Ultralow and glass-like lattice thermal conductivity in crystalline BaAg2Te2: Strong fourth-order anharmonicity and crucial diffusive thermal transport

Understanding the ultralow and glass-like lattice thermal conductivity (κ) of complex thermoelectric (TE) materials is challenging due to the intrinsic complexity that bridges glasses and crystals. Herein, we study the ultralow and glass-like κ of BaAg2Te2, a promising high-performance thermoelectric material with complex crystal structure. With a first-principles-based machine-learning potential, we thoroughly investigate the thermal transport mechanisms of BaAg2Te2 using the perturbation theory up to the fourth-order anharmonicity and molecular dynamics simulations. We find the strong four-phonon processes are crucial to determine the ultralow κ. We demonstrate the breakdown of conventional Peierls-Boltzmann theory and the dominance of diffusive thermal transport in BaAg2Te2. This two channel thermal transport behavior directly unearths the nature of the nearly temperature-independent κ of BaAg2Te2. The origin of the dominant diffusive thermal transport is further revealed using the unified theory. Our work paves an avenue to better understand the ultralow and glass-like κ of complex crystals.