Anomalous in-plane thermal conductivity suppression and dominant optical phonon transport in SnSe2/SnTe2 superlattice

Superlattice engineering is a well-established strategy for reducing lattice thermal conductivity (κL), typically achieving stronger suppression perpendicular to the interfaces. In this study, we employ density functional theory and the phonon Boltzmann transport equation to investigate thermal transport in bulk SnSe2 and SnSe2/SnM2 (M = S, Te) superlattices. Counterintuitively, our results reveal that the SnSe2/SnTe2 superlattice exhibits a more pronounced in-plane κL reduction (83.68%) than that in the out-of-plane direction (77.3%), reversing the conventional out-of-plane-dominated κL suppression phenomenon. Furthermore, optical phonons rather than acoustic ones dominate the thermal transport in SnSe2/SnTe2 superlattice, contributing up to 73.5% and 64.1% to κL along the in-plane and out-of-plane directions, respectively. These anomalous behaviors are attributed to additional phonon scattering channels arising from an in-plane avoided-crossing point in the phonon dispersion, as well as the marked enhancement of in-plane acoustic phonon anharmonicity. This work not only advances the understanding of phonon transport mechanisms in superlattices but also provides a novel perspective to effectively suppress in-plane lattice thermal conductivity in layered materials.