Within the framework of the unified theory thermal transport model, the competing contributions of coherent and incoherent terms create a trade-off relationship, posing substantial challenges to achieving a reduction in overall κL. In this work, we theoretically demonstrate that the superionic crystals X6Re6S8I8 (X = Rb, Cs) exhibit ultralow glass-like and particle-like thermal conductivities. The weak interactions between free alkali metal ions X+ (X = Rb, Cs) and I− anions induce pronounced lattice anharmonicity, which enhances phonon scattering and suppresses group velocities, thereby reducing the particle-like thermal conductivity (κp). Concurrently, the significant bonding heterogeneity within the [Re6S8I6]4− clusters promotes phonon dispersion flattening and low-frequency phonon localization. The resulting discretized phonon flat bands substantially diminish the glass-like thermal conductivity (κc). At room temperature, the total κL of X6Re6S8I8 (X = Rb, Cs) falls below 0.2 Wm−1K−1. Furthermore, the bonding characteristics between X+ and I−1 anions induce an anomalous cation mass-independent stiffening of low-frequency phonon branches in this system, resulting in counterintuitive thermal transport behavior. This work elucidates fundamental mechanisms governing heat transfer in ultralow κL materials and establishes novel pathways for transcending conventional thermal conductivity limitations.
@article{arxiv.2504.02187,
title = {Decoupling Coherent and Particle-like Phonon Transport through Bonding Hierarchy in Soft Superionic Crystals},
author = {Wenjie Xiong and Hao Huang and Yu Wu and Xinji Xu and Geng Li and Zonglin Gu and Shuming Zeng},
journal= {arXiv preprint arXiv:2504.02187},
year = {2025}
}