Edge and surface effects in the measurement of electrical conductivity in ionic crystals

Attention is drawn to two edge effects, which if neglected, introduce substantial error in the analysis of electrical conductivity data for ionic crystals. Firstly, where the area of the electrodes does not exceed the total specimen area the effect of field fringing must be accounted for, and secondly, because of the importance of surface conduction the use of guard electrodes is essential to ensure that the conductance measured is that of the crystal bulk alone. A simple expression for the field fringing correction is developed for the first case, and in the second it is shown that the suppression of surface conductivity results in a decrease in the calculated enthalpy of motion of a cation vacancy and an increase in the calculated enthalpy of formation of a Schottky vacancy pair.