This behaviour is consistent with previous experimental studies in other solid electrolytes where the superionic transition has been found to be accompanied by an increase in phonon linewidth . In contrast to these experimental studies, where the phonon breakdown at the superionic transition was associated with the breakdown of a particular mode, we are not able to assign a single-phonon breakdown at this stage. Instead, as shown in Figure (SI), our model predicts a general phonon breakdown that involves all phonon modes at all q. In short, the lattice dynamics description has broken down in α-Li 3 N at the superionic transition.