Structure-Dependent Thermoelectric Properties of GeSe1-xTex (0 ≤ x ≤ 0.5)

Stereoactive Lone-Pair Manipulation for High Thermoelectric Performance of GeSe-Based Compounds

Materials with high crystallographic symmetry are supposed to be good thermoelectrics because they have high valley degeneracy (NV) and superb carrier mobility (μ). Binary GeSe crystallizes in a low-symmetry orthorhombic structure accompanying the stereoactive 4s lone pairs of Ge. Herein, we rationally modify GeSe into a high-symmetry rhombohedral structure by alloying with GeTe based on the valence-shell electron-pair repulsion theory. We demonstrate that the substitution of Se by Te weakens the stereoactivity of the Ge lone-pair electrons, resulting in robust rhombohedral structures of GeSe1-xTex for x ≥ 0.3 at room temperature. The increase of crystal symmetry not only boosts NV from 2 for orthorhombic GeSe to 9 for rhombohedral GeSe1-xTex but greatly enhances μ from <5 to >10 cm2 V-1 s-1 (room-temperature values), thereby remarkably elevating the power factors by 2 orders of magnitude (26.9 μW cm-1 K-2 at 638 K for x = 0.5). Surprisingly, despite their higher crystallographic symmetry, rhombohedral GeSe1-xTex compounds display even lower lattice thermal conductivities (∼1.0 W m-1 K-1 at 300 K for x = 0.5) than binary GeSe (∼2.5 W m-1 K-1 at 300 K) due to abundant alloying defects in the Se-Te sublattice and ferroelectric instability. Altogether, a maximum ZT value of ∼1.1 at 638 K is achieved in rhombohedral GeSe0.5Te0.5, which already outperforms GeTe. This work provides an avenue for engineering the thermoelectric properties of low-symmetry compounds containing lone-pair electrons.

Two-Dimensional-Like Phonons in Three-Dimensional-Structured Rhombohedral GeSe-Based Compounds with Excellent Thermoelectric Performance

The coupling of charge and phonon transport in solids is a long-standing issue for thermoelectric performance enhancement. Herein, two new narrow-gap semiconductors with the same chemical formula of GeSe0.65Te0.35 (GST) are rationally designed and synthesized: one with a layered hexagonal structure (H-GST) and the other with a non-layered rhombohedral structure (R-GST). Thanks to the three-dimensional (3D) network structure, R-GST possesses a significantly larger weighted mobility than H-GST. Surprisingly, 3D-structured R-GST displays an extremely low lattice thermal conductivity of ∼0.5 W m-1 K-1 at 523 K, which is comparable to that of layered H-GST. The two-dimensional (2D)-like phonon transport in R-GST stems from the unique off-centering Ge atoms that induce ferroelectric instability, yielding soft polar phonons, as demonstrated by the Boson peak detected by the low-temperature specific heat and calculated phonon spectra. Furthermore, 1 mol % doping of Sb is utilized to successfully suppress the undesired phase transition of R-GST toward H-GST at elevated temperatures. Consequently, a peak ZT of 1.1 at 623 K is attained in the rhombohedral Ge0.99Sb0.01Se0.65Te0.35 sample, which is 1 order of magnitude larger than that of GeSe. This work demonstrates the feasibility of exploring high-performance thermoelectric materials with decoupled charge and phonon transport in off-centering compounds.

Two-step phase manipulation by tailoring chemical bonds results in high-performance GeSe thermoelectrics

In thermoelectrics, phase engineering serves a crucial function in determining the power factor by affecting the band degeneracy. However, for low-symmetry compounds, the mainstream one-step phase manipulation strategy, depending solely on the valley or orbital degeneracy, is inadequate to attain a high density-of-states effective mass and exceptional zT. Here, we employ a distinctive two-step phase manipulation strategy through stepwise tailoring chemical bonds in GeSe. Initially, we amplify the valley degeneracy via CdTe alloying, which elevates the crystal symmetry from a covalently bonded orthorhombic to a metavalently bonded rhombohedral phase by significantly suppressing the Peierls distortion. Subsequently, we incorporate Pb to trigger the convergence of multivalence bands and further enhance the density-of-states effective mass by moderately restraining the Peierls distortion. Additionally, the atypical metavalent bonding in rhombohedral GeSe enables a high Ge vacancy concentration and a small band effective mass, leading to increased carrier concentration and mobility. This weak chemical bond along with strong lattice anharmonicity also reduces lattice thermal conductivity. Consequently, this unique property ensemble contributes to an outstanding zT of 0.9 at 773 K for Ge0.80Pb0.20Se(CdTe)0.25. This work underscores the pivotal role of the two-step phase manipulation by stepwise tailoring of chemical bonds in improving the thermoelectric performance of p-bonded chalcogenides.

Doping by Design: Enhanced Thermoelectric Performance of GeSe Alloys Through Metavalent Bonding

Doping is usually the first step to tailor thermoelectrics. It enables precise control of the charge-carrier concentration and concomitant transport properties. Doping should also turn GeSe, which features an intrinsically a low carrier concentration, into a competitive thermoelectric. Yet, elemental doping fails to improve the carrier concentration. In contrast, alloying with Ag-V-VI₂ compounds causes a remarkable enhancement of thermoelectric performance. This advance is closely related to a transition in the bonding mechanism, as evidenced by sudden changes in the optical dielectric constant ε_∞ , the Born effective charge, the maximum of the optical absorption ε₂ (ω), and the bond-breaking behavior. These property changes are indicative of the formation of metavalent bonding (MVB), leading to an octahedral-like atomic arrangement. MVB is accompanied by a thermoelectric-favorable band structure featuring anisotropic bands with small effective masses and a large degeneracy. A quantum-mechanical map, which distinguishes different types of chemical bonding, reveals that orthorhombic GeSe employs covalent bonding, while rhombohedral and cubic GeSe utilize MVB. The transition from covalent to MVB goes along with a pronounced improvement in thermoelectric performance. The failure or success of different dopants can be explained by this concept, which redefines doping rules and provides a "treasure map" to tailor p-bonded chalcogenides.

Metavalent Bonding in GeSe Leads to High Thermoelectric Performance

Orthorhombic GeSe is a promising thermoelectric material. However, large band gap and strong covalent bonding result in a low thermoelectric figure of merit, zT≈0.2. Here, we demonstrate a maximum zT≈1.35 at 627 K in p-type polycrystalline rhombohedral (GeSe)_0.9 (AgBiTe₂ )_0.1  , which is the highest value reported among GeSe based materials. The rhombohedral phase is stable in ambient conditions for x=0.8-0.29 in (GeSe)_1-x (AgBiTe₂ )_x  . The structural transformation accompanies change from covalent bonding in orthorhombic GeSe to metavalent bonding in rhombohedral (GeSe)_1-x (AgBiTe₂ )_x  . (GeSe)_0.9 (AgBiTe₂ )_0.1 has closely lying primary and secondary valence bands (within 0.25-0.30 eV), which results in high power factor 12.8 μW cm^-1  K^-2 at 627 K. It also exhibits intrinsically low lattice thermal conductivity (0.38 Wm^-1  K^-1 at 578 K). Theoretical phonon dispersion calculations reveal vicinity of a ferroelectric instability, with large anomalous Born effective charges and high optical dielectric constant, which, in concurrence with high effective coordination number, low band gap and moderate electrical conductivity, corroborate metavalent bonding in (GeSe)_0.9 (AgBiTe₂ )_0.1 . We confirmed the presence of low energy phonon modes and local ferroelectric domains using heat capacity measurement (3-30 K) and switching spectroscopy in piezoresponse force microscopy, respectively.