Interplay between metavalent bonds and dopant orbitals enables the design of SnTe thermoelectrics

Engineering the electronic band structures upon doping is crucial to improve the thermoelectric performance of materials. Understanding how dopants influence the electronic states near the Fermi level is thus a prerequisite to precisely tune band structures. Here, we demonstrate that the Sn-s states in SnTe contribute to the density of states at the top of the valence band. This is a consequence of the half-filled p-p σ-bond (metavalent bonding) and its resulting symmetry of the orbital phases at the valence band maximum (L point of the Brillouin zone). This insight provides a recipe for identifying superior dopants. The overlap between the dopant s- and the Te p-state is maximized, if the spatial overlap of both orbitals is maximized and their energetic difference is minimized. This simple design rule has enabled us to screen out Al as a very efficient dopant to enhance the local density of states for SnTe. In conjunction with doping Sb to tune the carrier concentration and alloying with AgBiTe2 to promote band convergence, as well as introducing dislocations to impede phonon propagation, a record-high average ZT of 1.15 between 300 and 873 K and a large ZT of 0.36 at 300 K is achieved in Sn0.8Al0.08Sb0.15Te-4%AgBiTe2.

Ab initio study of elastic, electronic, and vibrational properties of SnTe and PbTe

CONTEXT AND RESULTS

The elastic, electronic, and vibrational properties of the ground state of the rocksalt SnTe and PbTe are investigated. The deduced elastic constants, namely, shear modulus, Young's modulus, and Poisson's ratio are in very good agreement with the experimental and other theoretical data. The electronic band structure and density of states are obtained with and without considering the spin-orbit coupling. The bandgaps of SnTe and PbTe with (without) spin-orbit coupling are 0.11 (0.05) eV and 0.01 (0.78) eV, respectively. The bandgaps with spin-orbit interactions are nearer to experimental data. The hybrid functionals give higher values of bandgaps for both the SnTe and PbTe. In both compounds, the bandgap increases with volume. The valence bandwidths, however, decrease with increasing volume. The vibrational frequencies are found in reasonable agreement with the experiment. The frequencies increase with pressure.

COMPUTATIONAL METHOD

In this work, the ab initio calculations of SnTe and PbTe crystals are carried out applying plane wave pseudopotential method using the QUANTUM ESPRESSO package. The PBE exchange and correlation functional based on GGA is considered. The fully relativistic norm-conserving pseudopotentials for Sn, Pb, and Te are used. The self-consistent field calculations are performed over a dense MP net of 18 × 18 × 18 k-points. The energy cut-off of 70 Ryd was found sufficient to achieve convergence of 10-6 Ryd in total energy of the crystals.

Band Modification and Localized Lattice Engineering Leads to High Thermoelectric Performance in Ge and Bi Codoped SnTe-AgBiTe2 Alloys

SnTe, emerging as an environment-friendly alternative to conventional PbTe thermoelectrics, has drawn significant attention for clean energy conversion. Here, a high peak figure of merit (ZT) of 1.45 at 873 K in Ge/Bi codoped SnTe-AgBiTe₂ alloys is reported. It is demonstrated that the existence of Ge, Bi, and Ag facilitate band convergence in SnTe, resulting in remarkable enhancement of Seebeck coefficient and power factor. Simultaneously, localized lattice imperfections including dislocations, point defects, and micro/nanopore structures are caused by incorporation of Ge, Bi, and Ag, which can effectively scatter heat carrying phonons with different wavelengths and contribute to an extremely low κ_L of 0.61 W m^-1  K^-1 in Sn_0.92 Ge_0.04 Bi_0.04 Te-10%AgBiTe₂ . Such high peak ZT is achieved by decouples electron and phonon transport through band modification and localized lattice engineering, highlighting promising solutions for advancing thermoelectrics.

