Evaluation of the mode-specific Grüneisen parameters (Supplemental Note 5) lends an explanation for the strong decline of the phonon gas channel (Figure ). Here, large Grüneisen parameters are found in the same frequency range in which the phonon gas channel contributions are the strongest (Figure ). Therefore, the extensive anharmonicity at low frequencies is believed to be the driving factor for the strong suppression of the phonon-channel and the subsequent transition to diffuson-dominated thermal transport. This was captured phenomenologically by our analytical scattering function. It is not surprising that such large Grüneisen parameters exist since anharmonic vibrations have been linked to ionic transport. Having assessed the vibrational character of phonon modes from thermal transport behavior, it is also possible to characterize the vibrational character directly from the mode eigenvectors.