Decoupling Coherent and Particle-like Phonon Transport through Bonding Hierarchy in Soft Superionic Crystals

Abstract

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 $\hat{\mathrm{I}}{º}_{\mathrm{L}}$. In this work, we theoretically demonstrate that the superionic crystals X${}_6$Re${}_6$S${}_8$I${}_8$ (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 ($\hat{\mathrm{I}}{º}_{\mathrm{p}}$). Concurrently, the significant bonding heterogeneity within the [Re${}_6$S${}_8$I${}_6$]${}^{4-}$ clusters promotes phonon dispersion flattening and low-frequency phonon localization. The resulting discretized phonon flat bands substantially diminish the glass-like thermal conductivity ($\hat{\mathrm{I}}{º}_{\mathrm{c}}$). At room temperature, the total $\hat{\mathrm{I}}{º}_{\mathrm{L}}$ of X${}_6$Re${}_6$S${}_8$I${}_8$ (X = Rb, Cs) falls below 0.2 Wm${}^{-1}$K${}^{-1}$. 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 $\hat{\mathrm{I}}{º}_{\mathrm{L}}$ materials and establishes novel pathways for transcending conventional thermal conductivity limitations.