CONTRIBUTION OF VACANCY DEFECTS TO MASS-TRANSPORT IN ALKALI-HALIDES - ASSESSMENT USING THEORETICAL CALCULATIONS OF DEFECT ENERGIES

The authors have made detailed calculations of the energies of formation and migration of vacancy defects in those alkali halides with the rock-salt structure, using recently derived potentials based on the shell model. The computed Schottky and cation-vacancy activation energies are in good agreement with experiment. The calculated cation and anion activation energies are very similar; the anion values thus agree more nearly with the results of diffusion measurements than with the larger values deduced by assuming that the separate contributions of the two types of vacancy can determine the curvature of the plot of log( sigma T) against T-1 ( sigma is the conductivity and T the temperature). The observed curvature of the conductivity plot may therefore indicate that some other process contributes to ion transport.