Growth and characteristics of the NaCl crystal surface grown from solution

Abstract

The morphology of {1 1 1} faces grown from water-formamide solutions as well as from pure water solutions was investigated. Surface patterns were examined ex situ and in situ using bright field and differential interference contrast optical microscopy and ex situ atomic force microscopy. It was shown that formamide and urea stabilize the {1 1 1} NaCl faces, whereas larger homologous molecules do not. For the {1 1 1} NaCl crystals growing from water-formamide solutions, it was observed that growth proceeds by monomolecular, stabilized layers of height d f1 1 1g , with most probably Na þ ions on top of Cl À ions. Steps originate from spiral-dislocation growth as well as from 2D nucleation starting from the edges of the crystal. Atomic resolution imaging of NaCl {1 1 1} showed no surface reconstruction. The {1 1 1} surfaces grown from pure water solutions showed developing of shallow growth hillocks with rounded tops. It is presumed that these hillocks are related to dislocation outcrops and growth proceeds close to the roughening temperature. Growth pits develop after a longer period of {1 1 1} surface growth in water solution. Their formation is explained by the presence of a semipermeable particle at the pit bottom, which locally retards the fast {1 1 1} growth.

Stability of the polar {111} NaCl crystal face

“…Using AFM we found the ͕111͖ NaCl surface to be atomically smooth and covered with steps of height d 111 , which means that the surface is either Cl − or Na + terminated. 3 The surface patterns did not change when the crystals were kept dry in a desiccator.…”

“…3 After separation from the growth solution, the crystals were quickly dried using a paper tissue. AFM was used to check the surface prior to x-ray diffraction.…”

“…The experiments were carried out at room temperature. The addition of 30% formamide to an aqueous solution is sufficient to stabilize the ͕111͖ face, 3 but in these vapor pressure experiments we used pure vapors in order to have an unambiguous composition of the liquid film. The experimental setup consists of a cell in which the crystal surface can be examined in a well-controlled atmosphere.…”

“…This agrees with our AFM data. 3 In order to fit the data, we need a model that describes the entire interface. Since our data set is limited, we further need to keep our model as simple as possible.…”

“…We have found that the addition of at least 20 wt % of formamide to the aqueous growth solution has the same effect. 3,4 This transition from the common ͕100͖ face to the octahedral or ͕111͖ face is an intriguing process, because the polar ͕111͖ face is not expected to be stable. Along the ͓111͔ direction the NaCl crystal consists of alternating layers of cations and anions.…”