Influence of impurities on the etching of NaCl crystals

Abstract

The effect of CdCl₂, CuCl₂·2H₂O, MnCl₂·4H₂O and FeCl₃·6H₂O impurities and undersaturation on the rates of macroscopic dissolution,v _p, lateral etching away from a dislocation line,v _t, and normal etching along the dislocation line,v _n, and on the surface micromorphology of the {100} face of NaCl single crystals in water, methanol and 96% ethanol is investigated. The dependence of etch rates on impurity concentration,c _i, showed that the addition of a salt to the solvent always leads to a decrease inv _p, which attains a minimum value after a particular value ofc _i. The concentration dependence ofv _t andv _n is relatively complex, but often both decrease or increase simultaneously. A change in etch-pit morphology is caused by increasing the concentrations of all additives in ethanol and methanol. The dependence of etch rates on the undersaturation of methanol and methanol containing 10⁻³ M CdCl₂ showed that dislocation etch pits are formed only for undersaturations greater than 0.02 and 0.06, respectively. These results as well as the roughening of etched surfaces at low impurity concentrations, the formation of terraced etch pits and the difference between etch pits at aged and fresh dislocations are discussed.