Abstract:
The possibility to obtain {111} surfaces on NaCl crystals grown from
pure water solutions was investigated in order to verify some early observations.
Repeating the same experiments as reported by Kern in 1953 (Bull. Soc.
Fr. Mineral Cristallogr. 76 (1953) 391) followed by detailed examination
using optical microscopy and optical goniometry revealed that, independent
of supersaturation, {001} is the only stable form. We also studied the
morphology of the alkali halide crystals obtained from formamide solutions.
We noticed that the appearance of the octahedral form is strictly related
to unit cell size. Octahedrons appear starting from NaF crystals with unit
cell size 0.462 nm up to KCl with a unit cell of 0.628 nm. All the alkali
halide crystals with dimensions outside this range are cubes. In addition
to rocksalt structure crystals, a new, non-cubic pseudo-polymorph was obtained
for several alkali halide crystals grown from formamide solution. For sodium
iodide this new compound was identified as NaI-1 (formamide). In addition
(to this and our recent paper (Surf. Sci. 523 (2003) 307)), we tried to
obtain NaCl crystals with {111} faces using pyridine, aniline and glycine.
In the case of pyridine and anyline we only observed cubes, whereas from
aqueous solutions containing glycine we obtained crystals with well-developed
faces with an average {110} orientation.