Formation of Beaks at Dislocation Sites in Sodium Chloride Crystals

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Abstract
Beaks, solution channels, etc., can be obtained on NaCl cleavage faces by an etchant consisting of methanol and cadmium chloride. The beak formation is described in detail and the mechanism for beak formation is discussed. Etching parameters, such as the concentration of the poison and understuration of the etchant, have been varied, and their effect on the beak formation is studied. At higher concentrations of the poison, as well as at low undersaturations of the etchant, beak formation was found to be predominant. Vigorous agitation of the etchant was found to suppress the beak formation. Beaks are always associated with the aged dislocations. The etch pits due to fresh dislocations do not have beaks, and therefore it is concluded that segregated impurities at the aged dislocations help in the rapid dissolution along the dislocations. If the dissolution rate along the aged dislocation VAD exceeds the velocity of steps VS across the surface, beaks can be formed. Etch pits associated with the beaks appear like funnels when observed from the side.