Formamide adsorption and habit changes of alkali halide crystals grown from solutions

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. (C) 2003 Elsevier B.V. All rights reserved.