Diffuson-mediated thermal transport. The ultra-low thermal conductivities of superionic conductors are often attributed to (static and dynamic) atomic disorder, soft bonding, lowfrequency optical phonon modes, anharmonicity and complex crystal structure. All of these parameters are associated with either lowering the phonon group-velocity or increasing the phonon scattering rate, in context of the phonon-gas model. While these structural and vibrational features are likely to factor into the characteristics of thermal transport, the vicinity to the Ioffe-Regel limit and the associated concept of minimum lattice thermal conductivity suggests the phonon-gas model is likely an incomplete description of thermal transport in these systems. 12 Two-channel modeling based on the calculated lattice dynamics for Ag8GeSe6 is utilized to explain these results. Here, the heat current operator matrix is analyzed regarding its diagonal (phonon-like) and off-diagonal (diffuson-like) elements such that the total lattice thermal conductivity of the two-channel model is = !" + $ %% . Given the computational cost of third and fourth order force constants, an analytical scattering model was implemented to describe the temperature dependence of the phonon lifetimes (details in Supplemental Note 9). The analytical model considers phonon-phonon scattering (& ! '( ) and boundary scattering (& ) '( ) resulting in a total scattering rate of