In this work, we first compute harmonic phonon spectra using finite difference methods for both α-Li 3 N and β-Li 3 N. Within the harmonic approximation we find that α-Li 3 N is the thermodynamically preferred phase above 300 K, and we therefore discount β-Li 3 N as playing a role in the superionic transition at T s ≈ 678 K. We then show that simple descriptors derived from this harmonic model, such as the phonon band-centre, are not able to describe the superionic transition, even when including a temperature-weighting. We then consider thermal expansion effects within the quasi-harmonic approximation (QHA). We again find that simple descriptors derived from the QHA are unable to describe the superionic transition of Li 3 N and conclude that thermal expansion is not sufficient to explain the superionic transition behaviour of Li 3 N. In contrast, within an anharmonic model, we observe a phonon breakdown above 678 K, which we interpret as a signature of the superionic transition, following previous experimental studies showing phonon breakdown at superionic transitions in CuCrSe 2 , AgCrSe 2 , and γ-LiAlO 2 . We propose that the origin of this phonon breakdown within our computational model is due a dramatically decreased residence time for the mobile lithium within individual sites, with this becoming comparable to their characteristic vibrational time period, which prevents the calculation of a well-defined harmonic restoring force for these ions.