The activation barrier 𝐸 A for ionic transport that is defined from the temperature dependence of ionic diffusion (e.g. the diffusion coefficient 𝐷), 𝐸 A = -d ln(𝐷)/d(1/k B 𝑇), is a thermodynamic quantity. That is, as a macroscopic quantity, it represents some average barrier to transport considering all possible ion jump conditions under well-defined experimental parameters. In general, the microscopic activation barriers for each ionic jump can vary widely depending on factors such as the ion coordination environment, diffusion pathways and ion-ion correlation effects. Thus, any observed changes in 𝐸 A inherently reflect the underlying microscopic mechanisms (Figure ). In many cases, these changes in 𝐸 A can be attributed to structural changes, such as changes in the interatomic distances that define the jump pathway. However, there is growing evidence that changes in the dynamical properties of solids (e.g. bonding) are important in some fast ionic conductors. In this case, 𝐸 A is correlated to frequencies of atomic vibrations (i.e. phonon modes), particularly those of the mobile ion. The general finding is that lower frequency vibrations coincide with lower activation barriers to ion transport. This observation has been rationalized by considering that a more polarizable anionic host lattice should lead to weaker (softer) bonding interactions that govern the mobile ion. Weaker bonds are easier to stretch/break (contributing to a lower activation barrier) and also result in overall lower vibrational frequencies.