Structure of
the {1 1 1} NaCl crystal surface grown from solution in the presence of
CdCl2
Neda Radenović, Willem van Enckevort, , Daniel Kaminski, Maurits Heijna
and Elias Vlieg
Abstract
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.