Roles of phonon amplitude and low-energy optical phonons on superionic conduction

From a simple equation of motion for a linear chain, the phonon amplitudes for mobile and cage ions are represented in terms of their ionic radius and atomic mass. Estimated values of the amplitude in superionic conductors are larger for the mobile ions than the cage ions. Based on this result, many binary and ternary ionic conductors are classified into superionic or normal-ionic conductors. The role of low-energy optical (LEO) phonon is to enhance the phonon amplitude and assist the jump of mobile ions. This is ascertained from the relationship between the frequency of the LEO phonon and the activation energy, which is similar to self-diffusion in bcc metals.