Impact of Quartic Anharmonicity on Lattice Thermal Transport in SnSe

Layered SnSe has demonstrated exceptional thermoelectric properties with record high energy conversion efficiency, majorly owning to the ultralow lattice thermal conductivity. However, a fundamental understanding of the lattice dynamics and thermal transport properties is still lacking, particularly for the high temperature phase. The theoretical challenge originates from the second-order phase transition from low-temperature Pnma to high-temperature Cmcm phases between 700-800~K, wherein the Cmcm-SnSe displays lattice instability and imaginary phonon frequencies. To overcome this limitation, we go beyond harmonic approximation by further incorporating anharmonic phonon renormalization due to quartic anharmonicity. Moreover, both three- and four-phonon scatterings are accounted for in solving Phonon Boltzmann transport equation. We apply this strategy to perform a comparative study of lattice dynamics and thermal transport properties of SnSe at 300~K (Pnma) and 800~K (Cmcm). We reveal in detail the impacts of quartic anharmonicity on lattice stability, phonon quasiparticle energies and lattice thermal conductivities of both Pnma- and Cmcm-SnSe. Our theoretical calcualtions are in good agreement with experimental measurements performed on SnSe single crystals.