in which 𝜔 ̅ H is the average frequency of the heavier isotope, 𝜔 ̅ L is the average frequency of the lighter isotope, 𝐸 A is the reference activation barrier of the lighter isotope and 𝐶 is a constant that phenomenologically relates the average frequencies to the transport-weighted average frequencies (Supplemental Note 4). Our experimental data is well-described using the theoretical ratio for 𝜔 ̅ 7 /𝜔 ̅ 6 (Eq. 1) and the value 𝐶 = 2 in Eq. 6 (dashed line in Figure ). Further work is required to determine the frequency-specific contributions to ionic transport in these materials, but it has been demonstrated in Li3PO4 that not all frequencies contribute equally. The temperature dependence of average phonon occupations is also likely a factor. Nevertheless, Eqs. 5 and 6 provide new insights for understanding ion transport from a phonon perspective, as well as a starting point for continued investigations of isotope effects in Li + conductors.