Anharmonic Phonon Vibrations of Ag and Cu for Poor Lattice Thermal Conductivity in Superionic AgCrSe₂ and CuCrSe₂

Solids generally possess higher thermal conductivity compared to liquids, as heat energy in solids is conducted through both transverse and longitudinal vibrations, whereas, the heat conduction in liquids is mainly reliant on the longitudinal vibrations. Here, we report the ultralow lattice thermal conductivity (k_l) and high thermoelectric (TE) performance in layered Ag­(Cu)­CrSe₂ attributed to the strong crystal anharmonicity caused by the weakly bounded Ag­(Cu)⁺ ions, following phonon-liquid electron-crystal concept. The low-frequency Raman vibrations vanish as the kinetically disordered state due to Ag­(Cu)⁺ ions starts appearing, showing the influence toward the vibrational properties. The strong acoustic-low energetic optical interactions, poor transverse and mean sound velocities, low-frequency rattling-like vibrations, a broad boson-like peak, and high Grüneisen parameter make AgCrSe₂ a poorer thermal conductor than CuCrSe₂. Overall, at high temperatures, we observed ultralow k_l ∼ 0.3 W m^–1 K^–1 for AgCrSe₂ and ∼0.5 W m^–1 K^–1 for CuCrSe₂, along with high TE figures of merit ∼0.83 and ∼0.61, respectively, indicating their potential suitability for applications requiring poor thermal conductivity and high TE performance.