Decoupling Ultralow Coherent and Particle-Like Phonon Transport via Bonding Hierarchy in Soft Superionic Crystals

Within the unified theoretical framework for thermal transport, the inherent interplay between coherent tunneling and propagative phonon mechanisms establishes an antagonistic relationship, thereby imposing fundamental limitations on suppressing lattice thermal conductivity ${\kappa }_L$ . In this work, it is 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 ( ${\kappa }_p$ ). Concurrently, the significant bonding heterogeneity within and between the [Re₆S₈I₆]^{4 -} clusters promotes phonon band flattening and low-frequency phonon localization. The resulting discretized phonon flat bands substantially diminish the glass-like thermal conductivity ( ${\kappa }_c$ ). At room temperature, the total ${\kappa }_L$ of X₆Re₆S₈I₈ (X = Rb, Cs) falls below 0.2 W m^-1 K^-1. Furthermore, the bonding characteristics between X⁺ and I^- 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 ${\kappa }_L$ materials and establishes novel pathways for transcending conventional thermal conductivity limitations.