A theoretical study of H-centre migration in alkali halide crystals KCl and NaCl

Two different theoretical techniques, ab initio Hartree-Fock cluster calculations, and that of atom-atom potentials, as implemented in the CASCADE code, are employed for simulating on-site and saddle-point positions of primary Frenkel defects (H centres) in NaCl and KCl crystals. In both types of calculations H centres are found to be oriented along <111>, which agrees with experimental data for NaCl but contradicts that for KCl. Reasons for the latter result are discussed. The activation energy for H-centre migration obtained by both methods agrees with experiment. The conformation of the saddle point is analysed.