Activation Energy of Substitutional Defects in Alkali Chlorides

Based on a semi‐continuum model, the electrostatic, the polarisation, and the repulsive energy change of a lattice are calculated numerically, as functions of four different types of relaxations for the first fourteen ions of a substitutional defect in the saddle point position in an alkali chloride crystal. It is found that for a particular value of displacement, the total energy change of the lattice is minimum. A study of variation of the angle for the relaxed ions suggests further lowering of the lattice energy corresponding to a particular value of the angular displacement. These results are utilised in calculating the saddle point energy, the migration energy, and the activation energy for the motion of a host ion and a defect ion in NaCl, KCl, and RbCl crystals.