Dislocation Energies in NaCl

The energies for both screw and edge dislocations in rock salt have been investigated. The effect of elastic anisotropy has been incorporated into the contribution from the region outside the core. Detailed calculations have been carried out for the energies of the cores themselves as a function of radius, and joined smoothly to the curves of the elastic theory. The core calculations are based on the Born-Mayer model and employ the formulas of Madelung for the potentials of rows of uniformly spaced charges.

For dislocations in the observed plane of slip (110), the constant term associated with the core energy is 1.0×${10}^7$ ev/cm more for the edge than for the screw. Approximate calculations show this term to be appreciably larger for the edge dislocation in the (100) plane. Also, there appears to be large lattice potential barrier for dislocation motion in this plane arising from anion closed shell repulsion. This result may explain why these planes, though close packed, are generally not active in glide for alkali halides. The stability of dislocations with Burger vector longer than the minimum lattice translation is investigated. The possibility of hollow dislocations is also considered.