Ionic transport in conventional (non-superionic) solids is usually considered as being effected by a sequence of discrete "hops", where individual mobile ions undergo stochastic moves between available crystallographic sites . While this model is usually appropriate for ionic solids with low-to-medium ionic conductivities, fast ionic conduction is often associated with highlyconcerted ionic motion, in which groups of ions undergo cooperative near-simultaneous motion . This observational correlation between fast-ion conduction and cooperative ion-transport mechanisms suggests that ionic motion in superionic conductors might be better described in terms of appropriate collective degrees of freedom, rather than independent single-atom coordinates . One appealing choice of basis for describing collective dynamics in a crystal is the set of normal modes of vibration, which correspond to phonon eigenvectors. Because phonons can be directly probed using established experimental methods, theoretical descriptions of superionic conduction in terms of phonon behaviour can, in principle, also be compared directly to experimental data for real materials.