Association Energy of Vacancies and Impurities with Edge Dislocations in NaCl

Abstract

Calculations have been made of the energies of association of ${\mathrm{Sr}}^{++}$ ions, ${\mathrm{Cd}}^{++}$ ions, and positive-ion vacancies with the core of a (110) edge dislocation in NaCl. It is found that the vacancy has an association energy of 0.4 ev, but that other impurities have smaller association energies. On the basis of this energy of association, the numbers of vacancies and divalent ion-vacancy complexes pinned in the cores of the (110) edge dislocations in a well-annealed crystal were calculated. These numbers are large enough at room temperature to suggest that impurity and vacancy pinning might be the agent determining the critical yield stress in annealed NaCl. The elastic field interaction between point defects and a dislocation is tabulated and compared with the interaction obtained within the core. It is shown that the elastic energy of interaction does not necessarily project into the core in a smooth fashion, but that in the case of the vacancy, an elastically "large" inclusion behaves like a "small" inclusion in the core. Several possible suggestions are made which may explain the strong hardening effect of divalent impurity additions to NaCl.