Using the average phonon energy ℏ𝜔 ̅ of the mobile ion and corresponding average critical phonon occupation number 𝑛 c seems to explain the origin of the isotope effect on activation barrier in these fast Li + conducting compounds (see schematic in Figure ). Foremost, phonon occupations of the vibrational modes of the mobile ions are fundamental to ion transport in these materials. This supports the concept that stimulated excitation of phonons can lead to higher ionic diffusivities. Within a harmonic oscillator perspective (Figure ), the isotope effect on 𝐸 A can be understood by the fact that lower frequency vibrations require more phonons to overcome the same energetic barrier. To demonstrate the concept behind Eq. 5, consider a fictitious material that has the same phonon spectrum as Li10SnP2S12. The average phonon energy for the Li compound is ℏ𝜔 ̅ 6 = 35.5