out the past five decades. Features related to phonon frequencies are useful for screening purposes because they can be measured relatively easily. Phonon frequencies can relate to both the pre-exponential factor in the Arrhenius expression for ion migration, which is often referred to as the "attempt frequency," and the migration barrier. In some cases, these two quantities are positively correlated with each other, such that increases in conductivity associated with reductions in the migration barrier are offset to some extent by reductions in the pre-exponential factor, though the strength of this correlation can vary. This phenomenon is known as the Meyer-Neldel rule. The present work focuses only on the migration barrier, as previous work suggests that this is the primary factor determining ionic conductivity in multivalent ion conductors, though further investigation is needed to confirm this. There are physical reasons to expect lower phonon frequencies to be associated with lower migration barriers (Figure ). According to the Clausius-Mossotti and Lyddane-Sachs-Teller relations, lattices comprised of more polarizable atoms exhibit lower frequencies. Increased polarizability results in weaker interactions within the lattice, which can reduce ion migration barriers. It has also been argued that lower phonon frequencies are associated with larger displacements of ions from their equilibrium positions, which can facilitate migration. This argument is supported by the equipartition theorem, which states that the average thermal displacement of a harmonic phonon mode is inversely proportional to its frequency. Relationships between phonons and ion migration have been investigated both from a data-driven perspective, i.e. by assessing correlations among many materials between phonon features, ion migration barriers, and pre-exponential factors, and from a mechanistic perspective, i.e. through detailed atomistic simulations. Correlations between frequencies of specific phonons and migration barriers have been observed in a variety of materials. More recent work on Li-ion conductors has focused on the phonon band center, i.e. the mean frequency of all phonons. Muy et al. and Krauskopf et al. reported linear relationships between experimentally measured phonon band cen-ters and ion migration enthalpies among materials derived from LISICON. Motivated by these findings, Muy et al. used calculated phonon band centers to screen a database of Li-containing materials for candidates with high ionic conductivity. In contrast, Sagotra et al. did not find such a correlation among a more diverse set of five Li-ion conductors, which raises questions about the generality of these trends. The reader is referred to refs 67 and 58 for a more comprehensive review of this history.