Once in the superionic regime, Li ions are not restricted to their regular lattice sites and diffuse, Figure . The mobile Li ions only vibrate for a short time at each individual lattice site before undergoing a diffusion event; this picture is supported by the large calculated average linewidth at 678 K. In a conventional "hopping" regime, where the average residence time for the mobile ions at individual sites is expected to be significantly greater than the average vibrational period , long-range vibrational order is preserved and the average motion of ions can still be described by a set of normal-modes. We also note that ions occupying interstitial positions may disrupt the long-ranged coherence of any lattice vibrations in a supercell model, which will further contribute to the reduction in mode lifetimes.