CC BY 4.0 UnportedShen, XingchenOuyang, NiuchangHuang, YulingTung, Yung‐HsiangYang, Chun‐ChuenFaizan, MuhammadPerez, NicolasHe, RanSotnikov, AndreiWilla, KristinWang, ChenChen, YueGuilmeau, Emmanuel2024-05-102024-05-102024https://oa.tib.eu/renate/handle/123456789/14594https://doi.org/10.34657/13625Due to their amorphous-like ultralow lattice thermal conductivity both below and above the superionic phase transition, crystalline Cu- and Ag-based superionic argyrodites have garnered widespread attention as promising thermoelectric materials. However, despite their intriguing properties, quantifying their lattice thermal conductivities and a comprehensive understanding of the microscopic dynamics that drive these extraordinary properties are still lacking. Here, an integrated experimental and theoretical approach is adopted to reveal the presence of Cu-dominated low-energy optical phonons in the Cu-based argyrodite Cu7PS6. These phonons yield strong acoustic-optical phonon scattering through avoided crossing, enabling ultralow lattice thermal conductivity. The Unified Theory of thermal transport is employed to analyze heat conduction and successfully reproduce the experimental amorphous-like ultralow lattice thermal conductivities, ranging from 0.43 to 0.58 W m−1 K−1, in the temperature range of 100–400 K. The study reveals that the amorphous-like ultralow thermal conductivity of Cu7PS6 stems from a significantly dominant wave-like conduction mechanism. Moreover, the simulations elucidate the wave-like thermal transport mainly results from the contribution of Cu-associated low-energy overlapping optical phonons. This study highlights the crucial role of low-energy and overlapping optical modes in facilitating amorphous-like ultralow thermal transport, providing a thorough understanding of the underlying complex dynamics of argyrodites.enghttps://creativecommons.org/licenses/by/4.0500600624amorphous-like ultralow thermal transportargyrodite Cu7PS6crystal structureCu Diffusionlattice dynamicsArticle