The activation entropy for transport in ionic conductors

The magnitude of the entropy of activation for conduction in a variety of solid electrolytes has been estimated from data on ion hopping rates. In materials that show a disordering transition in the mobile ion sublattice, a correlation exists between the magnitudes of the enthalpy and entropy of activation obtained from conductivity data for the low temperature, ordered region. This correlation accounts for the Meyer-Neldel rule, an exponential correlation between conductivity pre-exponential factor and activation enthalpy observed in families of related materials. In the low temperature, ordered region of solid electrolyte behaviour, significant entropy terms give rise to enhanced ion hopping rates: it is proposed that this phenomenon be taken as an operational definition of “fast ion conduction”.