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 (<i>N</i>_V) 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 GeSe_1-<i>x</i>Te_<i>x</i> for <i>x</i> ≥ 0.3 at room temperature. The increase of crystal symmetry not only boosts <i>N</i>_V from 2 for orthorhombic GeSe to 9 for rhombohedral GeSe_1-<i>x</i>Te_<i>x</i> 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 <i>x</i> = 0.5). Surprisingly, despite their higher crystallographic symmetry, rhombohedral GeSe_1-<i>x</i>Te_<i>x</i> compounds display even lower lattice thermal conductivities (∼1.0 W m^-1 K^-1 at 300 K for <i>x</i> = 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 <i>ZT</i> value of ∼1.1 at 638 K is achieved in rhombohedral GeSe_0.5Te_0.5, which already outperforms GeTe. This work provides an avenue for engineering the thermoelectric properties of low-symmetry compounds containing lone-pair electrons.