Entropy of Vacancies in Ionic Crystals

The entropy of formation of vacancies in alkali halides has been calculated by computing the changes in lattice frequencies as a result of ionic displacements produced by the creation of vancancies using the simple Einstein model. The contributions of both the electric and the elastic parts have been considered in the calculation of ionic displacements. The calculated values of entropy for LiF, NaCl, KCl, and KBr are in fair agreement with experimental values. The contributions of the surface effects and the dipole interactions to the entropy value are also discussed, and found to be negligible.