecule to the other. Because of this generic feature, a resonance transport in the vicinity of Fermi energy of electrodes is formed leading to a significant improvement of Seebeck coefficient and ZT of all derivatives. For example, the Seebeck coefficient enhances from 10.8 μV/K in BDT to −470 μV/K in dinitro-BDT (DBDT). In addition, destructive PI due to the nitro groups suppresses phonon thermal conductance, for example, from 20 pW/K in BDT to 11 pW/K in DBDT at room temperature. We also demonstrate that quantum and PI-enhanced single-molecule thermoelectric efficiency is conserved when parallel molecules are placed between gold electrodes. These results promise to remove the key roadblocks and open new avenues to exploit functionalized organic molecules for thermoelectric energy harvesting and cooling.

Divalent doping-induced thermoelectric power factor increase in p-type Bi2Te3 via electronic structure tuning

We use first-principles calculations to reveal the effects of divalent Pb, Ca, and Sn doping of Bi2Te3 on the band structure and transport properties, including the Seebeck coefficient, α, and the reduced power factor, α2σ/τ, where σ is the electrical conductivity and τ is the effective relaxation time. Pb and Ca additions exhibit up to 60%–75% higher peak α2σ/τ than that of intrinsic Bi2Te3 with Bi antisite defects. Pb occupancy and Ca occupancy of Bi sites increase σ/τ by activating high-degeneracy low-effective-mass bands near the valence band edge, unlike Bi antisite occupancy of Te sites that eliminates near-edge valence states in intrinsic Bi2Te3. Neither Pb doping nor subatomic-percent Ca doping increases α significantly, due to band averaging. Higher Ca levels increase α and diminish σ, due to the emergence of a corrugated band structure underpinned by high-effective-mass bands, attributable to Ca–Te bond ionicity. Sn doping results in a distortion of the bands with a higher density of states that may be characterized as a resonant state but decreases α2σ by up to 30% due to increases in the charge carrier effective mass and decreases in both spin–orbit coupling and valence band quasidegeneracy. These results, and thermal conductivity calculations for nanostructured Bi2Te3, suggest that Pb or Ca doping can enhance the thermoelectric figure of merit ZT to values up to ZT ∼ 1.7, based on an experimentally determined τ. Our findings suggest that divalent doping can be attractive for realizing large ZT enhancements in pnictogen chalcogenides.