Schottky Defects in Alkali Halides: Their Creation, Interaction, and Migration

Schottky defect energies of alkali halides are calculated by the force balance method employing a modified Born model with higher than van der Waals terms and appropriately adjusted repulsive terms. The energies for various potassium and rubidium halides of NaCl structure show good agreement with the experimental values. Schottky defect energies of CsCl are calculated for the CsCl and the NaCl structures. The calculated value (≈︁ 1 eV) for the CsCl structure is in excellent agreement with the recent experimental value. The Schottky energy of CsCl in the NaCl structure is about 2 eV. The ground state interaction energy between a positive and a negative ion vacancy is determined with the inclusion of van der Waals terms. The values obtained for various alkali halides are in fair agreement with the previous ones. The migration barriers for single and pairs of vacancies in KCl and RbCl are calculated by an energy minimization procedure. The barrier heights compare nicely with the experimental value and reflect the expected trend in various parameters. The values also show that the activation energies for the migration of double vacancies and single vacancies are nearly the same; this result contradicts the generally accepted statement that migration of pairs is less energetic, but supports the recent proposal of Lidiard who obtained this result by employing different repulsive energy expressions in the different interionic regions.