Anisotropic lattice thermal conductivity in Sn2S3: Role of rattling modes in phonon transport

The interplay between structure distortion and lattice dynamics provides a compelling strategy to suppress phonon transport in thermoelectric materials. In this work, we explore the origin of highly anisotropic thermal conductivity in ${\mathrm{Sn}}_{2}{\mathrm{S}}_{3}$ through first-principles calculations combined with the Wigner transport equation (WTE). We find that the stereochemically active lone-pair electrons of ${\mathrm{Sn}}^{2+}$ pronounced local structural distortions, characterized by asymmetric Sn--S bond angles and weakened bonding configurations. These distortions activate rattling modes, which flatten vibrational branches, enhance phonon scattering, and reduce group velocities, particularly along the $x$ and $z$ directions. In contrast, the $y$ direction supports more coherent and dispersive phonon transport, leading to markedly higher thermal conductivity. Additionally, the influence of rattling extends to coherent phonon transport, yielding a complex frequency-dependent interplay between particlelike and wavelike heat conduction. Our results establish a direct link between lone-pair-induced anharmonicity and anisotropic phonon transport, offering a design framework for low-$\ensuremath{\kappa}$ materials beyond van der Waals or interlayer-driven strategies.