Amorphous‐Like Ultralow Thermal Transport in Crystalline Argyrodite Cu₇PS₆

Abstract

Due 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.