This study systematically investigates the lattice thermal transport properties of layered ternary chalcogenides ${X\text{Sc}Y}_2$ ($X$ = Li, Na; $Y$ = Se, Te) using a unified thermal transport theory that combines self-consistent phonon calculations with the Wigner transport equation. An anomalous mass effect is first identified, where the lighter ${\mathrm{LiScTe}}_2$ exhibits a lower particle-like thermal conductivity (${\kappa }_{\mathrm{p}}$) than ${\mathrm{NaScTe}}_2$, originating from enhanced low-frequency anharmonicity associated with weakened Sc–Te bonding, acoustic–optical coupling, and enlarged phonon gaps. Beyond this particle-like picture, we uncover a distinct high-frequency mechanism governing cross-plane heat transport. Cross-gap hybridization enhances the Born effective charges, reinforcing lattice anharmonicity, while the rattling of light Li ions together with a large phonon band gap induces strong high-frequency phonon scattering, particularly four-phonon scattering, resulting in pronounced phonon linewidth broadening that significantly enhances wave-like coherent phonon transport (${\kappa }_c$) along the out-of-plane direction. Consequently, in ${\mathrm{LiScTe}}_2$ at 700 K, ${\kappa }_c$ contributes nearly 50% of the total lattice thermal conductivity ${\kappa }_L$ and becomes dominant at higher temperatures. As a result, the anomalous mass effect breaks down along the out-of-plane direction at elevated temperatures, despite persisting in plane. Incorporating ${\kappa }_c$ leads to good agreement between calculated and experimental ${\kappa }_L$ for ${\mathrm{NaScTe}}_2$ and ${\mathrm{NaScSe}}_2$. These findings highlight the critical role of rattling-induced high-order anharmonicity and coherent phonon transport in governing cross-plane heat transport in layered systems.

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• Lattice thermal conductivity
• Thermal transport
• Chalcogenides
• Layered crystals
• Density functional calculations