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 T-MN 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 T-MN. Our model for the MNR is supported by experimental data and it also can explain the inverse MNR for low activation energies. (C) 2002 American Institute of Physics.