Discovery of novel high-performance materials with earth-abundant and environmentally friendly elements is a key task for civil applications based on advanced thermoelectric technology. Advancements in this area are greatly limited...

PbTe is one of the highest-performing known thermoelectric materials. Much of its promising thermoelectric performance can be attributed to high valley degeneracy due to having a valence band minimum (VBM) and conduction band maximum (CBM) at the L-point in the first Brillouin zone, which has 4-fold degeneracy, instead of at Γ, which has 1-fold degeneracy. The existence of the VBM at L has been explained by the contribution of Pb-s states that make up the valence band edge. However, the dominance of Te-p states and the presence of Pb-p states near the VBM suggest that the Pb-s orbitals may not be as crucial as previously thought. The tight-binding (TB) or linear combination of atomic orbitals (LCAO) method of calculating electronic structures is ideally suited to gain qualitative insights to explain how simple chemistry and bonding principles lead to complex electronic structures of materials. In this study, we use a physically self-consistent TB model to understand the extent to which various atomic orbital interactions contribute to having a VBM at L instead of Γ. Based on the dominant interactions at play, a simple molecular orbital (MO) picture is developed that when extended into the periodic crystal explains the shape of the valence band dispersion between L and Γ. We find that there is sufficient interaction between Pb-p and Te-p states to provide the MO with the proper s-type symmetry to place the VBM at L rather than the usual p-type symmetry of the VB in rocksalt structures, where the VBM is at Γ. Furthermore, we show that the VBM would be at L even if the Pb-s states were removed and that the Pb-p states are at least as critical of a factor in dictating the position of the VBM in PbTe and in the other lead chalcogenides.

Semiconducting half-Heusler (HH, XYZ) phases are promising thermoelectric materials owing to their versatile electronic properties. Because the valence band of half-Heusler phases benefits from the valence band extrema at several...

Half‐Heusler (hH) compounds are promising candidates for inexpensive, low‐toxicity thermoelectric materials. It is well known that engineering electronic bands with high valley degeneracy is an effective approach for enhancing the performance of thermoelectric materials, and there are several routes for achieving high valley degeneracy in hH systems. For instance, there are multiple locations in the first Brillouin zone where the valence band maximum can be found (at the Γ‐, L‐, or W‐point), and there are two competing low‐lying conduction bands at the X‐point, where the conduction band minimum is located. By converging the multiple valence band and conduction band extrema, the valley degeneracy, and hence, performance of these materials can be improved. Here, group theoretical and tight‐binding approaches, in addition to first‐principles density functional theory calculations, are used to study the chemical origins of various band extrema in both the n‐type and p‐type compounds, with particular focus on ZrNiSn and NbFeSb. Specifically, the importance of avoided crossings is explained. The results of this work can be used to better understand and develop design strategies for engineering better performing hH thermoelectrics.

The tight-binding method provides insight into the orbital interactions that lead to the exceptional thermoelectric performance of PbTe. Using this framework, we can predict strategies to achieve enhanced thermoelectric performance in new alloys.

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SnTe is an attractive candidate for applications as a p-type thermoelectric semiconductor. The pristine SnTe compound exhibits poor thermoelectric performance at high temperatures because of its high hole concentration, small band gap, and large energy difference between the light and heavy bands (ΔE(L – Σ)). To overcome these problems, we investigate band structure changes upon the addition of trivalent dopants based on the tight-binding (TB) model and density functional theory (DFT) calculations. We find that tuning the relative on-site energies of the cation and anion s and p orbitals is a potential route for engineering band convergence. Codoping with Ge in addition to trivalent substitutions further enhances thermoelectric performance. We find that a low concentration of the isovalent Ge as well as As, which also acts as a donor (Sn0.952Ge0.016As0.016Te), induces band convergence (ΔE(L – Σ) = 0.12 eV) and enlarges the band gap (0.20 eV). This band convergence results in a remarkable increase of the peak power factor, while the increased band gap energy suppresses detrimental bipolar effects. We find that the theoretical and experimental results are in good agreement here, and the high power factor (high weighted mobility) can be attributed to the increased band convergence. Our work can efficiently screen the promising trivalent substitutions in SnTe-based materials codoped with Ge and find promising candidates for improved thermoelectric performance.

Valley degeneracy is a key feature of the electronic structure that benefits the thermoelectric performance of a material. Despite recent studies which claim that high valley degeneracy can be achieved...