To calculate phonon lifetimes at different temperatures from the MLMD trajectories, we used DY-NAPHOPY . The methodology for projection onto bulk harmonic phonon eigenvectors and the associated linewidth calculations is described in Supplementary Note I. We find that this approach, used in our previous report for shorter MD timescales, is not suitable when non-negligible diffusion occurs in the simulation. These projections onto bulk phonon modes cannot be used to discriminate between normal and superionic diffusion regimes. This point is discussed further in the Supplementary Note I.