Significantly Enhanced Thermoelectric Performance Achieved in CuGaTe2 through Dual-Element Permutations at Cation Sites

CuGaTe₂ has become a widely studied mid-temperature thermoelectric material due to the advantages of large element abundance, proper band gap, and intrinsically high Seebeck coefficient. However, the intrinsically high lattice thermal conductivity and low room-temperature electrical conductivity result in a merely moderate thermoelectric performance for pristine CuGaTe₂. In this work, we found that Cu deficiency can significantly reduce the activation energy E_a of Cu vacancies from ∼0.17 eV for pristine CuGaTe₂ to nearly zero for Cu_0.97GaTe₂, thus leading to dramatic improvements in hole concentration and power factor. More remarkably, element permutations (Ag/Cu and In/Ga) at both cation sites can effectively reduce the lattice thermal conductivity at the entire testing temperatures by producing intensive atomic-scale mass and strain fluctuations. Eventually, an ultrahigh peak ZT_max value of ∼1.5 at 873 K is achieved in the composition of Cu_0.72Ag_0.25Ga_0.6In_0.4Te₂, while a large average ZT_avg value of ∼0.7 (323-873 K) is obtained in the Cu_0.67Ag_0.3Ga_0.6In_0.4Te₂ sample, both of which are significant improvements over pristine CuGaTe₂.

Enhanced thermoelectric performance of In-doped and AgCuTe-alloyed SnTe through band engineering and endotaxial nanostructures

AgCuTe alloying introduces endotaxial nanostructures in SnTe, which significantly decrease thermal conductivity and enhance ZT.

Multiple Effects Promoting the Thermoelectric Performance of SnTe by Alloying with CuSbTe2 and CuBiTe2

In a SnTe-based thermoelectric material, the naturally high hole concentration caused by cation vacancies and high total thermal conductivity seriously hinder its thermoelectric performance. A recent work shows that alloying SnTe with other compounds from the I-V-VI2 family (I = Ag, Na; V = Sb, Bi; VI = Te) can be considered an effective strategy to boost the figure of merit efficiently via the synergy of manipulating hole concentration and lowering lattice thermal conductivity. Herein, we present a markedly enhanced thermoelectric performance in p-type SnTe through CuPnTe2 (Pn = Sb, Bi) alloying. Moreover, we found that the alloying with both CuSbTe2 and CuBiTe2 can facilitate the valence band convergence of SnTe, but their relative influence is different. Interestingly, compared to CuBiTe2, alloying with CuSbTe2 increases the carrier concentration of SnTe, which suppresses the bipolar effect. Ultimately, under the positive effect of valence band convergence, increased vacancy concentration, and decreased lattice thermal conductivity, compounds with a nominal composition of (SnTe)0.90(CuSbTe2)0.10 attains a peak zT of ∼1.26 at 823 K. In contrast, the thermoelectric performance of (SnTe)1-x(CuBiTe2)x is restricted by the reduced carrier concentration and diminished band gap, showing only a humble maximum zT value of ∼0.91 at 823 K in the sample with a nominal composition of (SnTe)0.96(CuBiTe2)0.04. These results demonstrate the multiple effects on thermoelectric transport during the formation of complex solid solutions.

Enhanced Thermoelectric Performance Achieved in SnTe via the Synergy of Valence Band Regulation and Fermi Level Modulation

SnTe is deemed a promising mid-temperature thermoelectric material for low toxicity, low cost, and decent performance. Sole doping/alloying on Sn sites was reported to result in either modified band alignment or reduced lattice thermal conductivity, thus contributing to an enhanced overall thermoelectric figure of merit. However, this strategy alone is always unable to take full use of the material's advantage, especially considering that it simultaneously pushes the hole concentration off the optimal range. In this work, we adopted a two-step approach to optimize the thermoelectric performance of SnTe in order to overcome the limitation. First, Mn was alloyed into Sn sites to increase the density of state effective mass of SnTe by regulating the valence bands; the Fermi level was further regulated by iodine doping, guided by a refined two-band model. Additionally, the lattice thermal conductivity was also suppressed by the microstructure optimizing via Mn doping and additional phonon scattering at I_Te mass/strain fluctuation. As a result, a high ZT of 1.4 at 873 K was achieved for Sn_0.91Mn_0.09Te_0.99I_0.01. This study provides a way to refine the single doping stratagem used in other thermoelectric materials.

Enhanced Band Convergence and Ultra-Low Thermal Conductivity Lead to High Thermoelectric Performance in SnTe

