Alignments are defined to be nonnegative, such that phonon mode displacement vectors that are parallel to a given displacement field will yield equal alignments as those that are antiparallel. Antiparallel and parallel displacement vectors are equally favorable for ion migration, since any displacement that is antiparallel will be parallel at a different time or in a different unit cell. The data in (a) presented with additional data from Muy et al. and Sagotra et al. The black dashed trend lines were obtained by least-squares linear regression.