The Meyer–Neldel rule for a property determined by two transport mechanisms

We propose that the Meyer–Neldel rule (MNR) arises naturally for a quantity where both an intrinsic process as well as a process involving impurities contribute. The strength of the latter depends solely on the density of the impurities. This leads to a spread in the apparent activation energy of the measured quantity and the observation of the MNR, even though the intrinsic processes have fixed activation energies. A consequence of the MNR is the occurrence of a temperature TMN where a measured parameter is independent of the activation energy. For the system studied, the MNR does not accurately predict the results at temperatures larger than TMN. Our model for the MNR is supported by experimental data and it also can explain the inverse MNR for low activation energies.