Lone-pair-driven suppression of out-of-plane phonon transport and excellent near-room-temperature thermoelectric performance in layered hexagonal γ-GeSe.

Intrinsically low lattice thermal conductivity is essential for high-performance thermoelectrics. Recently, the hexagonal GeSe polymorph has been reported to host stereochemically active lone-pair electron states (LPEs), offering a promising platform for exploring ultralow thermal transport. However, the specific role of these LPEs in phonon dynamics remains unexplored. In this work, we report the excellent near-room-temperature thermoelectric performance of layered hexagonal γ-GeSe using first-principles calculations combined with Boltzmann transport theory. The lattice thermal conductivity is highly anisotropic, with an ultralow out-of-plane value of 0.12 W m-1 K-1 and a much larger in-plane value of 3.33 W m-1 K-1 at 300 K. The maximum out-of-plane ZT values are 2.43 at 400 K for n-type doping and 2.26 at 450 K for p-type doping. Our analysis identifies stereochemically active LPEs as the microscopic origin of the strongly suppressed out-of-plane phonon transport in γ-GeSe. Specifically, lone-pair-driven weak interlayer restoring forces and bond softening jointly give rise to extremely low phonon group velocities and strong anharmonicity, leading to the ultralow thermal conductivity. These results establish γ-GeSe as a promising candidate for near-room-temperature thermoelectric applications and clarify the important role of lone pairs in suppressing phonon heat transport, offering useful guidance for the design of thermoelectric materials with intrinsically low lattice thermal conductivity.