SnTe, a structural analogue of champion thermoelectric (TE) material PbTe, has recently attracted wide attention for TE energy conversion. Herein, we demonstrate a co-doping strategy to improve the TE performance of SnTe via simultaneous modulation of electronic structure and phonon transport. The electrical transport is optimized by 3 mol % Ag doping in self-compensated SnTe (i.e., Sn_1.03 Te). Further, Mg doping in SnAg_0.03 Te resulted in highly converged valence bands, which enhanced the Seebeck coefficient markedly. The energy gap between two uppermost valence bands (ΔE_v ) decreases to 0.10 eV in Sn_0.92 Ag_0.03 Mg_0.08 Te compared to 0.35 eV in pristine SnTe. The optimized p-type carrier concentration and highly converged valence bands gave a high power factor of ca. 27 μW cm^-1  K^-2 at 865 K in Sn_0.92 Ag_0.03 Mg_0.08 Te. The lattice thermal conductivity of Sn_0.92 Ag_0.03 Mg_0.08 Te reached to an ultra-low value of ≈0.23 W m^-1  K^-1 at 865 K due to the formation of MgTe nanoprecipitates in SnTe matrix. These combined effects resulted in a high TE figure of merit, zT≈1.55 at 865 K in Sn_0.92 Ag_0.03 Mg_0.08 Te.

Optimized Electronic Bands and Ultralow Lattice Thermal Conductivity in Ag and Y Codoped SnTe

As a lead-free thermoelectric material, SnTe is inhibited by its inherent high carrier concentration and high thermal conductivity. This work describes the synergistic effect on the modulation of band structure and microstructural defects of SnTe by Ag and Y codoping, which gives rise to band convergence and multiple microstructural defects (secondary phases, dislocations, and boundaries) in the matrix and endows Sn_0.94Ag_0.09Y_0.05Te with an increased power factor of ∼2485 μW m^-1 K^-2, an extremely low lattice thermal conductivity of ∼0.61 W m^-1 K^-1, and a peak zT as high as ∼1.2 at 873 K. This work reveals that the combination of Ag and Y could play a role in the synergistic optimization of electronic and phonon transport properties of SnTe by modifying the band structure and microstructures, providing guidance for enhancing the thermoelectric performance of the relevant materials.

Advanced Thermoelectric Design: From Materials and Structures to Devices

The long-standing popularity of thermoelectric materials has contributed to the creation of various thermoelectric devices and stimulated the development of strategies to improve their thermoelectric performance. In this review, we aim to comprehensively summarize the state-of-the-art strategies for the realization of high-performance thermoelectric materials and devices by establishing the links between synthesis, structural characteristics, properties, underlying chemistry and physics, including structural design (point defects, dislocations, interfaces, inclusions, and pores), multidimensional design (quantum dots/wires, nanoparticles, nanowires, nano- or microbelts, few-layered nanosheets, nano- or microplates, thin films, single crystals, and polycrystalline bulks), and advanced device design (thermoelectric modules, miniature generators and coolers, and flexible thermoelectric generators). The outline of each strategy starts with a concise presentation of their fundamentals and carefully selected examples. In the end, we point out the controversies, challenges, and outlooks toward the future development of thermoelectric materials and devices. Overall, this review will serve to help materials scientists, chemists, and physicists, particularly students and young researchers, in selecting suitable strategies for the improvement of thermoelectrics and potentially other relevant energy conversion technologies.

Enhancement of Thermoelectric Properties in Pd-In Co-Doped SnTe and Its Phase Transition Behavior

SnTe has attracted more and more attention due to the similar band and crystal structure with high performance thermoelectric materials PbTe. Here, we introduced Pd into SnTe and the valence band convergence was confirmed by first-principles calculation. In the experimental process, we found that Pd-doped SnTe exhibit a reduced thermal conductivity because of softening chemical bonds and grain refining effects. To further improve the thermoelectric performance, Pd-In codoped SnTe samples were prepared, and the abnormal change of thermal conductivity was observed. The results of synchrotron powder diffraction suggest that the local phase transition (local structural distortions) near 400 K results in the first turn on thermal conductivity. Similarly, the second local phase transition in near 600 K observed by neutron powder diffraction lead to a decrease thermal conductivity of the sample. Finally, a peak thermoelectric figure of merit (ZT) ≈ 1.51 has been obtained in Sn_0.98Pd_0.025In_0.025Te at 800 K.

