Structure of the {1 1 1} NaCl crystal surface grown from solution in the presence of CdCl2

NaCl crystals grown from solution with small concentrations of CdCl2 as impurity have a shape determined by {1 1 1} faces instead of the normal {1 0 0} morphology. Optical microscopy, atomic force microscopy and surface X-ray diffraction (SXRD) observations show the {1 1 1} surface to be atomically smooth without surface reconstruction. The growth proceeds by monomolecular and higher steps, that originate from 2D nucleation starting from the edge of the crystal as well as from spiral growth. In situ SXRD observation also shows that at an atomic scale the {1 1 1} NaCl surface is similar to its bulk face truncated along the {1 1 1} plane. Analysis based on the SXRD results and electrical double layer theory leads to the conclusion that the polar {1 1 1} NaCl surface is stabilized by a mixed monolayer of Cd2+ (occupancy 0.25) and water (occupancy 0.75) in direct contact with the top Cl− layers of the NaCl {1 1 1} surface underneath.