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.

由于连接玻璃和晶体的内在复杂性，了解复杂热电 (TE) 材料的超低和玻璃状晶格热导率 ( κ ) 具有挑战性。在此，我们研究了BaAg ₂ Te ₂的超低和玻璃状κ，这是一种具有复杂晶体结构的高性能热电材料。凭借基于第一性原理的机器学习潜力，我们使用高达四阶非谐和分子动力学模拟的微扰理论彻底研究了 BaAg ₂ Te ₂的热传输机制。我们发现强大的四声子过程对于确定超低κ. 我们证明了传统 Peierls-Boltzmann 理论的崩溃和 BaAg ₂ Te ₂中扩散热传输的优势。这两个通道的热传输行为直接揭示了 BaAg ₂ Te ₂几乎与温度无关的κ的性质。使用统一理论进一步揭示了主要扩散热传输的起源。我们的工作为更好地理解复杂晶体的超低和玻璃状κ铺平了道路。