Ligand-Mediated Band Engineering in Bottom-Up Assembled SnTe Nanocomposites for Thermoelectric Energy Conversion

The bottom-up assembly of colloidal nanocrystals is a versatile methodology to produce composite nanomaterials with precisely tuned electronic properties. Beyond the synthetic control over crystal domain size, shape, crystal phase, and composition, solution-processed nanocrystals allow exquisite surface engineering. This provides additional means to modulate the nanomaterial characteristics and particularly its electronic transport properties. For instance, inorganic surface ligands can be used to tune the type and concentration of majority carriers or to modify the electronic band structure. Herein, we report the thermoelectric properties of SnTe nanocomposites obtained from the consolidation of surface-engineered SnTe nanocrystals into macroscopic pellets. A CdSe-based ligand is selected to (i) converge the light and heavy bands through partial Cd alloying and (ii) generate CdSe nanoinclusions as a secondary phase within the SnTe matrix, thereby reducing the thermal conductivity. These SnTe-CdSe nanocomposites possess thermoelectric figures of merit of up to 1.3 at 850 K, which is, to the best of our knowledge, the highest thermoelectric figure of merit reported for solution-processed SnTe.

Thermoelectric GeTe with Diverse Degrees of Freedom Having Secured Superhigh Performance

Driven by the ability to harvest waste heat into reusable electricity and the exclusive role of serving as the power generator for deep spacecraft, intensive endeavors are dedicated to enhancing the thermoelectric performance of ecofriendly materials. Herein, the most recent progress in superhigh-performance GeTe-based thermoelectric materials is reviewed with a focus on the crystal structures, phase transitions, resonant bondings, multiple valance bands, and phonon dispersions. These features diversify the degrees of freedom to tune the transport properties of electrons and phonons for GeTe. On the basis of the optimized carrier concentration, strategies of alignment of multiple valence bands and density-of-state resonant distortion are employed to further enhance the thermoelectric performance of GeTe-based materials. To decrease the thermal conductivity, methods of strengthening intrinsic phonon-phonon interactions and introducing various lattice imperfections as scattering centers are highlighted. An overview of thermoelectric devices assembled from GeTe-based thermoelectric materials is then presented. In conclusion, possible future directions for developing GeTe in thermoelectric applications are proposed. The achieved high thermoelectric performance in GeTe-based thermoelectric materials with rationally established strategies can act as a reference for broader materials to tailor their thermoelectric performance.

Prediction for electronic, vibrational and thermoelectric properties of chalcopyrite AgX(X=In,Ga)Te2: PBE + U approach

The electronic, vibrational and thermoelectric transport characteristics of AgInTe₂ and AgGaTe₂ with chalcopyrite structure have been investigated. The electronic structures are calculated using the density-functional theory within the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof functional considering the Hubbard-U exchange correlation. The band-gaps of AgInTe₂ and AgGaTe₂ are much larger than previous standard GGA functional results and agree well with the existing experimental data. The effective mass of the hole and the shape of density of states near the edge of the valence band indicate AgInTe₂ and AgGaTe₂ are considerable p-type thermoelectric materials. An analysis of lattice dynamics shows the low thermal conductivities of AgInTe₂ and AgGaTe₂. The thermoelectric transport properties' dependence on carrier concentration for p-type AgInTe₂ and AgGaTe₂ in a wide range of temperatures has been studied in detail. The results show that p-type AgInTe₂ and AgGaTe₂ at 800 K can achieve the merit values of 0.91 and 1.38 at about 2.12 × 10²⁰ cm^-3 and 1.97 × 10²⁰ cm^-3 carrier concentrations, respectively. This indicates p-type AgGaTe₂ is a potential thermoelectric material at high temperature.

ZnGeSb2: a promising thermoelectric material with tunable ultra-high conductivity

First principles calculations predict the promising thermoelectric material ZnGeSb₂ with a huge power factor (S²σ/τ) on the order of 3 × 10¹⁷ W m^-1 K^-2 s^-1, due to the ultra-high electrical conductivity scaled by a relaxation time of around 8.5 × 10²⁵ Ω^-1 m^-1 s^-1, observed in its massive Dirac state. The observed electrical conductivity is higher than the well-established Dirac materials, and is almost carrier concentration independent with similar behaviour of both n and p type carriers, which may certainly attract device applications. The low range of thermal conductivity is also evident from the phonon dispersion. Our present study further reports the gradual phase change of ZnGeSb₂ from a normal semiconducting state, through massive Dirac states, to a topological semi-metal. The maximum power factor is observed in the massive Dirac states compared to the other two states.

Element-selective resonant state in M-doped SnTe (M = Ga, In, and Tl)

SnTe is found to exhibit element-selective resonant state and its thermoelectric performance is predicted to be improved by In-X codoping.

Enhanced thermopower in rock-salt SnTe–CdTe from band convergence

The rock-salt type SnTe–CdTe alloys have been synthesized by the zone-melting method and show enhanced thermoelectric performance due to the improved band convergence.