The role of impurities on the morphology of NaCl crystals

Abstract

It is well known that crystal growth and morphology are largely influenced by the presence of impurities in the growth solution. However, little is known about the actual process of impurity interaction with the growing crystal surface. In this thesis we study this influence in detail using the NaCl crystal as a model system. Attention is mainly focused on the morphology and atomic scale structure of the {111} faces, which from a theoretical point of view should not be stable. In fact, NaCl is one of the first crystals for which the influence of an impurity was observed and investigated. Already in 1783 Rome de l’Isle reported that octahedrons instead of normal cubes are formed if rock salt is grown in the presence of urea. NaCl crystals grown from solution with formamide (or with small concentrations of CdCl2) as impurity have the same morphology as in the presence of urea. Our atomic force microscopy and surface X-ray diffraction observations show this {111} surface to be without surface reconstruction, thus the stability mechanism proposed earlier do not occur. Our observations lead to the conclusion the main stabilizing mechanism for the polar {111} faces of NaCl in solution is the formation of a charge-compensating ion layer in the first liquid monolayer of the solution on top of an unreconstructed crystal surface. To achieve stabilization, nature thus chooses the formation of an electrochemical double layer. Since formamide is structurally similar to urea we suggest that an analogous model explains the old observation of Rome de l’Isle.