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 (kappa) 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 kappa 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 kappa. 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 kappa 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 kappa of complex crystals. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

The study investigates the thermal transport mechanisms of the high-performance thermoelectric material BaAg2Te2, revealing the crucial role of strong four-phonon processes in determining the ultralow thermal conductivity. The breakdown of conventional Peierls-Boltzmann theory and the dominance of diffusive thermal transport in BaAg2Te2 are demonstrated, shedding light on the nearly temperature-independent nature of its kappa.